Spatial filtering of wall pressure fluctuations beneath a turbulent boundary layer

Methods of experimental spatial filtering of wall pressure fluctuations beneath a turbulent boundary layer are developed with the aim of obtaining information on the wave number-frequency spectrum. The spatial filtering of the pressure field components by wave-vector filters is studied. The method of spatial filtering of pressure fluctuations by an acoustic array, i.e., a periodic structure with a finite number of elementary transducers, is analyzed. The relation between the wave number characteristic of the acoustic array and the wave number spectrum of the amplitude distribution of transducer’s local sensitivity is determined. Quantitative estimates are obtained for the sensitivity of the array to the wave number spectrum of turbulent boundary-layer pressures, which is necessary for measuring the wall pressure fluctuations in a turbulent boundary layer by wave-vector filters.     

Spatial filtering of wall pressure fluctuations. Methods of direct measurements of wave number-frequency spectra

The problems of spatial filtering of turbulent aerohydrodynamic noise sources are considered in connection with the problem of the direct measurements of wave number-frequency spectra of turbulent pressure fluctuations. The methods of wave-vector filtering of turbulent pressure fluctuations with the use of an acoustic array, i.e., a periodic structure with a finite number of elementary rectangular pressure transducers, are analyzed. Original versions of the wave number-frequency spectrum analyzer that allows the reconstruction of the wave number spectrum from the results of measurements are developed. The filtering characteristics of such analyzers are studied, and the relation between the wave number characteristic of an acoustic array and the wave number spectrum of the amplitude distribution of transducer’s local sensitivity over the aperture is determined.     

Experimental estimate of wave spectra of wall pressure fluctuations of the turbulent boundary layer in the subconvective region

A technique is developed for measuring the intensity of the frequency-wave spectrum components of wall pressure fluctuations of the turbulent boundary layer in a quiet aeroacoustic installation with the use of wave filters in the form of rectangular plates. Aluminium-alloy and organic-glass plates of various thickness under a fine-meshed screen are used, set up rigidly flush with the polished wall of the working part of the installation. The experimental data testify to the fundamental possibility of determining the field components of wall pressure fluctuations of the turbulent boundary layer using similar wave filters in the subconvective region, where a substantially lower pressure fluctuation intensity is observed in comparison to the intensity in the region of the convective maximum of the frequency-wave spectrum at a small flow velocity.     

Wave membrane filters for estimating the wave spectra of near-wall pressure pulsations of a turbulent boundary layer in the subconvective region

We have obtained results on measuring the levels of the frequency-wave spectrum of the field of near-wall pressure pulsations of a turbulent boundary layer on the smooth wall of a wind tunnel with the use of stretched plastic membranes as wave filters. We compare the experimental data with the classical model.     

Measurement of wind noise levels in streamlined probes

This paper investigates the wind noise pressure spectra measured by aerodynamically designed devices in turbulent flow. Such measurement probes are often used in acoustic measurements in wind tunnels to reduce the pressure fluctuations generated by the interaction of the devices with the incident flow. When placed in an outdoor turbulent environment however, their performance declines noticeably. It is hypothesized that these devices are measuring the stagnation pressures generated by the cross flow components of the turbulence. Predictions for the cross flow contribution to the stagnation pressure spectra based on measured velocity spectra are developed, and are then compared to the measured pressure spectra in four different probe type devices in windy conditions outdoors. The predictions agree well with the measurements and show that the cross flow contamination coefficient is on the order of 0.5 in outdoor turbulent flows in contrast to the published value of 0.15 for measurements in a turbulent jet indoors.     

Experimental studies of flow noise around a surfacing device

The wall pressure fluctuations in turbulent boundary layers play an important role in acoustic measurements carried out in moving media. Results of measuring the frequency spectra of wall pressure fluctuations around a surfacing device are presented. The spatial resolution achieved in measuring the wall pressure fluctuations is investigated. It is demonstrated that the results of hydrodynamic flow noise measurements strongly depend on the aperture size of the measuring acoustic transducer and its orientation in the turbulent boundary layer. The pseudosound pressure fluctuation spectra observed in a series of experiments with surfacing devices show that the resolution of the pressure receivers operating in the turbulent boundary layers considerably varies. On the basis of systematic measurements of wall pressure fluctuations by miniature and distributed receivers at high Reynolds numbers, the effect of the geometric dimensions of a pressure receiver on its resolution in the flow noise measurements is studied. An experimental method is proposed for estimating the receiver-induced distortions.     

Self-noise of molecular electronic transducers

The effect of hydrodynamic fluctuations on noise in molecular electronic transducers is studied. It is shown that turbulent pulsation also makes a considerable contribution to the self-noise of molecular elec-tronic transformers, along with laminar flow fluctuations. A method for qualitative and quantitative calculation of the noise induced by turbulent pulsation that arises when a liquid flows along the electrode surface is proposed. A quantitative relationship that relates the rms pressure pulsation to the liquid head and an expression for the total spectral density of the hydrodynamic noise in molecular electronic transducers are obtained.     

