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Aerodynamic characteristics of flying fish in gliding flight
Abstract
The flying fish (family Exocoetidae) is an exceptional marine flying vertebrate, utilizing the advantages of moving in two different media, i.e. swimming in water and flying in air. Despite some physical limitations by moving in both water and air, the flying fish has evolved to have good aerodynamic designs (such as the hypertrophied fins and cylindrical body with a ventrally flattened surface) for proficient gliding flight. Hence, the morphological and behavioral adaptations of flying fish to aerial locomotion have attracted great interest from various fields including biology and aerodynamics. Several aspects of the flight of flying fish have been determined or conjectured from previous field observations and measurements of morphometric parameters. However, the detailed measurement of wing performance associated with its morphometry for identifying the characteristics of flight in flying fish has not been performed yet. Therefore, in the present study, we directly measure the aerodynamic forces and moment on darkedged-wing flying fish (Cypselurus hiraii) models and correlated them with morphological characteristics of wing (fin). The model configurations considered are: (1) both the pectoral and pelvic fins spread out, (2) only the pectoral fins spread with the pelvic fins folded, and (3) both fins folded. The role of the pelvic fins was found to increase the lift force and lift-to-drag ratio, which is confirmed by the jet-like flow structure existing between the pectoral and pelvic fins. With both the pectoral and pelvic fins spread, the longitudinal static stability is also more enhanced than that with the pelvic fins folded. For cases 1 and 2, the lift-to-drag ratio was maximum at attack angles of around 0 deg, where the attack angle is the angle between the longitudinal body axis and the flying direction. The lift coefficient is largest at attack angles around 30∼35 deg, at which the flying fish is observed to emerge from the sea surface. From glide polar, we find that the gliding performance of flying fish is comparable to those of bird wings such as the hawk, petrel and wood duck. However, the induced drag by strong wing-tip vortices is one of the dominant drag components. Finally, we examine ground effect on the aerodynamic forces of the gliding flying fish and find that the flying fish achieves the reduction of drag and increase of lift-to-drag ratio by flying close to the sea surface.
... Among many species exhibiting multi-modal behaviour, we are especially interested in the flying fish, which is a marine fish of the Exocoetidae family with unique aerial-aquatic transition capability. The studies of the flying fish in Breder (1929), Fish (1990), Davenport (1994), Park and Choi (2010), and Nelson et al. (2016) inspire researchers to develop robots that can fly after swimming, or swim after flying (Siddall and Kovač, 2014;Siddall et al., 2017;Zeng et al., 2022). As discussed in Davenport (1994), flying fish jump out of water with a speed of about 10.0 m/s, which is equivalent to (10− 20) body lengths per second. ...
... Although the unique dual-modal locomotion of flying fish has drawn much attention from scientists, only a few studies have addressed the aerodynamic characteristics of flying fish models. For example, Park and Choi (2010) performed a series of wind-tunnel experiments to investigate the aerodynamic performance of the gliding flight for several real flying fish models (Cypselurus hiraii). For the experiments, to maintain the body and wing geometries, the bodies of the fish models were hardened through injecting urethane foam inside the body, while fully-spread wings were fixed. ...
... This agrees with the previous observations on the body angle of flying fish during leaping from the sea surface and performing gliding, in which the largest lift is required, as reported by Hertel (1966) and Davenport (1994). Inspired by this work, the aerodynamic characteristics of the artificially designed flying fish models, whose geometric parameters were designed based on the real flying-fish models tested in the wind-tunnel experiment by Park and Choi (2010), were numerically studied using the Spalart-Allmaras Delayed Detached Eddy Simulation model (SA− DDES) (Deng et al., 2019a). They reported that the maximum C L of 1.03 is computed at an AoA of 35 • , while the maximum L/D of 4.7 is obtained at an AoA of 6 • . ...
Mimicking the unpowered gliding capabilities of flying fish is challenging, due to the various technical limitations that are involved in creating a dual-modal robot that can both swim underwater and fly in the air. In this work, we suggest a modified KUFish design equipped with a pair of foldable wings for gliding flight in the air. Although the current water-leaping speed of the KUFish is lower than that of flying fish, the robot may be able to lift off by taking advantage of a head wind and forces produced by tail-beating motion, compensating for its weight, and overcoming the drag. Our series of computational fluid dynamics simulations has shown that with the unfolded wings and fully submerged tail-beating motion, when the wing and body angles are maintained in specific ranges under the head wind speeds of (9.5 and 6.5) m/s, the robotic fish after water-leaping can perform efficient gliding flight without generating pitching moment. This work can also be used to explain how flying fish perform gliding flight under tail-beating motion, and to develop an actual model of the dual-modal robot that mimics flying fish in the future.
... To overcome these challenges, some researchers have opted for numerical and lab-based experimental methods. The lift, drag, and pitching moment coefficients of flying fish were first experimentally studied by catching and preserving a flying fish for wind tunnel measurements (Park and Choi 2010). A numerical study of flying fish aerodynamics that allowed more detailed visuals of the flow field was later developed (Deng et al. 2019). ...
... We evaluate biological relevance by comparing results from our own experiments to previously existing literature on flying fish aerodynamics. More specifically, we compare our lift, drag, and pitching moment results to numerical simulations from Deng et al. (2019) and experiments from Park and Choi (2010). Park and Choi's study is the only experimental aerodynamics study performed on biological flying fish. ...
... The RMO C L and C D data ( Fig. 5A and B) agree most with the numerical study (Deng et al. 2019). There is notably less agreement with the experimental study on the preserved fish (Park and Choi 2010), especially at lower angles of attack α < 8 ...
Flying fishes (family Exocoetidae) are known for achieving multi-modal locomotion through air and water. Previous work on understanding this animal’s aerodynamic and hydrodynamic nature has been based on observations, numerical simulations, or experiments on preserved dead fish, and has focused primarily on flying pectoral fins. The first half of this paper details the design and validation of a modular flying fish inspired robotic model organism (RMO). The second half delves into a parametric aerodynamic study of flying fish pelvic fins, which to date have not been studied in-depth. Using wind tunnel experiments at a Reynolds number of 30,000, we investigated the effect of the pelvic fin geometric parameters on aerodynamic efficiency and longitudinal stability. The pelvic fin parameters investigated in this study include the pelvic fin pitch angle and its location along the body. Results show that the aerodynamic efficiency is maximized for pelvic fins located directly behind the pectoral fins and is higher for more positive pitch angles. In contrast, pitching stability is neither achievable for positive pitching angles nor pelvic fins located directly below the pectoral fin. Thus, there is a clear a trade-off between stability and lift generation, and an optimal pelvic fin configuration depends on the flying fish locomotion stage, be it gliding, taxiing, or taking off. The results garnered from the RMO experiments are insightful for understanding the physics principles governing flying fish locomotion and designing flying fish inspired aerial-aquatic vehicles.
... Direct studies on aerodynamic performance of flying fish have rarely been reported. For the first and only time, Park and Choi [1] carried out wind-tunnel experiments to investigate the aerodynamic properties of a darkedged-wing flying fish (Cypselurus hiraii). In their experiments, real flying fish were used, of which the wings were fully spread and fixed, and urethane foam was injected into the body to maintain the geometries of wings and body. ...
... The studies on aerodynamic performance of flying fish are very few, despite the growing curiosities of people on their unique capability of aerial-aquatic locomotion, and the requirement of knowledge for the design of artificial aerial-aquatic robots. In this paper, we propose flying fish models, mimicing the darkedged-wing Cypselurus hiraii used in the previous wind tunnel experiments [1]. The aerodynamic coefficients and gliding performance will be investigated. ...
... Here, we consider three different models, as shown in figure 1. For model A, pelvic fins are removed, and the pectoral fins, which mimic that used in the previous experiments [1], are with an angle of leading edge sweep. The pectoral fins of both model B and model C are with straight leading edges, while the pelvic fins for mode C are kept, resembling the morphology of the 'four-winger' or 'biplane-type' [13]. ...
Flying fish is a family of unique aerial-aquatic animals, which can both swim in the water and glide over the sea surface. Most previous studies on their aerodynamic characteristics were based on field observations or measurements of their morphometric parameters. In the present study, we consider three different flying fish models, of which the preliminary one mimics the cypselurus hiraii in the pectoral fin morphology, following a previous wind tunnel experiment \citep{park2010aerodynamic}. Their aerodynamic performances are numerically studied by the computational fluid dynamics (CFD) method. The maximum lift force coefficient of 1.03 is reached at the angle of attack $\alpha=35^o$, and the maximum lift-to-drag ratio of 4.7 is achieved at $\alpha=6^o$. By choosing appropriately the center of gravity, the flying fish model is proved to be longitudinally stable, according to the negative slope of pitching moment profile. Furthermore, we build a three-degree-of-freedom (3-DOF) dynamic model in the longitudinal plane based on the aerodynamic coefficients obtained in our simulations, to predict its gliding performance. The results show that the flying fish can achieve a distance up to 45.4m, and reach a height of 13.2m, indicating an extraordinary gliding performance. Our numerical simulations are consistent with previous experimental results and theoretical prediction, which can be taken as the basis of further research on robotic flying fish.
... There are two categories of flying fish, namely, two-wingers and four-wingers. The difference, mainly, is two-wingers have enlarged pectoral fins, whereas four-wingers have both their pectoral and pelvic fins enlarged, as shown in Fig. 1 [7,8]. Fig. 2 shows the species of each category of diving birds and flying fish. ...
... The wings are rigid near the leading edge and flexible near the trailing edge. The flexible area is supported by fin rays making the area has a more uneven texture [8]. It should be noted that the length of the pectoral fins never surpasses about 78% of the flying fish's body length because while swimming, exiting and entering water the fish must keep its fins flushed against its body without disturbing the thrust of the tail [137]. ...
... During flight, the flying fish remains close to and almost parallel to the sea surface to obtain optimal and efficient performance [8]. The pectoral fins in two-wingers help increase the lift coefficient and both the pectoral and pelvic fins in four-wingers help maintain stability and augment the lift-to-drag ratio [8]. ...
... Since the aerodynamic performance of flying fish is comparable to that of birds in nature, the team led by Hyungmin Park utilized flying fish specimens, as depicted in Fig. 3d to conduct qualitative and quantitative analysis of the different angles of attack and lift coefficients generated by the pectoral and pelvic fins during gliding [43]. The folding of the pectoral and ventral fins before and after entering the water can significantly reduce the resistance of the entry process. ...
... The internal cavity of the flying squid generates a pressure difference through the contraction of surrounding muscles, allowing it to store and jet water. Although the efficiency of cross-medium locomotion achieved by jetting water is [43]. e Flying squid, ref [47] not as energy saving as the method used by dolphins, which involves flapping their pectoral fins and tail to leap out of the water [46], the mechanism of storing energy in jetting water enables the flying squid to perform cross-medium locomotion at faster speeds, exhibiting high maneuverability. ...
With the rapid development of unmanned aerial and underwater vehicles, various tasks, such as biodiversity monitoring, surveying, and mapping, as well as, search and rescue can now be completed in a single medium, either underwater or in the air. By systematically examining the water–air cross-medium locomotion of organisms, there has been growing interest in the development of aerial-aquatic vehicles. The goal of this review is to provide a detailed outline of the design and cross-medium theoretical research of the existing aerial-aquatic vehicles based on the research on the organisms capable of transiting between water and air. Although these designs and theoretical frameworks have been validated in many aerial-aquatic vehicles, there are still many problems that need to be addressed, such as inflexible underwater motion and unstable medium conversion. As a result, supplementation of the existing cross-medium biomimetic research, vehicle design, power selection, and cross-medium theory is urgently required to optimize the key technologies in detail. Therefore, by summarizing the existing designs and theoretical approaches on aerial-aquatic vehicles, including biomimetic research on water–air cross-medium locomotion in nature, different power selections, and cross-medium theoretical research, the relative problems and development trends on aerial-aquatic vehicles were thoroughly explored, providing significant help for the subsequent research process.
