Hao Liu

Chiba University · Department of Mechanical Engineering

It has been reported that flying organisms like birds and insects have wind-sensing organs on their body surfaces that help them not only sense the subtle wind but also control their body posture in natural environment. In this study, we have developed a wind sensor that is inspired from birds feathers due to the disturbances during flight. The bir...

Heart failure (HF) is a significant global public health issue, and accurate diagnosis and effective management are crucial for improving patient outcomes. This study aims to identify potential HF patient subgroups based on the pulse wave time series via unsupervised clustering algorithms and to quantify the importance ratio (IR) of pulse wave morp...

In recent years, the application of bio-inspired structures has garnered attention for enhancing the performance of fluid machinery. In this study, we experimentally investigated the effects of introducing a bio-inspired cutout structure to the propellers of drones, aiming to improve thrust efficiency and reduce noise levels. Our results demonstrat...

Flying animals such as insects and birds use wing flapping for flight, occasionally pausing wing motion and transitioning into gliding to conserve energy for propulsion and achieve high flying efficiency. In this study, we have investigated the gliding performance of a gliding model based on a flapping-wing robot developed in a previous study, with...

Odours used by insects for foraging and mating are carried by the air. Insects induce airflows around them by flapping their wings, and the distribution of these airflows may strongly influence odour source localisation. The flightless silkworm moth, Bombyx mori, has been a prominent insect model for olfactory research. However, although there have...

Background Left ventricular enlargement (LVE) is a common manifestation of cardiac remodeling that is closely associated with cardiac dysfunction, heart failure (HF), and arrhythmias. This study aimed to propose a machine learning (ML)-based strategy to identify LVE in HF patients by means of pulse wave signals. Method We constructed two high-qual...

With the rapid industrialization utilizing multi-rotor drones in recent years, an increase in urban flights is expected in the near future. This may potentially result in noise pollution due to the operation of drones. This study investigates the near- and far-field acoustic characteristics of low-noise propellers inspired by Gurney flaps. In addit...

Flying insects can achieve remarkable and robust flapping aerodynamic performance and flight stability in various environments partially owing to the passive mechanisms of their wings and bodies, but a careful analysis associated with flexible wing hinge for forward flight is missing. Here we develop a fluid–structure interaction (FSI) model that c...

The skin-friction drag generated by wall-bounded turbulent flows can potentially be reduced by a wall-parallel oscillatory motion. Inspired by microvibrations and the high sensitivity of dolphin skin, we examine whether wall-normal undulating motion actuated by longitudinal micro-ultrasonic waves (LMUWs) with ultrasonic-frequency oscillations and m...

Insects, birds, and bats that power and control flight by flapping their wings perform excellent flight stability and maneuverability by rapidly and continuously varying their wing motions. This article provides an overview of the state of the art of vortex-dominated, unsteady flapping aerodynamics from the viewpoint of diversity and uniformity ass...

Pulse wave, as a message carrier in the cardiovascular system (CVS), enables inferring CVS conditions while diagnosing cardiovascular diseases (CVDs). Heart failure (HF) is a major CVD, typically requiring expensive and time-consuming treatments for health monitoring and disease deterioration; it would be an effective and patient-friendly tool to f...

Insects are able to fly stably in the complex environment of the various gusts that occur in nature. In addition, many insects suffer wing damage in their lives, but many species of insects are capable of flying without their hindwings. Here, we evaluated the effect of hindwings on aerodynamics using a Navier–Stokes-based numerical model, and then...

Background/Purpose In patients with univentricular heart disease, the Fontan operation has played a significant role in improving cyanosis and reducing the volume load on the systemic ventricle. However, Fontan patients may develop various complications, and their hemodynamics may deteriorate gradually, resulting in a poor long-term survival rate....

As one of the unique owl-wing morphologies, trailing-edge (TE) fringes are believed to play a critical role in the silent flight of owls and have been widely investigated using idealized single/tandem airfoils. However, the effect of TE fringes and associated mechanisms on the aeroacoustics of owl wings, which feature curved leading edges, wavy TEs...

