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Numerical-experimental evaluation and modelling of aerodynamic ground effect for small-scale tilted propellers at low Reynolds numbers
Abstract
In recent years, aerial manipulators with fully-actuated capabilities are gaining popularity for being used in aerial manipulation operations such as critical infrastructure inspection or aerial manipulation tasks. Those scenarios usually demand the aerial platform to operate in constrained and narrow scenarios. It is well known that in these situations, the interaction of the wake generated by the propellers with the environment can significantly alter and change the performance of the rotors. Most studies have addressed this problem by considering the ground effect in hover conditions or during the landing maneuver for co-planar multirotor. However, few works analyze the behavior of tilted rotors, which are used in fully actuated multirotor configurations thanks to their omnidirectional motion capabilities. This paper presents a numerical-experimental evaluation of the aerodynamic ground effect for small-scale tilted propellers at low Reynolds numbers. This aerodynamic effect has been experimentally evaluated through an extensive testing campaign in a testbench designed for this purpose which has been complemented by a CFD-based study. CFD results have been validated through a mesh independence study and a CFD-experimental propeller performance comparison. A numerical model has been also proposed to capture the dependence of thrust with distance to the ground and angle of inclination between the propeller and ground planes. We demonstrate that the proximity to the ground of tilted rotors decreases the thrust increment due to the ground effect as the tilt angle (θ) increases. This means that Cheeseman's classical theory is inapplicable, as it only considers the distance from the ground without reference to how the thrust increment changes with the tilt angle. This outcome enables future aerial robotic applications that strongly demand accurate aerodynamic effect models to operate close to obstacles and narrow environments.
... to their novel concept. Along these lines, the ground effect of a single-tilted rotor is examined in [21]. In addition, a model is proposed in which the thrust increase depends on both the rotor height and the rotor inclination. ...
... Previous theories do not consider the tilt angle of the rotors in a multirotor. However, the first ground effect model was proposed in [21] for single-tilted rotors. On the one hand, in the upper right-hand side of the Fig. 4, it is shown in shading how the thrust ratio changes when an isolated propeller is tilted with respect to the ground (θ). ...
... This model will be used as a starting point for considering the inclination of the rotors in a similar way to that in [21]. ...
This paper investigates the ground proximity effects of a fully-actuated multirotor unmanned aerial vehicle (UAV). Different configurations of tilted rotors and a wide range of UAV heights relative to the ground are considered. In order to characterize the effect, an extensive experimental study was conducted on a static test-bench. The experimental results display a behavior differing from that previously observed for multirotors with co-planar propellers and single-tilted rotors. We demonstrate that the ground effect increases with angle when the UAV is near the ground, while the opposite occurs for areas further away. Two new ground effect models are proposed, one for co-planar and another for fully-actuated configuration. In both cases, the models are dependent on the propeller radius, the distance between the rotors, the height of the UAV and the fountain effect. In addition, in the fully-actuated configuration, the model also depends on the inclination of the rotors. Finally, the proposed models are integrated into the UAV control architecture using the Aerodynamic Power Model Inversion (APMI) module, showing a significant improvement in flight accuracy when the ground effect is considered.
... A simple relationship between rotor thrust and downwash flow can be easily obtained in the streamtube using the two physical laws of conservation of mass and conservation of momentum. Taking into account the geometry of the rotor blade, the expression for thrust given the inflow distribution across the rotor disk is derived by introducing the blade element theory on the basis of the momentum theory ( Figure 5) [21]. A blade element is one small portion of the blade distance with r, from the center of rotation with a spanwise dimension,∆r. ...
... Also, the figure of merit (FM) is presented to characterize the hovering performance [21]. Mathematically, the thrust of the rotor is described by [22]: ...
... Also, the figure of merit (FM) is presented to characterize the hovering performance [21]. ...
This paper investigates the aerodynamic performance of an overlapping octocopter with the effect of horizontal wind ranging from 0 to 4 m/s using both low-speed wind tunnel tests and numerical simulations. The hovering efficiency and the potential control strategies of the octocopter under the effect of horizontal wind are also validated using blade element momentum theory. The velocity distribution, rotor pressure and vortex of the downwash flow with the horizontal wind are presented using the Computational Fluid Dynamics (CFD) method. Finally, wind tunnel tests were performed to obtain the thrust and power consumption with the rotor speed ranging from 1500 to 2200 rpm for horizontal winds at 0 m/s, 2.5 m/s and 4 m/s. The results showed that horizontal wind decreased the flight efficiency of the planar octocopter and had little effect on the coaxial octocopter. It is also interesting to note that horizontal wind is beneficial for thrust increments at a higher rotor speed and power decrements at a lower rotor speed for the overlapping octocopter. Specifically, the horizontal wind of 2.5 m/s for a lower rpm is presented with a power decrement with proper aerodynamic interference between the rotor blades. Additionally, the overlapping octocopter obtains a higher hover efficiency at 4 m/s compared to traditional octocopters, which is more suitable for flying in a cross wind with a more compact structure.
... 21 25 further evaluated the GE of ducted rotors in forward flight and found that it is difficult to detect the GE for angles of attack less than 30 , even if the rotor-ground gap decreases to 0.5 times the duct exit diameter when the advance ratio is relatively large. In addition to the GE in the infinite parallel ground plane, the impact of the GE with a limited ground plane, 26 square obstacles, 27 inclined surfaces, [28][29][30] and tilted rotors 31 were also explored. ...
... The equations outlined in Sec. II A are discretized and solved using the finite volume method (FVM), which is widely used in the numerical simulation of the rotor aerodynamic performance 16,17,[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]36,39 due to its suitability for unstructured grids. 42,43 In this paper, Star-CCMþ, a commercial CFD solver, is utilized for its advanced meshing and parallel capabilities, 44 ideal for aerospace aerodynamics 25,45,46 and proximity free-surface flow [47][48][49][50] simulations. ...
The aerodynamic performance of the rotor hovering on the air–water free-surface, which is significant for cross-medium unmanned aerial vehicles, is merely studied. In this study, a compressible two-phase flow model is used to compare the aerodynamic performance in the free-surface effect (FSE) and the ground effect (GE) with various dimensionless distances, γ, between the rotor and the ground (or free-surface). According to the results, the vortex core in FSE moves further in both vertical and radial directions than in GE for the early stages. Additionally, the blade surface is separated into three parts. In zone I, the aerodynamic performance is mostly determined by proximity effects. For both FSE and GE, the downward induced velocity at the rotor disk rises with increasing γ, leading to a decrease in the sectional thrust coefficient CT,S. By the way, CT,S is larger in FSE. In zone III, the aerodynamic performance is mostly governed by the blade tip vortex. The trend of aerodynamic performance with γ is reversed compared with zone I. The above-mentioned two opposing tendencies result in a smaller rotor thrust in FSE than in GE within the range of 0.60≤γ≤3.00, but a higher rotor thrust in FSE within the range of γ≤0.60.
... An increase of 60% in the pitch moment when approaching the obstacles was observed. The same results were obtained by Garofano-Soldado et al. [23] when they examined the effect of a tilted propeller on the ground. They also compared the CFD results to the experimental data. ...
... The objective of the studies mentioned above diverges from the performance of commercial and already-made drones propellers in a free hovering state [1,6,7,17,18] to the ground/obstacle effect [22,23]. They also investigated the flow interaction between the propellers [24] and its effect on the drone structure, with [19] or without the duct [8,9,12,14]. ...
The current project uses computational studies to design and improve a new type of small propeller used in unmanned aerial vehicles. The design process started with selecting the appropriate airfoils for the new propeller and then investigating the effects of changing the airfoil, angle of attack and rotational speed on its aerodynamic performances. The numerical simulation of the turbulent flow produced by the propeller’s rotation is carried out using the commercial calculation code ANSYS Fluent 14.0. Numerical results show that the newly developed propeller has a 116% improvement in thrust force compared to the commercial propellers used in today’s UAVs. Furthermore, the values of the critical angles strongly depend on the propeller's rotational speed. Higher critical angles of attack could be reached (without the boundary layer getting separated) in low RPMs than in high RPMs. On the other hand, the thrust force increases when the rotation speed increases. Therefore, the selection of the angles of attack strongly depends on the rotation speed range desired.
... Many attempts and considerations have been made to improve the design and control iteratively to keep the multi-rotor UAV stable in the air [14][15][16][17][18]. For instance, UAV rotors were tilted for disturbance rejection on an octocopter [12,19], where the authors demonstrated the horizontal thrust vector produced by the rotortilt could keep the UAV in more stable condition, i.e. demonstrated in the wind tunnel testing with the optimized control system. ...