... Finally, kinematically and anatomically precise finlet models were constructed using yellowfin tuna video data [24]. Park H et al. [2010] aimed to offer an optimum design for a Caribbean-focused AUV. Deep sea divers' risks constitute a barrier to human exploration. ...
... Drag prediction was affected. This research presented a revolutionary design for the AUV's torpedo body, giving new possibilities for deep-water operations [25]. Benedetto Allotta et al. [2012] investigated flying fish, referred to as Exocoetidae. ...
The conceptual design, component selection, and deployment experiments of an unmanned amphibious system (US) with a unique Becker in vertical stabilizer based on hydrodynamic research are included in this work. The use of UAS is currently expanding significantly, and they are used for fish detection, oceanographic mapping, mining detection, monitoring marine life, and navy purposes. With a maximum forward speed of 30 m/s, the UAS's hull is largely built with criteria for identifying and researching marine species. The significant lifetime decline of ocean species drives the deployment of unmanned vehicles for species monitoring from the water's surface to 300 m below the surface. In addition, the medical team can help the species with health problems using this planned US because they have been identified. The conceptual design and estimated analytical equations encompass the fuselage, Becker rudder, propeller, and other sub-components. The locations of sensors, primarily used to locate mobile marine life, are also considered. A Becker rudder has been imposed to make sharp turns when the US is submerged in water. An advanced hydro propeller produces the propulsion with a 20 cm base diameter. Additionally, a piezoelectric patching-based energy-extracting approach is used for the hydro-outside propeller's surface. As a result, the electrical power generation for different lightweight materials is computed for the performance of US manoeuvrings. With the help of CATIA modelling of the intended USs and ANSYS Fluent hydrodynamic simulations, appropriate high-speed configurations are selected. Various stages of its mission profile, including the US in steady-level flight, the US in climb, and the UAS over the ocean surface, are subjected to computational simulations. Using an advanced computational technique and previously established experimental correlations, the reliability of these various computational solutions is examined and kept at an appropriate level. This UAS is highly suggested for marine-based real-time applications due to its acceptable output.
... Gliding pelicans have also been found to achieve significant energy savings from the ground effect (Hainsworth, 1988). A similar aerodynamic advantage was found for flying fish gliding close to the sea surface (Park and Choi, 2010). The ground effect is also observed in systems undergoing undulatory swimming with a deforming body or fin. ...
... The hydrodynamic benefits of the steady ground effect are a large lift force induced by a slow flow beneath a static lifting object and a smaller drag induced by the vortices between a flexible propulsor and the ground. These benefits of the steady ground effect are widely utilized by gliding birds (Park and Choi, 2010). The ground effects decrease the induced drag and increase the lift-to-drag ratio of the wings for birds gliding horizontally near the ground, with the proximity of the ground influencing the gliding performance. ...
Flapping motions of wings and fins are common in nature. Living organisms use such motions to float in a fluid or to propel themselves forward. Some entities, such as tadpoles, use distinct flexible components to generate propulsion. Here we introduce a propulsor consisting of a rigid circular head containing an energy source and a flexible fin for propulsion. The head imparts a sinusoidal torque to the leading edge of the fin and the flexible fin flaps along the leading edge. The flexible propulsor thus moves via an oscillating relative angle between the head and the leading edge of the fin. Unlike a self-propelled heaving and pitching fin, our ‘autonomous’ flexible propulsor has no prescribed motion or constraint referenced from outside coordinates. The immersed boundary method was used to model the interaction between the flexible propulsor and the surrounding fluid. A penalty method, in which the head and fin imparted a periodic torque to each other, was used to connect the head and the fin. The cruising speed and propulsive efficiency of the propulsor were explored as a function of the ratio of the head size to the fin length (D/L), the pitching amplitude (θp) and the pitching frequency (f). The cruising speed and the equilibrium position (geq) of the flexible propulsor near the ground were also examined. The optimal propulsive efficiency was achieved at the head ratio of D/L = 0.2 at θp = 30° and f = 0.2. The cruising speed of the flexible propulsor increased when operating near the ground. The gap distance between the propulsor and the ground was dynamically determined by the pitching motion.
... In addition, their lower caudal fins are all longer than their upper caudal fins, which facilitates their exit from the water and provides additional power for some species without fully re-entering the water (Davenport, 1994;Lewallen et al., 2011). The flight behavior of flying fishes has attracted extensive attention from scholars in many fields, including ecology, morphology, genetics, and aerodynamics (Davenport, 1992;Park and Choi, 2010;Lewallen et al., 2018;Daane et al., 2021). ...
Flying fishes, which use their wing-like pectoral fins and hypocercal caudal fin to glide through the air to avoid underwater predators, have independently evolved flight behavior, making them ideal for the study of adaptive evolution. To investigate the adaptation of flight behavior in flying fishes and the origin of Beloniformes fishes, this study obtained the complete mitochondrial genomes of Cheilopogon nigricans and Oxyporhamphus micropterus and constructed the DNA sequences extracted from these newly sequenced mitochondrial genomes with the DNA sequences of 32 previously published mitochondrial genomes into a dataset for reconstructing the phylogenetic relationships of Beloniformes fishes. The phylogeny that emerged strongly supported the possibility that flying fishes developed from halfbeaks and the progressive transition of flying fishes from two-wing to four-wing gliding. The divergence time analysis showed that the split between the suborder Belonidei and the family Adrianichthyidae occurred roughly 77.08 Mya, which fell within the period of evolution of the Indian plate in the late Cretaceous. Selection analyses revealed that flying fishes have a lower dN/dS ratio than the other members of Beloniformes, indicating that flying fishes experienced stronger purifying selection to eliminate deleterious mutations to maintain efficient energy metabolism to adapt to flight behavior. Moreover, this work found the positively selected signal in the ND4 gene, suggesting that different mitogenomic genes might have undergone different selective patterns during adaptive evolution.
... In the gliding stage, fully utilizing of the ground effect contributes . to further distance movement [14]. It's said that flying fishes can continue this process in the air for a total distance of 400m. ...
The concept of aerial-aquatic robots has emerged as an innovative solution that can operate both in the air and underwater. Previous research on the design of such robots has been mainly focused on mature technologies such as fixed-wing and multi-rotor aircraft. Flying fish, a unique aerial-aquatic animal that can both swim in water and glide over the sea surface, has not been fully explored as a bionic robot model, especially regarding its motion patterns with the collapsible pectoral fins. To verify the contribution of the collapsible wings to the flying fish motion pattern, we have designed a novel bio-robot with collapsible wings inspired by the flying fish. The bionic prototype has been successfully designed and fabricated, incorporating collapsible wings with soft hydraulic actuators, an innovative application of soft actuators to a micro aquatic-aerial robot. We have analyzed and built a precise model of dynamics for control, and tested both the soft hydraulic actuators and detailed aerodynamic coefficients. To further verify the feasibility of collapsible wings, we conducted simulations in different situations such as discharge angles, the area of collapsible wings, and the advantages of using ground effect. The results confirm the control of the collapsible wings and demonstrate the unique multi-modal motion pattern between water and air. Overall, our research represents the study of the collapsible wings in aquatic-aerial robots and significant contributes to the development of aquatic-aerial robots. The using of the collapsible wings must a contribution to the future aquatic-aerial robot.
... As the recent interest in the enhancement of the aerodynamic performance of several systems including SSWTs continues to grow, the aerodynamic characteristics of biological structures became a major source of inspiration. Several research efforts keenly investigated the innate traits of biological beings including plants and animals with the aim of examining the main geometric aspects allowing a remarkable set of structural and/or aerodynamic or aptitudes [24][25][26][27][28][29][30][31]. Structurally, a variety of biological geometric features and material properties that are common among several animal taxa are now being adopted to improve different designs' mechanical characteristics, for instance flexibility, strength, fracture toughness, resistance to wearing and energy absorption to name a few [31][32][33][34]. ...
... Fig. 3 shows the final assembly of the KUFish, and Table 2 describes its morphological parameters. The robot size and mass are comparable to those of the flying fish reported in Breder (1929), Davenport (1992), and Park and Choi (2010). Typically, the length of an adult flying fish ranges (150-400) mm, and the body mass is from (0.05-0.4) kg. ...
Natural underwater species outperform man-made underwater vehicles in many aspects. In this work, as our first
effort to eventually mimic flying fish, a robotic fish capable of swimming fast and leaping out of water was developed, named KUFish. The thrust of KUFish was produced by a tail-beating mechanism driven by a DC motor in combination with reduction gears, four-bar linkage, and pulley-string mechanisms. The passive dynamic stability was implemented by the symmetric mass distribution, positive buoyancy, and lower center of gravity. A series of swimming experiments indicated that the KUFish could swim 0.68 m and leap out of the water with a speed of 1.35 m/s (6.1 BL/s) at an instant of 0.68 s after release. In addition to experimentation, a two-dimensional dynamic model was developed to predict the swimming behavior of the robot. The proposed dynamic model could reasonably capture the measured swimming performance of the robot before water leaping. The water leaping capability of the KUFish can be well supported by the computed Froude number of (1.08 or 0.92) in terms of the body faring length or robot length, respectively. The results from the current work can be useful for developing a flying-fish-like swimming robot in the future.
... The phenomenon that flying fish in gliding flight [1,2] and benthic rays [3] achieve excellent locomotion performance by ground effect is ubiquitous in nature. Inspired by this, the ground effect of the airfoil or wing is widely studied [4,5]. ...
In this paper, a pitching airfoil near flat and wavy ground is studied by numerical simulations. The kinematic features of the airfoil and the flow field around it are analyzed to reveal unsteady vorticity dynamics of the self-propelled airfoil in ground effect. The optimal pitching periods at different initial heights above flat ground are obtained, which make the pitching airfoil achieve the maximum lift-to-drag ratio. Compared with flat ground, at the same initial height, the optimal pitching periods vary with the shape of ground. The structure and the strength of the wake vortices shedding from the airfoil are adjusted by the wavelength of ground. This leads to the changes of amplitude and occurrence times of the peak and valley of lift and drag force. The results obtained in this study can provide some inspiration for the design of underwater vehicles in the ground effect.
... Indeed, it is known that ground effects are used by bird to improve their efficiency in flight and gliding, this includes herring gulls [143], brown pelicans [144], black skimmers [145]. Fish also use ground effects for improved efficiency in swimming, including the mandarin fish [146], steelhead trout [147] and flying fish [148]. Our understanding of ground effects, while qualitatively known, is largely limited to steady flow, perhaps highlighting the reason that the porpoising effect was missed in the 2022 formula one regulations. ...
Damped-driven systems are ubiquitous in engineering and science. Despite the diversity of physical processes observed in a broad range of applications, the underlying instabilities observed in practice have a universal characterization which is determined by the overall gain and loss curves of a given system. The universal behavior of damped-driven systems can be understood from a geometrical description of the energy balance with a minimal number of assumptions. The assumptions on the energy dynamics are as follows: the energy increases monotonically as a function of increasing gain, and the losses become increasingly larger with increasing energy, i.e. there are many routes for dissipation in the system for large input energy. The intersection of the gain and loss curves define an energy balanced solution. By constructing an iterative map between the loss and gain curves, the dynamics can be shown to be homeomorphic to the logistic map, which exhibits a period doubling cascade to chaos. Indeed, the loss and gain curves allow for a geometrical description of the dynamics through a simple Verhulst diagram (cobweb plot). Thus irrespective of the physics and its complexities, this simple geometrical description dictates the universal set of logistic map instabilities that arise in complex damped-driven systems. More broadly, damped-driven systems are a class of non-equilibrium pattern forming systems which have a canonical set of instabilities that are manifest in practice.
... Fins on planes increased lift versus drag. The authors estimated the ground effect g-forces per ton to reduce drag when flying low [39]. This study developed Tifone, an AUV, to monitor submerged archaeological sites. ...