In this study, we propose an insect-inspired design of unmanned aerial vehicle (UAV), which is developed with a novel thrust vectoring mechanism. Like two-winged insects, our UAV is equipped with two propellers and two sets of thrust vectoring mechanisms, which enable controlling each propeller's position and attitude independently. By designing a...

Aeroacoustic noise in multiple rotor drones has been increasingly recognized as a crucial issue. This study focuses on addressing this challenge by introducing a novel low-noise looprop propeller design with three blades. Unlike traditional propellers, the looprop propeller utilizes three closed-loops to generate thrust. Through comprehensive exper...

Insects, birds, and bats that power and control flight by flapping their wings perform excellent flight stability and maneuverability by rapidly and continuously varying their wing motions. This article provides an overview of the state of the art of vortex-dominated, unsteady flapping aerodynamics from the viewpoint of diversity and uniformity ass...

From the perspective of aeronautical engineers, this paper gives a systematical summary of the technical aspects of bird flight that should be considered in the analysis and design of flapping unmanned and micro air vehicles (UAVs and MAVs). The relevant aspects include the scaling laws, avian wing geometry, avian wing kinematics, aerodynamics mode...

Flying insects exhibit outperforming stability and control via continuous wing flapping even under severe disturbances in various conditions of wind gust and turbulence. While conventional linear proportional derivative (PD)-based controllers are widely employed in insect-inspired flight systems, they usually fail to deal with large perturbation co...

This paper presents a novel integrated study of the aerodynamic performance and acoustic signature of multirotor propellers with a specific focus on the blade twist angle effect. Experimental measurements and computational fluid dynamic (CFD) simulations were utilized to examine and compare the aerodynamic performance and noise reduction between tw...

Objective: This study aims to accurately identify the effects of respiration on the hemodynamics of the human cardiovascular system (CVS), especially the cerebral circulation. Approach: We have developed a machine learning (ML)-integrated 0–1D multiscale hemodynamic model combining a lumped-parameter 0D model for the peripheral vascular bed and a...

In this paper, the motion modes transition and dynamic performance of the flow past a rigid-flexible coupling system were investigated at low Reynolds numbers. The coupling system consisted of a rigid plate and a trailing closed flexible filament and was simulated numerically using the immersed boundary method. According to whether the filament mov...

Aeroacoustic noise in multiple rotor drones has been increasingly recognized as a crucial issue, while noise reduction is normally associated with a trade-off between aerodynamic performance and sound suppression as well as sound quality improvement. Here, we propose an integrated methodology to evaluate both aeroacoustics and psychoacoustics of a...

Intermittent swimming, also termed “burst-and-coast swimming,” has been reported as a strategy for fish to enhance their energetical efficiency. Intermittent swimming involves additional control parameters, which complexifies its understanding by means of quantitative and parametrical analysis, in comparison with continuous swimming. In this study,...

Hemodynamic prediction of carotid artery stenosis (CAS) is of great clinical significance in the diagnosis, prevention, and treatment prognosis of ischemic strokes. While computational fluid dynamics (CFD) is recognized as a useful tool, it shows a crucial issue that the high computational costs are usually required for real-time simulations of com...

It has been reported that flying organisms like birds and insects have wind-sensing organs on their body surfaces that help them predict and control their body posture before being affected by wind gusts. This study developed a wind sensor that is inspired from the deflection of birds feathers due to the effect of wind during flight. The bird-inspi...

Left ventricular enlargement (LVE) typically indicates a common occurrence in cardiac remodeling, closely associated with heart failure (HF). Pulse waves, serving as messengers of the cardiovascular system, have been confirmed to reflect cardiac conditions. In this study we developed a machine learning (ML) methodology to predict LVE in patients wi...

