Fluid-structure-acoustic coupling for a flat plate

The present work deals with the aeroacoustic sound radiated by a forward–backward facing step in combination with a flexible wall behind the step. A numerical flow computation with coupled aeroacoustic and vibroacoustic simulation was carried out. The structural deformations of the oscillating plate like structure in the wake of the forward–backward facing step were considered to be small and therefore not affecting the flow field. The presented approach enables a separate consideration for the aeroacoustic as well as the structural borne noise. The influence of the interactions of the acoustic medium with the flexible structure on the vibroacoustic sound radiation is investigated. One-sided and two-sided coupling approaches for the vibroacoustic analysis are introduced. The two-sided vibroacoustic computation allows for considering the damping influence of the ambient fluid on the flexible plate vibration and therefore on the sound radiation. Additional to the simulations, aeroacoustic measurements in an acoustic wind tunnel were performed for validation purposes.     

A simulation-based analysis of the effect of a reflecting surface on aeroacoustic time-reversal source characterization and comparison with beamforming

This paper presents a simulation-based analysis of the effect of a reflecting surface on aeroacoustic Time-Reversal (TR) source localization/characterization and compares the results of TR with those obtained using cross-spectral Conventional Beamforming (CB). The TR technique is shown to require the use of at least two line arrays of microphones to accurately characterize the nature of aeroacoustic sources. This work, however, shows that in the presence of a rigid surface, only one line array of microphones is sufficient to accurately localize and characterize idealized aeroacoustic sources. Forward simulations were carried out using the 2-D Linearized Euler Equations on a rectangular domain with a rigid bottom boundary (modeling a 2-D semi-infinite space) for the test-cases of stationary idealized tonal aeroacoustic (monopole, dipole and lateral quadrupole) sources located in a fully-developed mean shear flow field wherein the acoustic pressure time–history was stored at the computational boundaries. A set of TR simulations are implemented that show for each test-case that only the top line array is required to accurately characterize the idealized aeroacoustic sources in the presence of a reflecting bottom boundary, thereby suggesting the redundancy of acoustic pressure measurement at the rigid surface. The test-case of convecting (moving) idealized aeroacoustic source was also considered and the TR simulation using only the top line array in the presence of reflecting bottom boundary was able to accurately retrieve the source trajectory and simultaneously characterize its nature. This numerical experiment demonstrates in principle that when a rigid surface is mounted on the floor of an Anechoic Wind Tunnel, the use of only one (top) line array of microphones should be sufficient to characterize the nature and location of experimental flow-induced noise source. Acoustic source maps were also obtained using the CB method based on the Method of Images (to model the reflecting surface) and incorporation of the Ray-Tracing algorithm necessary to account for the effect of mean flow. The CB results were found to be highly comparable to those obtained using TR for the test-cases of non-convecting sources; thereby demonstrating the conceptual equivalence of the Method of Images and directly implementing the rigid-wall condition during TR for source localization/characterization.     

High‐order numerical simulations of flow‐induced noise

In this paper, the flow/acoustics splitting method for predicting flow‐generated noise is further developed by introducing high‐order finite difference schemes. The splitting method consists of dividing the acoustic problem into a viscous incompressible flow part and an inviscid acoustic part. The incompressible flow equations are solved by a second‐order finite volume code EllipSys2D/3D. The acoustic field is obtained by solving a set of acoustic perturbation equations forced by flow quantities. The incompressible pressure and velocity form the input to the acoustic equations. The present work is an extension of our acoustics solver, with the introduction of high‐order schemes for spatial discretization and a Runge–Kutta scheme for time integration. To achieve low dissipation and dispersion errors, either Dispersion‐Relation‐Preserving (DRP) schemes or optimized compact finite difference schemes are used for the spatial discretizations. Applications and validations of the new acoustics solver are presented for benchmark aeroacoustic problems and for flow over an NACA 0012 airfoil. Copyright © 2010 John Wiley & Sons, Ltd.     