Recent large-scale operations, including frequent maritime transportation and unauthorised as well as unlawful collisions of drainage wastes, have polluted the ocean’s ecology. Due to the ocean’s unsuitable ecology, the entire globe may experience drastic aberrant conditions, which will force illness onto all living things. Therefore, an advanced system is very necessary to remove the undesired waste from the ocean’s surface and interior. Through the use of progressive unmanned amphibious vehicles (UAV), this study provides a dynamic operational mode-based solution to damage removal. In order to successfully handle the heavy payloads of ravage collections when the UAV reveals centre of gravity concerns, a highly manoeuvrable-based design inspired by nature has been imposed. The ideal creatures to serve as the inspiration for this piece are tropical birds, which have a long tail for navigating tricky situations. The design initialization was carried out by focusing on the outer body of tropical birds. Following this, special calculations were conducted and the full design parameters of the UAV were established. This study proposes a unique mathematical formulation for the development of primary and secondary design parameters of an UAV. The proposed mission profile of this application is computationally tested with the aid of sophisticated computational methodologies after the modelling of this UAV. The computational methods that are required are one-way coupling-based hydro-structural interaction assessments and computational hydrodynamic analyses. Computing is used to determine the aerodynamic and hydrodynamic forces over the UAV, the lightweight materials to withstand high fluid dynamic loads, and the buoyancy forces to complete the UAV components. These computational methods have been used to produce a flexible and fine-tuned UAV design for targeted real-time applications.
... At present, the submersibles (Park and Choi, 2010), including autonomous underwater vehicles (AUVs), underwater gliders (UGs) as well as remotely operated vehicles (ROVs), are essential tools for the human to explore, understand and utilize the ocean. These conventional submersibles are underwater platforms that carry different exploration equipment or operational tools for underwater observation, detection, and operation tasks. ...
Recently, there has been growing interest in developing hybrid aerial vehicles (HAUV) capable of operating in air and water and performing continuous and uninterrupted high-quality observation and sampling. This paper provides a detailed literature review of existing HAUVs characterizing their designs, and describes the current state of the art in methodologies to enable cross-domain mobility of HAUVs. The Control system is identified as the core and crucial component to ensure repeated smooth air-water transitions for HAUV. The complete dynamic model and cross-domain motion control of HAUV under the complex sea state with wind and wave interferences have received much attention from the research community. This paper presents a review of the leading research works focusing on these technologies. In this survey paper, we introduce a systematic categorization of the HAUV platform designs and discusses their cross-domain mobility and characteristics. We also present a review of recent developments in the transition control of HAUV and the growth of related research. Finally, the paper discusses the key observations and highlights challenges that need to be addressed in developing HAUV with persistent autonomy.
... The first difference is that the flying fish's caudal fins are heterocercal (one lobe is bigger) (Fish 1990) while the mudskipper's caudal fins are homocercal (lobes are equal in size). The second difference is that the flying fish extends both of its paired fins (pectoral and pelvic) during take-off to provide the Kutta condition and bring the flying fish into the air (Park and Choi 2010). This makes the flying fish's pectoral fins function like airplane wings and the flying fish's pelvic fins function like the aircraft elevator (horizontal stabilizer). ...
Mudskippers constitute a group of amphibious fish that have adapted for terrestrial locomotion. Some species in this group possess a tree-climbing ability. This study looks at the biomechanics behind this unique locomotion to assess their prospects for engineering applications. Our main objective in this study was to characterise the mudskippers’ body features that relate to the terrestrial behaviour. We achieved this by developing simulation-based models to replicate the locomotive functions for engineering purposes.
We sampled the mudskippers through on-site (in situ) recording—employing cameras to capture both moving and still images—in order to conduct mechanical and kinematical analyses. We also collected off-site (ex situ) data, recording pelvic fin features to examine individual fin flexibility through finite element modelling (FEM). Additionally, we dissected fish to acquire data for an FEM focused on the skeletal system and musculature of the pectoral and pelvic fins. Furthermore, we obtained and analysed mucus samples from the mudskippers using Fourier transformation infrared (FTIR) spectroscopy. Following FTIR, we modelled the mucus chemical component through molecular dynamics simulations to assess its biomolecular interactions with the substrates on which the fish are commonly found. We conducted these simulations to test the hypothesis that the mucus provides the mudskipper with additional adherence support during vertical substrate attachment (i.e., tree-climbing).
The results of the molecular dynamics simulations show that the mucus chemical compound (hyaluronic acid [HA]) attracts the substrate compounds (calcium carbonate of the limestone, silica, and plant cell wall components: cellulose, hemicellulose, and lignin), indicating that the mucus positively supports mudskippers’ adhesion. Complementing the mucus, the high structural-flexibility pelvic fins enable the fish to grip the surface of the substrate. The mudskippers are unique in that their scales are covered by skin and mucus—most fish have skin covered by scales. Furthermore, their pelvic fins can be dispatched downward, like a piston passively using the inward push of pectoral fins as an efficient energy-saving method. Finally, the mudskipper can hop over water surfaces. It is able to conserve its kinetic energy throughout the water-hopping sequence to perform a long, efficient hop as an escape mechanism.
Learning from the results of this study, the biomechanics of the mudskipper can be modelled into various applications, some of which are detailed in this study. First, we consider the development of a controlled adhesion surface by applying the Stefan adhesion used by the mudskippers during climbing. The adhesion could be activated through mucus production at the interface area and deactivated using water, as the adhesion is governed by fluid viscosity.
Second, we consider the creation of a mudskipper-inspired robot/drone capable of walking on land and sticking to inclined surfaces. Third, we consider the development of elastic materials inspired by the skin-covered scales, though this would require further examination of mudskippers.
... In addition, shape sensors could be relevant for identifying morphological changes for impulsive and dynamic aquatic locomotion, such as in squid where it has been shown that changes in body cross-section can lead to thrust peaks of up to a factor of 2.6 (Steele et al. 2017). Similarly, flying fish exhibit very fast undulation of their tail for aquatic escape (with speeds of up to 10-20 m/s (Park and Choi 2010)). This shape morphing happens relatively quickly (in less than 1 s) and will require appropriately matched sensor dynamics to measure the time-independent changes in body shape. ...
We propose the use of bio-inspired robotics equipped with soft sensor technologies to gain a better understanding of the mechanics and control of animal movement. Soft robotic systems can be used to generate new hypotheses and uncover fundamental principles underlying animal locomotion and sensory capabilities, which could subsequently be validated using living organisms. Physical models increasingly include lateral body movements, notably back and tail bending, which are necessary for horizontal plane undulation in model systems ranging from fish to amphibians and reptiles. We present a comparative study of the use of physical modeling in conjunction with soft robotics and integrated soft and hyperelastic sensors to monitor local pressures, enabling local feedback control, and discuss issues related to understanding the mechanics and control of undulatory locomotion. A parallel approach combining live animal data with biorobotic physical modeling promises to be beneficial for gaining a better understanding of systems in motion.
... The phenomenon of increased lift generated over static surfaces moving parallel to a solid boundary is termed 'steady ground' or 'steady wall' effect. This process has been previously studied and reviewed [1] leading to literature describing the advantages in air [2][3][4][5] and water [6,7]. The increase in lift near the ground is largely attributed to decelerated flow beneath the lifting surface resulting in higher underside pressures. ...
It has been well documented that animals (and machines) swimming or flying near a solid boundary get a boost in performance. This ground effect is often modelled as an interaction between a mirrored pair of vortices represented by a true vortex and an opposite sign ‘virtual vortex’ on the other side of the wall. However, most animals do not swim near solid surfaces and thus near body vortex–vortex interactions in open-water swimmers have been poorly investigated. In this study, we examine the most energetically efficient metazoan swimmer known to date, the jellyfish Aurelia aurita , to elucidate the role that vortex interactions can play in animals that swim away from solid boundaries. We used high-speed video tracking, laser-based digital particle image velocimetry (dPIV) and an algorithm for extracting pressure fields from flow velocity vectors to quantify swimming performance and the effect of near body vortex–vortex interactions. Here, we show that a vortex ring (stopping vortex), created underneath the animal during the previous swim cycle, is critical for increasing propulsive performance. This well-positioned stopping vortex acts in the same way as a virtual vortex during wall-effect performance enhancement, by helping converge fluid at the underside of the propulsive surface and generating significantly higher pressures which result in greater thrust. These findings advocate that jellyfish can generate a wall-effect boost in open water by creating what amounts to a ‘virtual wall’ between two real, opposite sign vortex rings. This explains the significant propulsive advantage jellyfish possess over other metazoans and represents important implications for bio-engineered propulsion systems.
... Animals (and machines) swimming or flying near a solid boundary get a boost in performance (Baudinette & Schmidt-Nielsen 1974, Blake 1983, Hainsworth 1988, Nowroozi et al. 2009, Park & Choi 2010, Rayner 1991. The phenomenon of increased lift generated over static surfaces moving parallel to solid boundary is termed the steady wall or steady ground effect. ...
Jellyfish have provided insight into important components of animal propulsion, such as suction thrust, passive energy recapture, vortex wall effects, and the rotational mechanics of turning. These traits are critically important to jellyfish because they must propel themselves despite severe limitations on force production imposed by rudimentary cnidarian muscular structures. Consequently, jellyfish swimming can occur only by careful orchestration of fluid interactions. Yet these mechanics may be more broadly instructive because they also characterize processes shared with other animal swimmers, whose structural and neurological complexity can obscure these interactions. In comparison with other animal models, the structural simplicity, comparative energetic efficiency, and ease of use in laboratory experimentation allow jellyfish to serve as favorable test subjects for exploration of the hydrodynamic bases of animal propulsion. These same attributes also make jellyfish valuable models for insight into biomimetic or bioinspired engineering of swimming vehicles. Here, we review advances in understanding of propulsive mechanics derived from jellyfish models as a pathway toward the application of animal mechanics to vehicle designs.
Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... Breaching, or leaping out of the water, is a well-documented behavior exhibited by many different marine vertebrates, including pelagic rays (Medeiros et al., 2015), flying fish (Fish, 1990;Park and Choi, 2010), squid (O'Dor et al., 2013) sharks [Brunnschweiler et al., 2005;Johnston et al., 2018;Martin et al., 2005;Semmens et al., 2019], and cetaceans Waters and Whitehead, 1990;Whitehead, 1985a;Whitehead, 1985b). When coupled with high-speed horizontal travel and streamlined re-entry, low-angle breaching can be further classified as porpoising (Weihs, 2002), a behavior that is frequently observed in dolphins and pinnipeds. ...
The considerable power needed for large whales to leap out of the water may represent the single most expensive burst maneuver found in nature. However, the mechanics and energetic costs associated with the breaching behaviors of large whales remain poorly understood. In this study we deployed whale-borne tags to measure the kinematics of breaching to test the hypothesis that these spectacular aerial displays are metabolically expensive. We found that breaching whales use variable underwater trajectories, and that high-emergence breaches are faster and require more energy than predatory lunges. The most expensive breaches approach the upper limits of vertebrate muscle performance, and the energetic cost of breaching is high enough that repeated breaching events may serve as honest signaling of body condition. Furthermore, the confluence of muscle contractile properties, hydrodynamics, and the high speeds required likely impose an upper limit to the body size and effectiveness of breaching whales.
... Davenport, 1994 created by the pectoral and pelvic fins. These fins have an angle of incidence of 12°and 5°, respectively, and are used to control the lift while the tail movements generate a forward thrust, which subsequently enables an airborne ascent (Park and Choi, 2010). The kinematics of this behaviour might be similar to the pre-hop taxiing behaviour observed in P. variabilis, though flying fish taxi and are airborne for longer durations than mudskippers. ...
