Siyang Zhong

The Hong Kong Polytechnic University | PolyU

This study presents an acoustic metasurface (AM) consisting of coupled Helmholtz resonators with extended necks (HRENs) to effectively absorb broadband sound waves at both low and high amplitudes. A nonlinear acoustic model is proposed to characterize the AM performance based on the equivalent medium theory and the transfer matrix method. The nonli...

This research investigates the aerodynamic forces and moments acting on a windsurfing sail during steady sailing. We performed wind tunnel experiments on a scaled-down sail model to measure the aerodynamic forces and moments at varying angles of attack and rig-back (swept-back). The study found that increasing the rig-back angle resulted in lower l...

In cycling races, overtaking is a crucial maneuver that involves drafting behind and moving to the side of another cyclist. The flow interaction between the cyclists has a very significant impact on the aerodynamic performance. Previous experimental and numerical studies have shown the aerodynamic drag changes at certain relative positions between...

A unique challenge of the urban air mobility noise assessment is the configuration-dependent acoustic signature of different designs, of which the installed noise sources must be considered. This work presents a computational aeroacoustics study of a four-rotor urban air mobility vehicle, focusing on near-field propagation considering the installat...

The rapidly emerging market of urban air mobility has raised considerable public concern about noise exposure and annoyance, which is likely to limit the development of future aerial transportation. This study presents a comprehensive approach to evaluate the flight noise of an urban air mobility vehicle under various operating conditions and to pr...

While various aerodynamic noise generation mechanisms and characteristics have been extensively studied for drone research, the noise features during the descending flight state have not been well addressed. In this work, we investigate the aerodynamics and aeroacoustics of a two-bladed drone rotor operating in descent using both experimental and n...

The multi-rotor propulsion system has become one of the popular configurations for urban air mobility vehicles due to its capability of high-density vertical take-off and landing operations. The noise characteristics, as a critical criterion for urban air mobility, usually differ for a multi-rotor configuration from a single-rotor configuration. In...

In this work, we conduct a numerical investigation of aerodynamics and aeroacoustics of propeller blades with vibrations, which could occur in practical unmanned aerial vehicles as the blades are often made of lightweight materials. The simulations resolve the sound generation from the unsteady turbulent flows using an acoustic-wave preserved artif...

Through anechoic wind tunnel tests, this study comprehensively investigates the noise and drag reductions on a circular cylinder with dimples. Dimples built on a surface pattern fabric cover the cylinder surface as one of the passive flow control methods. The force, noise, and flow field measurements are performed at diameter-based Reynolds numbers...

This study experimentally investigates the potential of using surface pattern fabrics for the cylinder noise and drag control in different Reynolds number ranges. The aerodynamic and aeroacoustic effects were evaluated through the noise and force measurements in an anechoic wind tunnel. It was observed that the noise and drag reductions take place...

The aeroacoustic characteristics of flying vehicles with pitch-fixed rotors differ from traditional helicopters with pitch-controlled rotor blades. Accurate predictions of rotor noise are still challenging because many uncertainty factors and unsteadinesses exist. This work investigates the aeroacoustic effects of rotational speed deviation, rotati...

The geometry of the sectional airfoil has a determinative impact on the aeroacoustic characteristics of propellers. However, there are always slight deviations in the practical profiles due to manufacturing tolerance, wear loss, and limitations of processing techniques, which can potentially introduce uncertainties to aeroacoustic measurements. To...

This paper presents an experimental investigation on improving the noise reduction capability of the trailing edge serrations using vortex generators. Experiments were conducted on a 150 mm chord flat plate at four angle of attack between 0° and 9° and flow speeds of 20-50 m/s. The Reynolds number based on the chord length ranges from 2.1×10 ⁵ to 5...

An acoustic liner design based on metamaterial is proposed for broadband duct noise reduction. The material is constructed by four periodically arranged partitions-embedded units covered by a perforated plate, forming a linear reflected phase-shifting within 0 to 2π in a wide target frequency range. The sound absorption performance under normal inc...

This work presents an aerodynamic noise prediction model for counter-rotating coaxial rotors. The aerodynamic interactions due to the flow accelerations and wake flow properties are considered in an efficient blade element and momentum theory method to compute the mean flow variables. Then, the sectional aerodynamic lifts are represented as an arra...

This study focuses on methods of processing the noise data of propulsive propellers commonly found on ubiquitous multi-rotor flying vehicles. The noise signals recorded in experiments typically contain periodic tonal and random broadband components produced by the rotational motion of the propellers, the interaction between the propellers and turbu...

The reflection performance of periodic acoustic metasurfaces (AMs) subjected to a grazing flow, consisting of a uniform region and a finite sheared layer, is studied. A convective generalized Snell’s law (CGSL) is derived to account for the effect of uniform mean flows on the reflection of AMs. Then, an AMs design method considering the mean flow p...

