Yi Fang

The Hong Kong University of Science and Technology | UST · mechanical and aerospace engineering

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...

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

The Helmholtz resonator with an extended neck (HREN) has shown effective sound absorption with a small thickness. It is therefore a promising candidate for noise mitigation in aeroengines where the installation space for the acoustic treatment is strictly constrained. In aeroengine appli-cations, the noise intensity is extremely high, resulting in...

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...

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...

A phase-gradient acoustic metasurface is designed based on wavefront manipulation for achieving broadband noise reduction in a flow duct. It is constructed by eight periodically arranged varying-depth units filled with porous materials, forming a linear phase shift of reflected wave at 3000Hz. The linear reflected phase-shifting can be roughly main...

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...

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...

Effective broadband duct sound propagation control is highly required in many practical engineering applications. In this study, a compact structure constituted by multiple detuned resonators is proposed for broadband duct noise transmission control. The coupling characteristics of two detuned resonators flush-mounted on the sidewall of a duct are...

Based on a level set-based topology optimization method, this study designs an acoustic metasurface capable of tailoring reflection wave arbitrarily. The present metasurface is composed of a periodic array of supercells and the supercell is constructed by eight inhomogeneous units possessing a full 2π reflection phase coverage with an interval of π...

This study proposes an ultra-thin sound-absorbing metasurface for low-frequency sound absorption, which is composed of resonators with a spiral extended neck and a coiling-up backing cavity. The analytical, numerical and experimental results show that the proposed metasurface can achieve excellent absorption (absorption coefficient being 0.98) at 1...

In a recent work by the authors, an optimal sound absorber with a compact scale consisting of multiple inhomogeneous Helmholtz resonators with extended necks (HRENs) was designed and demonstrated for effective sound absorption in a prescribed frequency range. The development of HREN-based absorber provides a promising potential for sound absorption...

Herein, electrospinning and paper folding (i.e., origami) techniques are integrated to fabricate a novel ultralight metamaterial for sound absorption based on Miura‐ori tessellation structure, which exists at an extremely low mass density of only 10.64 mg cm⁻³. Sound absorption tests using an impedance tube demonstrate a significant improvement of...

Based on multi-layer Helmholtz resonators with extended necks (HREN), a compact sound-absorbing structure is developed for extra-broadband sound absorption. The structure of HREN with a single perforation is beneficial for low-frequency absorption under a thin thickness. However, it faces the problem of effectively attenuating noise only within a n...

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...

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...

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...

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...

This study develops a structure with thin thickness targeting for wideband low-frequency sound absorption, which consists of multi-layer Helmholtz resonators with extended necks (HRENs). An analytical prediction model is established based on the classic transfer matrix method to study the acoustic characteristics of the multi-layer HREN absorber. R...

The noise attenuation properties of an acoustic liner consisting of Helmholtz resonators with extended necks (HRENs) are investigated. An optimal liner constructed by 16 inhomogeneous HRENs is designed to be effective in sound absorption in a prescribed frequency range from 700 to 1000 Hz. Its quasi-perfect absorption capability (average absorption...

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...

This Letter reports an acoustic metasurface for low-frequency sound absorption with a compact size in both vertical and lateral directions, which is constructed by a series of Helmholtz resonators with spiral extended necks. Analytical, numerical, and experimental results show that the metasurface with a thickness of 13.5 mm possesses total sound a...

This study presents a thin absorber for low-frequency noise mitigation based on a Helmholtz resonator with an extended neck (HREN). An analytical model is established based on the transfer matrix method and the equivalent medium model to predict the acoustic characteristics of the HREN-based absorber, and is validated by experiments and simulations...

This study systemically applies porous acoustic metamaterials in an inverted wedge shape on the planar interface. The porous acoustic metamaterials have been studied analytically, numerically, and in the laboratory. Results indicate that the new porous acoustic metamaterials based on melamine foams demonstrate anomalous reflection phenomena and pos...

This study systemically investigates a porous labyrinthine type of acoustic metamaterials (LAMs), a sort of acoustic metasurface, analytically, numerically, and in laboratory tests. The LAMs are composed of a series of porous elements, where stainless steel plates with various lengths are inserted into the melamine foam. At the frequency of interes...

To enhance the sound absorption capability of porous materials, an acoustic metaporous layer is proposed. The sound absorption is enhanced based on the capability of reflected wave manipulation using the metaporous layer. The designed structure is periodic and one period of the metaporous layer contains four slits filled with the porous materials a...

Maintenance workers on offshore platforms are usually exposed to a high level of noise from the working environment as most of the daily operations of oil and gas process machines generate noise over 85 dBA, causing substantial health and safety issues. Avoiding exposure of workers to the modules that generate high sound power during maintenance ac...

Making use of the phase gradient method for diffracted wave manipulation, we propose an acoustic metasurface with a periodic structure backed with a rigid wall for sound absorption. The system consists of four slits filled with a metal-based fibrous material with various thicknesses in one period enabling a linear phase gradient of the reflected wa...

In this work, we develop a deep subwavelength metasurface which is capable of reflected wave manipulation arbitrarily. Each unit cell of the metasurface is constructed of a Helmholtz resonator with an extended neck. The possibility of creating a phase shift offered by different unit cells is analytically explored based on characteristic mode analys...

A duct acoustics model is an essential component of an impedance eduction technique and its computation cost determines the impedance measurement efficiency. In this paper, a model is developed for the sound propagation through a lined duct carrying a uniform mean flow. In contrast to many existing models, the interface between the liner and the du...

Acoustic liners, consisting of a perforated panel affixed to a honeycomb core with a rigid back plate, are widely used for noise attenuation purpose. In this study, by exploiting inhomogeneous impedance properties, we report an experimental and numerical study on a liner-type acoustic metasurface, which possesses the functionality of both reflected...

An analytical model is built to predict the acoustic fields of acoustic metasurfaces. The acoustic fields are investigated for a Gaussian sound beam incident on the acoustic metasurfaces. The Gaussian sound beam is decomposed into a set of discrete elementary plane waves. The diffraction caused by the acoustic metasurfaces can be obtained using thi...

A composite structure made by a metal-based porous material, namely porous metasurface here, is designed for good sound absorption performance considering both reflection and transmission. The proposed configuration is a periodic structure containing four slits filled with the porous samples with different thicknesses in one period. Through designi...

We propose a two-dimensional (2D) acoustic metasurface fabricated using porous material. An analytical model is developed to predict the three-dimensional sound field produced by an incident acoustic wave interacting with acoustic porous metasurface. The validity of analytical model is validated by comparing with numerical simulations. The directiv...

We report an analytical, numerical, and experimental study of sound transmission through a metasurface fabricated by porous materials, detailing systematically the factors that influence acoustic properties. The design of the metasurface is composed of four elements with varying properties, which are aligned in a periodic manner. The structures are...