In this communication, we describe the water-hopping kinematics of the dusky-gilled mudskipper (Periophthalmus variabilis), and by doing so elucidate an entirely new form of fish locomotion that has yet to be reported in the public domain. Water-hopping is defined herein as an ability to hop once, or in succession, on the surface of water without full submergence and without a fin-guided glide. We find that taxiing on the water surface is the predominating kinematic movement used for the execution of successful water-hops. We observe that an initial concentric ripple forms as the mudskipper impacts the water, and that subsequent taxiing on the water surface generates a sinusoid-like ripple pattern in the water prior to take off. Interestingly whilst airborne, the pectoral fins of P. variabilis appear to remain stationary, only to be deployed upon contact with the water. When landing back onto the surface of the water, P. variabilis makes the initial contact via its pelvic region, occasionally extending its pectoral fins during its descent. The reasons for pectoral and pelvic fin extension are unclear, however, there may be either aerodynamic or hydrodynamic benefits in its doing so. This motion furthermore prepares the mudskipper for either, a follow-on water-hop, or a discontinuation of movement altogether, as the body of the mudskipper becomes aligned in a way conducive to either. P. variabilis will launch and land using both, horizontal surfaces such as littorals, and inclined-to-vertical surfaces such as rocks and trees.
... Diving birds, for instance, fold back their wings to reduce drag and limit water impact forces [2]. Flying fish extend their fins to improve their gliding performance [8], some spiders adapt their shape to sail [9], and mute swans have been observed to arch their wings to sail at speeds up to 1.3 m/s [10]. Two recently developed adaptive aerial-aquatic robots are presented in the remainder of this paper, where we discuss the design of each robot, as well as the advantages and limitations of adaptation for aerial-aquatic locomotion. ...
Aerial-aquatic mobility is envisaged to significantly facilitate applications involving aquatic sampling or underwater surveying. Allowing water vehicles to take flight would allow for rapid deployment, access to remote areas, over-flying of obstacles and easy transitioning between separate bodies of water. The use of a single vehicle capable of reaching distant locations rapidly, conducting measurements and returning to base, would greatly improve upon the current solutions, which often involve integrating different types of vehicles (e.g. vessels carrying deployable submarines), or rely heavily on manpower (e.g. sensors deployed manually from ships). The usage of single adaptive multi-modal robots for such applications could significantly improve the efficiency, costs and safety of such operations.
... animals such as birds and flying fish use steady ground effect to improve their cost of transport or gliding distance (Hainsworth 1988;Rayner 1991;Park & Choi 2010). In contrast, some fish exploit unsteady ground effect to improve their cost of transport or cruising speed when swimming near substrates and sidewalls (Blake 1983;Webb 1993Webb , 2002Nowroozi et al. 2009;Blevins & Lauder 2013), and it is important in the take-off and landing of insects (Van Truong et al. 2013a,b). ...
Experiments and computations are presented for a foil pitching about its leading edge near a planar, solid boundary. The foil is examined when it is constrained in space and when it is unconstrained or freely swimming in the cross-stream direction. It was found that the foil has stable equilibrium altitudes: the time-averaged lift is zero at certain altitudes and acts to return the foil to these equilibria. These stable equilibrium altitudes exist for both constrained and freely swimming foils and are independent of the initial conditions of the foil. In all cases, the equilibrium altitudes move farther from the ground when the Strouhal number is increased or the reduced frequency is decreased. Potential flow simulations predict the equilibrium altitudes to within 3 %-11 %, indicating that the equilibrium altitudes are primarily due to inviscid mechanisms. In fact, it is determined that stable equilibrium altitudes arise from an interplay among three time-averaged forces: a negative jet deflection circulatory force, a positive quasistatic circulatory force and a negative added mass force. At equilibrium, the foil exhibits a deflected wake and experiences a thrust enhancement of 4 %-17 % with no penalty in efficiency as compared to a pitching foil far from the ground. These newfound lateral stability characteristics suggest that unsteady ground effect may play a role in the control strategies of near-boundary fish and fish-inspired robots.
... The waterfall-climbing gobies (e.g., sicydiine gobiids) present an excellent model to examine how anatomical structure and functional performance scale during ontogeny in the course of their distinct life history to enable these gobies to overcome particular physical forces. As modified fins allow teleosts to achieve various locomotor functions (e.g., aerial gliding, hitchhiking on larger swimming animals, and station-holding in an aquatic environment: Gibson, 1969;Green and Barber, 1988;Das and Nag, 2004;Park and Choi, 2010;Britz and Johnson, 2012), the waterfall-climbing gobies use modified pelvic fins as a suction disc (see Fig. 1) enabling them to exit the water and climb the rock surface of waterfalls during their upstream migration after completing an oceanic larval phase (McDowall, 2004). These gobies exhibit an amphidromous life history, in which post-larvae migrate from the ocean and sexually matured adults spawn in freshwater. ...
An amphidromous sicydiine goby, Sicyopterus japonicus, exhibits rock-climbing behavior during upstream migration along rivers and streams. Using a pelvic sucker formed by fused pelvic fins, S. japonicus generates suction adhesion on the climbing surface. By measuring performance variables that correlate with successful rock-climbing capability, we evaluated scaling relationships of adhesive suction force generated by the pelvic sucker and fatigue during climbing in S. japonicus during ontogeny. In continuous climbing on the experimental 60°-inclined surface, the pelvic sucker of S. japonicus exhibited strong positive allometry in generating suction force for adhesion during ontogeny. In contrast, fatigue time of the pelvic sucker muscles for sustained adhesion scaled non-linearly with body mass during ontogeny. In addition, fatigue time and body mass showed the best fit to a quadratic regression, which predicted intermediate-sized individuals (large juveniles to small adults) to have better performance in adhesive endurance than smaller or larger individuals. Our experimental results indicate that different sizes of waterfall-climbing gobies have different performance capacities for rock climbing perhaps because of physiological differences in their pelvic muscles. In addition, our data from S. japonicus indicates that selection pressures on the locomotor capacities of waterfall-climbing gobiids vary during ontogeny.
... Gliding pelican was found to achieve significant energy savings from ground effect [2]. Similar aerodynamics advantage was found in flying fish when gliding close to the sea surface [3]. Ground effect is also observed in undulatory swimming with deforming body or fin. ...
... Park and Choi studied the flying fish during flight and aerodynamic characteristics using lift and drag measurements in a wind tunnel experimental. It was found that the maximum lift coefficient is largest at angles of attack of 30°-35°above the horizontal plane (Park and Choi 2010). ...
Biology inspired inventions have been of great interest to the researcher and engineer. Biomimicry offers special insight into nature’s methods of motion control, with significance in thrust and drag control for swimming and flying lifeforms. An array of actuators designed in an artificial wing could be used to control aerodynamic effects to adjust drag or lift according to given wind conditions for improved flight, or to control stability prior to touchdown for a smooth landing, providing an additional means of aerodynamic stability control. In this study, a method of generating a travelling wave motion in attempt to mimic that observed in the wings of flying-fish (Exocoetidae) during descent are presented. Ionic polymer-metal composite actuators were arranged in an array and oscillated in a travelling wave motion. The arrays were held rigid between glass slides and embedded into a flexible substrate to create the soft “wing” surface for free-end displacement measurements. Using a microcontroller and motor drivers, a controllable travelling wave motion was created. Additionally, an array of actuators was connected to a 3D printed wing skeleton based on the dimensions of a four-wing flying-fish like structure. The results indicate the travelling wave motion can be controlled with ionic polymer-metal composite actuators as arranged in several configurations. This offers an experimental platform for further study of the aerodynamic effects of a travelling wave across a wing during flight.
Developing hybrid aerial-aquatic vehicles that can interact with water surfaces while remaining aloft is valuable for various tasks, including ecological monitoring, water quality sampling, and search and rescue operations. Storm petrels are a group of pelagic seabirds that exhibit a unique locomotion pattern known as "pattering" or "sea-anchoring," which is hypothesized to support forward locomotion and/or stationary posture at the water surface. In this study, we use morphological measurements of three storm petrel species and aero/hydrodynamic models to develop a computational storm petrel model and interact it with a hybrid fluid environment. Using Deep Reinforcement Learning (DRL) algorithms, we find that the storm petrel model exhibits high maneuverability and stability under a wide range of constant wind velocities after training. We also verify in the simulation that the storm petrel can use its “pattering” or “sea-anchoring” behavior to achieve different biomechanical sub-tasks (e.g., weight support, forward locomotion, stabilization) and adapt it under different wind speeds and optimization objectives. Specifically, we observe an adjustment in storm petrel’s movement patterns as wind velocity increases and quantitively analyze its biomechanics underneath. Our results provide new insights into how storm petrels achieve efficient locomotion and dynamic stability at the air-water interface and adapt their behaviors to different wind velocities and tasks in open environments. Ultimately, our study will guide the design of next-generation biomimetic petrel-inspired robots for tasks requiring proximity to the water interface and efficiency.
The natural history of birds is summarized. Account of what contemporary birds are, when and how they came to be what they are, and why and how they evolved exceptional physiognomies are given. The evolution of birds from reptilian stock, their domestication that resulted in some of the species becoming leading food animals and the sociocultural impacts of birds on organizations of many human societies are outlined. The evolution of the lung-air sac system of birds, which among the air-breathing vertebrates is the most structurally complex and efficient gas exchanger, is described. Unique properties, capacities, and activities such as long distant migration, flight under the extremely hypoxic conditions of high altitude, anthropogenic impacts of climate change (global warming) on the ecology and biology of birds, sound production (vocalization), birds as bioindicator animals of environmental health, and the cognitive prowess of birds in exploits such as dropping hard food objects on firm surfaces to break them and that way access otherwise unobtainable food and caching of food in various ways and places and shrewdly accessing it for use during adverse conditions are presented. The biology of birds can only be well understood by considering them from various perspectives that include the habitats they occupy and the lifestyles that they lead.
Seabirds are expected to increase their flight height in tailwind and to increase their airspeed in headwind during goal-oriented flight to minimize their cost of transport. To understand how flapping birds respond to variability in wind speed and direction experienced during their commuting flights between their breeding colony and foraging areas, we measured the flight height and speed of black-tailed gulls Larus crassirostris using GPS loggers. We analyzed the relationships between these flight parameters and local wind speed and direction. Over the course of the 2016 to 2018 breeding seasons, we tagged 105 birds at 2 colonies in northern Hokkaido, Japan. A total of 90 flight track-lines within a 500 m radius of 5 coastal meteorological stations were analyzed. The median flight height ranged from 0 to 153.8 m, and the median ground speed and airspeed were between 18.6-82.1 and 19.5-93.0 km h ⁻¹ , respectively. Gulls flew higher with greater tailwind speed, supporting the hypothesis that birds utilize greater wind assistance at higher altitudes. Furthermore, gulls increased their airspeed under strong headwind conditions, suggesting they adjust airspeed to achieve the most cost-effective speed to reach their destination. Better understanding the drivers of seabird flight height is key to assessing the potential for collisions with wind turbines in coastal and offshore wind farms. These findings provide useful information to reduce collisions with coastal and offshore wind facilities.
Some research on aircraft is largely inspired by birds. Among them, aerial-aquatic amphibians with trans-media locomotion capabilities have greatly promoted the development of aquatic unmanned aerial vehicles (AquaUAV). In this article, the studies of AquaUAV are sorted out by their biological counterpart and summarized in chronological order from 2005 to 2021. To further understand the key technologies of AquaUAV, we focus on the structural compatibility design of wing and aerial-aquatic propulsion methods by analyzing their advantages and disadvantages. In addition, the analysis methods of kinematics and dynamics performance of AquaUAV for simulation and experiment are involved in the process of studying the kinematics, lift/drag, and propulsion of prototypes. Finally, we present several challenges and propose some potential solutions to improve the ability of AquaUAV in the future.
Monitoring, sensing, and exploration of over 70% of the Earth’s surface that is covered with water
is permitted through the deployment of underwater bioinspired robots without afecting the
natural habitat. To create a soft robot actuated with soft polymeric actuators, this paper describes
the development of a lightweight jellyfsh-inspired swimming robot, which achieves a maximum
vertical swimming speed of 7.3 mm/s (0.05 body length/s) and is characterized by a simple design.