In this work, the aerodynamic characteristics of isolated bicycle wheels, including a disc wheel and a five-spoke wheel, were investigated using an advanced test rig in a wind tunnel and high-fidelity numerical simulations. The effects of velocity, wheel rotation and yaw angle were systematically investigated. The wheel rotating speed and the frees...

Acoustic metasurfaces (AMs) used for reflected sound wavefront manipulation are generally designed based on the generalized Snell’s law (GSL) at a single frequency, suffering from inferior broadband properties. Herein, an acoustic meta-porous layer (AMPL) with periodic structures is designed to realize reflected wavefront manipulation and effective...

Multiple propellers arranged in overlapping configurations can reduce the sizes of unmanned aerial vehicles for compact architectures. However, the resulting aerodynamic interaction between the propellers could lead to extra noise and calls for extra studies on acoustic physics. In this work, we conducted aeroacoustic tests of two overlapping prope...

This work presents a high-efficiency low-noise propeller design for unmanned aerial vehicles. Three different blade configurations are first investigated, using the computational aeroacoustic approach to recognize the flow around propellers and the noise emissions. The flow simulation is obtained by an acoustic-wave preserved artificial compressibi...

This work investigates the turbulent flow of an urban air mobility (UAM) vehicle and its far-field acoustic feature. The vehicle is equipped with six propellers, which can generate a total thrust of about 5000 N. Each propeller has a radius of 1 m. The near-field flows are simulated using the acoustic wave preserved artificial compressibility (APAC...

This work studies the trailing edge noise reduction potentials of combining trailing edge serrations and porous membrane/velvet structures made of non-woven fabrics. Anechoic wind tunnel experiments were conducted based on a flat plate model, at a chord-based Reynolds number between 2×10^5 a and 5×10^5 and zero angle of attack, with fully tripped b...

This work presents a parametric study of the effects of rotational speed variation and blade vibration on the aeroacoustic characteristics of the fixed-pitch rotors. The study is based on an efficient prediction of the rotor noise with the effect of unsteady motions, including the fluctuations in the rotation speed and blade vibration, considered....

Acoustic metasurfaces (AMs) are artificial structures that can realize exotic properties, such as negative refraction and perfect absorption, etc., in sub-wavelength thickness scales. Most of the previous studies on AMs were conducted in laboratory conditions with a static medium, limiting their applications in practical conditions, e.g., the aircr...

The Gaussian beam tracing method has advantages in computing long-distance noise propagation in outdoor environments due to its high efficiency. Usually, repeated computations are required if the target sound wave is broad-band or contains multiple frequencies because the method was developed in the frequency domain. This paper presents an efficien...

This paper presents an efficient sliding mesh method to simulate the noise emission from aero-engines. The vortexes shed from the rotating blade and interact with the downstream stationary outlet guide vanes (OGVs), producing noise. The challenges in simulating the problem are the accurate modelling of the wake turbulence, and the capabilities to c...

This study experimentally investigates the potential of using combinations of trailing edge serrations and thin porous membrane/velvet structures for turbulent boundary layer trailing edge noise reduction. The experiments were conducted in an anechoic wind tunnel, with a flat plate model as the baseline model. The chord-based Reynolds number ranged...

This paper focuses on the radiation modes and efficiency of propeller tonal noise. The thickness noise and loading noise model of propellers has been formulated in spherical coordinates, thereby simplifying numerical evaluation of the integral noise source. More importantly, the radiation field can be decomposed and projected to spherical harmonics...

Drones and urban air mobility vehicles, which are mostly intended for urban operations, have attracted intensive research interest. Their environmental noise impact should not be ignored and should be a certification consideration. Numerical simulations can provide means to efficiently assess the noise impact on the complex urban environments, than...

Broadband noise due to the turbulence-aerofoil interaction, which is also called the leading edge noise, is one of the major noise sources of aircraft (including the engine). To study the noise properties numerically is a popular approach with the increasing power of computers. Conventional approaches of using body-fitted grids at the boundaries wo...

Multi-rotor powered drones and urban mobility vehicles (UMV) have received considerable attention over recent years and attracted ever-increasing interest in their aerodynamic noise. Physics-based prediction tools for aerodynamic noise are of importance to facilitate quiet drone designs. In this work, a boundary element method (BEM) based solver is...

This paper reports an experimental study on the aerodynamic noise generated by a two-dimensional large obstacle in a turbulent boundary layer. Square and triangular obstacles with varying heights of h/δ=0.48,0.8,1.2,1.6, and 2 (where δ is the boundary layer thickness measured without the obstacle present) are tested at various flow speeds ranging f...

