吹吸扰动对壁湍流相干结构的影响

采用热线风速仪测量受吹吸扰动的壁湍流边界层的流向速度，用傅里叶变换和子波变换研究 吹吸扰动对壁湍流能谱的影响，结果显示施加的低频扰动使边界层内层大尺度结构的能量减 少，小尺度结构的能量有所增强，远离壁面时扰动强度逐步衰减直到在外层中消失；通过VITA 法和子波变换法检测猝发事件，表明该扰动降低了猝发强度，使猝发周期延长，条件平均速 度波形的幅值降低、持续时间变短，说明扰动明显抑制了相干结构的猝发过程. 利用子波变 换可以实现湍谱分析，能有效检测猝发中的湍流结构，是一种客观的分析工具.     

Direct numerical simulation of a turbulent plane Couette-Poiseuille flow with zero-mean shear

Direct numerical simulations of a turbulent Couette-Poiseuille flow with zero-mean-shear at the moving wall (SL-flow) is performed to examine flow features compared to those for a turbulent pure Poiseuille flow (P-flow). Profiles of the streamwise mean velocity, indicator function and ratio of production to dissipation show that the logarithmic region is significantly elongated for the SL-flow compared to that for the P-flow at a similar Reynolds number. In addition, the magnitudes of the Reynolds stresses are found to be larger in both inner and outer layers for the SL-flow than those for the P-flow. The spanwise spectra of the production term in the turbulent kinetic energy equation are examined to provide a structural basis for explaining the statistical behaviors. In addition, because the growth of the energy-containing motions extends to the outer layer further for the SL-flow due to the presence of a positive mean shear throughout the entire wall layer, the self-similar behavior of the energy balance between the production and transport terms with respect to the self-similar wavenumber is found far from the wall. We also find the increase in the number of uniform momentum zones in the SL-flow, revealing the hierarchical distribution of the energy-containing eddies which are composed of multiple uniform momentum zones. These coherent motions lead to the elongation of the logarithmic region for the SL-flow. Finally, investigation of the turbulent energy transfer process in a spectral domain for the SL-flow demonstrates importance of outer layer very-long structures, and these structures attribute to the energy transport process in an entire flow field.     

壁湍流相干结构的辨识

本文评价了辨识湍流边界层近壁区域相干结构的条件采样法、自相关函数法、数字滤波法和子波分析法。结果表明：条件采样方法对门限值和平均窗宽等检测参数具有较强的依赖性，可以通过调整这些参数来选择最佳的参数值使得相干结构的误判概率达到最小。自相关函数法不需要任何参数就能够得出确定的结论，因此是一种比较客观的方法。但是，这种方法只能从统计上确定平均猝发周期，并不能实时地辨识相干结构。数字滤波法能够将湍流脉动信号分解为大尺度涡和近似各向同性的小尺度涡两部分，但是如何去掉大尺度信号中的非相干部分是仍然需要解决的问题。子波分析是分解湍流信号辨识湍流相干结构的有效手段，它能够将湍流信号在时域空间和频域空间进行双局部化分解，用能量最大准则确定相干结构对应的时间尺度参数，用子波逆变换提取相干结构对应的速度信号波形     

Burst event detection in wall turbulence by WVITA method

Wavelet Variable Interval Time Average (WVITA) is introduced as a method incorporating burst event detection in wall turbulence. Wavelet transform is performed to unfold the longitudinal fluctuating velocity time series measured in the near wall region of a turbulent boundary layer using hot-film anemometer. This unfolding is both in time and in space simultaneously. The splitted kinetic of the longitudinal fluctuating velocity time series among different scales is obtained by integrating the square of wavelet coefficient modulus over temporal space. The time scale that related to burst events in wall turbulence passing through the fixed probe is ascertained by maximum criterion of the kinetic energy evolution across scales. Wavelet transformed localized variance of the fluctuating velocity time series at the maximum kinetic scale is put forward instead of localized short time average variance in Variable Interval Time Average (VITA) scheme. The burst event detection result shows that WVITA scheme can avoid erroneous judgement and solve the grouping problem more effectively which is caused by VITA scheme itself and can not be avoided by adjusting the threshold level or changing the short time average interval. The project supported by the National Natural Science Foundation of China (19732005) and the National Climbing Project of China     