... Reynolds number Another important parameter while determining the performance variation of any propeller is based on its operating Reynolds number (Re) [9,48,49]. The corresponding Re of a propeller is typically estimated by employing the tangential velocity and the chord length at the 3/4 radial position of the propeller blade [18,34]. Therefore, the equation of the Re is written as below - Moreover, there is a further variation in Re while the free stream velocity is introduced. ...
... Re is an important parameter for understanding the behaviour of the flow in its vicinity, as the propeller performance will vary with Re. According to the previous studies [40,41], the typical Re of the propeller is given as: ...
... The purpose of such settings is to simulate an environment under conventional operation. The geometric boundaries for the stationary and rotating zones are defined in accordance with several earlier CFD studies [13,41,52]. ...
... Studies have extensively applied MRF techniques to study complex flows around rotating propeller structures. These computational developments create new opportunities for understanding propeller performance characteristics through enhanced prediction capabilities and detailed flow analysis methods [6]. ...
This research work demonstrates computational prediction approaches to evaluate compact UAV propeller characteristics through full size domain and symmetrical domain assessments. The investigation employs flow simulation software ANSYS FLUENT to perform detailed calculations. Research conducted thorough evaluations between computed results and experimental test measurements to examine differences in thrust and power coefficients across both computational regions. The examined propeller device labeled APC 9x45 contains two rotating blades measuring 9 inches across plus 4.5 inches in pitch angle making it widely selected for validation research projects. The study utilized polyhexcore mesh arrangements combined with standard k-omega turbulence modeling techniques throughout computational analysis. Additionally Multiple Reference Frame calculations enabled proper assessment of blade rotation effects within specific coordinate systems at numerous operating speeds measured through RPM values. The findings reveal accurate thrust and power coefficient predictions while demonstrating that this computational method proves beneficial for systems with restricted computing power by reducing calculation duration when employing symmetrical domain analysis techniques
... A consensus has been reached that ground effects can result in increased rotor thrust and reduced duct thrust, while ceiling effects lead to increases in both thrusts. 25 However, the existing literature has mainly focused on the effects of hovering in proximity. 26 The aerodynamic behavior of ducted fans under dynamic proximity effects has not been thoroughly examined. ...
Ducted fans are widely employed in unmanned aerial vehicles (UAVs) for civil and military uses because they offer low noise and high efficiency. However, dynamic motions in close proximity can disrupt the flow patterns and aerodynamic behaviors of ducted fans, posing a significant stability risk. In this research, numerical simulations were performed using the unsteady Reynolds-averaged Navier–Stokes method and dynamic mesh technique to assess the dynamic performance of ducted fans when rising and descending in proximity effects. The findings reveal that, on the one hand, the rising motion enhances ground effects and diminishes ceiling effects. The thrust losses in ground effect can reach up to 80% of the hovering thrust, while the thrust gains in the ceiling effect can decrease to as low as zero as the translational velocity rises from 1 to 8 m/s. Karman vortex streets are observed in the fan wake at high-speed rise. On the other hand, the descending motion enhances both ground and ceiling effects, leading to increased losses and gains in thrust. The maximum changes account for 63% and 165% of the hovering thrust, respectively. Evolving vortex ring structures are observed during descent. These insights are crucial for optimizing ducted-fan aerodynamic designs and enhancing UAV flight control to ensure safe and efficient operations in varying dynamic conditions.
... The model was later on compared to real single-rotor helicopter flights, stating that the theoretical approximation is acceptable for values of distance to the ground greater than 0.6 times the radius of the rotor and low values of aircraft speed. Since then, many experimental tests have been performed for both single-rotors [5,6] and complete multi-copters [7][8][9], comparing real data to the theoretical model for helicopters. In [8] and [9], a test bench capable of measuring the thrust of the complete vehicle is used. ...
Multicopters are used for a wide range of applications that often involve approaching buildings or navigating enclosed spaces. Opposed to the open spaces in obstacle-free environments commonly flown by fixed-wing unmanned aerial vehicles, multicopters frequently fly close to surfaces and must take into account the airflow variations caused by airflow rebound. Such disturbances must be identified in order to design algorithms capable of compensating them. The evaluation of ground, ceiling and wall effects using two different test stands is proposed in this work. Different propellers and sensors have been considered for testing. The first test setup used was placed inside terraXcube’s large climatic chamber allowing a precise control of temperature and pressure of around 20°C and 1000 hPa, respectively. The second test setup is located at the University of Denver (DU) Unmanned Systems Research Institute (DU2SRI) laboratory with a stable pressure of around 800 hPa. Two different fixed 6 degrees of freedom force-torque sensors have been used for the experiments, allowing to sample forces and moments in three orthogonal axes. The tests simulate a hovering situation of a quadcopter at different distances to either the ground, the ceiling or a wall. The influence of the propeller size, rotation speed, pressure and temperature have also been considered and used for later dimensionless coefficient comparison. A thorough analysis of the measurement uncertainty is also included based on experimental evaluations and manufacturer information. Experimental data collected in these tests can be used for the definition of a mathematical model in which the effect of the proximity to the different surfaces is evaluated.
... Vertiport ground effects on a UAM aircraft is imperative to ensure safe operation and to avoid accidents that may occur during vertical take-off and landing near the ground. Several studies have previously investigated the influence of ground effects on the performance of helicopters through computational and experimental approaches [29][30][31][32]. ...
Urban air mobility (UAM) aircraft has emerged as the solution to the growing traffic congestion problems and increasing demand for efficient air mobility. However, noise pollution is one of the major concerns for gaining social acceptance as UAM is being designed for future transport in highly populated urban areas at low altitudes. The noise generated by UAM aircraft can exceed the acceptable noise level due to the ground effect when it approaches a vertiport. This study investigates the ground effects on the aerodynamic and noise performance of side-by-side UAM aircraft in full configuration by utilizing coupled vortex methods and acoustic analogy, respectively. The simulation results show that fuselage and ground directly influence the aerodynamic loads of the rotor blade, wake structure, acoustic signature, and noise directivity. As the aircraft approaches the ground, the sound pressure level (SPL) increases, and the impact of the fuselage becomes more noticeable, especially above the rotor system, due to the stronger upwash wake by the airframe and the ground. Moreover, the most pronounced ground effect on the noise characteristics of the UAM aircraft is the high-frequency tonal noise, and the overall sound pressure level (OASPL) in the aft quadrant of the UAM aircraft is higher than the forward quadrant due to the higher loading in the rear of the rotor plane. The results of the noise hemisphere analysis show that the maximum OASPL increases by more than 3 dBA as the altitude of UAM aircraft gets closer to the ground.
... For instance, during ultralow altitude cargo delivery, precise hovering or slow landing is essential. At altitudes less than 2-3 times the size of the UAV, the ground effect becomes noticeable [6], leading to downwash airflow and lateral crosswinds caused by the Bernoulli effect, which impact flight stability. Furthermore, rotorcraft logistic UAVs need to achieve high-speed and high-maneuverability flight while carrying a payload [7]. ...
This paper proposes a cascaded dual closed-loop control strategy that incorporates time delay estimation and sliding mode control (SMC) to address the issue of uncertain disturbances in logistic unmanned aerial vehicles (UAVs) caused by ground effects, crosswind disturbances, and payloads. The control strategy comprises a position loop and an attitude loop. The position loop, which functions as the outer loop, employs a proportional–integral–derivative (PID) sliding mode surface to eliminate steady-state error through an integral component. Conversely, the attitude loop, serving as the inner loop, utilizes a fast nonsingular terminal sliding mode approach to achieve finite-time convergence and ensure a quick system response. The time-delay estimation technique is employed for the online estimation and real-time compensation of unknown disturbances, while SMC is used to enhance the robustness of the control system. The combination of time-delay estimation and SMC offers complementary advantages. The stability of the system is proven using Lyapunov theory. Hardware-in-the-loop simulation and flight tests demonstrate that the control law can achieve a smooth and continuous output. The proposed control strategy can be effectively applied in complex scenarios, such as hovering, crash recovery, and high maneuverability flying, with significant practicality in engineering applications.
... It was observed that a high-pressure region was formed on the lower surface of the fuselage located between the front and rear wings, which is the main reason for the increase in total lift. Garofano-Soldado et al. [16] performed experimental and numerical evaluations of the aerodynamic ground effect for a small-scale tilted propeller. Results demonstrate that the thrust increment due to the ground effect decreases as the tilt angle increases when approaching the ground. ...