Aerodynamic and aeroacoustic characteristics of a cylinder with longitudinal grooves were studied in an anechoic wind tunnel, emphasizing on drag and noise attenuation. The parallel mounted load cells and a single microphone measured the aerodynamic resistance and noise of a circular cylinder with longitudinal grooves at Reynolds number ranging fro...

This study presents a framework to analyze the propulsive performance of a sail under oscillatory pumping motion, which should be modified from the analysis framework for conventional flapping wings. A new set of metrics, including the propulsive efficiency ηs and efficacy ΔCx¯, which depend not only on the aerodynamic coefficient but also on kinem...

Drones and urban air mobility vehicles, which are mostly intended for urban operations, have attracted intensive research interest. Their environmental noise impact should not be ignored and should be a certification consideration. Numerical simulations can provide means to efficiently assess the noise impact on the complex urban environments, than...

Urban aerial mobility (UAM) is a promising approach to improve the traffic situation in gigantic cities, which, however, may encounter significant noise pollution issues. An integrated research platform, which is being established at HKUST, to include noise generation, long-distance propagation, and perception at the observers is timely to assess t...

Multi-copters or drones are engaged in a wide range of industrial applications for their flexibility, safety and low-cost. The noise emission is becoming an issue with the expanding applications, among which the propellers that drive the drones are the major sources of noise. In this work, the noise characteristics of small-scale propellers is expe...

The operation of the rapidly growing unmanned aerial vehicles (UAV) and the promising urban aerial mobility (UAM) could have a significant noise impact on the environment. In this work, we developed a cloud-based noise simulator to efficiently assess the environmental impact of UAM and UAV. The noise sources and long-distance propagation are comput...

Aeroacoustic and aerodynamic characteristics of the turbulent boundary layer encountering a large obstacle are experimentally investigated in this paper. Two-dimensional obstacles with a square and a semi-circular cross-section mounted on a flat plate are studied in wind tunnel tests, with particular interests in the shear layer characteristics, wa...

Acoustic metasurfaces are artificial 2D structures with a sub-wavelength thickness that can realize some exotic properties such as non-trivial refraction, broadband and low frequency absorption. However, most relevant studies are still in a static medium, hindering their realistic applications in aviation, where background flow exists. To address i...

The Hong Kong University of Science and Technology (HKUST) has designed and assembled a new facility, a grazing flow tube, for aeroacoustic characteristics measurement of acoustic liners, e.g. transmission loss, impedance, etc., under a high-speed grazing flow. The cross-section of the test section of the tube has a dimension of 50 mm × 50 mm, and...

This study is inspired by the velvety structures on an owl's upper wing surface. Anechoic wind tunnel experiments were conducted to study the effect of the velvety structures on trailing edge noise as well as the boundary layer flow of a flat plate model. The tests were conducted in The Hong Kong University of Science and Technology low-speed wind...

Multiple propellers are usually used in unmanned aerial vehicles. This work studies the aerodynamic and acoustic properties of dual propellers in close vicinity. Aerodynamic and acoustic tests of the side-by-side propellers were conducted in an anechoic chamber, and the flow recirculation effect was considered in the analysis to reduce uncertaintie...

The Hong Kong University of Science and Technology (HKUST) has installed and commissioned an Ultra-quiet Noise Injection Test and Evaluation Device (UNITED). This facility is the first closed-circuit anechoic wind tunnel in Hong Kong. Calibrations and improvements have been made to improve the aerodynamic and aeroacoustic performance since its comm...

Turbulence generated broadband noise is an important source of noise that can be found in many applications. The characteristics of noise are defined by a combination of turbulence statistics, flow and geometry, imposing challenges in developing efficient and robust prediction methods. Direct simulations of flow governing equations are currently to...

For large-distance sound propagation with complex obstacles, the Gaussian beam tracing (GBT) method is often applied. An omnidirectional source model is commonly implemented in GBT, which, however, neglects the influence of generic directivity patterns of practical acoustic problems. There have been efforts to synthesize or reproduce the target dir...

The periodic variation of the flow pattern and the boundary layer characteristics near the trailing edge of a pitching NACA (National Advisory Committee for Aeronautics) 0012 airfoil are examined experimentally in this study. The mean pitching angle and pitching amplitude were 0° and 7.5°, respectively, and the reduced frequency varied from 0.094 t...

The acoustic transfer function is a well-known concept for efficient computations of acoustic analysis with various boundary conditions and sound sources. It is first proposed to accelerate the boundary element analysis by interpolating instead of directly assembling influence matrices. To further improve its computational efficiency, a new frequen...

The accurate evaluation of the broadband noise generated by fan wakes interacting with the outlet guide vanes (OGVs) in the aero-engines remains challenging due to the stochastic nature of sound and the complex flow conditions. A systematic study of the design parameters from full-scale and three-dimensional turbulence simulations is still expensiv...

