On the effect of absorption on multiple wave-scattering in a magnetized turbulent plasma

Multiple scattering of electromagnetic waves by a plane layer of a turbulent magnetized collision plasma is considered. The influence of the distance between both the emitter and the receiver and the layer boundaries is analysed. It is found that the width of the angular spectrum of the received radiation for sufficiently strong absorption in the plasma is greater than in the collisionless plasma; the spectral maximum is substantially displaced with respect to the direction of the source. It is shown that these effects are weakened when the emitter approaches the layer. The relationship between the spectral width and also the displacement of its maximum and the distance from the receiver to the layer boundary may be substantially non-monotonic.     

Statistical characteristics of radiation of a point source scattered by a layer of turbulent collisional magnetized plasma

We consider multiple scattering of electromagnetic waves by a layer of turbulent magnetized collisional plasma. The influence of the distances from the layer's boundaries to the emitter and receiver is analyzed. We find out that, provided absorption in the plasma is strong enough, the width of the angular spectrum of the received radiation can be larger than in the collisionless plasma, and the maximum of the spectrum is substantially shifted with respect to the direction to the source. It is shown that these effects weaken as the emitter approaches the layer. The dependence of the width of the spectrum and the shift of its maximum on distance from the receiver to the layer's boundary can be substantially nonmonotonic. Lobachevsky State University, Nizhny Novgorod, Russia; Georgian Technical University, Tbilisi, Georgia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol.42, No. 4, pp. 340–354, April 1999.     

On the sensitivity of sound power radiated by aircraft panels to turbulent boundary layer parameters

The objective of the present study is to investigate and quantify how sensitive the response of an aircraft panel is to the change of the turbulent flow parameters. Several empirical models currently exist that provide the turbulent boundary layer wall pressure cross spectrum. These wall pressure cross spectrum models are usually dependent on four parameters: the reference power spectrum, the flow convective velocity, and the coherence lengths in streamwise and spanwise directions. All the proposed models provide different predictions for the wall pressure cross spectrum. Also, real flow conditions over aircraft do not conform to the ideal behavior of the turbulent boundary layer pressure predicted by the models. In this context, the questions that this work aims to explore are “What is the impact of different wall pressure estimates in the radiated sound power?” and “What is the effect of the range of possible flow conditions on the radiated sound power?”. For that objective, data from flight tests and estimates provided by the empirical models are used to predict radiated sound power, and the results are compared. A sensitivity analysis is performed and the relative contribution of each boundary layer parameter to the radiated sound power is obtained.     

On the effect of absorption on multiple wave-scattering in a magnetized turbulent plasma

Abstract

Multiple scattering of electromagnetic waves by a plane layer of a turbulent magnetized collision plasma is considered. The influence of the distance between both the emitter and the receiver and the layer boundaries is analysed. It is found that the width of the angular spectrum of the received radiation for sufficiently strong absorption in the plasma is greater than in the collisionless plasma; the spectral maximum is substantially displaced with respect to the direction of the source. It is shown that these effects are weakened when the emitter approaches the layer. The relationship between the spectral width and also the displacement of its maximum and the distance from the receiver to the layer boundary may be substantially non-monotonic.     

Multiscaling dynamics of impurity transport in drift-wave turbulence

Intermittency effects and the associated multiscaling spectrum of exponents are investigated for impurities advection in tokamak edge plasmas. The two-dimensional Hasagawa-Wakatani model of resistive drift-wave turbulence is used as a paradigm to describe edge tokamak turbulence. Impurities are considered as a passive scalar advected by the plasma turbulent flow. The use of the extended self-similarity technique shows that the structure function relative scaling exponent of impurity density and vorticity follows the She-Leveque model. This confirms the intermittent character of the impurities advection in the turbulent plasma flow and suggests that impurities are advected by vorticity filaments.     

On a Modification of the Scintillation Method

We solve the problem of diffraction of fluctuating radiation by an optically thin irregular layer (phase screen) with developed turbulent structure. It is shown that in the case of diffraction of radiation with saturated fluctuations and a narrow-band frequency spectrum by a weakly turbulent moving phase screen, the measured frequency spectrum of intensity fluctuations in the observation plane allows one to obtain information on the form of the spectrum of irregularities of an optically thin irregular layer in a wide size range significantly exceeding the size of the first Fresnel zone. Similarly to the well-known phase method of diagnostics of randomly irregular media, the conventional scintillation method modified in such a way yields undistorted information on the form of the irregularity spectrum. However, in contrast to the phase method, it also allows one to obtain data on the drift velocity of irregularities in the studied irregular layer.     

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.     

Rayleigh-Taylor turbulence is nothing like Kolmogorov turbulence in the self-similar regime

An increasing number of numerical simulations and experiments describing the turbulent spectrum of Rayleigh-Taylor (RT) mixing layers came to light over the past few years. Results reported in recent studies allow to rule out a Kolmogorov-like turbulence as a mechanism acting on a self-similar RT turbulent mixing layer. A different mechanism is presented, which complies with both numerical and experimental results and relates RT flow to other buoyant flows.     