Cavity Tones by Computational Aeroacoustics

The flow past open cavities is a problem that is encountered in many engineering applications and can result in intense acoustic tones. The flow physics and acoustics of cavity configurations are complex and computational simulation techniques provide an opportunity to gain further understanding and provide a tool to predict not only cavity tone frequencies but their amplitude. In this paper, we describe the available techniques for performing computational aeroacoustic simulations of cavity flows, and review recent applications for the prediction and control of cavity tones in subsonic, transonic and supersonic regimes.     

可渗透壁对翼型声涡相互作用下流场及声场的影响

由仿生学原理构建的可渗透翼型对湍流气动噪声抑制作用已展现良好的应用前景。对NACA 0012可渗透翼型和实体翼型进行了数值计算,得到了声涡相互作用下气动噪声声场和流场,分析了可渗透壁对翼型流场和声场的影响。研究表明,相对实体翼型,可渗透壁通过减小声源强度降低了主纯音噪声声压级幅值和远场总声压级,消除了高阶离散纯音,但对噪声的指向性没有较大改变。进一步的流场分析表明,可渗透壁对翼型气动性能影响不大的情况下能够降低边界层扰动和翼型后缘大尺度涡旋强度,并推迟分离泡转捩和再附位置。     

基于比拟理论的翼型扰流声场数值模拟

从二维模型方程的全离散形式出发,重点分析了差分格式的色散特性和各向异性效应,证实迎风紧致格式比对称格式有更好的色散和各向同性特性,故有利于声场的数值模拟,并采用三阶迎风紧致格式（ＵＣＤ３）和四阶对称紧致格式（ＳＣＤ４）计算了绕ＮＡＣＡ００１２翼型的可压缩非定常流场,并将此流场作为近场声源,运用声学比拟理论对气动声进行模拟。     

Lagrangian and Eulerian pressure field evaluation of rod-airfoil flow from time-resolved tomographic PIV

This work investigates the rod-airfoil air flow by time-resolved Tomographic Particle Image Velocimetry (TR-TOMO PIV) in thin-light volume configuration. Experiments are performed at the region close to the leading edge of a NACA0012 airfoil embedded in the von Kármán wake of a cylindrical rod. The 3D velocity field measured at 5 kHz is used to evaluate the instantaneous planar pressure field by integration of the pressure gradient field. The experimental data are treated with a discretized model based on multiple velocity measurements. The time separation used to evaluate the Lagrangian derivative along a fluid parcel trajectory has to be taken into account to reduce precision error. By comparing Lagrangian and Eulerian approaches, the latter is restricted to shorter time separations and is found not applicable to evaluate pressure gradient field if a relative precision error lower than 10% is required. Finally, the pressure evaluated from tomographic velocity measurements is compared to that obtained from simulated planar ones to discuss the effect of 3D flow phenomena on the accuracy of the proposed technique.     

Inflow broadband noise from an isolated symmetric airfoil interacting with incident turbulence

Inflow noise from a symmetric airfoil interacting with homogeneous and isotropic turbulence is investigated focusing on the effects of airfoil geometry. The numerical method employed is based on computational aeroacoustic techniques using the high-order dispersion-relation-preserving finite-difference schemes for solving two-dimensional linearized Euler equations. Effects on inflow noise of the airfoil thickness, leading-edge radius, and freestream Mach number are examined by comparing the acoustic power spectrum of the airfoils and their flow field characteristics. Acoustic power levels of airfoils are found to exponentially decrease in the high-frequency range as airfoil thickness increases because incident turbulent velocities are more distorted in the larger stagnation region near the leading edge. This distortion is shown to be related to the slope angle of the streamline of steady mean flow near the leading edge. However, this high-frequency reduction weakens as the Mach number increases due to the decreasing slope angle. In addition, the chordwise velocity component in the incident turbulence contributes more to the radiating acoustic pressure level as the freestream Mach number increases, which also results in less high-frequency reduction at higher freestream Mach number. At fixed airfoil thickness, increasing the leading-edge radius leads to decreases in the acoustic power level, which may also be explained by size variation of the stagnation region around the leading edge. An approximate algebraic formula for acoustic power spectra is derived on the basis of these observations. Acoustic power spectra predicted using this formula are shown to closely follow the numerical results. Finally, the applicability of the algebraic formula and the current numerical methods to more realistic problems are confirmed by comparing their predictions with the measured data.     