The paper presents the results of supercomputer simulations of turbulent flow around a rotor of a small-scale unmanned aerial vehicle. Rotor aerodynamics and near-field acoustics are modeled using both RANS and scale-resolving hybrid RANS-LES approaches. The underlying numerical method uses an edge-based reconstruction scheme for higher-accuracy di...

In this study, an asymptotic analysis of the frequency-domain formulation to compute the tonal noise of the small rotors in the now ubiquitously multi-rotor powered drones is conducted. Simple scaling laws are proposed to evaluate the impacts of the influential parameters such as blade number, flow speed, rotation speed, unsteady motion, thrust, an...

In this study, we investigated the performance of flexible trailing edge serrations of various shapes for airfoil self-noise reduction through anechoic wind tunnel experiments. A flat plate model was tested at zero angle of attack. The chord-based Reynolds number was between 1.6 × 10⁵ and 4 × 10⁵. The boundary layers were fully tripped near the lea...

Large-distance sound propagation with high-frequency noise sources, multiple obstacles/geometry with varying acoustic impedance is common in real-life applications. To resolve the acoustic governing equations directly is often computationally costly, especially in three-dimensional space. Methods based on geometric acoustics can be more rapid. Howe...

Extended-resolution acoustic imaging of low-frequency wave sources by acoustic analogy-based tomography - Volume 899 - Wangqiao Chen, Siyang Zhong, Xun Huang

In this study, the far-field noise and near-field flow properties from a heaving NACA 0012 airfoil at the Reynolds number of 6.6×104 were investigated experimentally in a 0.4 m2 anechoic wind tunnel. The airfoil had an incident angle of 0° and followed a sinusoidal heaving motion. The Strouhal number, controlled by changing the heaving frequency an...

Leading edge noise of airfoil is a significant source of broadband noise for both aircraft and wind turbines. In this work, the noise generated by the interaction of incoming turbulence and a heaving airfoil was studied. Numerical experiments were performed using a hybrid computational aeroacoustics method, where the mean flow was computed by using...

Gust/turbulence-leading edge interaction is a significant source of airfoil broadband noise. An approach often used to predict the sound is based on Amiet's flat-plate solution. Analytical studies have been conducted to investigate the influences of airfoil geometries, non-uniform mean flows and turbulence statistics, which, however, were often too...

Spurious wave contaminations may occur in the computations of acoustic far-field directivities in turbulent flows with only surface integrations. Two methods could be used to tackle the problem: (1) to take average of the solutions using multiple integration surfaces based on the Ffowcs-Williams and Hawkings equation that was widely applied to vari...

Most of the acoustic cloak designs are based on the stationary medium, which, however, may be inaccurate in many practical applications with non-uniform flows. The optimization of the acoustic cloaking performance was described as an active noise control problem, and the theoretical model by Huang, Zhong, and Stalnov [J. Acoust. Soc. Am. 135(5), 25...

Usually the prediction of airfoil-gust interaction noise is mainly based on the flat plate solution, and only the transverse velocity disturbance that is perpendicular to the flat plate surface is considered. However, the solution might underestimate the sound generation in non-uniform mean flows for finite thickness airfoils, in which the velocity...

A frequency domain formulation of the generalized sound extrapolation method is proposed. The Fourier transform is performed to the flow variables to prepare the source terms as the input of the sound extrapolation solver, and the inhomogeneous convected Helmholtz equation is solved using the Green's functions in the frequency domain. The proposed...

The sound produced by airfoil-gust interaction is a significant source of broadband noise in turbofan engines or contra-rotating open rotors (CRORs). There are competing mechanisms in this regime because of the presence of shocks that were seldom considered in the previous subsonic studies. A numerical investigation of airfoil-gust interaction nois...

In time-domain simulations of sound propagation, solutions obtained from linearised Euler equations may suffer from numerical Kelvin–Helmholtz instabilities in the presence of a sheared mean flow. The Kelvin–Helmholtz instabilities are vortical disturbances that can grow boundlessly and eventually contaminate the solution field. Several methods wer...

Sound extrapolation methods are often used to compute acoustic far-field directivities using near-field flow data in aeroacoustics applications. The results may be erroneous if the volume integrals are neglected (to save computational cost), while non-acoustic fluctuations are collected on the integration surfaces. In this work, we develop a new so...

This paper presents an innovative stability analysis and design approach for time-domain impedance boundary conditions to simulate noise propagation and radiation from a lined turbomachinery duct in the presence of a mean flow. A control-oriented model is developed for the stability analysis of the impedance boundary condition by using generalized...

Almost all reconstruction methods in photoacoustic tomography (PAT) have been developed by assuming that sound propagation is linear, which is valid for ordinary PAT applications but would become inappropriate when the sound amplitude is higher than a certain threshold level. In the current study, we investigate the effect of nonlinear sound propag...