The aerodynamic performances and control characteristics of vertical takeoff and landing aircraft will be significantly affected by the ground effect during takeoff and landing. The longitudinal aerodynamic characteristics and main flow features of a fixed-wing vertical takeoff and landing aircraft hovering in ground effect are investigated in this paper, using Multiple Reference Frame based numerical simulations. The aircraft is propelled by three ducted fans. Results show that the overall morphology of the flow is characterized by a fountain, ground vortex, reingestion, and recirculation. When the thrust distribution between the front and aft ducted fans changes, the flow features change as well. As the aircraft approaches the ground, the ducted fan thrust decreases, the fuselage lift increases, and the total lift first decreases and then increases. The maximum lift increases by 9.6% and the minimum lift decreases by 3.0% compared with that out-of-ground effect. The magnitude of the drag is about 1% of the lift, which has little influence on the aircraft’s performance. The pitching moment gradually changes from the equilibrium state to a significant nose-up moment. The total power consumed increases at a specific rotational speed.
... As observed from the wind tunnel experiments of Airbus A400M military transport aircraft [17], four 6-component Rotating Shaft Balances were used to measure the thrust and torque plus the forces and moments contained in the propeller rotating plane (so called 1P-force). In addition, Garofano et al. [18] and Yang et al. [19] applied the 6-component balance to the ground effect experiment and aeroacoustics experiment of the propeller, which indicated that the balance provided an effective way to understand the aerodynamic characteristics of various configurations of propellers. Therefore, we designed a set of kinematic mechanisms to generate the coupled motion of the propeller in order to simulate the conditions of random wing vibration or encountering gust, and then measured the propeller follower forces with a 6-component balance. ...
Considering the vibration generated by a propeller-driven UAV or encountering gust, the propeller will perform a very complex follower motion. A pitch and rotating coupled motion is proposed in the present work that can take more complex unsteady performance of follower force than a regular fixed-point rotating motion. In order to evaluate the unsteady follower force and conduct parametric study, an extensive ground test bench was designed for this purpose where the whole test system was driven by a linear servo actuator and the follower force was measured by a 6-component balance. For CFD simulation, coupled motion in particular needs detailed unsteady aerodynamic model; therefore, a high-fidelity CFD-based study integrated with the overset mesh method was complemented to solve the unsteady fluid of varying conditions. The results suggest that a significant influence on unsteady follower force is observed, and the mean value of in-plane force does not equal to zero during the coupled motion process. Compared with the regular fixed-point rotation of propeller, the fluctuation frequency of follower force in present work couples the rotation and pitch motion frequencies. In addition, the oscillation amplitude of out-plane force and torque is positively related with the pitch frequency, pitch amplitude, and relative length from leading edge of wing to the rotation center. For example, the oscillation amplitude of 1-blade’s out-plane force and torque increases by 57.122% and 66.542% for the 5 Hz-5 deg case compared with the 5 Hz-3 deg case, respectively. However, the torque is not sensitive to frequency of pitch motion. The generally excellent agreement evident between the ground test and numerical simulation results is important as guidance for our future investigation on “dynamic” aerodynamic performance of a propeller-driven UAV.
... Studying the physics of unsteady aerodynamic phenomena in the low-Reynolds number (low-Re) flow regimes is crucial in many industrial applications. Small-sized aero-hydrodynamic mechanisms such as wind turbines, 1 rotor blades, 2 propellers, 3 flapping wings, 4 and energy harvesting devices 5,6 are among the examples of low-Re regimes. A better understanding of unsteady flow physics of the low-Re oscillating airfoil could be important in the selection of the flow separation control scenarios 7 and noise reduction methods. ...
This paper investigates the flow field around a NACA (National Advisory Committee for Aeronautics) 0012 airfoil undergoing pure pitching motion using continuous wavelet transform. Wind tunnel experiments were performed with a test-stand that provides a wide range of oscillation frequencies (f = 0–10 Hz). Sinusoidal pure pitching motion was considered with respect to the quarter chord for five reduced frequencies (K = 0.05, 0.1, 0.15, 0.2, and 0.3) at a Reynolds number of Re = 6 × 104. Mean angle of attack and pitch amplitude for all the cases were considered 0° and 6°, respectively. Unsteady surface pressure measurement was conducted, and the lift coefficient was calculated based on the phase-averaged surface pressure coefficient. The unsteady velocity distributions in the airfoil wake have been measured employing a pressure rake. The results indicate that the maximum value of the lift coefficient decreases by increasing the reduced frequency due to the “apparent mass” effects. For K = 0.05, close to the quasi-steady regime, the cl-α loop approximately follows the trend of the static case. Wavelet transform was used as a tool to examine the surface and wake pressure time series. Surface pressure wavelet transform plots indicate the presence of oscillation frequency and its superharmonics. Moreover, surface pressure wavelet analysis shows that the third and higher superharmonic frequencies are sensitive to the airfoil pitch angle during the oscillation cycle. Wavelet transform on wake reveals that the effective wake width gets smaller by increasing the reduced frequency. Furthermore, the trailing edge vortices get weaker by increasing the reduced frequency.
... En primer lugar, se necesitan nuevas estrategias de control, al tener el sistema de translación y de rotación desacoplados (Rajappa et al., 2015). Así mismo, al tener los rotores inclinados aparecen interacciones aerodinámicas entre los rotores, las cuales no aparecen en la configuración convencional (Garofano-Soldado et al., 2022). ...
Con el desarrollo de la robótica aérea han aparecido nuevas plataformas de multirotores de actuación completa (fully-actuated en inglés), las cuales tienen la capacidad de desplazarse sin inclinar la plataforma. Este artículo presenta una comparación en cuanto a capacidades de movimiento entre un hexarotor de rotores coplanarios, configuración estándar, y un hexarotor de rotores inclinados, configuración fully-actuated. Para ello, se presenta el diseño, modelo y control de ambas configuraciones. Tras el montaje de las plataformas, se comparan con diferentes trayectorias, mediante simulaciones y experimentos. Así mismo, se muestran capacidades exclusivas de la plataforma fully-actuated, como la capacidad de mantenerse en hover con un ángulo de inclinación. Finalmente, se presenta la aplicación de la plataforma fully-actuated para inspección visual de techos de puentes. Vídeo del artículo: https://youtu.be/d95Qvz5hba4
... The propeller cavity enclosed by the rotating fluid zone is defined as the smooth free-slip wall. The geometric boundaries for these zones are defined in accordance with several earlier CFD studies [21][22][23]. The computational domain is meshed using the ANSYS meshing tool and is shown in Fig. 3(b); the computational domain is discretized into unstructured meshes with tetrahedral-shaped elements. ...
... It was also observed that the separation at the leading edge could enhance the noise generated from these rotors. Garofano-Soldado A. at.al. [11] studied the ground effect on small-scale tilted rotors using experiments and CFD simulations. Three different rotors were used with diameters from 9 to 18 inches and Reynolds numbers from 0.46 × 10 5 to 2.2 × 10 5 . ...
The use of small rotors has increased due their applications in drones and UAVs. In order to improve the global performance of these aerial vehicles, it is necessary to understand the aerodynamics of small rotors, since this is related to the global energy consumption of such vehicles. Most of the computational fluid dynamics (CFD) studies found in the literature that are related to the analysis of small rotors employ fully turbulent models, despite the low-to-moderate Reynolds numbers of these applications. This paper presents CFD simulations for a small rotor at hover at different Reynolds numbers using fully turbulent and transitional SST k−ω turbulence models. Numerical results show that thrust and torque are close to experimental measurements, showing differences of less than 5% for both fully turbulent and transitional models. However, significant differences were observed between the fully turbulent and the transitional models when studying the boundary-layer development and separation. As the Reynolds number was increased, it was observed that at the tip of the blade, these differences were reduced, but at mid-span, the differences were more obvious.
... On the other hand, 3D RANS models have been used as a more accurate physical model with a reasonable computational cost for propeller design and optimization [11,12], typically at higher Reynolds numbers [13,14] or at low Reynolds numbers [15,16]. In the context of 3D RANS, aerodynamics can be modeled at low Reynolds numbers using transition turbulence models such as the γ − Re θ [17]. ...
Performance evaluations for propellers operating at high altitudes are subject to increased uncertainty due to scarce experimental or flight data and difficulties in modeling low Reynolds number flows.
For this reason, the Polynomial Chaos Expansion (PCE) method is used in this paper to assess the performance uncertainty of propellers operating at high altitudes. Aleatoric (i.e. linked to the geometry or operating conditions) and epistemic (i.e. linked to the mathematical model describing the flow) uncertainty variables are included in this study to estimate the total uncertainty related to performance predictions made by two physical models, namely 3D RANS with the use of γ − Reθ transition model and Blade Element Momentum Theory (BEMT). In order to validate the proposed method, multipoint uncertainty quantification (UQ) studies are performed for two benchmark propeller geometries under various operating conditions for which experimental data are available. The UQ method is further illustrated on a propeller operating at high altitude. The efficacy of UQ with Computational Fluid Dynamics (CFD) and BEMT is compared and the most influential uncertain variables are found using Sobol’s total order indices. As a result of the CFD-based uncertainty quantification studies, two major uncertain variables are identified, providing a direction for more computationally affordable UQ studies.