Direct numerical simulation of flow over a forward-facing step – Flow structure and aeroacoustic source regions

A direct numerical simulation of the flow over a forward-facing step at a Reynolds number of 8000 based on the step height is presented. Calculations were performed using second-order finite volume discretisation in space on co-located meshes. A hybrid calculation approach based on Lighthill’s acoustic analogy is explained. Results of the simulation are intended to be used as a database for the validation of different discretisation schemes for the flow computation and simulation approaches for the calculation of sound radiation using a hybrid approach. Turbulent statistics are presented along with aeroacoustic source regions. Strong and weak forms of the aeroacoustic source term are presented and compared. For visualization purposes, the strong form is more suitable, whereas for the calculation of sound radiation both forms can be used. From the visualization of the aeroacoustic sources, it can be seen that they mainly concentrate on the region of the leading edge of the step and the shear layer close to the step.     

Finite element modelling of sheared flow effects on the radiation characteristics of acoustic sources in a circular duct

The recent interest in propeller noise generation, stimulated by development of new propeller types for commerical propjets, has generated a need for the ability of measure the noise characteristics of propellers. However, wind tunnel noise measurements are affected by reflections from the wind tunnel walls. Computer codes predicting the free-field noise of a propeller and its noise field in a circular wind tunnel allow validating the use of wind tunnel measurements to predict free-field noise characteristics. A wind tunnel contains flow which is uniform in the duct axial direction, but can vary in the radial direction. It can be shown that a third-order differential equation governs the acoustic pressure field for such a duct containing radially sheared subsonic flow. This third-order problem is then posed as a coupled pair of equations which are second-order in terms of acoustic density and first-order in terms of an artificial variable which represents the effects of the flow being sheared. It is shown that this form of the problem allows a natural extension of the existing numerical solution techniques for non-sheared flow. The sheared flow problem is presented, and a finite element method is developed to yield a solution for propeller-type acoustic forces. The finite element code and method are refined with numerical experiments, and results are presented for a specific propeller and duct geometry. Good agreement is shown between this method and an alternate approach to the sheared flow problem using a piecewise constant representation of the velocity in the boundary layer. This validates both the numerical methods.     

Determination of two-dimensional space–time correlations in jet flows using simultaneous PIV and LDV measurements

The two-dimensional space–time turbulence statistics of free shear jet flows in the form of the two-point velocity correlation tensor are important for aeroacoustic noise source modelling based on the acoustic analogy approach. This paper presents a direct application of the Point-Referenced Global Correlation (PRGC) technique to measure the components of this correlation tensor for a sector of the flow field in two jet configurations. The PRGC approach combines single point and global measurement techniques and enables two-point space–time correlations over a region of the flow to be obtained. The technique is applied to a single stream jet and a co-axial coplanar jet at a Mach number of 0.24 using commercial Laser Doppler velocimetry (LDV) and low-speed particle image velocimetry (PIV) systems. Results for the one-dimensional correlations are shown to compare well with two-point measurements. The results for the two-dimensional space–time correlations are presented and the characteristics for both configurations discussed.     