再入体缩比模型湍流等离子体电磁散射特性测量

介绍了在弹道靶上利用二级轻气炮发射再入体缩比模型开展湍流等离子体电磁散射特性模拟试验方法、湍流等离子体的雷达散射截面积(RCS)测量方法。给出了模型尾迹湍流等离子体的电磁散射特性测量典型试验结果,对获得的试验结果进行了分析,并与一阶畸变波Born方法计算结果进行了比较。结果表明:合理选择缩比模型发射速度和飞行环境压力,在弹道靶上能够模拟产生湍流等离子体;利用激光阴影成像技术获得的锥模型尾迹流场图像证实了尾迹转捩的出现及其湍流形成;在给定的试验条件下,锥模型及其绕流RCS测量值比其尾迹RCS测量值高3个数量级,比背景散射电平高0.5~2.5个数量级,且信号没有周期性,幅度脉动范围为1~15dB,频率脉动范围0.4~40kHz;锥模型湍流尾迹RCS的脉动可能是尾迹电子密度的脉动引起的;单站X波段雷达系统测量的锥模型尾迹亚密湍流等离子体的散射信号测量值与计算结果变化规律一致;弹道靶RCS测量技术可用于再入体缩比模型湍流等离子体电磁散射特性研究,为开展相关研究提供了一种有效的地面模拟实验研究途径。     

Measured wavenumber: frequency spectrum associated with acoustic and aerodynamic wall pressure fluctuations

Direct measurements of the wavenumber-frequency spectrum of wall pressure fluctuations beneath a turbulent plane channel flow have been performed in an anechoic wind tunnel. A rotative array has been designed that allows the measurement of a complete map, 63×63 measuring points, of cross-power spectral densities over a large area. An original post-processing has been developed to separate the acoustic and the aerodynamic exciting loadings by transforming space-frequency data into wavenumber-frequency spectra. The acoustic part has also been estimated from a simple Corcos-like model including the contribution of a diffuse sound field. The measured acoustic contribution to the surface pressure fluctuations is 5% of the measured aerodynamic surface pressure fluctuations for a velocity and boundary layer thickness relevant for automotive interior noise applications. This shows that for aerodynamically induced car interior noise, both contributions to the surface pressure fluctuations on car windows have to be taken into account.     

Reconstruction of the wind velocity profile from the intensity fluctuations of a reflected spherical wave in the turbulent atmosphere

The problem of reconstructing the wind velocity profile from the spatiotemporal statistics of turbulent reflected optical radiation intensity fluctuations is considered in the article. Expressions for the spatiotemporal correlation function and the spectrum of weak intensity fluctuations of the wave scattered on a diffusive screen are derived. An algorithm for reconstructing the wind velocity profile from the spatiotemporal spectra of the intensity of a reflected spherical wave in the turbulent atmosphere is suggested. The results of closed numerical experiments are presented that confirm the efficiency of the suggested algorithm.     

Suppression of acoustic noise in the measurements of wall pressure fluctuations

It is shown that, in hydrodynamic noise measurements in the presence of acoustic noise acting upon the pressure fluctuation receiver, spatial filtering methods should provide the best results. Active methods are developed for suppressing the acoustic noise that affects a miniature receiver in the course of turbulent pressure fluctuation measurements. The methods are based on complicating the structure of the measuring transducer by introducing an extra compensating sensing element whose characteristics are identical with those of the main sensing element. The spatial filtering of small-scale turbulent pressure fluctuations by a finite-size electroacoustic transducer is used as the basis for the development of noise-compensated measuring systems, as well as methods of measuring the turbulent pressure fluctuations by receivers with noise compensation. A numerical study of the wave-number filtering of acoustic noise in wall pressure measurements by a noise-compensated receiver is performed.     

Acoustic Transducer of Turbulent Pressure Fluctuations in a Temperature-Stratified Medium

The operation of an acoustic transducer in a temperature-stratified medium is investigated. The formation of a response of piezoceramic transducers of pressure fluctuations under the action of temperature fluctuations in a working medium on the sensor element is considered. The attenuation of the temperature signal of a pressure transducer in a turbulent boundary layer is calculated numerically. The effect of distortions of the spectral levels of pressure fluctuations detected by a sound transducer in the field of temperature inhomogeneities is investigated for the example of measurements of turbulent pressure fluctuations in a boundary layer during vertical ascent of the device to the surface from a specified depth in a deep sea.     

Measuring surface pressure with far field acoustics

This paper presents measurements of the wavenumber frequency spectrum of wall pressure fluctuations under a turbulent boundary layer made using sound radiated from hydrodynamically smooth ridges in the surface. The measurements also serve as a test of the scattering theory of roughness noise. The radiated sound reveals a cut through the full three-dimensional wavenumber frequency spectrum of the wall pressure at the wavenumber of the surface. Since ridges can be made with very small wavelengths, this technique can be used to probe the structure of the wall pressure spectrum on scales far smaller than those that can be reached using conventional wall-mounted transducers. Furthermore, the method reveals the wavenumber frequency spectrum directly, without the need for multi-point measurements or the spatial Fourier transforming of data. Measured spectra bear a close similarity to Corcos’ and Chase's model forms, and confirm the applicability of the theory of roughness noise and its prediction of roughness noise directivity.     