Avian wings are composed of flexible feathers, which can deflect during flapping and gliding flights owing to its flexibility and change its shape passively in response to wind gust. While the crucial role that the feathers play in passive flow control during various flights and in detecting wind variations, it remains yet poorly studied. In this s...

Flying insects perform remarkable flapping flights of hovering and rapid maneuvers under complex natural environments. The flapping flight of insects is achieved through a complex musculoskeletal system to transmit the thorax vibration/deformation to the wing hinge, which is considered as the most complicated dynamic system in biological systems bu...

Owls are widely known for their silent flight, which is attributed to their unique wing morphologies comprising leading-edge (LE) serrations, trailing-edge (TE) fringes, and a velvety surface. The aeroacoustic characteristics of owl-inspired TE fringes have been widely investigated through two-dimensional (2D) modeling, but remain yet poorly studie...

Flying animals such as insects, bats, and birds have acquired the ability to achieve diverse and robust flight patterns in various natural environments. Their sophisticated morphologies, kinematics, and dynamics have motivated engineers to develop bioinspired flying robots. Particularly, the capabilities of morphing wing and tail controls in birds...

Flying insects are capable of hovering and rapid maneuver under unpredictable environments. The principal wing-beat is generated by transmitting the rhythmical contractions of power muscles to the exoskeleton and wing-base articulation. Fine-tuning of the flapping wing kinematics is achieved by deforming the articulation with tiny steering muscles....

Leading-edge (LE) serrations on owls’ outermost remiges play a crucial role in the silent flight of owls. While the aeroacoustic characteristics of LE serrations have been widely studied using single feathers/airfoils, how they affect feather–feather (feather slots) interactions during flight remains unclear. Here, we present a numerical analysis o...

The utilization of small unmanned aerial vehicles (SUAVs), commonly known as drones, has increased drastically in various industries in the past decade. Commercial drones face challenges in terms of safety, durability, flight performance, and environmental effects such as the risk of collision and damage. Biomimetics, which is inspired by the sophi...

Drones, which have become increasingly popular in recent years, produce a lot of noise due to the movement of their propellers. When flying near humans, especially as in urban situations, noise suppression is critical. It has been demonstrated that noise can be minimized by increasing propeller lift per unit rotation speed and decreasing propeller...

In city-wide weather prediction, wind gust information can be obtained using unmanned aerial vehicles (UAVs). Although wind sensors are available, an algorithm-based active estimation can be helpful not only as a weightless substitute but also as feedback for robust control. This paper aims to estimate the wind gusts affecting the quadrotors (a typ...

Due to the differences between simulations and the real world, the application of reinforcement learning (RL) in drone control encounters problems such as oscillations and instability. This study proposes a control strategy for quadrotor drones using a reference model (RM) based on deep RL. Unlike the conventional studies associated with optimal an...

The hovering in ground effect (IGE) of a quadcopter is investigated numerically in terms of flow patterns and aerodynamic characteristics with two different rotor configurations with two different rotor-to-rotor distances. A model of variable-pitch-controlled rotors revealed complicated outwash flow patterns around the quadcopter, showing a pronoun...

This study proposes a low noise loop-type propeller that provides lift by loops rather than blades, aiming to reduce the noise sound pressure level (SPL) and improve sound quality. Noise data were acquired via a hover stand test in an anechoic chamber. We then made a comparison of the SPL and psychoacoustic metrics (loudness, sharpness, roughness,...

This paper presents an integrated study of the aerodynamic and acoustic performance of a shrouded / ducted propeller with a specific focus on unveiling the underlying mechanisms associated with the aeroacoustic interplay between duct and propeller. A hover-stand experiment of the ducted propeller was undertaken to measure both aerodynamic and acous...

Although the aerodynamics and energetics associated with single or paired flapping wings of insects have attracted significant attention, the aerodynamic interaction between the flapping wings and the flying body as a function of flight velocity remains an open question. Here we present a computational fluid dynamic (CFD) study of hawkmoth aerodyna...