带制退器的膛口射流噪声数值模拟与实验研究

为了研究膛口装置对膛口噪声气动特性的影响,对带膛口制退器的某小口径武器的膛口射流噪声进行了数值模拟和实验研究。采用计算流体力学CFD (computational fluid dynamics)-计算气动声学CAA (computational aeroacoustics)耦合算法对膛口噪声进行数值模拟,即对膛口流场进行瞬态CFD模拟,获取流场数据,然后利用所得到的结果采用声学方程模拟声源信息求解声场。基于数值模拟结果,分析了膛口流场变化及噪声的指向性分布,并与实验结果进行了对比。研究表明:膛口制退器的安装改变了膛口流场结构,影响了膛口射流噪声的指向性分布。计算结果与实验结果的误差小于9%,验证了该计算方法的可行性。研究结果可为膛口射流噪声的预测及膛口制退器的设计提供一定的参考。     

COMPARISON OF THEORY AND EXPERIMENT ON AEROACOUSTIC LOADS AND DEFLECTIONS

The correlation of acoustic pressure loads induced by a turbulent wake on a nearby structural panel is considered: this problem is relevant to the acoustic fatigue of aircraft, rocket and satellite structures. Both the correlation of acoustic pressure loads and the panel deflections, were measured in an 8-m diameter transonic wind tunnel. Using the measured correlation of acoustic pressures, as an input to a finite-element aeroelastic code, the panel response was reproduced. The latter was also satisfactorily reproduced, using again the aeroelastic code, with input given by a theoretical formula for the correlation of acoustic pressures; the derivation of this formula, and the semi-empirical parameters which appear in it, are included in this paper. The comparison of acoustic responses in aeroacoustic wind tunnels (AWT) and progressive wave tubes (PWT) shows that much work needs to be done to bridge that gap; this is important since the PWT is the standard test means, whereas the AWT is more representative of real flight conditions but also more demanding in resources. Since this may be the first instance of successful modelling of acoustic fatigue, it may be appropriate to list briefly the essential “positive” features and associated physical phenomena: (i) a standard aeroelastic structural code can predict acoustic fatigue, provided that the correlation of pressure loads be adequately specified; (ii) the correlation of pressure loads is determined by the interference of acoustic waves, which depends on the exact evaluation of multiple scattering integrals, involving the statistics of random phase shifts; (iii) for the relatively low frequencies (one to a few hundred Hz) of aeroacoustic fatigue, the main cause of random phase effects is scattering by irregular wakes, which are thin on wavelength scale, and appear as partially reflecting rough interfaces. It may also be appropriate to mention some of the “negative” features, to which may be attached illusory importance; (iv) deterministic flow features, even conspicuous or of large scale, such as convection, are not relevant to aeroacoustic fatigue, because they do not produce random phase shifts; (v) local turbulence, of scale much smaller than the wavelength of sound, cannot produce significant random phase shifts, and is also of little consequence to aeroacoustic fatigue; (vi) the precise location of sound sources can become of little consequence, after multiple scattering gives rise to a diffuse sound field; and (vii) there is not much ground for distinction between unsteady flow and sound waves, since at transonic speeds they are both associated with pressures fluctuating in time and space.     

Computation of Vortex Shedding and Radiated Sound for a Circular Cylinder: Subcritical to Transcritical Reynolds Numbers

The Lighthill acoustic analogy combined with Reynolds-averaged Navier–Stokes flow computations are used to investigate the ability of existing technology to predict the tonal noise generated by vortex shedding from a circular cylinder for a range of Reynolds numbers (100 < Re < 5 million). Computed mean drag, mean coefficient of pressure, Strouhal number, and fluctuating lift are compared with experiment. Two-dimensional calculations produce a Reynolds number trend similar to experiment but incorrectly predict many of the flow quantities. Different turbulence models give inconsistent results in the critical Reynolds number range (Re≈ 100000). The computed flow field is used as input for noise prediction. Two-dimensional inputs overpredict both noise amplitude and frequency; however, if an appropriate correlation length is used, predicted noise amplitudes agree with experiment. Noise levels and frequency content agree much better with experiment when three-dimensional flow computations are used as input data. Received 5 May 1998 and accepted 28 September 1998     