输气管道壁面涂料减阻机理的实验研究

用IFA-300热线风速仪以高于对应最小湍流时间尺度的分辨率精细测量了风洞中不同壁面涂料的管道湍流边界层不同法向位置流向速度分量的时间序列信号,利用湍流边界层近壁区域对数律平均速度剖面与壁面摩擦速度、流体黏性系数等内尺度物理量的关系和壁面摩擦速度与壁面摩擦切应力的关系,在准确测量湍流边界层近壁区域对数律平均速度剖面的基础上,间接测量湍流边界层的壁面摩擦阻力．对不同壁面涂料的壁湍流脉动速度信号用子波分析进行多尺度分解,用子波系数的瞬时强度因子和平坦因子检测管道湍流边界层中的多尺度相干结构,提取不同尺度相干结构的条件相位平均波形,对比研究输气管道壁面涂料的减阻机理．     

用能量最大准则确定VITA法的平均周期

引入了子波变换的方法来确定检测壁湍流猝发事件的VITA法的短时间平均周期T。为了确定VITA法的短时间平均周期T,用子波变换对热膜测速仪测量得到的湍流边界层近壁区域的流向脉动速度的时间序列进行了分解,分解在时域和频域同时进行,根据每一个尺度的子波系数的模的平方在时域的积分得到壁湍流每一个尺度的脉动动能随尺度的分布,用能量最大准则确定与壁湍流猝发事件的时间尺度对应的能量最大的尺度,该尺度也就是VITA法的短时间平均周期T。     

Experimental study of wall pressure fluctuations in rigid and elastic pipes behind an axisymmetric narrowing

Wall pressure fluctuations, p_t, in rigid and elastic pipes behind a local axisymmetric narrowing are studied. A sharp increase in their rms level in a finite region immediately downstream of the narrowing, leading up to a pronounced maximum upstream of the point of jet reattachment, is found. Approximate estimates both for the distance from the narrowing to the point of maximum rms pressure and for the rms magnitude at this point are obtained. Inspection of the wall pressure power spectrum, P, reveals the presence of low-frequency maxima. The maxima are found to be associated with the large-scale eddies in the regions of separated and reattached flow, and their frequencies are close to the characteristic frequencies of the eddies’ formation. These maxima are the main distinguishing features of the spectrum under investigation compared to the power spectrum of the wall pressure fluctuations in a fully-developed turbulent flow in a pipe without narrowing. A comparative analysis of the data for rigid and elastic pipes shows that changes in the pipe wall bending stiffness cause alterations in the flow structure near the wall and the corresponding redistribution of flow energy among the vortices. This results in an increase in the wall pressure amplitude and the low-frequency level of the wall pressure power spectrum, as well as the appearance of new frequency components in this domain.     

Separation between coherent and turbulent fluctuations: what can we learn from the empirical mode decomposition?

The performances of a new data processing technique, namely the empirical mode decomposition, are evaluated on a fully developed turbulent velocity signal perturbed by a numerical forcing which mimics a long-period flapping. First, we introduce a “resemblance” criterion to discriminate between the polluted and the unpolluted modes extracted from the perturbed velocity signal by means of the empirical mode decomposition algorithm. A rejection procedure, playing, somehow, the role of a high-pass filter, is then designed in order to infer the original velocity signal from the perturbed one. The quality of this recovering procedure is extensively evaluated in the case of a single tone perturbation (sine wave) by varying both the amplitude and the frequency of the perturbation. An excellent agreement between the recovered and the reference velocity signals is found, even though some discrepancies are observed when the perturbation frequency overlaps the frequency range corresponding to the energy-containing eddies as emphasized by both the energy spectrum and the structure functions. Finally, our recovering procedure is successfully performed on a non-stationary perturbation (linear chirp) covering a broad range of frequencies.     