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.     

Turbulent boundary layer at the border of geomagnetic trap

A new phenomenon was discovered on the basis of analysis of the Interball project data. A hot plasma flow is thermalized through the formation of “long-operating” vortex streets and local discontinuities and solitons in a distributed region over polar cusps. Plasma percolation through the structured boundary and secondary reconnection of fluctuating magnetic fields in a high-latitude turbulent boundary layer account for the main part of solar wind plasma inflow into the magnetospheric trap. Unlike local shocks, the ion thermalization is accompanied by the generation of coherent Alfvén waves on the scales ranging from ion gyroradius to the radius of curvature of the averaged magnetic field, as well as by the generation of diamagnetic bubbles with a demagnetized heated plasma inside. This “boiling” plasma has a frequency region where the spectrum is different from the Kolmogorov law (with slopes 1.2 and 2.4 instead of 5/3 or 3/2). The fluctuation self-organization in the boundary layer (synchronization of three-wave decays) was observed on certain frequency scales.     

电磁力控制湍流边界层分离圆柱绕流场特性数值分析

在亚临界区高雷诺数Re=1.4×105下,采用脱体涡模拟结合湍流分离的方法对弱电解质中电磁力作用下湍流边界层分离圆柱绕流场及其升(阻)力特性进行了数值模拟和分析.结果表明,电磁力可以提高圆柱体湍流边界层内的流体动能,延缓圆柱体湍流边界层的流动分离,减弱圆柱体湍流绕流场中在流向和展向上大尺度漩涡的强度,减小圆柱体阻力时均值及其升力脉动幅值.当电磁力作用参数大于某个临界值后,湍流边界层流动分离消失,在圆柱体尾部产生射流现象,从而电磁力对圆柱体产生净推力作用,出现负阻力现象,而且升力脉动幅值接近于零,出现圆柱体升力消失现象.     

超声速层流/湍流压缩拐角流动结构的实验研究

在Ma=3.0的超声速风洞中, 分别对上游边界层为超声速层流和湍流, 压缩角度为25°和28°的压缩拐角流动进行了实验研究. 采用纳米粒子示踪平面激光散射(NPLS)技术获得了流场整体和局部区域的精细结构, 边界层、剪切层、分离激波、回流区和再附激波等典型结构清晰可见, 测量了超声速层流压缩拐角壁面的压力系数. 从时间平均的流场结构中测量出分离激波、再附激波的角度和再附后重新发展的边界层的增长情况, 通过分析时间相关的流场NPLS图像, 可以发现流场结构随时间的演化特性. 实验结果表明: 在25°的压缩角度下, 超声速层流压缩拐角流动发生了典型的分离, 边界层迅速增长失稳转捩, 并引起一道诱导激波, 流场中出现了K-H涡、剪切层和微弱压缩波结构, 而超声速湍流压缩拐角流动没有出现分离, 湍流边界层始终表现为附着状态; 在28° 的压缩角度下, 超声速层流压缩拐角流动进一步分离, 回流区范围明显扩大, 诱导激波、分离激波向上游移动, 再附激波向下游移动, 分离区流动结构复杂, 相比之下, 超声速湍流压缩拐角流动的回流区范围明显较小, 边界层增长缓慢, 流场中没有出现诱导激波、K-H涡和压缩波, 流动分离区域的结构也相对简单, 但分离激波的强度则明显更强. 关键词： 压缩拐角 层流 湍流 流动结构     

Scaling of turbulent wall pressure fluctuations downstream of a rearward facing step

The turbulent boundary layer that forms downstream of a surface discontinuity is locally inhomogeneous. Here, we consider the boundary layer that occurs downstream of a rearward facing step discontinuity on a flat plate that is exposed to a zero incidence, uniform subsonic flow. The wall pressure "point" spectra are measured by small, flush-mounted pressure transducers located at various locations downstream of the step. A new form of the non-dimensional point wall pressure auto-frequency spectrum is proposed that includes the statistical variations of the reattachment location. This form is shown to collapse quite well all of the spectra measured slightly upstream, within, and downstream of the flow reattachment location.     

压缩拐角激波/湍流边界层干扰特性分析

用大涡模拟方法对Mach数3.0下的压缩拐角激波/湍流边界层干扰问题进行数值研究.对拐角上游平板区域边界层转捩及湍流进行模拟,设定平板区域长度,使得转捩过程于平板区域发生并充分完成,从而在拐角处产生激波/湍流边界层相互干扰,研究激波/湍流边界层的作用机理.研究表明:流场能够在非定常扰动激励下迅速转捩,并于平板区发展为完全湍流;湍流边界层与激波相互作用过程中,拐角附近分离区较层流情况明显减小;展向不同区域分离区大小差异较大,局部区域分离现象消失.     