Keywords Drones have a major drawback which prevents them from being used extensively-the excessive noise they produce. This article presents an optimization process of aerodynamic noise reduction for a novel design called the Toroidal Propeller, which consists of two blades looping joined in a manner where the end of one blade arches back into the other, has been designed by the Aerospace Propulsion Systems (APSs) research team at School of Mechanical Engineering, Hanoi University of Science and Technology. In addition, four toroidal propellers with different curved shapes (pitch) and Number of Blades (NOB) are considered. The goal of this research is to evaluate the effects of modifying the geometry design of the Toroidal Propeller on the intensity of blade tip vortex and aerodynamic flow characteristics passing through the blade. The ultimate goal is to decrease blade tip vortex and turbulence produced by the blade, which can help estimate the sound pressure level and minimize it without causing significant performance losses. Based on the outcome results, the models of the four geometry studies are compared in terms of Acoustic Power Level (APL), Surface Acoustic Power Level (SAPL), Thrust, Torque, and Power. In general, the propeller model with NOB of 3 provides the most optimal efficiency in terms of both Thrust and APL. At the output cross-section, the APL dropped from nearly 139 dB to 121 dB, while thrust increased from almost 6.2N to 8.7N compared to the first version of the Toroidal Propeller model.
Wall-modeled large-eddy simulation (WMLES) is an advanced mathematical model for turbulent flows which solves for the low-pass filtered numerical solution. A subgrid-scale (SGS) model is used to account for the effects of unresolved small-scale turbulent structures on the resolved scales (i.e. for the dissipation of the smaller scales), while the flow behavior near the walls is modeled by wall functions (thus reducing the requirements for mesh fineness/ quality). This paper investigates the possibilities of applying WMLES in the estimation of aerodynamic performance of small-scale propellers, as well as in the analysis of the wake forming downstream. Induced flows around two propellers designed for unmanned air vehicles (approximately 25 cm and 75 cm in diameter) in hover are considered unsteady and turbulent (incompressible or compressible, respectively). Difficulties in computing such flows mainly originate from the relatively low values of Reynolds numbers (several tens to several hundreds of thousands) when transition and other flow phenomena may be present. The choice of the employed numerical model is substantiated by comparisons of resulting numerical with available experimental data. Whereas global quantities, such as thrust and power (coefficients), can be predicted with satisfactory accuracy (up-to several percents), distinguishing the predominant flow features remains challenging (and requires additional computational effort). Here, wakes forming aft of the propeller rotors are visualized and analyzed. These two benchmark examples provide useful guidelines for further numerical and experimental studies of small-scale propellers.
Grasping lightweight objects with aerial manipulators remains challenging due to the rotorwash interference caused by spinning rotor blades. This letter details a rotorwash-aware motion planning method for Chat-PM, a hybrid aerial-terrestrial vehicle equipped with a robotic arm. With proposed method's adjusting the vehicle's thrust and pose on flat surfaces timely, the rotorwash can be reduced and directed away from targets. To achieve this, we first model the rotorwash velocity distribution of Chat-PM in aerial and surface modes with computational fluid dynamics (CFD) simulations. The model further functions as an optimization objective of nonlinear model predictive control (NMPC) method, alongside other objectives including aerial-surface reference tracking and target grasping. An aerial-surface path searching method for the vehicle is then introduced, which takes into consideration both rotorwash velocity and kinodynamic feasibility to generate a high-quality reference for NMPC to track. Experimental results showcase the effectiveness of our proposed planning approach for aerial-surface grasping with rotorwash optimization.
The present work is aimed at analyzing and modelling the aerodynamic interactions of two small-scale tilted propellers operating at low Reynolds numbers near the ground. Two configurations of counter-rotating propellers found in fully-actuated multirotors are investigated: Face-to-Face, where the propeller flows are opposite and non-overlapping, and Back-to-Back, where the interaction between the rotor wakes occurs downstream. Moreover, an aerodynamic ground effect model is proposed for each non-planar rotor pair. To this end, CFD and test-bench experiments are used with different propellers and aerial platforms. Good agreement is found between the output of the proposed models and the numerical-experimental data. We demonstrated that the ground effect depends on rotor spacing, tilt angle, propeller radius, and rotor height, extending previous theories that do not consider these parameters. The results reveal that the near-ground aerodynamic effect for Face-to-Face propellers decreases with increasing inclination, but the opposite behaviour is observed for Back-to-Back propellers. Aerial robotics applications require a deep understanding of the effects of multirotors proximity to the environment to reliably and safely perform inspection and maintenance tasks in confined spaces or near infrastructure.
The ground effect is a phenomenon that takes place when an air vehicle is flying or hovering in vicinity of another surface, as this alters the airflow. Ground effect impacts inter alia flight stability which is a negative factor when landing. In this research we investigated a landing platform with a grid surface for a drone. 4 different textures for a landing platform were tested, a solid surface, a grid surface with hexagon cut-outs hovering in the air and the same grid with a solid surface 6cm below. We compared the vertical trust data of these to no surface within ground effect distance. The grid surface hovering in the air proved to have a 13% reduction of ground effect compared to the solid surface. While using the grid surface it is important to keep the distance of an underlying solid surface in mind. If the surface below the grid was too close, the positive effect was greatly reduced, making it no longer a preferable option to a solid surface. Therefore additionally, the minimal distance between the grid and the surface below was checked, for the second surface to be of no influence. This being 2 times the diameter of the rotor. This research shows potential for a grid surfaced landing platform, however due to stability issues while testing, further research on this topic is required.
Experimental tests and numerical simulations were typically presented with a rotor spacing of 1.2 times the diameter to investigate the effect of disc angle (DA) on the hover performance of non-planar multi-rotor unmanned aerial vehicles (UAVs). Torque, thrust coefficient, power coefficient, and power loading measurements were performed to evaluate the performance of the vehicles with different rotor rational speeds and DAs. The hovering performance exhibits a maximum improvement of 10.3 % when DA = 50° compared with the traditional planar muti-rotor system. The flow fields were also simulated by unsteady Reynolds-averaged Navier-Stokes solver. The downwash shifts, flow contractions, pressure difference enlargements, and wake inclinations were discovered and contributed to the interference reduction with the increasing DA. Based on these observations, the tilt-rotor configuration has a noticeable enhancement on the hovering performance of UAVs with proper DA. The insights gained can be applied as guidance for further UAV design.
Detailed studies of propeller flows are regaining both interest and significance worldwide, as the number of their different design and applications (particularly for futuristic urban air vehicles) continues to grow. An additional distinctive characteristic of small-scale unmanned air vehicles (UAV) propellers is that they are meant to operate in a wide range of (previously considered atypical) operating conditions, including backward flight, flight in the vicinity of obstacles, hard/ground surfaces, etc. These specific requirements raise the issue of the effects of ground proximity on their aerodynamic performance. This paper computationally investigates flows around a small-scale, custom-made propeller in ground effect. Different ground distances are considered and novel thrust and power relations (dependencies) on them are proposed. In order to obtain sufficiently reliable and accurate results and capture the most significant flow features, Reynolds-averaged Navier-Stokes (RANS) equations are solved by finite volume method. In addition, interesting flow visualizations are presented. Although the obtained thrust trend correlates well with the conventionally used semi-empirical formula, more realistic estimations are obtained for small ground distances. Furthermore, the positive effects of ground vicinity on rotor aerodynamic performances are once again confirmed and quantified.
The computational fluid dynamics (CFD) simulation is gaining attraction in the development of modern unmanned aerial vehicles (UAV), but few research has been made on quadcopters and the characterisation of the flows generated by the propellers, which determine the thrust capacity. Therefore, the purpose of this study is to assess the performance in the 3D flow simulation of the most promising methods: multiple reference frames (MRF) and sliding meshes. Additionally, the effect of the ground proximity has been included. The results for a sole propeller revealed both models as equivalent with respect to the evaluation of the ground effect, even though a noticeable deviation was observed in the thrust quantification. In the case of quadcopters, the relative position between blades and frame was proved as a key factor. Thus, similar rates of thrust change were obtained when minimising the superposition of the blade over the body arms in the MRF case. However, the thrust magnitude differed at least 11% at any simulated position. Assuming this deviation, the significantly lower computational cost of the MRF turns this model into a very interesting option. Finally, the influence of the relative blade-to-blade position in the sliding simulation was also assessed.
