Xiaodong Jing

A straightforward method for wall impedance eduction in a flow duct

The development of the advanced liner technology for aeroengine noise control necessitates the impedance measurement method under realistic flow conditions. Currently, the methods for this need are mainly based on the inverse impedance eduction principle, confronting with the problems of initial guess, high computation cost, and low convergence. In view of this, a new strategy is developed that straightforwardly educes the impedance from the sound pressure information measured on the duct wall opposing to the test acoustic liner embedded in a flow duct. Here, the key insight is that the modal nature of the duct acoustic field renders a summed-exponential representation of the measured sound pressure; thus, the characterizing axial wave number can be readily extracted by means of Pronys method, and further the unknown impedance is calculated from the eigenvalue and dispersion relations based on the classical mode-decomposition analysis. This straightforward method is simple in its basic principle but remarkably has the advantages of ultimately overcoming the drawbacks inherent to the inverse methods, incorporating the realistic multimode nonprogressive wave effects, high computational efficiency, possibly reducing the measurement points, and even avoiding the necessity of the duct exit impedance that bothers perhaps all the existing waveguide methods.

Sound-excited flow and acoustic nonlinearity at an orifice

In this paper a potential model is developed to study the sound-excited flow and the acoustic nonlinearity at an orifice. It is assumed that the vortices shed from the orifice edge are simply pushed to move by the fluid slug coming out of the orifice. The fluid slug is either cylindrical or in the shape of an inviscid steady jet through the orifice. The strength of the shed vortices is determined by applying the Kutta condition at the orifice edge. For a sinusoidal applied sound pressure, the fluctuating orifice flow is computed, and some basic flow features such as the velocity distortion in the orifice and the asymmetry of velocity between the inflow and outflow half cycle at positions away from the orifice are described. The nonlinear acoustic resistance as well as reactance of the orifice is also obtained from the average flow velocity through the orifice. The present theory has been compared with the existing studies for the acoustic nonlinearity of an orifice. For the purpose of further validation an experiment is carried out. The theoretical predictions agree fairly well with the experimental data despite the simplifications having been made in this model. It is thereby concluded that the nonlinear acoustic behavior of an orifice depends mainly on the vortex shedding rate at the orifice edge and the convection speed of the shed vortex in the vicinity of the orifice rather than the fine details of the shed vortices.

Vortex Shedding and its Nonlinear Acoustic Effect Occurring at a Slit

This paper presents a theoretical study of the nonlinear acoustic properties of a slit in a thin plate subjected to highintensity sound excitation. In the present model, the conventional discrete vortexmethod is employed to simulate the near-field two-dimensional unsteady flow in order to capture the mechanism of the sound–vortex interaction, which is in combination with a spanwise-averaged three-dimensional Green’s function method used to connect the nearfield flow quantities with the far-field sound pressure. This model is compared with an existing particle image velocimetry flow visualization and a direct numerical simulation model, showing good qualitative agreement. It is revealed that the oscillatory slitflow is dominated by a pair of spiral-like counter-rotating vorticesmoving away from the slit and eventually colliding into each other. Because of the vortical flow effect, increases in the sound pressure level result in significant increases in acoustic resistance and modest decreases in acoustic reactance. For the parametric range in this study, increases in the aspect ratio of the slit result in slight increases in acoustic resistance but unnegligible reductions in acoustic reactance. However, the influence of the aspect ratio on acoustic impedance tends to be less important when the sound pressure level exceeds a certain high value.

Discrete vortex model of a Helmholtz resonator subjected to high-intensity sound and grazing flow

In this paper, a theoretical model is developed to study the acoustical response of a Helmholtz resonator as a duct-branched acoustic absorber subjected to both high-intensity sound and grazing flow. The present model is comprised of a discrete vortex model in combination with a one-dimensional duct sound propagation model. The present work is to study the overall effect of incident sound interacting with grazing flow but putting emphasis on the nonlinear or intermediate regime where the sound intensity has a marked or non-negligible influence on the acoustic behavior of the Helmholtz resonator. The numerical results reveal that the flow field around the orifice is dominated by the evolution of the vortex sheet and the flow pattern is influenced by the ratio of the orifice flow velocity to the grazing flow velocity. When the incident sound pressure is high or the resonance occurs, the resonator shows nonlinearity, i.e., the acoustic impedance and absorption coefficient vary not only with duct flow Mach number buy also with incident frequency and incident sound pressure level.

An investigation on the characteristics of a non-locally reacting acoustic liner

A finite element acoustic model was proposed for investigating the characteristic of a novel non-locally reacting acoustic liner. This non-locally reacting acoustic liner was based on the so-called multiple cavity resonance mechanism which could broaden the absorption bandwidth. The basic idea of it was that when the back cavity of the liner was partitioned into sub-volumes of different size, multiple resonances relating to different length were produced so that the attenuation peaks came closer and worked jointly to provide high attenuation over a broad frequency band. There were strong coupling effects of the resonances inside the back cavities and in the duct when the sound propagated in the duct installed this non-locally acoustic liner. The proposed finite element acoustic model could simulate the entire sound field both in the duct and inside the back non-uniform cavities of the liner including the strong coupling effects. The comparison between the numerical and the experimental results demonstrated that, the finite element acoustic model was an accurate, stable and rapid method for investigating the acoustic characteristic of this novel non-locally reacting acoustic liner. The numerical study was also targeted for developing an optimal non-locally acoustic liner for engineering applications.