Interaction of a Low Frequency Coherent Magneto-Acoustic Wave with a Turbulent Spectrum of Extra-Ordinary Waves

Starting from nonlinear evolution equations of a three-dimensional extra-ordinary wavepacket in a magnetized plasma and introducing a correlation function, equations are derived that describe interaction of a coherent low-frequency magneto-acoustic wave with a turbulent spectrum of high frequency extra-ordinary waves. These equations are used to derive the dispersion relation for a low frequency magnetoacoustic wave in the presence of a spectrum of turbulent high frequency extra-ordinary waves. In the narrow spectrum case, it is found that a small but finite spread in the spectrum of turbulence has a stabilizing influence on the instability which exists for vanishing spectral width. In the wide spectrum case, the effect of a weak turbulent spectrum of extra-ordinary waves is to produce a slight shift in frequency of the coherent low frequency magneto-acoustic wave and to damp or excite the wave.     

Noise of a turbulent boundary layer flow over smooth and rough plates at low mach numbers

The spectral levels of the quadrupole noise generated by a boundary layer flow over a smooth surface are calculated. Explicit dependences of the noise levels on the Reynolds number are obtained for the low-frequency and high-frequency ranges. It is shown that the logarithmic zone of the velocity profile is responsible for the region of the quadrupole noise spectrum with a hyperbolic dependence on frequency. A method of calculating the dipole noise of a boundary layer flow over a rough surface is developed. The method is based on the use of the combined probability density for the turbulent velocity fluctuations and the random dimensions of protuberances of the rough surface. The two constants involved in this theory are determined from a special experiment. It is shown that the surface roughness noticeably increases the radiation levels of a boundary layer flow in a certain frequency range.     

Frequency Characteristics of the Satureted Gain in Nonequilibrium Gas Flow and Doppler-Broadened Chemical Laser Amplifiers

We present "convection-nonequilibrium cross relaxation" model for a gas flow and chemical lasers, Teh experessions of the saturated gain spectrum are obtained for the two-type flow models with layer or turbulent flow mixture respectively. The numerical results show that the gain profile of a saturated amplifier with Doppler-broadening is a mixture of inhomogeneous and homogeneous profiles     

Scattering of sound waves by a turbulent wake

In this investigation, the Doppler shifted power spectrum of the scattering cross-section is obtained for plane acoustic waves scattered by fluid flow fluctuations appropriate to a turbulent wake. The wake considered in this paper is assumed almost homogeneous and isotropic and of low Reynolds number.It is shown that the evaluation of the Doppler scattering cross-section essentially reduces to the calculation of the wave number converted and frequency shifted energy spectrum function of the turbulent flow fluctuations. In prescribing the low Reynolds number turbulence spectrum, inertial forces are assumed negligible. Convective effects of the macro-eddies, which cause a Doppler shift in the scattered waves, are considered using a Lagrangian-type of space-time velocity correlation.After finding the spectrum of turbulent fluctuations, the Doppler shifted power spectrum of the scattering cross-section, which characterizes the scattered waves, is obtained explicitly for the far field approximation.     

Mean velocity behaviour in the near wake of long slender cylinder with a sharp trailing edge at a high Reynolds number

A simple eddy viscosity model is applied to the governing equations to establish the behaviour of the mean velocity in the turbulent axisymmetric near wake. The near wake develops from a long slender cylinder which is kept parallel to the flow and is developing under zero streamwise pressure gradient. The upstream turbulent boundary layer on the body of revolution is fully developed. In the inner layer of the flow downstream of the trailing edge, the turbulent inner layer of the upstream boundary layer grows into the initial logarithmic layer, and as a consequence, the centreline velocity in the near wake is shown to increase logarithmically with streamwise distances for large streamwise distances. The analysis further leads to two regions of the near wake flow (the inner near wake and the outer near wake), similar to that of a fully developed turbulent boundary layer, for which the governing equations have been derived. The matching between these two regions leads to a logarithmic variation in the normal direction. Also shown is the variation of the square of the wake width which varies logarithmically with streamwise distance in the near wake. These features are validated by comparison with available experimental data.     

Turbulent boundary layer over a flat plate with uniform wall suction

A complete theory of turbulent boundary layer flow over a flat plate with uniform wall suction is proposed. The theory relies on an asymptotic analysis of the Reynolds equations and dimensional considerations and does not involve any special closure hypotheses. Characteristics of the turbulent boundary layer with suction are calculated for the entire range of flow parameters by using the known characteristics of a reference flow (turbulent boundary layer over an impermeable flat plate). The velocity and shear stress profiles, the distribution of skin friction along the plate, and integral flow characteristics are obtained by using only the known velocity profile in the reference flow. The normal Reynolds stresses are calculated by using analogous characteristics of the reference flow. Results are presented in terms of scaling variables.