The robot, named Jelly-Z, utilizes a contraction–expansion mechanism for swimming similar to the
motion of a Moon jellyfsh. The objective of this paper is to understand the behavior of soft silicone
structure actuated by novel self-coiled polymer muscles in an underwater environment by varying
stimuli and investigate the associated vortex for swimming like a jellyfsh. To better understand the
characteristics of this motion, simplifed Fluid–structure simulation, and particle image velocimetry
(PIV) tests were conducted to study the wake structure from the robot’s bell margin. The thrust
generated by the robot was also characterized with a force sensor to ascertain the force and cost of
transport (COT) at diferent input currents. Jelly-Z is the frst robot that utilized twisted and coiled
polymer fshing line (TCPFL) actuators for articulation of the bell and showed successful swimming
operations. Here, a thorough investigation on swimming characteristics in an underwater setting is
presented theoretically and experimentally. We found swimming metrics of the robot are comparable
with other jellyfsh-inspired robots that have utilized diferent actuation mechanisms, but the
actuators used here are scalable and can be made in-house relatively easily, hence paving way for
further advancements into the use of these actuators
The aquatic-aerial robot with the free interface crossing can enhance adaptability in complex aquatic environments. However, its design is extremely challenging for the striking discrepancies in propulsion principles. The flying fish in nature exhibits remarkable multi-modal cross-domain locomotion capability, such as high-maneuvers swimming, agile water-air crossing, and long-distance gliding, providing extensive inspiration. In this paper, we present a unique aquatic-aerial robotic flying fish with powerful propulsion and a pair of morphing wing-like pectoral fins to realize cross-domain motion. Furthermore, to explore the gliding mechanism of flying fish, a dynamic model with a morphing structure of pectoral fins is established, and a double deep Q-network (DQN)-based control strategy is proposed to optimize the gliding distance. Finally, experiments were conducted to analyze the locomotion of the robotic flying fish. The results suggest that the robotic flying fish can successfully perform the ``fish leaping and wing spreading'' cross-domain locomotion with an exiting speed of 1.55~m/s (5.9 body lengths per second, BL/s) and a crossing time of 0.233~s indicating its great potential in cross-domain. Simulation results have validated the effectiveness of the proposed control strategy and indicated that the dynamical adjustment of morphing pectoral fins contributes to improving the gliding distance. The maximum gliding distance has increased by 7.2%. This study will offer some significant insights into the system design and performance optimization of aquatic-aerial robots.
Gliding arboreal lizards in the genus Draco possess a pair of patagia, which are thin wing membranes supported by highly-elongated thoracic ribs and can be actively folded and unfolded. The uniqueness of Draco gliding flight is that the forelimbs of Draco can move freely independent of the patagia which are the main lifting surfaces. During the main glide phase, the entire forelimbs are straightened, abducted from the body and held very close to the patagial leading edges. The reasons for adopting this abducted pose have not been investigated before, especially from the perspective of fluid physics. In this study, wind tunnel experiments and computational simulations are conducted to compare the aerodynamic performances of the abducted pose with two other poses, which have the forelimbs held away from the patagial leading edges. The results show that the abducted pose leads to the highest maximum lift coefficient. This aerodynamic advantage is caused by the larger leading-edge radius due to the abducted forelimbs and small gaps between the abducted forelimbs and the patagial leading edges. Furthermore, it is found that the low aspect ratio of the patagium (0.985) allows the wingtip vortex to energise the flow over the top patagial surface at high angles of attack which leads to a gentle stall characteristic. The current results also show the existence of distinct leading-edge vortices up to moderate angles of attack. Overall, this work deepens our understanding of the gliding flight aerodynamics of Draco lizards, and is useful for future artificial flying machine applications.
When swimming near a solid planar boundary, bio-inspired propulsors can naturally equilibrate to certain distances from that boundary. How these equilibria are affected by asymmetric swimming kinematics is unknown. We present here a study of near-boundary pitching hydrofoils based on water channel experiments and potential flow simulations. We found that asymmetric pitch kinematics do affect near-boundary equilibria, resulting in the equilibria shifting either closer to or away from the planar boundary. The magnitude of the shift depends on whether the pitch kinematics have spatial asymmetry (e.g. a bias angle, $\theta_0$) or temporal asymmetry (e.g. a stroke-speed ratio, $\tau$). Swimming at stable equilibrium requires less active control, while shifting the equilibrium closer to the boundary can result in higher thrust with no measurable change in propulsive efficiency. Our work reveals how asymmetric kinematics could be used to fine-tune a hydrofoil's interaction with a nearby boundary, and it offers a starting point for understanding how fish and birds use asymmetries to swim near substrates, water surfaces, and sidewalls.
Fish movement investigations typically assume motion through unbound fluids. However, benthic organisms experience ground effects when moving near substrates. To identify the internal ground-effect mechanisms on self-propelled stingrays, simulations applying an in-house developed fluid-structure interaction algorithm were performed. A geometric model was developed from three-dimensional (3D) laser-scan data of a live freshwater stingray. The ratios of distances between the stingray and solid boundary to its disc width and unbounded fluid were investigated at two swimming frequencies: 2.385 and 3.384 Hz. Velocity fluctuation amplitude during one cycle near a substrate was approximately 30% less than that in unbounded fluid. The two peaks of the force coefficient in one cycle decreased and increased, respectively, with decreased power loss near the substrate; this is related to the pressure and wake distributions on the lower stingray surface at two typical instances. Pressure regions on the front and rear lower wing-crest surfaces decreased when approaching the substrate at the first typical instance, whereas that of the wing trough increased for the second typical instance. The vortex size on the lower wing surface decreased near the substrate for both instances.
To investigate the role of the amplitude of an object under the unsteady wall effect in fluid dynamics, we modelled an undulating fin near a wall in a two-dimensional Cartesian coordinate system. The fin was tethered in a uniform flow and controlled by a user-defined function program. The unsteady wall effect improved the propulsion force and propulsion efficiency at different amplitudes, but the lift force behaved differently. We determined the critical amplitude for the model, below which the lift force is positive within an appropriate off-wall distance range. At amplitudes larger than the critical amplitude and as the off-wall distance decreases, the average lift force is, however, always negative, causing the undulating fin to overturn towards the wall and lose stability. The essence of propulsion and lift variation lies in the change in the shape of the space between the wall and the fin, which affects the fluid flow structure and pressure distribution. In addition, some interesting phenomena related to the vortex core arrangement and pressure distribution were introduced at different amplitudes caused by the unsteady wall effect. The present results may provide new insights into the behaviours of benthic fish, reducing their undulating amplitudes and pitch angles near walls.
Gliding animals change their body shape and posture while producing and modulating aerodynamic forces during flight. However, the combined effect of these different factors on aerodynamic force production, and ultimately the animal’s gliding ability, remains uncertain. Here, we quantified the time-varying morphology and aerodynamics of complete, voluntary glides performed by a population of wild gliding lizards (Draco dussumieri) in a seven-camera motion capture arena constructed in their natural environment. Our findings, in conjunction with previous airfoil models, highlight how three-dimensional (3D) wing shape including camber, planform, and aspect ratio enables gliding flight and effective aerodynamic performance by the lizard up to and over an angle of attack (AoA) of 55° without catastrophic loss of lift. Furthermore, the lizards maintained a near maximal lift-to-drag ratio throughout their mid-glide by changing body pitch to control AoA, while simultaneously modulating airfoil camber to alter the magnitude of aerodynamic forces. This strategy allows an optimal aerodynamic configuration for horizontal transport while ensuring adaptability to real-world flight conditions and behavioral requirements. Overall, we empirically show that the aerodynamics of biological airfoils coupled with the animal’s ability to control posture and their 3D wing shape enable efficient gliding and adaptive flight control in the natural habitat.
Flying fish is a special marine fish that has inherent advantages of swimming in the sea and flying in the air. In this paper, by imitating the structure of the flying fish in nature, a kind of bionic flying fish is designed and implemented, which can obtain information under water, on the water surface, as well as in the air. Besides, this paper discusses the application of leaked oil tracking in multiple bionic flying fishes coordination based on a k-winner-take-all (k-WTA) model.
There are 71 species of flying fish (exocoetidae) in the world, 18 species in Indonesia, and ten species in Maluku. The southern waters of Ambon Island are potential areas for catching flying fish, but the species distribution has not been reported. This study aimed to determine the composition and distribution of flying fish species caught by gillnet in the southern Ambon Island waters. The study of flying fish species was carried out from February to June 2021. There were five dominant flying fish species distributed consistently based on their respective zones. The morphological identification revealed flying fish species which broadcast on the coast of the island respectively, namely: Cypselurus poecilopterus , Cheilopogon abei , Cheilopogon spilopterus with geographical position - 3°73′07″S – -3°92′01″S and 128°15′04″E – 128°44′08″E. The high seas were dominated by Cheilopogon furcatus dan Hirundichthys oxycephalus with position -4°14′08″S – -4°72′16″S and 128°28′05″E – 129°42′09″E. This information could be provided as a database for the management and sustainable use of flying fish resources to support Indonesia's National Fish Reserve or Lumbung Ikan Nasional (LIN).
Bio-mimicry is one of the leading fields of research that has been around since the early ages of science. Aerospace industry, especially, takes most of its design inspiration from the aquatic life. In this paper a breed of flying fish scientifically known as Exocoetidae, Cypselurus Hiraii has been chosen to study. The aerodynamic characteristics of the fish were studied experimentally in a wind tunnel previously, but the plausibility of its design application in the aerospace industry hasn’t been explored yet. Therefore, in the present study the cross-section of the fish in the side view has been traced and its aerodynamic performance as a 2D infinite wing has been examined in ANSYS R15 (FLUENT). The performance of this fish inspired shape has been compared to the properties of conventional aerofoils in terms of coefficient of lift and coefficient of drag.
Flying fish is a fascinating animal that has the abilities to swim and glide. These unique abilities have attracted interest amongst scientist and researchers alike. Several studies have been performed to better understand flying fish aerial locomotion and aerodynamics performance. However, understanding biological beings’ aerodynamics characteristics such as flying fish have always been more than educated estimation. Researchers have approximated the physics of flying fish based on known aerodynamics principle of other flying animals with similarly aerodynamics parameters. In the present study, we manufactured and tested a model of Exocoetus Volitans flying fish in the wind tunnel. The experiment evaluated the lift and drag coefficients generated by Exocoetus Volitans wings by subtracting the aerodynamics forces contribution from other fish parts. The experiment evaluated the performance at various angles of attacks starting from −15° to +45° and airspeeds ranging from 10 to 15 m/s. The results show that as the angle of attack increases, the lift and drag coefficients also increase even beyond 20°. The maximum lift coefficient is achieved when the angle of attack is at 40°.
Animals (and machines) swimming near walls obtain boosts in performance due to fluid vortex-wall interactions. Here, we examine a highly efficient metazoan swimmer, the jellyfish, to elucidate vortex interactions in open water. Using high-speed video, particle image velocimetry and a pressure algorithm, we measure the effect of near-body vortex-vortex interactions on swimming performance. We show that vortex rings from the previous swim cycle are critical for increasing thrust. This vortex acts in the same way as the virtual vortex during modeled wall-effect swimming, by helping converge fluid and generating significantly higher pressures resulting in greater thrust. These findings advocate that jellyfish generate a wall-effect boost in open water by creating what amounts to a virtual wall between two real, opposite sign vortex rings and helps explain the significant propulsive advantage jellyfish possess over other metazoans.