竖直平板间自然对流的湍流频谱特性

根据直接数值模拟的计算结果，对竖直平板间湍流自然对流的脉动动能、速度及温度等物理量的时间序列进行频谱分析．结果表明，流动达到充分发展状态后，小尺度到大尺度的能谱很宽，计算的分辨率足够．从能谱分布可以观察到含能区、惯性子区和耗散区的存在，文中对各区的特性进行分析．由于该流动的强各向异性，惯性子区很窄．并讨论了法向位置对脉动动能的影响以及大尺度结构的特性．     

结构损伤的小波分形神经网络检测

用神经网络进行结构损伤检测、分析的有效性在很大程度上取决于训练样本的好坏。小波变换在时域和频域都具有表征信号局部特征的能力,小波包分析利用可以伸缩和平移的可变视窗能够聚焦到信号的任意细节,因此对有损伤的结构的非线性动力特性能进行有效的分析。利用分形几何方法不依赖于系统的数学模型的特点,将分形维数与小波分析相结合,建立了结构损伤的小波分形神经网络检测方法。研究结果表明,结构不同状态下的振动信号的各频段分形维数有明显的不同,可以将振动信号的各频段分形维数作为结构损伤检测的特征量,并用神经网络将结构的不同状态模式识别出来。     

Higher-order moments and spectra of velocity fluctuations in adverse-pressure-gradient turbulent boundary layer

The characteristics of turbulent boundary layer flows with adverse pressure gradients (APGs) differ significantly from those of canonical boundary layers. We have investigated the effects of an APG on the higher-order moments and spectra of velocity fluctuations. The local wavelet spectra reveal a large difference in energy-containing frequencies of streamwise and wall-normal components of turbulent velocities, which results in smaller Reynolds shear stress production. Moreover, an analysis of bispectra in the Fourier space has revealed that non-local interactions, consisting of streamwise fluctuating velocity with low frequency and wall-normal velocity with high frequency, occur in the APG flow. However, the small-scale motions are not affected by imposing an APG.     

用子波变换检测壁湍流信号局部标度指数

本文用子波变换检测了刻画壁湍流脉动信号自相似性的局部标度指数，研究了不同尺度的湍流结构的自相似性，发现在湍流边界层猝发过程中，喷射和扫掠发生时刻小尺度脉动速度信号的局部标度指数为负值，说明在大尺度猝发事件发生的时刻小尺度结构具有奇异的自相似性，在猝发过程中其作用不仅仅是对湍能的耗散．     

Experimental investigation of streamwise velocity fluctuation based on the Reynolds-number dependency in turbulent viscoelastic-fluid flow

This paper describes an experimental verification of energy supply mechanisms for the streamwise component of the turbulent kinetic energy (TKE) at different Reynolds numbers in viscoelastic-fluid flow. We investigated the characteristics of the streamwise turbulent velocity fluctuation by analyzing the production and turbulent diffusion terms in the TKE transport equation. In addition, we reported on the Reynolds-number dependency in a high Reynolds-number regime where direct numerical simulation cannot demonstrate changes in fluid properties. Based on the experimental verification, we proposed a conceptual model of the energy-exchange term between the TKE and the elastic energy, with focusing on the dependency of the fluid properties on the shear stress. This model is indirectly reflected in the streamwise TKE, the instantaneous velocity field, and the wave number relevant to energy-containing eddies. The main gain term of the TKE switches between the energy-exchange term and the production term dependently on the Reynolds number: as the Reynolds number exceeds the value which provides the maximum drag reduction rate, the production term becomes dominant and the magnitude of streamwise TKE becomes high compared to the water flow case.     

