Direct computation of the noise radiated by a subsonic cavity flow and application of integral methods

The goal of this paper is to investigate the acoustic field generated by the flow over a cavity using two different and complementary numerical methods. First, a Direct Numerical Simulation of the 2-D compressible Navier-Stokes equations is performed to obtain directly the radiated noise. The results of the acoustic and aerodynamic fields are compared to the experimental data in the literature. Second, this reference solution is compared to solutions provided by hybrid methods using the flowfield computed inside the cavity combined with an integral formulation to evaluate the far-field noise. Numerical issues of three integral methods are studied: the Ffowcs Williams and Hawkings analogy that extends Lighthill's theory to account for solid boundaries and two Wave Extrapolation Methods from a control surface, the Kirchhoff and porous Ffowcs Williams and Hawkings methods. All methods show a good agreement with the Direct Numerical Simulation, but the first one is more expensive owing to an additional volume integral. However, the analogy can help in the analysis of wave patterns, by separating the direct waves from the reflected ones. The wave extrapolation methods from a surface are more efficient and provide a complementary tool to extend Computational Aeroacoustics near field to the very far field.     

圆柱绕流辐射声场的数值研究

本文从ψ-ω形式的不可压N-S方程出发,采用ADI-BGE格式及新的物面边界条件处理方法,并提出一种数值扰动模型,成功地计算了R_e=100时圆柱绕流卡门涡街流动,得到的流场结果比原有数值解准确,与实验结果符合良好,因而可用来进行声场数值研究。在声场计算方面,直接积分Curle方程,对R_e=100时的声源分布及其声辐射特性进行数值分析,并与涡声理论结果和流场中脉动力和脱体旋涡频谱进行比较,证明本文方法是成功的。     

具有旋涡的翼型绕流辐射声场的数值研究

本文运用面无法和涡声理论数值来研究具有旋涡的NASAGA（W）-1型翼型绕流的流场和辐射声场,这对于提高透平机械的效率及降低辐射噪声很有帮助。采用作者积累的关于面无法的一些计算技巧,首先成功地求解了不同攻角来流时的流场,再用涡声理论计算了涡旋绕过翼型时声压场的变化及声源的指向性,最后给出了有关这种流动噪声的发声机理和声源特性的一些有用的结论。     

Acoustic excitation of a square plate by turbulent flow noise

An experiment was conducted to measure the characteristics of flow noise in a bounded system with forced circulation. The flow noise facility is described herewith. Vortex shedding in the conducting rectangular ducting was experimentally controlled by the insertion of cylinders of various diameter and pitch perpendicular to the flow. The sound pressure level exciting an instrumented square plate parallel with the flow direction was measured, as well as the plate response.     

A numerical study of a turbulent mixing layer and its generated noise

A direct numerical simulation of a turbulent mixing layer with the Reynolds number 500 and the convective Mach number 0.6 is performed and the results obtained are used to study the turbulent flow field and its generated noise.In the present simulation,the numerical techniques of absorbing buffer zones,artificial convection velocity and spatial filtering are used to achieve nonreflecting boundary conditions.The self-similarity is used to validate the present numerical simulations.The large-scale coherent structures are plotted together with the acoustic waves,which demonstrates the directivity of acoustic waves.The Lighthill’s source and space-time correlations are further investigated.The main contributions to mixing noise are identified in terms of large-scale coherent structures,Lighthill’s source and space-time correlations.     

Influence of stiffeners on plate vibration and radiated noise excited by turbulent boundary layers

The influence of stiffeners on plate vibration and noise radiation induced by turbulent boundary layers is investigated by wind tunnel measurements. Plates with and without stiffeners are tested under the flow speed of 60 m/s, 71 m/s and 86 m/s, respectively. The stiffeners are set either perpendicular or parallel to the direction of the free stream. Measured vibration and noise levels are compared with theoretical calculations, where wall pressure cross-spectra are described by the Corcos model. For the plates tested, it is evident that stiffeners perpendicular to the direction of the free stream could increase noise radiation, but have almost no influence on vibration level of plates.     