In the present study, flow structure around a live rhinoceros beetle in a tethered flight is investigated experimentally using a smoke-wire visualization technique in a wind tunnel with a free-stream velocity of 1.2 m/s, which is close to that of a typical flight speed. While varying the body angle (from 5 to 85°), the flow structures generated by the hindwings, elytra, and body are visualized along the spanwise direction. During the flapping period, the complex flow structures comprised leading-edge, trailing-edge, and tip vortices generated on the hindwing, but the flow structure is quite simple on the elytra (attached flow) and body (separated flow). As the body angle increases, these vortices convect in the downward direction, which matches the observation that the body angle of a hovering flight is larger than that of a forward flight. When the body angle matches the condition of forward flight, it is also found that the Strouhal number of a flapping hindwing is tuned to 0.4, which is known as an optimal value for thrust efficiency. Further, the effect of free-stream velocity (i.e., advance ratio) on the formation and evolution of these coherent vortical structures are investigated.
Recent years have witnessed rapid progress in the bioinspired aquatic robots that usually use creatures as a source of inspiration.
It is challenging to emulate high-speed and short-duration surface piercing motions for a self-propelled robotic dolphin when it attempts to perform leaps in the context of bioinspired robotics. This paper presents motion control strategies for a repetitive leaping robotic dolphin serving as a platform for implementation and evaluation of modeling and control methods. Firstly, an integrative model that takes account of both kinematics and dynamics is established to explore the possibility of leaping with an untethered swimming robot. Then, a novel high-speed swimming control strategy is then put forward based on the angle of attack theory, followed by the proposal of orientation control strategy. Finally, leaping tests on the actual robot verify the effectiveness of the conducted leaping analysis along with the proposed control strategies. Remarkably, the robot was able to conduct three continuous leaps back-to-back for the first time in a confined swimming pool. Results from this study also have implications for bioinspired design, where high speeds and maneuverability are required.
We numerically examine the hydrodynamic interaction between a flexible fin and surrounding fluid near the ground when four relevant parameters of initial position, bending rigidity, mass ratio and Reynolds number are varied. The leading edge of the fin is fixed in the streamwise direction, whereas the lateral motion is freely movable by the fluid–flexible body interaction near the ground. When the fin is initially positioned far from the ground, the fin passively migrates toward another wall-normal position near the ground for an equilibrium state due to larger positive deflection angle for the fin than the negative angle by the effects of vorticity generated by the lateral velocity gradient near the ground. In addition, as the flapping amplitude of the fin is small for large bending rigidity and small mass ratio, the great asymmetry between the positive and negative deflection angles reduces the transient time of the fin to reach the equilibrium position near the ground, and thus the fins can quickly take the hydrodynamic benefits with low drag at an equilibrium state without any energy consumption for lift force due to local balance between the flapping motion and the ground. The most important observation is that the equilibrium position of the fin is invariant to the initial position, bending rigidity and mass ratio of the fin. However, the equilibrium position of the fin is dramatically affected by the Reynolds number. The present results provide new insights into the functional role of the relevant parameters in passively flapping-based locomotion near the ground.
The effect of asymmetric heaving motion on the aerodynamic performance of a two-dimensional flapping foil near a wall is studied numerically. The foil executes the heaving and pitching motion simultaneously. When the heaving motion is symmetric, the mean thrust coefficient monotonically increases with the decrease in mean distance between foil and wall. Meanwhile, the mean lift coefficient first increases and then decreases sharply. In addition, the negative mean lift coefficient appears when the foil is very close to the wall. After the introduction of asymmetric heaving motion, the influence of wall effect on the force behavior becomes complicated. The mean thrust coefficient is enhanced when the duration of upstroke is reduced. Moreover, more and more enhancement can be achieved when the foil approaches the wall gradually. On the other hand, the positive mean lift coefficient can be observed when the duration of downstroke is shortened. By checking the flow patterns around the foil, it is shown that the interaction between the vortex shed from the foil and the wall can greatly modify the pressure distribution along the foil surface. The results obtained here might be utilized to optimize the kinematics of the Micro Aerial Vehicles (MAVs) flying near a solid wall.
From millimeter-scale insects to meter-scale vertebrates, several animal species exhibit multimodal locomotive capabilities in aerial and aquatic environments. To develop robots capable of hybrid aerial and aquatic locomotion, we require versatile propulsive strategies that reconcile the different physical constraints of airborne and aquatic environments. Furthermore, transitioning between aerial and aquatic environments poses substantial challenges at the scale of microrobots, where interfacial surface tension can be substantial relative to the weight and forces produced by the animal/robot. We report the design and operation of an insect-scale robot capable of flying, swimming, and transitioning between air and water. This 175-milligram robot uses a multimodal flapping strategy to efficiently locomote in both fluids. Once the robot swims to the water surface, lightweight electrolytic plates produce oxyhydrogen from the surrounding water that is collected by a buoyancy chamber. Increased buoyancy force from this electrochemical reaction gradually pushes the wings out of the water while the robot maintains upright stability by exploiting surface tension. A sparker ignites the oxyhydrogen, and the robot impulsively takes off from the water surface. This work analyzes the dynamics of flapping locomotion in an aquatic environment, identifies the challenges and benefits of surface tension effects on microrobots, and further develops a suite of new mesoscale devices that culminate in a hybrid, aerial-aquatic microrobot.
The kinematics and aerodynamics of flapping and gliding flight by the black skimmer were investigated to evaluate the significance of ground effect to the foraging and daily energy budget of skimmers. Ground effect is an increase in lift and decrease in drag of an aerofoil when close to the ground.
The duration of upstroke and downstroke, the wing movements and the pronation/supination of the wings during flapping flight of the skimmer are similar to other birds. Wing-beat frequency was 3.1 s-1 and flight velocity was 9.1 m s-1. The wing stroke was markedly asymmetric, with the majority of the stroke occurring above the plane of the body. During skimming, wing beats are intermittent and of low amplitude; flight velocity is 10.3 m s-1.
Induced power, parasite power and profile power of skimmers were calculated after Tucker (1973) in the absence of ground effect, and the glide angle and sink velocity were calculated for gliding skimmers. Ground effect was shown to markedly reduce induced power requirements, and hence total power requirement, of flapping flight, and reduce the glide angle and sink velocity during gliding.
The hydrodynamic drag of the lower mandible was estimated to be 10-4 N, which is insignificant compared to the total aerodynamic drag (0.4 N).
Ground effect was shown to markedly increase foraging efficiency and facilitate the attainment of a positive daily energy balance. The significance of ground effect to other flying vertebrates was discussed.
Note:
Department of Zoology, University of Cape Town, Cape Town, Republic of south Africa.
The aerodynamic properties of bird wings were examined at Reynolds numbers of 1·5 × 104 and were correlated with morphological parameters such as apsect ratio, camber, nose radius and position of maximum thickness. The many qualitative differences between the aerodynamic properties of bird, insect and aeroplane wings are attributable mainly to their differing Reynolds numbers. Bird wings, which operate at lower Reynolds numbers than aerofoils, have high minimum drag coefficients (0·03–0·13), low maximum lift coefficients (0·8–1·2) and low maximum lift/drag ratios (3–17). Bird and insect wings have low aerofoil efficiency factors (0·2–0·8) compared to conventional aerofoils (0·9–0·95) because of their low Reynolds numbers and high profile drag, rather than because of a reduced mechanical efficiency of animal wings.
For bird wings there is clearly a trade-off between lift and drag performance. Bird wings with low drag generally had low maximum lift coefficients whereas wings with high maximum lift coefficients had high drag coefficients. The pattern of air flow over bird wings, as indicated by pressure-distribution data, is consistent with aerodynamic theory for aeroplane wings at low Reynolds numbers, and with the observed lift and drag coefficients.
Summary This paper examines the aerodynamics and power requirements of forward flight in bumblebees. Measurements were made of the steady-state lift and drag forces acting on bumblebee wings and bodies. The aerodynamic force and pitching moment balances for bumblebees previously filmed in free flight were calculated. A detailed aerodynamic analysis was used to show that quasi-steady aerodynamic mechanisms are inadequate to explain even fast forward flight. Calculations of the mechanical power requirements of forward flight show that the power required to fly is independent of airspeed over a range from hovering flight to an airspeed of 4-5ms~1.
Fin and body dimensions of six genera of flying fish (Exocoetidae) were examined to study variation in morphological parameters in relation to aerodynamics performance. The fins are modified as wings for gliding flight. Fin area and fin span increase with increasing body mass, whereas the percentage of wing area contributed by the pectoral fins and the percentage of the caudal fin area contributed by the hypocaudal lobe remain constant. The aerodynamic design of flying fish approximates the monoplane-biplane classification proposed by Breder (1930). Scaling relationships for wing loading and aspect ratio indicate that wing morphology in the Exocoetidae is more similar to birds and bats than to other gliders. The flight performance of flying fish is a high-speed glide with a relatively flat trajectory. The wing, as indicated by the aspect ratio, is designed for high lift with low drag characteristics.
The flight path of flying fish is analyzed by two methods, a simple analytical method and a numerical method based on optimal control theory. It has been observed that flying fish fly over the sea to escape from predators. Therefore, two possible strategies are considered, flight for the maximum range and flight for the longest time.The simple analytical method is based on energy consumption during flight. Using this method, the flight distance and the flight time of unsteady level gliding flight were compared to those of steady gliding flight. The steady gliding flight achieves a longer flight distance and time than the unsteady level gliding flight when the ground effect is not considered, although the differences are small. When the ground effect is taken into consideration, the unsteady level gliding flight is superior both in the length and the duration of flight.In order to obtain detailed flight behaviors the numerical method based on optimal control theory was then used. This theory has the capability of giving the optimal solution for a given performance index. The calculated optimal flight path for the maximum flight range agrees well with the observed flight paths of the flying fish, Cypselurus agoo agoo and Cypselurus heterurus doederleini. In contrast, the calculated optimal flight path for the longest flight time is similar to that of the steady gliding flight and is different from the observed flight paths. Therefore, it is concluded that the flight of flying fish corresponds closely to the optimal solution for achieving the maximum flight range.
A lifestyle influenced by the conflicting pressures of two mediums can impose awkward constraints on animals. However, there are morphological and behavioural adaptations which permit animals to capitalize on the advantages of living and feeding in water and of moving (that is, of flying) in air, while at the same time avoiding many of the disadvantages they might face in one medium alone. This article considers these adaptations, and explores their mechanical and energetic consequences for pleustonic animals which have evolved to move in both water and air.
It has been proposed elsewhere that flap-bounding, an intermittent flight style consisting of flapping phases interspersed with flexed-wing bounds, should offer no savings in average mechanical power relative to continuous flapping unless a bird flies 1.2 times faster than its maximum range speed (Vmr). Why do some species use intermittent bounds at speeds slower than 1.2Vmr? The 'fixed-gear hypothesis' suggests that flap-bounding is used to vary mean power output in small birds that are otherwise constrained by muscle physiology and wing anatomy to use a fixed muscle shortening velocity and pattern of wing motion at all flight speeds; the 'body-lift hypothesis' suggests that some weight support during bounds could make flap-bounding flight aerodynamically advantageous in comparison with continuous flapping over most forward flight speeds. To test these predictions, we studied high-speed film recordings (300 Hz) of wing and body motion in zebra finches (Taenopygia guttata, mean mass 13.2 g, N=4) taken as the birds flew in a variable-speed wind tunnel (0-14 m s-1). The zebra finches used flap-bounding flight at all speeds, so their flight style was unique compared with that of birds that facultatively shift from continuous flapping or flap-gliding at slow speeds to flap-bounding at fast speeds. There was a significant effect of flight speed on all measured aspects of wing motion except percentage of the wingbeat spent in downstroke. Changes in angular velocity of the wing indicated that contractile velocity in the pectoralis muscle changed with flight speed, which is not consistent with the fixed-gear hypothesis. Although variation in stroke-plane angle relative to the body, pronation angle of the wing and wing span at mid-upstroke showed that the zebra finch changed within-wingbeat geometries according to speed, a vortex-ring gait with a feathered upstroke appeared to be the only gait used during flapping. In contrast, two small species that use continuous flapping during slow flight (0-4 m s-1) either change wingbeat gait according to flight speed or exhibit more variation in stroke-plane and pronation angles relative to the body. Differences in kinematics among species appear to be related to wing design (aspect ratio, skeletal proportions) rather than to pectoralis muscle fiber composition, indicating that the fixed-gear hypothesis should perhaps be modified to exclude muscle physiology and to emphasize constraints due to wing anatomy. Body lift was produced during bounds at speeds from 4 to 14 m s-1. Maximum body lift was 0.0206 N (15.9 % of body weight) at 10 m s-1; body lift:drag ratio declined with increasing air speed. The aerodynamic function of bounds differed with increasing speed from an emphasis on lift production (4-10 m s-1) to an emphasis on drag reduction with a slight loss in lift (12 and 14 m s-1). From a mathematical model of aerodynamic costs, it appeared that flap-bounding offered the zebra finch an aerodynamic advantage relative to continuous flapping at moderate and fast flight speeds (6-14 m s-1), with body lift augmenting any savings offered solely by flap-bounding at speeds faster than 7.1 m s-1. The percentage of time spent flapping during an intermittent flight cycle decreased with increasing speed, so the mechanical cost of transport was likely to be lowest at faster flight speeds (10-14 m s-1).