Aeolian tones of a plate cascade

A model for the aeroacoustic resonance effects (aeolian tones) excited around a plate cascade in a gas flow is suggested. Methods of calculating the frequencies of natural acoustic oscillations near the cascade are developed. The effect of the cascade geometry and the Mach number of the main flow on the frequencies, abundance, and modes of the natural oscillations is investigated. Anomalous acoustic oscillations near a cyclic plate cascade are shown to exist and are studied. It is shown that there always exist no less than two natural oscillation frequencies in the gas flow near any nontrivial cyclic plate cascade. It has been found that the natural oscillation frequencies can be combined in bundles such that in the case where the number of plates in a period is large the frequencies pertaining to each bundle occupy a certain interval with arbitrary density. The natural oscillations are classified with respect to the form of the eigenfunctions; the classification is based on the theory of representations of groups of locally plane symmetries of the cyclic plate cascade in the solution space. The correctness of the proposed model of the aeroacoustic resonance effects (aeolian tones) excited near a plate cascade in a gas flow is supported by a comparison with the available experimental and theoretical data. On the basis of the investigation performed, some previously unknown physical phenomena are predicted. Thus, the existence of frequency zones or main-flow Mach number ranges on which aeroacoustic resonance phenomena exist near a cyclic cascade with a large number of plates in a period is proved; it is shown that for certain frequencies of the natural oscillations near the cyclic plate cascade the resonance oscillations may be localized in the vicinity of the source; and the existence of narrow-band wave packets slowly propagating along the cascade is demonstrated. Novosibirsk, e-mail: sukhinin@hydro.nsc.ru. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 171–186, March–April, 2000.     

Determination of complete velocity gradient tensor by using cinematographic stereoscopic PIV in a turbulent jet

Cinematographic stereoscopic PIV measurements were performed in the far field of an axisymmetric co-flowing turbulent round jet (Re _T ≈ 150, where Re _T is the Reynolds number based on Taylor micro scale) to resolve small and intermediate scales of turbulence. The time-resolved three-component PIV measurements were performed in a plane normal to the axis of the jet and the data were converted to quasi-instantaneous three-dimensional (volumetric) data by using Taylor’s hypothesis. The availability of the quasi-three-dimensional data enabled the computation of all nine components of the velocity gradient tensor over a volume. The use of Taylor’s hypothesis was validated by performing a separate set of time-resolved two component “side-view” PIV measurements in a plane along the jet axis. Probability density distributions of the velocity gradients computed using Taylor’s hypothesis show good agreement with those computed directly with the spatially resolved data. The overall spatial structure of the gradients computed directly exhibits excellent similarity with that computed using Taylor’s hypothesis. The accuracy of the velocity gradients computed from the pseudo-volume was assessed by computing the divergence error in the flow field. The root mean square (rms) of the divergence error relative to the magnitude of the velocity gradient tensor was found to be 0.25, which is consistent with results based on other gradient measurement techniques. The velocity gradients, vorticity components and mean dissipation in the self-similar far field of the jet were found to satisfy the axisymmetric isotropy conditions. The divergence error present in the data is attributed to the intrinsic uncertainty associated with performing stereoscopic PIV measurements and not to the use of Taylor’s hypothesis. The divergence error in the data is found to affect areas of low gradient values and manifests as nonphysical values for quantities like the normalized eigenvalues of the strain-rate tensor. However, the high gradients are less affected by the divergence error and so it can be inferred that structural features of regions of intense vorticity and dissipation will be faithfully rendered.     