Experimental investigation of supersonic turbulent flow over cylinders with various heights

Journal of Visualization - The unsteady flow structures and pressure fluctuations of the cylinder-induced shock wave/boundary layer interactions (SBLIs) were investigated at Mach 3.4 and 3.8. The...     

Correlation for Nusselt number in pure magnetic convection ferrofluid flow in a square cavity by a numerical investigation

Magnetic convection heat transfer in a two-dimensional square cavity induced by magnetic field gradient is investigated numerically using a semi-implicit finite volume method. The side walls of the cavity are heated with different temperatures, the top and bottom walls are isolated, and a permanent magnet is located near the bottom wall. Thermal buoyancy-induced flow is neglected due to the nongravity condition on the plane of the cavity. Conditions for the different values of non-dimensional variables in a variety of ferrofluid properties and magnetic field parameters are studied. Based on this numerical analysis, a general correlation for the overall Nusselt number on the side walls is introduced for a wide range of effective parameters. Results showed that maximum error produced by use of this correlation is about 6 percent.     

The numerical investigation of heat transfer and pressure drop of turbulent flow in a triangular microchannel

In this presentation, the flow and heat transfer inside a microchannel with a triangular section, have been numerically simulated. In this three-dimensional simulation, the flow has been considered turbulent. In order to increase the heat transfer of the channel walls, the semi-truncated and semi-attached ribs have been placed inside the channel and the effect of forms and numbers of ribs has been studied. In this research, the base fluid is Water and the effect of volume fraction of Al₂O₃ nanoparticles on the amount of heat transfer and physics of flow have been investigated. The presented results are including of the distribution of Nusselt number in the channel, friction coefficient and Performance Evaluation Criterion of each different arrangement. The results indicate that, the ribs affect the physics of flow and their influence is absolutely related to Reynolds number of flow. Also, the investigation of the used semi-truncated and semi-attached ribs in Reynolds number indicates that, although heat transfer increases, but more pressure drop arises. Therefore, in this method, in order to improve the heat transfer from the walls of microchannel on the constant heat flux, using the pump is demanded.     

Three-dimensional turbulent flow over cube-obstacles

In order to investigate the influence of surface roughness on turbulent flow and examine the wall-similarity hypothesis of Townsend, three-dimensional numerical study of turbulent channel flow over smooth and cube-rough walls with different roughness height has been carried out by using large eddy simulation(LES) coupled with immersed boundary method(IBM). The effects of surface roughness array on mean and fluctuating velocity profiles, Reynolds shear stress, and typical coherent structures such as quasi-streamwise vortices(QSV) in turbulent channel flow are obtained. The significant influences on turbulent fluctuations and structures are observed in roughness sub-layer(five times of roughness height).However, no dramatic modification of the log-law of the mean flow velocity and turbulence fluctuations can be found by surface cube roughness in the outer layer. Therefore, the results support the wall-similarity hypothesis. Moreover, the von Karman constant decreases with the increase of roughness height in the present simulation results. Besides, the larger size of QSV and more intense ejections are induced by the roughness elements, which is crucial for heat and mass transfer enhancement.     

流动管道内反声降噪数值研究

本文用流体力学方法建立了流动管道内反声降噪数值方法,其中,严格推导了反声源的数学模型.本文还给出了产逆流和声顺流传播时管内声场反声影响的数值算例,并讨论了流动速度、反声源强度和相位的影响.算例表明:反声降噪具有很好的工程应用前景.     