The flow characteristics around an elliptic cylinder with an axis ratio of AR=2 located near a flat plate were investigated experimentally. The elliptic cylinder was embedded in a turbulent boundary layer whose thickness is larger than the cylinder height. For comparison, the same experiment was carried out for a circular cylinder having the same vertical height. The Reynolds number based on the height of the cylinder cross-section was 14000. The pressure distributions on the cylinder surface and on the flat plate were measured for various gap distances between the cylinder and the plate. The wake velocity profiles behind the cylinder were measured using hot-wire anemometry. In the near-wake region, the vortices are shed regularly only when the gap ratio is greater than the critical value of G/B=0·4. The critical gap ratio is larger than that of a circular cylinder. The variation of surface pressure distributions on the elliptic cylinder with respect to the gap ratio is much smaller than that on the circular cylinder. This trend is more evident on the upper surface than the lower one. The surface pressures on the flat plate recover faster than those for the case of the circular cylinder at downstream locations. As the gap ratio increases, the drag coefficient of the cylinder itself increases, but the lift coefficient decreases. For all gap ratios tested in this study, the drag coefficient of the elliptic cylinder is about half that of the circular cylinder. The ground effect of the cylinder at small gap ratio constrains the flow passing through the gap, and restricts the vortex shedding from the cylinder, especially in the lower side of the cylinder wake. This constraint effect is more severe for the elliptic cylinder, compared to the circular cylinder. The wake region behind the elliptic cylinder is relatively small and the velocity profiles tend to approach rapidly to those of a flat plate boundary layer
This study investigates the effects of various ground clearances (h/D=0.14, 0.2, 0.5, 1.0), as well as the type of ground (stationary or moving ground plane), on the flow around a variety of two-dimensional bluff bodies. The measured base pressures with or without a moving ground were almost identical, suggesting that the drag remains almost constant. However, significant changes in the lift forces were observed when the ground plane was moving. The difference in pressure distribution between the top and bottom of the model increases as the ground clearance is reduced. This phenomenon is exaggerated with the introduction of a moving ground which concludes that larger lift forces and longer wakes are apparent with decreasing clearance. The measurement of Strouhal number show that the low ground clearance dampens the periodic flow behind the models, the effect is intensified with a moving ground plane. A comparison of the results with those of the flow around a two-dimensional car model (h/D=0.14, 0.2) underline the dependence of the results on the model geometry.
The purpose of this paper is to report test results which have been obtained on a high speed moving ground (belt) wind tunnel (60 m s−1). In this paper, we will do a comparison of results with “belt on” and “belt off”.We will also compare results with a slow wind tunnel with moving ground (25 m s−1) and with two other wind tunnels with stationary ground, one blowing at 30 m s−1, the other at 58 m s−1.The test model is a scale sedan car with simulation of all mechanical underbody and bonnet components. The car has a rear transmission and a front engine. It is a three volumes type.Starting from the basic shape of the car, we have a kit of components ready made and easy to fit on the car which makes it possible to change from one configuration to another quickly and with good accuracy. This enables us to test a lot of shapes, to compare belt on-belt off results and to obtain interesting conclusions.
The work presented here is the result of an effort by the authors to study and document the aerodynamic effect of varying aspect ratio, ground clearance, and underbody roughness for basic rectangular shapes with radiused edges in ground effect. Particular emphasis is placed on interpretation of wake flow survey results and the correlation to measured forces.A study of rectangular bodies in ground effect with different aspect ratios, varying ground clearances, and different levels of underbody roughness was performed in a wind tunnel experiment. Results of force measurements and wake surveys obtained using a seven-hole pressure probe system are analyzed.Results for drag and lift coefficient are presented and analyzed. Expanded interpretation of wake survey data is presented to assist in the development of physical rationale to explain the force results. Strong correlation between the wake flows and force results is demonstrated.
A live laggar falcon (Falco jugger) glided in a wind tunnel at speeds between 6·6 and 15·9 m./sec. The bird had a maximum lift to drag ratio (L/D) of 10 at a speed of 12·5 m./sec. As the falcon increased its air speed at a given glide angle, it reduced its wing span, wing area and lift coefficient. A model aircraft with about the same wingspan as the falcon had a maximum L/D value of 10. Published measurements of the aerodynamic characteristics of gliding birds are summarized by presenting them in a diagram showing air speed, sinking speed and L/D values. Data for a high-performance sailplane are included. The soaring birds had maximum L/D values near 10, or about one quarter that of the sailplane. The birds glided more slowly than the sailplane and had about the same sinking speed. The ‘equivalent parasite area’ method used by aircraft designers to estimate parasite drag was modified for use with gliding birds, and empirical data are presented to provide a means of predicting the gliding performance of a bird in the absence of wind-tunnel tests. The birds in this study had conventional values for parasite drag. Technical errors seem responsible for published claims of unusually low parasite drag values in a vulture. The falcon adjusted its wing span in flight to achieve nearly the maximum possible L/D value over its range of gliding speeds. The maximum terminal speed of the falcon in a vertical dive is estimated to be 100 m./sec.
Flight in ground effect above a flat, smooth surface may give an animal considerable performance advantages, including a reduction in cost of transport of up to 15%, and a reduction in mechanical flight power of as much as 35%, compared with values for flight out of ground effect. Previous theories modelling the phenomenon have either been incomplete or marred by typographical errors. A complete lifting line theory of flight in ground effect with a fixed wing is developed, and instructions are given so that it may be applied to animals such as skimmers, pelicans and myotid bats which fly and forage close above water. Several predictions are made about likely flight behaviour in ground effect, and about the appropriate flight morphology for taking advantage of the potential performance improvements. The most important conclusion, differing from previous analyses, is that slow flight performance in ground effect is very poor, owing to the horizontal air velocity induced around the wing in the presence of the ground.
The off-surface aerodynamic characteristics of a wing in ground effect are investigated using a number of methods including laser Doppler anemometry and particle image velocimetry. The study focuses on two aspects of the flow: turbulent wake and edge vortex. These features are closely associated with the behavior of the aerodynamic force in ground effect. The size of the wake increases in proximity to the ground. A downward shift of the path of the wake is also observed. Discrete vortex shedding is seen to occur behind the wing. As the wing height is reduced, separation occurred on the suction surface of the wing, and the spanwise vortex shedding is found to couple with a flapping motion of the wake in the transverse direction. An edge vortex is also observed off the edge of the end plate of the wing, which contributes to force enhancement and helps to define the force behavior in the force enhancement region. The rate of change in the downforce vs height curve is linked to the strength of the edge vortex. The vortex breakdown signals a slowdown in the force enhancement. When the maximum downforce height is reached, the. edge vortex breaks down completely.
This paper presents a review of theoretical and conventional experimental approaches to studies of wind-induced water currents and introduces a new method for simulating these motions in the laboratory. Mean velocity profiles obtained in conventional air-water systems when plotted in terms of the coordinates of the smooth inner law of velocity distribution, relegate all of the experiments into the region of transitional roughness. The shortcomings of laboratory air-water systems such as uncontrollable surface roughness and three-dimensionality of flow are overcome with novel experimental approach in which the actual water body is presented by the air volume contained inside a moving box, and the surface of a stationary wall constitutes the actual air-water interface. Mean velocity profiles obtained with this apparatus are used to verify an analytical solution based on the parabolic distribution of the eddy viscosity.
The purpose of this article is to provide a synthetic and comparative view of selected aircraft and rotorcraft (nearly 300 of them) from past and present. We report geometric characteristics of wings (wing span, areas, aspect-ratios, sweep angles, dihedral/anhedral angles, thickness ratios at root and tips, taper ratios) and rotor blades (type of rotor, diameter, number of blades, solidity, rpm, tip Mach numbers); aerodynamic data (drag coefficients at zero lift, cruise and maximum absolute glide ratio); performances (wing and disk loadings, maximum absolute Mach number, cruise Mach number, service ceiling, rate of climb, centrifugal acceleration limits, maximum take-off weight, maximum payload, thrust-to-weight ratios). There are additional data on wing types, high-lift devices, noise levels at take-off and landing. The data are presented on tables for each aircraft class. A graphic analysis offers a comparative look at all types of data. Accuracy levels are provided wherever available.
The accurate prediction of ground effect aerodynamics is an important aspect of wing-in-ground (WIG) effect vehicle design. When WIG vehicles operate over water, the deformation of the nonrigid surface beneath the body may affect the aerodynamic performance of the craft. The likely surface deformation has been considered from a theoretical and numerical position. Both two-dimensional and three-dimensional cases have been considered, and results show that any deformation occurring on the water surface is likely to be caused by the wing tip vortices rather than an increased pressure distribution beneath the wing.
‘Four-winged’ flying fish (in which both pectoral and pelvic fins are hypertrophied) reach greater maximum sizes than ‘two-winged’ forms in which only the pectoral fins are enlarged. Exocoetus obtusirostris shows negatively allometric growth in relation to standard length in terms of body mass (b=2·981), and lateral fin area (b=1·834). In consequence, wing-loading rises in positive allometric fashion with standard length (b=l·236). Pectoral fin length cannot be greater than 78–79% of standard length or swimming will be impaired, so the requirement for increased flying speed resulting from increased wing-loading during growth means that lift:drag ratios have to be improved by relatively narrowed wings and tapered wing tips; features which in turn increase wing-loading. Evidence is presented to show that hypertrophied pelvic fins in four-wingers are required to solve problems of stability in pitch, rather than to decrease wing-loading. The ‘non-flying’ flying fish, Oxyporhamphus micropterus, has very high wing-loadings, but the main reason that it cannot fly is that the centre of gravity of the fish is so far behind the pectoral fins that stalling on take-off would be inevitable. Flying fish possess reasonable quantities of red axial musculature, but no more than are used for cruising in fast-moving pelagic fish such as mackerel; it seems probable that acceleration to take-off speed in flying fish requires use of anaerobic white muscles.
Systematic measurements of drift currents below and of airflows above an air-water interface have been made under various wind conditions. The current near but not immediately below the water surface is found to follow a Kármán-Prandtl (logarithmic) velocity distribution. The current immediately below the water surface varies linearly with depth. The transitions of the current boundary layer to various regimes appear to lag behind, or to occur a t a higher wind velocity than, those of the airflow. The fraction of the wind stress supported by the wave drag seems to vary with the wind and wave conditions: a large fraction is obtained at low wind velocities with shorter waves and a small fraction is obtained a t high wind velocities with longer waves. At the air-water interface, the wind-induced current is found to be proportional to the friction velocity of the wind. The Stokes mass transport, related to wave characteristics, is only a small component of the surface drift in laboratory tanks. However, in terms of the fraction of the wind velocity, the mass transport increases, while the wind drift decreases, as the fetch increases. The ratio between the total surface drift and the wind velocity decreases gradually as the fetch increases and approaches a constant value of about 3·5% at very long fetches.