This paper describes a method for autonomous aerial cinematography with distributed lighting by a team of unmanned aerial vehicles (UAVs). Although camera-carrying multi-rotor helicopters have become commonplace in cinematography, their usage is limited to scenarios with sufficient natural light or of lighting provided by static artificial lights. We propose to use a formation of unmanned aerial vehicles as a tool for filming a target under illumination from various directions, which is one of the fundamental techniques of traditional cinematography. We decompose the multi-UAV trajectory optimization problem to tackle non-linear cinematographic aspects and obstacle avoidance at separate stages, which allows us to re-plan in real time and react to changes in dynamic environments. The performance of our method has been evaluated in realistic simulation scenarios and field experiments, where we show how it increases the quality of the shots and that it is capable of planning safe trajectories even in cluttered environments.
This article analyzes the evolution and current trends in aerial robotic manipulation, comprising helicopters, conventional underactuated multirotors, and multidirectional thrust platforms equipped with a wide variety of robotic manipulators capable of physically interacting with the environment. It also covers cooperative aerial manipulation and interconnected actuated multibody designs. The review is completed with developments in teleoperation, perception, and planning. Finally, a new generation of aerial robotic manipulators is presented with our vision of the future.
In the present work, the aerodynamic performance of the Caradonna and Tung and S-76 in hover were investigated using a simplified concept of multiple reference frame (MRF) technique in the context of high-order Monotone Upstream Centred Scheme for Conservation Laws (MUSCL) cell-centred finite volume method. In the present methodology, the frame of reference is defined at the solver level by a simple user input avoiding the use of mesh interface to handle the intersections between frames of reference. The calculations were made for both out-of-ground-effect (OGE) and in-ground-effect (IGE) cases and compared with experimental data in terms of pressure distribution, tip-vortex trajectory, vorticity contours and integrated thrust and torque. The predictions were obtained for several ground distances and collective pitch angle at tip Mach number of 0.6 and 0.892.
This article aims at collecting and discussing the results reached by the research community regarding the study of the ground effect on small rotorcraft unmanned aerial vehicles, especially from the modeling and control point of view. Rotorcraft performance is affected by the presence of the ground or any other boundary that alters the flow into the rotors. Specifically, the ground effect can induce perturbations in the flight stability, when operating near the ground. For a rotorcraft, an accident is likely to happen when the vehicle leaves or enters the ground effect region, which may cause crashes and property damages. Today, the use of unmanned aerial vehicles has grown widespread, which raises safety concerns when they are flying at very low altitudes and near the ground. Consequently, studying the influence of the ground over rotorcrafts is of paramount importance for general safety. Also, these investigations can be used to design systems of guidance, navigation, and control. In this review, we break down the most relevant works to date. We discuss aspects related to modeling, control, and application of the ground effect for small-scale multirotors, as well as other aerodynamic proximity effects, such as the ceiling and wall effects. We conclude by mentioning potential avenues of research when studying the ground effect from the point of view of the robotics and artificial intelligence fields.
This study presents the decays of three components of velocity for a ship twin-propeller jet associated with turbulence intensities using the Acoustic Doppler Velocimetry (ADV) measurement and computational fluid dynamics (CFD) methods. Previous research has shown that a single-propeller jet consists of a zone of flow establishment and a zone of established flow. Twin-propeller jets are more complex than single-propeller jets, and can be divided into zones with four peaks, two peaks, and one peak. The axial velocity distribution is the main contributor and can be predicted using the Gaussian normal distribution. The axial velocity decay is described by linear equations using the maximum axial velocity in the efflux plane. The tangential and radial velocity decays show linear and nonlinear distributions in different zones. The turbulence intensity increases locally in the critical position of the noninterference zone and the interference zone. The current research converts the axial momentum theory of a single propeller into twin-propeller jet theory with a series of equations used to predict the overall twin-propeller jet structure.
This Paper presents a new quasi-steady in-ground effect (IGE) model for small multirotor aerial vehicles such as quadcopter unmanned aerial vehicles. The model offers three major advances to the state of the art: 1) predicting the IGE thrust ratio without singularity, 2) relating the strength of the ground effect to the blade geometry (pitch angle, solidity, and radius), and 3) capturing the effect of multirotor configurations. Two IGE coefficients, based on blade element theory, are introduced and then analyzed using the method of images. Additionally, the thrust-loss phenomenon on multirotor IGE is observed and modeled empirically. Experiments are conducted to validate each component of the model, and the results suggest that the model can be used to predict the onset of the IGE for vehicle motion control and planning in the regime where IGE dominates.
This paper presents a novel paradigm for physical interactive tasks in aerial robotics allowing reliability to be increased and weight and costs to be reduced compared with state-of-the-art approaches. By exploiting its tilted propeller actuation, the robot is able to control the full 6D pose (position and orientation independently) and to exert a full-wrench (force and torque independently) with a rigidly attached end-effector. Interaction is achieved by means of an admittance control scheme in which an outer loop control governs the desired admittance behavior (i.e., interaction compliance/stiffness, damping, and mass) and an inner loop based on inverse dynamics ensures full 6D pose tracking. The interaction forces are estimated by an inertial measurement unit (IMU)-enhanced momentum-based observer. An extensive experimental campaign is performed and four case studies are reported: a hard touch and slide on a wooden surface, called the sliding surface task; a tilted peg-in-hole task, i.e., the insertion of the end-effector in a tilted funnel; an admittance shaping experiment in which it is shown how the stiffness, damping, and apparent mass can be modulated at will; and, finally, the fourth experiment is to show the effectiveness of the approach also in the presence of time-varying interaction forces.
This paper focuses on modeling and control of in-ground-effect (IGE) on multirotor unmanned aerial vehicles (UAVs). As the vehicle flies and hovers over, around, or underneath obstacles, such as the ground, ceiling, and other features, the IGE induces a change in thrust that drastically affects flight behavior. This effect on each rotor can be vastly different as the vehicle's attitude varies, and this phenomenon limits the ability for precision flight control, navigation, and landing in tight and confined spaces. An exponential model describing this effect is proposed, analyzed, and validated through experiments. The model accurately predicts the quasi-steady IGE for an experimental quadcopter UAV. To compensate for the IGE, a model-based feed-forward controller and a nonlinear-disturbance observer (NDO) are designed for closed-loop control. Both controllers are validated through physical experiments, where results show approximately 23% reduction in the tracking error using the NDO compared to the case when IGE is not compensated for.
There is a strong demand in the oil and gas industry to develop alternatives to manual inspection. This paper presents AeroX, a novel aerial robotic manipulator that provides physical contact inspection with unprecedented capabilities. AeroX has a semi-autonomous operation, which provides interesting advantages in contact inspection. In the free-flight mode, the pilot guides the robot until performing contact with its end-effector on the surface to be inspected. During contact, AeroX is in its fully-autonomous global navigation satellite system (GNSS)-free contact–flight mode, in which the robot keeps its relative position w.r.t. the surface contact point using only its internal sensors. During autonomous flight, the inspector can move—with uninterrupted contact—the end-effector on the surface for accurately selecting the points where to perform A-scan measurements or continuous B-scan or C-scan inspections. AeroX adopts an eight-tilted rotor configuration and a simple and efficient design, which provides high stability, maneuverability, and robustness to rotor failure. It can perform contact inspection on surfaces at any orientation, including vertical, inclined, horizontal-top or horizontal-bottom, and its operation can be easily integrated into current maintenance operations in many industries. It has been extensively validated in outdoor experiments including a refinery and has been awarded the EU Innovation Radar Prize 2017.
Recent mass commercialization of affordable Unmanned Aerial Vehicles (UAVs, or "drones") has significantly altered the media production landscape, allowing easy acquisition of impressive aerial footage. Relevant applications include production of movies, television shows or commercials, as well as filming outdoor events or news stories for TV. Increased drone autonomy in the near future is expected to reduce shooting costs and shift focus to the creative process, rather than the minutiae of UAV operation. This short overview introduces and surveys the emerging field of autonomous UAV filming, attempting to familiarize the reader with the area and, concurrently, highlight the inherent signal processing aspects and challenges .
The influence of time-varying ground effect (e.g., induced by ship deck motion) or even of static, in clined ground planes (e.g., hillsides) on the flow field and on the rotor inflow in hover is not yet understood. Therefore, experiments and CFD simulations were performed to study the flow field below a two-bladed 0.8 m-diameter rotor in hover over a parallel and a 15 degree inclined ground plane at a height of one rotor radius above the ground plane pivot point. Particle image velocimetry measurements were used to measure the rotor wake, and CFD simulations were correlated to the experimental results. To investigate the flow field, instantaneous, phase-averaged, and time-averaged data were used. The flow field was found to be sensitive to the ground plane inclination angle. It was found that the inclined ground plane reduced the unsteadiness in the flow field. The phase-averaged experimental results were predicted well by the numerical simulation. The computations captured the flow phenomenology well, but underestimated the influence of the inclined ground plane on the rotor inflow.