Three-dimensional direct numerical simulation of turbulent channel flow catalytic combustion of hydrogen over platinum

The turbulent catalytic combustion of a fuel-lean hydrogen/air mixture (equivalence ratio ? = 0.24) was investigated by means of three-dimensional direct numerical simulation (DNS) in a platinum-coated plane channel with a prescribed wall temperature of 960 K and an incoming Reynolds number, based on the channel height, of 5700. Heat transfer from the hot catalytic walls laminarized the flow, as manifested by the progressive suppression of the high vorticity components of the flow aligned parallel to the channel walls at increasing streamwise distances. The impact of turbulence suppression on the mass transfer towards or away from the catalytic wall was subsequently assessed. Far upstream where high turbulence fluctuations persisted, the instantaneous local transverse gradient of the limiting hydrogen reactant (a quantity proportional to the catalytic reaction rate) as well as the instantaneous hydrogen concentration at the wall exhibited strong fluctuations by up to 300%, a result of finite-rate chemistry induced by the high inrush events towards the catalytic walls. Fourier analysis of the reaction rate fluctuations yielded peak frequencies of less than 1 kHz, values comparable to the thermal response frequencies of typical materials in commercial catalytic geometries. This has direct implications on the thermal stress of the reactor walls as well as on the decoupling between flow and solid thermal modeling currently used in practical catalytic reactors. Far downstream, the dampening of turbulence resulted in weaker hydrogen concentration fluctuations with nearly symmetric distributions. Finally, computed transverse turbulent species fluxes indicated inherent weaknesses of near-wall turbulence models in describing turbulent transport of species with disparate molecular diffusivities.     

A numerical study of noise reduction in ducts with flow by anti-sound

I.IntroductionAnti-soundisalsocal1edactivenoisecontro1(ANC).Itsbasicidea,presentedintheLueg'spatentinl936l'l,isthatthenoisereductionisobtainedbyuseofthesignalpro-cessingofthepreliminarysoundsource(i.e.noisesource)toformcoherenceinfluencebe-twccnthepreliminarysoundsourceandthesecondarysoundsourcc(i.c.anti-soundsource).Therearesomeadvantagesofanti-soundsuchasactivecontro1,lcsseffectonthecharacteristicsofnoisesourccandmorereductionoflowfrequcncynoisc.Inrecentyears,therewerealotoftheoreticalande…     

Self-sustained oscillations of turbulent pipe flow terminated by an axisymmetric cavity

Turbulent flow through a long pipe terminated by an axisymmetric cavity can give rise to self-sustained oscillations exhibiting a very strong coherence, as evidenced by the narrow-band character of corresponding amplitude spectra. These oscillations, associated with the turbulent axisymmetric jet passing through the cavity, are strongly influenced by the acoustic modes of the pipe. The frequencies of oscillation lie within or near the range of most “unstable” frequencies of the turbulent jet previously predicted by using concepts of inviscid hydrodynamic stability theory; consequently, these experiments show truly self-excited and strongly coherent “instability” of a fully turbulent, low Mach number (～10^?2), axisymmetric flow undergoing separation, corroborating previous experiments involving the external forcing of free turbulent jets. As flow velocity or cavity length is varied, both upward and downward jumps in oscillation frequency are observed; the sign (up or down) of these jumps tends to systematically alternate with increase of velocity or length. The role of these frequency jumps is, in effect, to allow the oscillation of the flow to remain “locked-on” to a pipe mode over a wide range of impingement length or flow velocity. Moreover, these jumps exhibit two types of behavior: for the first kind, the predominant frequency makes a relatively continuous transition between stages and the frequency of the neighboring stage appears as a secondary component; for the second kind, there is a dead zone (where no oscillation occurs) between stages. The consequence of externally exciting the system is strongly dependent on whether the self-sustaining oscillation is relatively near, or well away from, a frequency jump. During excitation, the amplitudes of pressure fluctuations in the cavity substantially exceed the corresponding no-flow values only in regions away from the frequency jumps; at locations of jumps, there can be significant attenuation of the no-flow excitation amplitude. For the type of frequency jump involving a “dead zone”, enhancement of a given mode of oscillation can be achieved by externally exciting not only the given mode, but also neighboring modes. For the other type of jump, involving a relatively continuous transition from one stage to the next, the predominant mode of oscillation following the jump is that mode giving maximum amplitude response to excitation before the jump.     