Flying fish wing area and wing-loading both rise in strongly negative allometric fashion with increasing body length and mass. Evidence is presented to show that this occurs because: (1) the leading edge of the pectoral fin ‘wing’ is fixed at 24% of standard length ( LS) from the snout, (2) the wing length cannot exceed 76% of LS or the tips will interfere with propulsive tail beat and (3) increased mass demands faster flying and wings with better lift : drag ratios; this selects for tapered, higher aspect ratio wing shapes. A consequence of this situation is that larger flying fishes have centres of mass increasingly further behind the centre of wing pressure. Resultant longitudinal instability restricts the maximum size of the two-winged design and the pelvic fins of four-wingers act as a stabilizing tailplane. These data indicate that the accepted model of evolution of flight in flying fishes (by extension of ballistic leaps) is flawed; it is proposed that evolution of lift-supported surface taxiing in half-beaks with enlarged pectoral fins (enhanced by ground effect) was an essential preliminary; subsequent forward migration of the centre of mass to within the wing chord permitted effective gliding.
In the present study, we perform a wind-tunnel experiment to investigate the aerodynamic performance of a gliding swallowtail-butterfly
wing model having a low aspect ratio. The drag, lift and pitching moment are directly measured using a 6-axis force/torque
sensor. The lift coefficient increases rapidly at attack angles less than 10° and then slowly at larger attack angles. The
lift coefficient does not fall off rapidly even at quite high angles of attack, showing the characteristics of low-aspect-ratio
wings. On the other hand, the drag coefficient increases more rapidly at higher angles of attack due to the increase in the
effective area responsible for the drag. The maximum lift-to-drag ratio of the present modeled swallowtail butterfly wing
is larger than those of wings of fruitfly and bumblebee, and even comparable to those of wings of birds such as the petrel
and starling. From the measurement of pitching moment, we show that the modeled swallowtail butterfly wing has a longitudinal
static stability. Flow visualization shows that the flow separated from the leading edge reattaches on the wing surface at
α < 15°, forming a small separation bubble, and full separation occurs at α ≥ 15°. On the other hand, strong wing-tip vortices are observed in the wake at α ≥ 5° and they are an important source of the lift as well as the main reason for broad stall. Finally, in the absence of
long hind-wing tails, the lift and longitudinal static stability are reduced, indicating that the hind-wing tails play an
important role in enhancing the aerodynamic performance.
KeywordsGliding-Hind-wing tails-Lift-to-drag ratio-Longitudinal static stability-Low-aspect-ratio wing-Modeled swallowtail butterfly wing
In the present study, we experimentally investigate the aerodynamic capabilities of flying fish. We consider four different
flying fish models, which are darkedged-wing flying fishes stuffed in actual gliding posture. Some morphological parameters
of flying fish such as lateral dihedral angle of pectoral fins, incidence angles of pectoral and pelvic fins are considered
to examine their effect on the aerodynamic performance. We directly measure the aerodynamic properties (lift, drag, and pitching
moment) for different morphological parameters of flying fish models. For the present flying fish models, the maximum lift
coefficient and lift-to-drag ratio are similar to those of medium-sized birds such as the vulture, nighthawk and petrel. The
pectoral fins are found to enhance the lift-to-drag ratio and the longitudinal static stability of gliding flight. On the
other hand, the lift coefficient and lift-to-drag ratio decrease with increasing lateral dihedral angle of pectoral fins.
KeywordsFlying fish-Aerodynamic performance-Gliding-Static stability-Wing morphology
1.
The review is concerned mainly with exocoetid flying fish, because little reliable information is available concerning other groups.
2.
Adult flying fish are of variable size (150–500 mm maximum length) and may be broadly divided into two categories: two-wingers (e.g.Fodiator, Exocoetus, Parexocoetus) in which the enlarged pectoral fins make up most of the lifting surfaces, and four-wingers (e.g.Cypsilurus, Hirundichthys) in which both pectoral and pelvic fins are hypertrophied.
3.
The pectoral girdle of flying fish is considerably enlarged by comparison with most teleosts, the coracoid and scapula being particularly hypertrophied.
4.
The pectoral fins are controlled by two groups of muscles, the lateral muscles that extend the wings, and the medial muscles that furl them. Both groups appear from external appearances to be red (aerobic) muscles.
5.
A general picture of the flight of an adult four-wing flying fish is presented: fish swim toward the surface at very high speed (< 30="" body="" lengths="">-1) with the lateral fins furled, leap through the water surface at a shallow angle, accelerate to take-off speed by taxiing with the lateral fins expanded and the tail beating in the water at up to 50 beats s-1, and enter a free flight that may be prolonged by further taxiing.
6.
Flying fish do not flap their wings to gain lift, but a whirring noise oroduced during take-off is possibly due to fluttering caused by the coupling together of the contraction of the axial muscles in the production of tail movements, and the action of the pectoral muscles in moving the pectoral fin rays. Alternatively, the noise may be due to a passive, flag-like function of the wings, stemming from their relatively rigid leading edges and flexible trailing edges.
7.
Flying fish grow in slightly (but significantly) negatively allometric fashion, becoming slimmer with increased body length. On the other hand, wingloading has a markedly positive allometric relationship with standard length because flying fish cannot increase relative wingspan during growth, but have to narrow the wings to improve performance as they fly at greater speeds. Wingloading of flying fish is similar to that of birds and bats, and the largest of flying fish exhibit wingloadings similar to cormorants and pelicans.
8.
The expanded, flat pelvic fins of four-wingers have evolved, not to increase wing area, but to function as tailplanes or stabilizers well behing the centre of gravity, with an area some 20–35% of the total lateral fin area, and an angle of incidence less than that of the cambered pectoral fins.
9.
Flying fish start to exhibit flight at a length of around 50 mm; at smaller sizes surface tension is of importance, limiting flying fish to simple leaps with the fins held against the body by surface tension. Evidence is presented to indicate that smaller flying fish gain positive benefits to their swimming performance from possession of expanded lateral fins.
10.
For a flying fish of 0.3 m standard length, significant reduction of drag by ground effect will take place at heights below about 0.5 m, prolonging flights and helping take-off.
11.
Flying fish are in general limited to surface waters warmer than 20–23 C. Evidence is presented to show that it is unlikely that flying fish are capable of flight at temperatures below 20 C because of fundamental limitations of muscle function.
12.
The most recent cladistic analysis supports the view that flying fish evolved from half-beak-like ancestors. They probably developed from elongate epipelagic fishes with hypocercal tails that helped them to swim quickly in the near-surface high-drag zone.
13.
Flying fish probably fly mainly to escape from predators, particularly dolphin-fishes (Coryphaena hippurus) and ommastrephid squid. An alternative hypothesis of energy conservation is rejected; other possibilities (e.g. migration between food-poor and food-rich areas) are at present supported by limited evidence.
The vortices behind a bluff body equipped with an upswept aft section are studied in a model test. The bluff body operates in close proximity to ground. The principle measurement technique is laser Doppler anemometry (LDA), which is supported by surface flow, pressure and force measurements. The upswept surface has an angle of 17° to horizontal. With the presence of end-plates on the aft section, discontinuities in slope of the force curve exist at several heights in proximity to the ground. The characteristics of these changes are linked with edge vortices. The position and strength of the vortex are identified. Three main types of trailing vortices exist: (a) concentrated, symmetric with a high axial speed core, (b) diffused, symmetric with a low axial speed core, and (c) diffused and asymmetric. The study provides further clarification of major physics and a database for validating predictive methods.
The flow characteristics over a symmetrical airfoil––NACA 0015––are studied experimentally in a low speed wind tunnel. The pressure distribution on the airfoil surface was obtained, lift and drag forces were measured and mean velocity profiles were obtained over the surface. The wake region was also explored in detail and measurements of mean velocity and turbulence intensities were performed at two stations downstream of the trailing edge. Experiments were carried out by varying the angle of attack, α, from 0° to 10° and ground clearance of the trailing edge from the minimum possible value to one chord length. It was found that high values of pressure coefficient are obtained on the lower surface when the airfoil is close to the ground. This region of high pressure extended almost over the entire lower surface for higher angles of attack. As a result, higher values of lift coefficient are obtained when the airfoil is close to the ground. The flow accelerates over the airfoil due to flow diversion from the lower side, and a higher mean velocity is observed near the suction peak location. The pressure distribution on the upper surface did not change significantly with ground clearance for higher angles of attack. The upper surface suction causes an adverse pressure gradient especially for higher angles of attack, resulting in rapid decay of kinetic energy over the upper surface, leading to a thicker wake and higher turbulence level and hence a higher drag. The lift was found to drop at lower angles of attack at some values of ground clearance due to suction effect on the lower surface as the result of formation of a convergent–divergent passage between the airfoil and the ground plate. For the angle of attack of 12.5°, a very thick wake region was observed and higher values of turbulence intensity were recorded.
A model of the mechanics of gliding without loss of altitude (horizontal gliding) is developed. The model can be employed to assess the influence of the principal drag components (induced, profile and parasite drag), choice of initial and final glide velocities and height above the ground on glide distance. For birds gliding near to the ground the ground effect acts to decrease the induced drag and increase the lift to drag ratio of the wings. Minimum drag speed is reduced for birds gliding near to the ground. The model is applied to the gliding flight of the black skimmer (Rhyncops nigra). Glide distances for given initial and final velocities are significantly increased in the influence of the ground effect over out of ground effect values.
Stability is as essential to flying as lift itself, but previous discussions of how flying animals maintain stability have been limited in both number and scope. By developing the pitching moment equations for gliding animals and by discussing potential sources of roll and yaw stability, we consider the various sources of static stability used by gliding animals. We find that gliding animals differ markedly from aircraft in how they maintain stability. In particular, the pendulum stability provided when the centre of gravity lies below the wings is a much more important source of stability in flying animals than in most conventional aircraft. Drag-based stability also appears to be important for many gliding animals, whereas in aircraft, drag is usually kept to a minimum. One unexpected consequence of these differences is that the golden measure of static pitching stability in aircraft--the static margin--can only strictly be applied to flying animals if the equilibrium angle of attack is specified. We also derive several rules of thumb by which stable fliers can be identified. Stable fliers are expected to exhibit one or more of the following features: (1) Wings that are swept forward in slow flight. (2) Wings that are twisted down at the tips when swept back (wash-out) and twisted up at the tips when swept forwards (wash-in). (3) Additional lifting surfaces (canard, hindwings or a tail) inclined nose-up to the main wing if they lie forward of it, and nose-down if they lie behind it (longitudinal dihedral). Each of these predictions is directional--the opposite is expected to apply in unstable animals. In addition, animals with reduced stability are expected to display direct flight patterns in turbulent conditions, in contrast to the erratic flight patterns predicted for stable animals, in which large restoring forces are generated. Using these predictions, we find that flying animals possess a far higher degree of inherent stability than has generally been recognized. This conclusion is reinforced by measurements of the relative positions of the centres of gravity and lift in birds, which suggest that the wings alone may be sufficient to provide longitudinal static stability. Birds may therefore resemble tailless aircraft more closely than conventional aircraft with a tailplane.
Take-off and flight of the flying fish.
- Hertel
Hertel, H. (1966). Take-off and flight of the flying fish. In Structure-Form-Movement
(ed. M. S. Katz), pp. 218-224. New York: Reinhold Publishing Company.
Examples of three representative types of airfoil-section stall at low speed
- McCullough
McCullough, G. B. and Gault, D. E. (1951). Examples of three representative types of
airfoil-section stall at low speed. NACA TN-2502.
On a fin and a prayer
- Fish
Fish, F. E. (1991). On a fin and a prayer. Scholars 3, 4-7.
Flying-fish aerodynamics
- Latimer-Needham
Latimer-Needham, C. H. (1951). Flying-fish aerodynamics. Flight 26, 535-536.