An immersed boundary computational model for acoustic scattering problems with complex geometries

An immersed boundary computational model is presented in order to deal with the acoustic scattering problem by complex geometries, in which the wall boundary condition is treated as a direct body force determined by satisfying the non-penetrating boundary condition. Two distinct discretized grids are used to discrete the fluid domain and immersed boundary, respectively. The immersed boundaries are represented by Lagrangian points and the direct body force determined on these points is applied on the neighboring Eulerian points. The coupling between the Lagrangian points and Euler points is linked by a discrete delta function. The linearized Euler equations are spatially discretized with a fourth-order dispersion-relation-preserving scheme and temporal integrated with a low-dissipation and low-dispersion Runge-Kutta scheme. A perfectly matched layer technique is applied to absorb out-going waves and in-going waves in the immersed bodies. Several benchmark problems for computational aeroacoustic solvers are performed to validate the present method.

Aeroacoustic model of a modulation fan with pitching blades as a sound generator

This paper is to develop an aeroacoustic model for a type of modulation fan termed as rotary subwoofer that is capable of radiating low-frequency sound at high sound pressure levels. The rotary subwoofer is modeled as a baffled monopole whose source strength is specified by the fluctuating mass flow rate produced by the pitching blades that rotate at constant speed. An immersed boundary method is established to simulate the detailed unsteady flow around the blades and also to estimate the source strength for the prediction of the far-field sound pressure level (SPL). The numerical simulation shows that the rotary subwoofer can output oscillating air flow that is in phase with the pitching motion of the blades. It is found that flow separation is more likely to occur on the pitching blades at higher modulation frequency, resulting in the reduction of the radiated SPL. Increasing the maximum blade excursion is one of the most effective means to enhance the sound radiation, but this effect can also be compromised by the flow separation. As the modulation frequency increases, correspondingly increasing the rotational speed or using larger blade solidity is beneficial to suppressing the flow separation and thus improving the acoustic performance of the rotary subwoofer.

Acoustic properties of multiple cavity resonance liner for absorbing higher-order duct modesa)

This paper describes analytical and experimental studies conducted to investigate the acoustic properties of axially non-uniform multiple cavity resonance liner for absorbing higher-order duct modes. A three-dimensional analytical model is proposed based upon transfer element method. The model is assessed by making a comparison with results of a liner performance experiment concerning higher-order modes propagation, and the agreement is good. According to the present results, it is found that the performance of multiple cavity resonance liner is related to the incident sound waves. Moreover, an analysis of the corresponding response of liner perforated panel-cavity system is performed, in which the features of resonance frequency and dissipation of the system under grazing or oblique incidence condition are revealed. The conclusions can be extended to typical non-locally reacting liners with single large back-cavity, and it would be beneficial for future non-locally reacting liner design to some extent.

Flow-excited acoustic resonance of a Helmholtz resonator: Discrete vortex model compared to experiments

The acoustic resonance in a Helmholtz resonator excited by a low Mach number grazing flow is studied theoretically. The nonlinear numerical model is established by coupling the vortical motion at the cavity opening with the cavity acoustic mode through an explicit force balancing relation between the two sides of the opening. The vortical motion is modeled in the potential flow framework, in which the oscillating motion of the thin shear layer is described by an array of convected point vortices, and the unsteady vortex shedding is determined by the Kutta condition. The cavity acoustic mode is obtained from the one-dimensional acoustic propagation model, the time-domain equivalent of which is given by means of a broadband time-domain impedance model. The acoustic resistances due to radiation and viscous loss at the opening are also taken into account. The physical processes of the self-excited oscillations, at both resonance and off-resonance states, are simulated directly in the time domain. Results show that the shear layer exhibits a weak flapping motion at the off-resonance state, whereas it rolls up into large-scale vortex cores when resonances occur. Single and dual-vortex patterns are observed corresponding to the first and second hydrodynamic modes. The simulation also reveals different trajectories of the two vortices across the opening when the first and second hydrodynamic modes co-exist. The strong modulation of the shed vorticity by the acoustic feedback at the resonance state is demonstrated. The model overestimates the pressure pulsation amplitude by a factor 2, which is expected to be due to the turbulence of the flow which is not taken into account. The model neglects vortex shedding at the downstream and side edges of the cavity. This will also result in an overestimation of the pulsation amplitude.

Plate Thickness Effect on Orifice Impedance with Nonlinear Acoustic/Grazing Flow Interaction

This paper presents a theoretical study of the acoustic properties of a small rectangular orifice in a plate of finite thickness, in the presence of both grazing flow and high-intensity sound wave. The present investigation has three basic objectives. The first is to study the plate thickness effect on acoustic properties of the orifice. The second is to study the interaction of high-intensity sound with grazing flow. The third objective is to present flow details around the orifice. On the assumption of large aspect ratio, a 2D discrete vortex method is employed to provide the near field unsteady flow simulation results that are then substituted into a spanwise-averaged 3D Green function integral formula to develop a quasi3D model. Depending on this strategy, the nonlinear acoustic impedance of the orifice is calculated from the average flow velocity through the orifice. The reactance of the orifice increases markedly with the increase of the plate thickness, and the reactance results of the finite-thickness plate are apparently larger than the one of the zero-thickness plate. The calculated resistance reveals that the acoustic property of the orifice undergoes a transition from linear to nonlinear, as the sound pressure amplitude increases; the grazing flow velocity has influence on the transition SPL, resulting from the interaction. More detailed analyses of the effects of the grazing flow and the incident sound wave on the acoustic properties of the orifice are carried out. The vortex shedding process at the orifice edge and the evolution of the shed vortex sheet are simulated, the simulation results exhibit the major features of the vortex sheet, such as the rolling up nature and the “flapping” motion of the shear layer over the orifice.