In this paper, we shed light on two fundamental actuation capabilities of multirotors. The first is the degree of coupling between the total force and total moment generated by the propellers. The second is the ability to robustly fly completely still in place after the loss of one or more propellers, in the case of mono-directional propellers. These are formalized through the definition of some algebraic conditions on the control allocation matrices. The theory is valid for any multirotor, with arbitrary number, position, and orientation of the propellers. As a show case for the general theory, we demonstrate that standard star-shaped hexarotors with collinear propellers are not able to robustly fly completely still at a constant spot using only five of their six propellers. To deeply understand this counterintuitive result, it is enough to apply our theory, which clarifies the role of the tilt angles and locations of the propellers. The theory is also able to explain why, on the contrary, both the tilted star-shaped and the Y-shaped hexarotors can fly with only five out of six propellers. The analysis is validated with both simulations and extensive experimental results showing recovery control after rotor losses.
Multirotor drones are considered a new and innovative technology. Therefore, many fields are showing increasing interest in utilizing multirotor drones, such as mapping in mining and surveillance in transportation. The construction industry has been a slow adopter of novel technologies. However, multirotor drones have potential to facilitate construction in many aspects. There is, therefore, a need to extensively research their applications and analyze their roles in construction engineering and management. This paper aims to comprehensively investigate the current applications of multirotor drones, analyze their benefits and explore their potential in the future of the construction industry. Several main aspects are reviewed and discussed, namely land surveying, logistics, on-site construction, maintenance and demolition. The results reveal that the main contributions are work safety, cost-effectiveness and carbon emission reduction, while there are possible adverse impacts on the basis of current limitations of multirotor drones. However, it can be predicted that the usefulness of drones will continue to increase in the future of the construction industry. Thus, this study will benefit construction managers in raising awareness of the use of these emerging technologies and researchers in further exploring applications of multirotor drones in construction projects.
In model tests, propellers are usually operated in nominal ship wake to measure exciting forces, and the incoming flow to propellers is different from that when the propeller is working right after the hull, which may lead to different results. Numerical methods were applied for simulations of two experiments: one is to simulate the propeller after a specially designed stern, and the nominal ship wake is set to the velocity inlet boundary, and free surface is considered; the other is to simulate the propeller right after the entire hull model. Results show that the amplitude of axial force of a single blade from the simulation of propeller after the stern is under-predicted, while the amplitudes of transverse and vertical forces of the whole propeller are over-predicted, but the differences are small. The amplitudes of fluctuating pressures on the hull surface predicted by the two approaches are basically the same.
A well-known phenomenon in rotorcraft, the ground effect, occurs when a rotor is at close proximity to the ground creating an obstruction to the trailing wake from the rotor. This results in an increase in thrust and a decreased inflow through the rotor disk, in turn affecting handling qualities. Until recently, analytical modelling of ground effect involved using empirical factors or method of images to account for the variation in thrust as a function of the elevation. The Morillo-Peters dynamic inflow model has now been known to satisfactorily predict the variations in rotor thrust for a rotor in ground effect. Although the variation in thrust seem to agree with empirical formulations, the model needs to be evaluated with experimental or computational simulations by comparing the variation of inflow over the rotor disk. An investigation on how closely the model predicts the inflow distribution is beneficial to future developments, considering the possibility of extending the model to other dynamic environments involving ground effect. This paper draws a comparison between the inflow models of Morillo, Ke Yu and Peters, Nowak and He, and a CFD package, RotCFD, for a hovering rotor in ground effect.
This paper analyzes the ground effect in multirotors, that is, the change in the thrust generated by the rotors when flying close to the ground due to the interaction of the rotor airflow with the ground surface. This effect is well known in single-rotor helicopters but has been assumed erroneously to be similar for multirotors in many cases in the literature. In this paper, the ground effect for multirotors is characterized with experimental tests in several cases and the partial ground effect , a situation in which one or some of the rotors of the multirotor (but not all) are under the ground effect, is also characterized. The influence of the different cases of ground effect in multirotor control is then studied with several control approaches in simulation and validated with experiments in a test bench and with outdoor flights.
The current work presents the numerical prediction method to determine small-scale propeller performance. The study is implemented using the commercially available computational fluid dynamics (CFD) solver, FLUENT. Numerical results are compared with the available experimental data for an advanced precision composites (APC) Slow Flyer propeller blade to determine the discrepancy of the thrust coefficient, power coefficient, and efficiencies. The study utilized unstructured tetrahedron meshing throughout the analysis, with a standard k-ω turbulence model. The Multiple Reference Frame model was also used to consider the rotation of the propeller toward its local reference frame at 3008 revolutions per minute (RPM). Results show reliable thrust coefficient, power coefficient, and efficiency data for the case of low advance ratio and an advance ratio less than the negative thrust conditions.
Mobility of a hexarotor UAV in its standard configuration is limited, since all the propeller force vectors are parallel and they achieve only 4-DoF actuation, similar, e.g., to quadrotors. As a consequence, the hexarotor pose cannot track an arbitrary trajectory while the center of mass is tracking a position trajectory. In this paper, we consider a different hexarotor architecture where propellers are tilted, without the need of any additional hardware. In this way, the hexarotor gains a 6-DoF actuation which allows to independently reach positions and orientations in free space and to be able to exert forces on the environment to resist any wrench for aerial manipulation tasks. After deriving the dynamical model of the proposed hexarotor, we discuss the controllability and the tilt angle optimization to reduce the control effort for the specific task. An exact feedback linearization and decoupling control law is proposed based on the input-output mapping, considering the Jacobian and task acceleration, for non-linear trajectory tracking. The capabilities of our approach are shown by simulation results.
Inspection and maintenance tasks are increasingly being carried out by multi-rotor platforms in order to avoid certain risks being taken by humans. In this way, it is necessary to have a good knowledge of the aerodynamic effects that occur when tasks are performed in confined environments. In addition, the use of tilted rotors is becoming more and more widespread for tasks that require direct contact with a surface. In that sense, this paper presents several numerical simulations to analyse the aerodynamic performance of tilted rotors. In particular, the wall effect and the corner effect will be shown in detail. Two different configurations are considered in the corner effect: straight and curved corner. The influence of the corner and the wall has been studied for three tilt angles (θ=20∘,30∘,40∘) and various distances between the rotor and the wall. Flow field visualization is depicted to understand the physical behaviour of the airflow.
We present FAST-Hex, a micro aerial hexarotor platform that allows to seamlessly transit from an under-actuated to a fully-actuated configuration with only one additional control input, a motor that synchronously tilts all propellers. The FAST-Hex adapts its configuration between the more efficient but underactuated, collinear multi-rotors, and the less efficient but fullpose-tracking, which is attained by non-collinear multi-rotors. On the basis of prior work on minimal input configurable micro aerial vehicle, we mainly stress three aspects: mechanical design, motion control, and experimental validation. Specifically, we present the lightweight mechanical structure of the FAST-Hex that allows to only use one additional input to achieve configurability and full actuation in a vast state space. The motion controller receives as input any reference pose in R 3 ×SO(3) (3D position + 3D orientation). Full pose tracking is achieved if the reference pose is feasible with respect to actuator constraints. In case of unfeasibility, a new feasible desired trajectory is generated online giving priority to the position tracking over the orientation tracking. Finally, we present a large set of experimental results shading light on all aspects of the control of the FAST-Hex.
The increasing use of unmanned aerial vehicles and micro air vehicles creates a strong demand for the accurate aerodynamic performance of small-diameters fixed-pitch propellers. The techniques range from low-fidelity to high-fidelity methods. Among the low-fidelity techniques, there is the blade element momentum theory (BEMT). Computational fluid dynamics (CFD) is categorized as a high-fidelity method. This study aims to employ these methods for the analysis of an APC propeller 14 × 7e. Geometric modeling was performed in a 3D scanner and simulations via BEMT and CFD. Analyses using the BEMT model were conducted with two formulations. The first one considered a linear lift coefficient up to the stall limits, without specifying the post-stall lift coefficients. The second one incorporates three-dimensional flow equilibrium effects, and best represents the post-stall conditions. In the CFD models, two approaches for the compatibility of the rotating and stationary domains were evaluated, the stationary frozen rotor (FR) and the transient arbitrary mesh interface (AMI). The kω SST turbulence model was employed. To investigate the influence of the transition laminar-turbulent boundary layer, the Gamma Theta transitional model was also considered. In the comparison of the results with the experimental tests, it was observed that the appropriate choice of analysis method had a strong relationship with the Reynolds number. For low advanced ratio velocities, the best results were obtained with the BEMT models. BEMT linear model overpredicted the power and efficiency for high advance ratio, while BEMT with tridimensional flow equilibrium presented consistent results with a low computational cost. On the other hand, the CFD-AMI model shows better results for higher advanced ratio velocities but is computationally more expensive. The CFD-FR model maintains a constant pattern of error that, in the analysis of efficiency, did not compromise the results.