A numerical and experimental study of the high-enthalpy high-speed cavity flow

Results of an experimental and numerical study of supersonic turbulent high-enthalpy flow in a channel with cavity are reported. On the basis of wind-tunnel tests performed in the IT-302M short duration wind tunnel, data on the flow structure and on the distribution of static pressure along the model walls were obtained. These data were subsequently used to verify the numerical algorithm. In the calculations, a parametric study of the effects of Mach number, cavity configuration, and temperature factor on flow quantities was performed. It was numerically shown that variation of the above parameters leads to a transition of the flow regimes in the vicinity of the cavity.     

Analysis on the frequency-domain numerical method to compute the noise radiated from rotating sources

A frequency-domain solution of the Ffowcs Williams-Hawkings equation with a penetrable data surface is presented for the thickness, loading and quadrupole noise to avoid the singularities that exist in the time-domain methods. Since this method is based on the numerical integration over source time, there is no need to solve the retarded-time equation or to perform the interpolation on time-domain data, and the time-domain source information obtained by modern CFD codes can be utilized directly. The acoustic pressure spectra of monopole, dipole and quadrupole point sources in subsonic and supersonic rotation are calculated with the presented method, and the results agree well with those obtained by the retarded-time method and frequency-domain analytical method.     

Direct numerical simulation of a statistically stationary,turbulent reacting flow

An inhomogeneous, non-premixed, stationary, turbulent, reacting model flow that is accessible to direct numerical simulation (DNS) is described for investigating the effects of mixing on reaction and for testing mixing models. The mixture-fraction-progress-variable approach of Bilger is used, with a model, finite-rate, reversible, single-step thermochemistry, yielding non-trivial stationary solutions corresponding to stable reaction and also allowing local extinction to occur. There is a uniform mean gradient in the mixture fraction, which gives rise to stationarity as well as a flame brush. A range of reaction zone thicknesses and Damkohler numbers are examined, yielding a broad spectrum of behaviour, including thick and thin flames, local extinction and near equilibrium. Based on direct numerical simulations, results from the conditional moment closure (CMC) and the quasi-equilibrium distributed reaction (QEDR) model are evaluated. Large intermittency in the scalar dissipation leads to local extinction in the DNS. In regions of the flow where local extinction is not present, CMC and QEDR based on the local scalar dissipation give good agreement with the DNS.M This article features multimedia enhancements available from the supplemental page in the online journal.     

一种重构辐射噪声调制谱的方法

在进行辐射噪声信号的仿真研究中,希望利用经滤波、特征提取等处理所得到的反映目标特性的线谱、连续谱、调制谱来重构辐射噪声,以便于研究辐射噪声在声场中的传播特性。本文提出了利用迭代算法估算调制深度谱的算法,给出了时域辐射噪声的重构模型,以实测数据的调制谱为出发点,来重构其时域信号,并对重构前后的调制谱进行了误差分析,取得了较好的效果。     

Experimental investigation of the power radiated by a monopole above a reflected plane

The presence of reflecting boundaries causes the acoustic power output of a point source to increase. As existing experimental data were found to be inadequate, experiments have been carried out in this study to determine the effect of a reflecting plane on the power radiated by a point source. A corona-type monopole was used in this study. Both source and receiver positions are varied; the statistical results of power radiated are compared with the theoretical predictions.     

Numerical investigation of the passive control of cavity flow oscillations by a dimpled non-smooth surface

Computational investigations are conducted to determine the effectiveness of a passive control technique, which was employed to decay the pressure oscillations induced by a subsonic flow over a cavity. This work focuses on a cavity with a small opening but a large volume. The passive control technique is employed by introducing a dimpled non-smooth surface, which is installed at the upstream of the cavity. Large eddy simulation is used to investigate the flow field and flow instability around the cavity for the smooth and non-smooth cases. Experiments are conducted in an acoustic wind tunnel for the smooth case to validate the computational scheme. Flow visualizations revealed that the dimpled surface located upstream effectively suppresses cavity flow oscillations. Finally, the control mechanism of cavity oscillation with the dimpled non-smooth surface is also determined based on the comparison of the flow field structure between the smooth and non-smooth cases.