SWT-120风洞稳定段的性能测量

在普通超音速风洞中,由于受到噪声干扰,很难进行有效的边界层转捩特性试验研究,针对超声速流动特点发展较低噪声风洞十分必要,而稳定段设计的好坏直接影响到下游试验段噪声水平。本文介绍一座低噪声风洞稳定段的结构和性能测量结果。先对稳定段的结构设计做了简单介绍,然后对实验结果进行分析,实验结果表明在大角度扩散段内装置孔锥,稳定段安装消音夹层和阻尼网组等部件后,气流的速度脉动和压力脉动明显降低,其中压力脉动降低一个量级,速度脉动为1%。进一步优化设计和改进工艺,速度脉动还可进一步降低。测量结果表明SWT-120稳定段的设计是成功的,对我们以后发展更高性能的静风洞有借鉴和参考价值。     

High speed PIV applied to aerodynamic noise investigation

In this paper, we study the acoustic emissions of the flow over a rectangular cavity. Especially, we investigate the possibility of estimating the acoustic emission by analysis of PIV data. Such a possibility is appealing, since it would allow to directly relate the flow behavior to the aerodynamic noise production. This will help considerably in understanding the noise production mechanisms and to investigate the possible ways of reducing it. In this study, we consider an open cavity with an aspect ratio between its length and depth of 2 at a Reynolds number of 2.4 × 10⁴ and 3.0 × 10⁴ based on the cavity length. The study is carried out combining high speed two-dimensional PIV, wall pressure measurements and sound measurements. The pressure field is computed from the PIV data. Curle’s analogy is applied to obtain the acoustic pressure field. The pressure measurements on the wall of the cavity and the sound measurements are then used to validate the results obtained from PIV and check the range of validity of this approach. This study demonstrated that the technique is able to quantify the acoustic emissions from the cavity and is promising especially for capturing the tonal components on the sound emission.     

An aeroacoustic hybrid approach for non‐isothermal flows at low Mach number

This paper presents an aeroacoustic hybrid technique for the study of non‐isothermal flows at low Mach number. The flow dynamics and the acoustic production and propagation are computed separately. The fully compressible Navier–Stokes equations are modified through an expansion of the physical quantities using a low Mach number approximation. Compressibility effects are thus removed in the CFD while inhomogeneities of the flow related to heat transfer are preserved. One advantage is a reduction of the time step constraint. Another advantage is that the Mach number does not appear explicitly and a simple rescaling allows a study over a relatively wide band of subsonic Mach number flows with a single dynamic simulation. Compatible acoustic source terms for LEE based propagation have been defined and the procedure is implemented in the case of a temporal mixing layer. Compressible simulations for Mach numbers of 0.2, 0.3 and 0.4 are compared with the numerical results obtained using the proposed method. Very good agreement is obtained even at relatively high subsonic Mach number demonstrating the efficiency of the proposed technique. Copyright © 2004 John Wiley & Sons, Ltd.     

Toward accurate hybrid prediction techniques for cavity flow noise applications

A large variety of hybrid computational aeroacoustics (CAA) approaches exist differing from each other in the way the source region is modeled, in the way the equations are used to compute the propagation of acoustic waves in a non-quiescent medium, and in the way the coupling between source and acoustic propagation regions is made. This paper makes a comparison between some commonly used numerical methods for aeroacoustic applications. The aerodynamically generated tonal noise by a flow over a 2D rectangular cavity is investigated. Two different cavities are studied. In the first cavity (L/D=4, M=0.5), the sound field is dominated by the cavity wake mode and its higher harmonics, originating from a periodical vortex shedding at the cavity leading edge. In the second cavity (L/D=2, M=0.6), shear-layer modes, due to flow-acoustic interaction phenomena, generate the major components in the noise spectrum. Source domain modeling is carried out using a second-order finite-volume large eddy simulation. Propagation equations, taking into account convection and refraction effects, are solved using high-order finite-difference schemes for the linearized Euler equations and the acoustic perturbation equations. Both schemes are compared with each other for various coupling methods between source region and acoustic region. Conventional acoustic analogies and Kirchhoff methods are rewritten for the various propagation equations and used to obtain near-field acoustic results. The accuracy of the various coupling methods in identifying the noise-generating mechanisms is evaluated. In this way, this paper provides more insight into the practical use of various hybrid CAA techniques to predict the aerodynamically generated sound field by a flow over rectangular cavities. Copyright © 2009 John Wiley & Sons, Ltd.