Particle image velocity measurements were taken to document the wake generated by a rotor operating in ground effect above inclined surfaces. In particular, the current work focused on the average wake structure and axial velocity distribution through the rotor. A two-bladed rotor was operated at a height of one rotor radius above a ground plane that was inclined at angles from 0° to 30°. Rotor performance measurements were also taken, using a six-axis load cell, to examine the effect ground plane angle had on the thrust produced and power required. The wake structure was found to be very sensitive to ground plane angle causing the radial distribution of axial velocity through the rotor to increase inboard and decrease outboard with increasing ground plane angle. As compared to the level ground case, the peak figure of merit of the rotor decreased by approximately 10% for a ground plane inclination angle of 30°.
This paper describes progress made on the new design and analysis of a twin-propeller (front and rear) with duct. Major advantage of this design is that the Unmanned Aerial Vehicle (UAV) could complete the mission with only one engine, in the case of failure in one of the engines. Such a design also makes a fair weight distribution within the UAV, which makes it more stable. Furthermore, this design makes it possible to arm the UAV with missiles and other military equipment easily. Ducts traditionally increase the propulsive force produced by propeller at high speeds. Improper design of the duct can result in performance loss of the UAV. Therefore, the UAV model with Six different duct configurations are analyzed to find the most appropriate duct design. The Moving Reference Frame (MRF) method is implemented and examined in CFD simulations for flows around a moving rigid body. Variations included rotational speed of the propeller, incident angle allowing a comprehensive understanding of the overall aerodynamic characteristics of this design. The results of this study show good conformity with the experimental outcomes. Lift force and drag force are studied with different angles of attack. This work shows that rear and front propeller design with duct improves the efficiency by 6%, which offers better performance than two-propeller (without duct) designs.
Quadcopters are attracting a growing interest in many applications such as cargo delivery or surfaces inspection. These applications are often subjected to flights in the proximity of walls or ground that generate external forces and torques on the vehicle due to aerodynamic effects, because of what strong safety guarantees are required. In this research a methodology based on dynamic meshes was developed and applied to computational simulations to reproduce the flight of the drone over an obstacle. Thus, the effect of the ground proximity on the drone performance was assessed, and also its combination with the flow around the body at different translational velocities. The results shown a decrease of the drag force, and an increase of the lift and forward pitch moment due to the presence of the ground. These effects are magnified by the translational velocity, which also deviates the flow generated by the propellers and delays the interaction with the obstacle. Both in the approaching and leaving to the obstacle, increases of up to 60% in the pitch moment are observed. This sudden variations must be properly counteracted to guarantee the stability and safety of the drone operation.
Small-scale propellers, especially those used on conventional multicopter configurations or in the context of urban air mobility, may experience high inflow angles. In order to improve their aerodynamic efficiency, a better understanding of the flow-fields that occur under non-axial inflow conditions is required. Therefore, both isolated and ducted small-scale fixed-pitch propellers are analyzed at a range of inflow angles from zero to 180 degrees, to investigate their steady and temporal resolved loads. Furthermore, the influence of the inflow angle on the flow field is investigated. The load data is also used to simulate the efficiency of different configurative propulsion concepts. The aerodynamics coefficients obtained experimentally, as well as the flow field analysis by particle image velocimetry (PIV), are in close agreement with the unsteady numerical results obtained by URANS calculations.
This work presents the correlation between experimental measurements and numerical simulations concerning the aerodynamics of a hovering rotor operating in ground effect above parallel and inclined surfaces. The capability of a potential-flow, free-wake aerodynamic solver based on a boundary-element-method formulation to simulate in-ground-effect problems is examined in detail. In particular, two approaches for including ground effect are outlined and compared, namely the bounded domain method and the mirror image method. Analyses of rotor aerodynamic performance and flow-field visualization reveal that the experimental data and the numerical predictions given by the solver based on the mirror-image method are in good agreement, thus proving the suitability of this developed computational tool to simulate rotor aerodynamics in the presence of close ground. Concerning the bounded domain method, it has been proven that it is suitable for the evaluation of rotor thrust and wake shape close to the rotor, but does not provide accurate simulation of the flow-field in proximity to the ground and corresponding inflow over the rotor disk.
In the conventional turbulence models, the energy flux due to turbulent motion is often modeled by the gradient-diffusion approximation. In rotating turbulence, however, it is known that the turbulent energy is transfered faster in the direction of the rotation axis than that in non-rotating turbulence. In this study, we theoretically and numerically discuss a model describing the rapid energy transfer in rotating inhomogeneous turbulence. Theoretical analysis suggests that the rapid energy transfer due to the rotation is closely related to helicity. The numerical result shows that the proposed model gives a good performance at an early stage but the accuracy decreases at a later stage due to the wave-number dependence; that is, the low-wavenumber modes are transfered faster than the high-wavenumber modes.
This paper presents the design, modelling and control of a multirotor for inspection of bridges with full contact. The paper analyzes the aerodynamic ceiling effect when the aerial robot approaches the bridge surface from below, including its aerodynamic characterization using Computational Fluid Dynamics (CFD). The proposed multirotor design takes the modelled aerodynamic effects into account, improving the performance of the aerial platform in terms of the stability and position accuracy during the inspection. Nonlinear attitude and position controllers to manage the aerodynamic effects are derived and tested. Last, outdoor experiments in a real bridge inspection task have been used to validate the system, as well as, the controller and the aerodynamic characterization. The experiments carried out also include a full autonomous mission of the aerial platform during a structural assessment application.
This chapter presents the aerodynamic effects that appear when multirotors fly close to different structures, which is a common situation in aerial manipulation. The classical theory used to solve this problem in helicopters is explained, and a generalization of this method to quadrotors is introduced. The chapter also presents validation experiments in a test stand to measure these aerodynamic effects in different conditions.
An unmanned aerial vehicle (UAV), commonly known as a drone, offers the advantage of speed, flexibility, and ease in delivering goods to customers. They are particularly useful for tasks that are dull, hazardous, or dirty. Whether the use of drone delivery is beneficial to the environment and cost saving is still a topic under debate. Ideally, drones yield lower energy consumption and reduce greenhouse gas emissions, thus reducing the carbon footprint and enhancing environmental sustainability. In this research, we analytically study the impact of UAVs on CO 2 emission and cost. We propose a mixed-integer (0–1 linear) green routing model for UAV to exploit the sustainability aspects of the use of UAVs for last-mile parcel deliveries. A genetic algorithm is developed to efficiently solve the complex model, and an extensive experiment is conducted to illustrate and validate the analytical model and the solution algorithm. We find that optimally routing and delivering packages with UAVs would save energy and reduce carbon emissions. The computational results strongly support the notion that using UAVs for last-mile logistics is not only cost effective, but also environmentally friendly.
In ground effect conditions, the wake of Quad-copter rotors interacting with the ground causes significant perturbation to the flow near the rotor blades, as well as to the body. These interactions have serious effects on the handling qualities and cause instability in flight. Most studies in the ground effect are focused on hover and landing capabilities of rotorcraft. In this paper, a comprehensive non-linear model of quadrotor is provided in the state-space, which is suitable for all flight types near the ground. First of all, it is tried to add a ground surface quality coefficient in modified ground effect relation by experimental investigations. The experiments have been done in outdoor during calm days with no wind. In the next step, a robust non-linear control strategy is designed based on the modified model. The effects of ground effect and the increase of the advance ratio are then investigated on controller performance. The simulation results show that this model is closer to the actual flight conditions and the stability and the trajectory tracking are significantly improved by implementation of this control strategy. So, this strategy provides an appropriate platform to run other methods without the need for iterative learning techniques and high battery energy consumption.
The wind power contribution for the global energy matrix and its technological and commercial maturity becomes an important fact for the sustainable energy development. The CFD studies gain importance with the computational progress to improve the efficiency of wind turbines on the aerodynamic criteria. The RANS models show the best relation between accuracy and required computational effort. The present article investigates the application of Arbitrary Mesh Interface (AMI) in transient regime and k-ω SST turbulence model in its standard setting to obtain the Power Coefficient of a small HAWT by using OpenFOAM (pimpleDyMFoam). The numerical results were comparable with the field test results and the numerical results obtained with frozen rotor approach, in stationary regime (simpleFoam) and the same turbulence model. The findings showed good agreement between simulations and experiments. The moving mesh approach with layers addition over the blade's surface, for the adjustment of y+ values, was determinant for the results and reproduced well the three-dimensional dynamic effects of flow for this application. The frozen rotor approach resembled the condition of a stopped rotor and its weaknesses are presented and discussed. The numerical results lied between the highest and the mean experimental values and consistently within the confidence interval.