Flying insects could perform robust flapping-wing dynamics under various environments while minimizing the high energetic cost by using elastic flight muscles and motors. Here we propose a fluid-structure interaction model that couples unsteady flapping aerodynamics and three-torsional-spring-based elastic wing-hinge dynamics to determine passive a...

Leading-edge (LE) noise is a common source of broadband noise for fans that can be suppressed using appended LE serrations. We conduct an integrated study of the morphological effects of interval, length, and inclination angle of owl-inspired LE serrations on the aeroacoustic characteristics of a mixed flow fan using experiments, computational flui...

Three-dimensional computational fluid dynamics (CFD) simulations were performed in the anastomotic region of the Fontan route between the venae cava and pulmonary arteries to investigate the risk of thrombosis due to blood stasis in the Fontan circulation. The finite volume method based on the time-averaged continuity and Navier–Stokes equations co...

Background Although serum lactate levels are widely accepted markers of haemodynamic instability, an alternative method to evaluate haemodynamic stability/instability continuously and non-invasively may assist in improving the standard of patient care. We hypothesise that blood lactate in paediatric ICU patients can be predicted using machine learn...

This paper presents a numerical study on the aerodynamic performance of three-dimensional flapping wings hovering in ultra-low-density fluid by using an immersed boundary method with a focus on the effects of compressibility on force production and flapping efficiency. Simulations are conducted by varying Mach number, aspect ratio, stroke amplitude...

Leading-edge (LE) noise is a common source of broadband noise for fans that can be suppressed using appended LE serrations. We conduct an integrated study of the morphological effects of interval, length, and inclination angle of owl-inspired LE serrations on the aeroacoustic characteristics of a mixed flow fan using experiments, computational flui...

For fish, swimming in group may be favorable to individuals. Several works reported that in a fish school, individuals sense and adjust their relative position to prevent collisions and maintain the group formation. Also, from a hydrodynamic perspective, relative-position and kinematic synchronisation between adjacent fish may considerably influenc...

Flight speed is positively correlated with body size in animals ¹ . However, miniature featherwing beetles can fly at speeds and accelerations of insects three times their size ² . Here we show that this performance results from a reduced wing mass and a previously unknown type of wing-motion cycle. Our experiment combines three-dimensional reconst...

This paper presents an airflow vector sensor for drones. Drones are expected to play a role in various industrial fields. However, the further improvement of flight stability is a significant issue. In particular, compact drones are more affected by wind during flight. Thus, it is desirable to detect air current directly by an airflow sensor and fe...

Objective: Pulse wave has been considered as a message carrier in the cardiovascular system (CVS), capable of inferring CVS conditions while diagnosing cardiovascular diseases (CVDs). Clarification and prediction of cardiovascular function by means of powerful feature-abstraction capability of machine learning method based on pulse wave is of grea...

The silent flight achieved by owls is attributed to their unique wing morphologies, characterized by leading-edge (LE) serrations, trailing-edge (TE) fringes, and a velvet-like surface. The specific morphological effects of LE serrations and TE fringes on aeroacoustic performance have been widely studied, but the LE–TE aeroacoustic interaction rema...

To explore hemodynamic interaction between the human respiratory system (RS) and cardiovascular system (CVS), here we propose an integrated computational model to predict the CVS hemodynamics with consideration of the respiratory fluctuation (RF). A submodule of the intrathoracic pressure (ITP) adjustment is developed and incorporated in a 0-1D mul...

Coleoptera is characterized by hardened forewings, called elytra. While some beetles open the elytra during flight (four-wing flyer, 4WF), other beetles flap their wings without opening them (two-wing flyer, 2WF). We hypothesized that the status of the elytra affects the flight performances, such as efficiency and stability. Using synchronized high...

Instability of the outdoor flight of small UAVs (Unmanned Aerial Vehicle) is an obstacle to their application in industry, such as inspection and forestry. Flying insects however can achieve stable while robust flight performance, which provides an excellent biomimetic design for UAVs. In this study, inspired by insect flights, we developed a Dual...