An experimental test of the dual-frequency method for monitoring transverse currents

The dual-frequency method for monitoring the transverse component of the velocity of current is tested using the data obtained earlier in a tidally forced flow through a channel. The turbulent properties of the flow cause fluctuations in the acoustic signals propagating through it. The fluctuations of such signals emitted at two different frequencies are coherent in the low-frequency band and noncoherent in the high-frequency region of the spectrum. The frequency separating these spectral regions is determined as the cutoff frequency of the coherence function and is directly related to the component of the current in the direction perpendicular to that of signal propagation. The results of measuring this component of the current are consistent with the data obtained with other independent methods and measurements. Published in Russian in Akusticheskiĭ Zhurnal, 2006, Vol. 52, No. 2, pp. 269–274. The article was translated by the authors.     

Anisotropic scaling of magnetohydrodynamic turbulence

We present a quantitative estimate of the anisotropic power and scaling of magnetic field fluctuations in inertial range magnetohydrodynamic turbulence, using a novel wavelet technique applied to spacecraft measurements in the solar wind. We show for the first time that, when the local magnetic field direction is parallel to the flow, the spacecraft-frame spectrum has a spectral index near 2. This can be interpreted as the signature of a population of fluctuations in field-parallel wave numbers with a k(-2)_(||) spectrum but is also consistent with the presence of a "critical balance" style turbulent cascade. We also find, in common with previous studies, that most of the power is contained in wave vectors at large angles to the local magnetic field and that this component of the turbulence has a spectral index of 5/3.     

The use of distributed pressure receivers based on elastic piezoelectric composite materials for measuring the hydrodynamic noise produced by a near-wall turbulence

Receiving electroacoustic transducers with sensing elements made of an elastic piezoelectric composite material are described. The parameters of a composite material that exhibits a bulk piezoelectric effect are presented. Results obtained by measuring the turbulent noise in a hydrodynamic channel with the use of piezoelectric composite receivers are reported. The results are compared with those of the noise measurements by a miniature piezoceramic receiver and are considered in the light of the known models of near-wall turbulence.     

Space-time-wave number-frequency Z(x, t, k, f) analysis of SAW generation on fluid filled cylindrical shells

A new 4D space-time-wave number-frequency representation Z(x,t,k,f) is introduced. The Z(x,t,k,f) representation is used for processing 2D space-time signal collection issued from wave propagation along a 1D medium. This representation is an extension along the time dimension of the space-wave number-frequency representation. The Z(x,t,k,f) representation is obtained by short time-space 2D Fourier transforming the space-time collection. The Z(x,t,k,f) representation allows the characterization transient aspects of wave generation and propagation in both space and time dimensions. The Z(x,t,k,f) representation is used to experimentally investigate Lamb wave generation and propagation around a cylindrical shell (relative thickness is equal to 0.03) surrounded by water and excited by a pulse (0.1 micros duration with 1-5 MHz transducers). Three kinds of fluids have been used inside the shell: air, water, propanol. In all the cases, the Z(x,t,k,f) analysis clearly identify the reflected field on the insonified side of the shell and it allows the measurement of the local reflection coefficients R(x,t,k,f). The generation and the propagation of Lamb waves are also quantified. For the liquid filled shells, the multiple internal reflections are revealed by Z(x,t,k,f) analysis: the local transmission coefficients T(x,t,k,f) are also measured. When local matching conditions allows Lamb wave generation, the multiple regeneration of Lamb wave is observed. Based on these results, a link is establish toward the theoretical results obtained by steady state approach and Sommerfeld-Watson transform.     

Pressure fluctuations within a turbulent gas flow and their interaction with a shock wave

The interaction of a shock wave with a turbulent air flow is investigated experimentally. The turbulence was created with the aid of a grid. On its reflection from a perforated disc the wave propagated through a turbulent flow. The Mach number of the incident shock was equal to 1.9–4, the Mach number of the reflected wave was equal to 1.6–2.5. We found the autocorrelation functions of pressure fluctuations and their phase diagrams. The turbulent length scale of pressure fluctuations behind the incident shock was determined. The appropriate quantity behind the reflected wave is less of an order as compared with the previous case. It is established that the pressure behind the reflected wave in the turbulent flow is 7–8% higher as compared with the pressure in the laminar flow, if other conditions are the same.     

Wind velocity profile reconstruction from intensity fluctuations of a plane wave propagating in a turbulent atmosphere

Reconstruction of a wind profile based on the statistics of plane-wave intensity fluctuations in a turbulent atmosphere is considered. The algorithm for wind profile retrieval from the spatiotemporal spectrum of plane-wave weak intensity fluctuations is described, and the results of end-to-end computer experiments on wind profiling based on the developed algorithm are presented. It is shown that the reconstructing algorithm allows retrieval of a wind profile from turbulent plane-wave intensity fluctuations with acceptable accuracy.