The field of Civil Engineering has lately gained increasing interest in Unmanned Aerial Vehicles (UAV), commonly referred to as drones. Due to an increase of deteriorating bridges according to the report released by the American Society of Civil Engineers (ASCE), a more efficient and cost-effective alternative for bridge inspection is required. The goal of this paper was to analyze the effectiveness of drones as supplemental bridge inspection tools. In pursuit of this goal, the selected bridge for inspection was a three-span glued-laminated timber girder with a composite concrete deck located near the city of Keystone in the state of South Dakota (SD). A drone, a Dà-Jiāng Innovations (DJI) Phantom 4, was utilized for this study. Also, an extensive literature review to gain knowledge on current bridge inspection techniques using drones was conducted. The findings from the literature review served as the basis for the development of a five-stage drone-enabled bridge inspection methodology. A field inspection utilizing the drone was performed following the stages of the methodology, and the findings were compared to current historical inspection reports provided by the SD Department of Transportation (SDDOT). Quantified data using the drone such as a spalled area of 0.18 m², which is identical to the measurement provided by the SDDOT (0.3 m by 0.6 m), demonstrated the efficiency of the drone to inspect the bridge. This study detailed drone-enabled inspection principles and relevant considerations to obtain optimum data acquisition. The field investigation of the bridge demonstrated the image quality and damage identification capabilities of the drone to perform bridge inspection at a lower cost when compared to traditional methods.
Based on the detailed flow characteristics analyses of a base four-propeller/wing integration at a low Reynolds number of 3.0×10⁵, new multi-propeller/wing integrated aerodynamic design philosophy and methodology have been developed and validated numerically. The core of the present design philosophy is to make good use of coupling effects between two adjacent propellers to realize the low-Reynolds-number flow-field reconstruction, thus to improve the aerodynamic performance of the multi-propeller/wing integration at the operating power-on state. The multi-reference frame (MRF) technique which quasi-steadily solves the Reynolds-averaged Navier–Stokes (RANS) equations coupled with transition model is used to design the example four-propeller/wing integration at a low Reynolds number of 3.0×10⁵. As a result, the designed multi-propeller/wing integration yields a maximum lift-to-drag ratio of 72.81, which represents a 21.08% increase compared to the base four-propeller/wing integration.
Unmanned aerial vehicles are gaining a lot of popularity among an ever growing community of amateurs as well as service providers. Emerging technologies, such as LTE 4G/5G networks and mobile edge computing, will widen the use case scenarios of UAVs. In this article, we discuss the potential of UAVs, equipped with IoT devices, in delivering IoT services from great heights. A high-level view of a UAV-based integrative IoT platform for the delivery of IoT services from large height, along with the overall system orchestrator, is presented in this article. As an envisioned use case of the platform, the article demonstrates how UAVs can be used for crowd surveillance based on face recognition. To evaluate the use case, we study the offloading of video data processing to a MEC node compared to the local processing of video data onboard UAVs. For this, we developed a testbed consisting of a local processing node and one MEC node. To perform face recognition, the Local Binary Pattern Histogram method from the Open Source Computer Vision is used. The obtained results demonstrate the efficiency of the MEC-based offloading approach in saving the scarce energy of UAVs, reducing the processing time of recognition, and promptly detecting suspicious persons.
This paper presents the modeling of the performance of small propellers used for vertical takeoff and landing micro aerial vehicles operating at low Reynolds numbers and in oblique flow. The blade element momentum theory, vortex lattice method, and momentum theory for oblique flow are used to predict propeller performance. For validation, the predictions for a commonly used propeller for vertical takeoff and landing micro aerial vehicles are compared to a set of wind-tunnel experiments. Both the blade element momentum theory and vortex lattice method succeed in predicting correct trends of the forces and moments acting upon the propeller shaft, although accuracy decreases significantly in oblique flow. For the dataset analyzed here, combining the available data of the propeller in purely axial flow with the momentumtheory for oblique flow and applying a correction factor for the wake skew angle results in more accurate performance estimates at all elevation angles.
A new free-wake aerodynamic model of a rotor hovering in ground effect is briefly presented. The numerical characteristics of the model are investigated. The convergence between consecutive iterations and convergence with respect to different parameters that define the model, are presented and discussed. The model is then validated by comparing the numerical results with existing experimental data. Very good agreement of the thrust coefficient is obtained, whereas discrepancies in the torque coefficients are discussed. At the end of the paper the capabilities of the model are shown by considering various aspects of the ground effect. (A)
This paper describes a dynamic controller for rotorcraft landing and hovering in ground effect using feedback control based on flowfield estimation. The rotor downwash in ground effect is represented using a ring-source potential flow model selected for real-time use. Experimental verification of the flow model is also presented. A nonlinear dynamic model of the rotorcraft in ground effect is presented with open-loop analysis and closed-loop control simulation. Experimental results of the open-loop dynamics are presented and the effect of motor dynamics on the overall dynamics are investigated. Flowfield velocity measurements are assimilated into a grid-based recursive Bayesian filter to estimate height above ground in both simulation and experiment. Height tracking in ground effect and landing are implemented with a dynamic linear controller. Experimental validation of the closed-loop controller is ongoing.
The dynamic response and performance of a micro UAV is greatly influenced by its aerodynamics which in turn is affected by the interactions with features in the environment in close proximity. In the paper we address the modeling of quadrotor robots in different flight conditions that include relative wind velocity and proximity to the ground, the ceiling and other robots. We discuss the incorporation of these models into controllers and the use of a swarm of robots to map features in the environment from variations in the aerodynamics.
An experimental and computational study of the static performance of small microcoaxial twin rotors with a diameter of 7.5 cm and twisted blades within Reynolds range from 6000 to 15,000 at 75% span was carried out. A test bench, which is based on five-component sting balance with maximum load capacity of 2 N, was designed. The sting balance experimental validation, carried out before the tests, showed average relative errors of forces and moments were 1.589 and 3.777%, respectively. Loading unloading tests showed a relatively important hysteresis for low loads and a negligible hysteresis for higher loads. The static performance of the coaxial rotor was measured at three rotor axial stations, which are located 0.53, 1.07, and 1.60R from the center, with the maximum thrust at the middle station. A thrust of 0.112 N was measured at 6500 RPM for both rotors. The computational fluid dynamics solver Fluent was used in simulations. Flow models, such as laminar, Spalart-Allmaras, standard k-omega, and shear stress transport k-omega, were compared with different grid configurations. Results show that laminar model has the best fidelity for flowfield. Full simulations of the final coaxial rotor were performed with laminar model showing an overestimation of thrust and torque.
Flow visualization and phase-resolved particle image velocimetry (PIV) experiments were conducted on a hovering rotor to help understand the fluid dynamics of its vortical wake as it interacted with a horizontal ground plane. Visualization was performed by illuminating submicron tracer particles using a strobed laser sheet that was phase locked to the rotational frequency of the rotor. Two-component PIV measurements of the flow in radial planes were obtained at four rotor heights off the ground. Measurements at several wake ages were obtained to examine the morphology of the wake-surface interaction process. The results showed that the rotor wake and its downwash flow are initially subjected to powerful curvature and straining effects as it is deflected into a radially outward direction at the ground plane. An unsteady outward flow comprising a wall jet then developed in the region between the ground plane and the vortical remnants of the rotor wake. The viscous processes of diffusion, vorticity intensification by straining, as well as turbulence generation were all shown to be factors affecting the fluid dynamics of the resulting flow field. In general, the tip vortices were found to induce significantly unsteady flow velocities if they reached the ground plane without undergoing substantial diffusion, especially when they remained coherent enough for adjacent vortices to pair and possibly merge. Further away from the rotor, the turbulence and velocity gradients in the developing wall jet were noted to shear the tip vortices and accelerate the diffusion of vorticity. An intermediate rotor height off the ground was found where the diffusion of vorticity was countered by the straining effects in the flow, which in this case resulted in the highest flow velocities and shear stresses on the ground plane.
A fault detection and isolation scheme is presented to handle three successive faults in the skew-configured inertial sensors of an unmanned aerial vehicle. The skew-configured inertial sensors are composed of the primary inertial measurement unit and the redundant secondary inertial measurement unit. Since small unmanned aerial vehicles are restricted by cost and payload space, the secondary small and low-cost inertial measurement unit is installed with a skewed angle in addition to the primary inertial measurement unit. In the hybrid fault detection and isolation scheme, a parity space method and an in-lane monitoring method are combined to increase system tolerance to the occurrence of multiple successive faults during flight. The first and second faults are detected and isolated by the parity space method. The third fault is detected by the parity space method and isolated by the in-lane monitoring method based on the discrete wavelet transform. Hardware-in-the-loop tests and flight experiments with a fixed-wing unmanned aerial vehicle are performed to verify the performance of the proposed fault diagnosis scheme.