Statistical Structure at the Wall of the High Reynolds Number Turbulent Boundary Layer

The one and two-point statistical structure of very high Reynolds number turbulence in the surface layer near a rigid `wall' is analysed. The essential mechanisms for turbulent eddies impinging on the wall are studied using linearised rapid distortion theory, which show how the mean shear and blocking actions of the surface act first independently and then, over the life time of the eddy, interactively. Previous analytical results are reinterpreted and some new results are derived to show how the integral length scales, cross correlations and spectra of the different components of the turbulence are distorted depending on the form of the spectra of eddies above the surface layer and how they are related to motions of characteristic eddy structures near the surface. These results are applied to derive some quantitative and qualitative predictions in the surface layers (SL), where the eddies are affected by local shear dynamics, and in the `eddy surface layer' (ESL) where quasi independents loping elongated eddies interact directly with the wall, and where there is a large range of wave number within which the spectra of the horizontal velocity components are proportional to k ⁻¹. The longest eddies in the boundary layer occur near the wall. Field experiments agree with the theoretical model predictions for the quite different forms for the spectra, cospectra and cross correlations for the vertical and horizontal components of the velocity field. By showing that in SL the energy exchange between the large and small scale eddies is local(`staircase') energy cascade, whereas in ESL there is a direct nonlocal (`elevator-like')energy transfer to the small scales, it is shown why the thickness of the ESL increases over rougher surfaces and as the Reynolds number decreases. This revised version was published online in August 2006 with corrections to the Cover Date.     

Shock-vortex shear-layer interaction in the transonic flow around a supercritical airfoil at high Reynolds number in buffet conditions

This paper provides a conceptual analysis and a computational model for how the unsteady ‘buffeting’ phenomenon develops in transonic, low incidence flow around a supercritical aerofoil, the OAT15A, at Reynolds number of 3.3 million. It is shown how a low-frequency buffet mode is amplified in the shock-wave region and then develops upstream and downstream interaction with the alternating von Kármán eddies in the wake past the trailing-edge as well as with the shear-layer, Kelvin–Helmholtz vortices. These interactions are tracked by wavelet analysis, autoregressive (AR) modelling and by Proper Orthogonal Decomposition. The frequency modulation of the trailing-edge instability modes is shown in the spectra and in the wall-pressure fluctuations. The amplitude modulation of the buffet and von Kármán modes has been also quantified by POD analysis. The thinning of the shear layers, both at the outer edge of the turbulent boundary layers and the wake, caused by an ‘eddy-blocking’ mechanism is modelled by stochastic forcing of the turbulent kinetic energy and dissipation, by small-scale straining of the higher-order POD modes. The benefits from thinning the shear-layers by taking into account the interfacial dynamics are clearly shown in the velocity profiles, and wall pressure distribution in comparison with the experimental data.     

基于小波变换的水下连续爆炸声信号特征分析

为了研究水下连续爆炸声信号的特征,利用Mallat算法,采用离散小波变换对水下连续爆炸声信号进行了分层提取分析,讨论了水下连续爆炸声信号在各频带的能量分布状况,采用Welch方法实现了对水下连续爆炸声信号的功率谱特征提取,并采用离散小波变换对声信号进行时频谱特性分析。结果表明,水下连续爆炸声信号具有很强的声功率,声压级可以达到190 dB以上,声持续时间较长,频率范围宽、声信号的能量主要集中在频率48 kHz以下,其中在低频段能量更大,这些特点使其有望成为水声干扰源。     

基于连续小波变换的风速时程特性识别与分析

由于风速时程属于频域宽和频率变化剧烈的时变信号，需用具有良好时频局部化特性和弹性时．频窗口的小波变换进行分析。本文的目的是在风速时程的描述上较全面地了解风速的时频特性。利用小波分析方法在时域和频域的良好局部化性质，聚焦到风速时程的任意细节并加以分析，快速、准确地提取样本的局部谱密度特征，特别是对在整个时程记录中，具有相同功率谱但时频内容有差别的风速时程。用小波变换分析试验得到的风速时程，并研究和识别试验得到的曲线和实测风速曲线的时频特性、能量关系和局部谱密度特征。     

Characterization of near-bed coherent structures in turbulent open channel flow using synchronized high-speed video and hot-film measurements

High-speed video recordings (500 Hz) of flow visualizations in the near wall region of a turbulent open channel flow were synchronized with hot-film measurements of flow velocity and bed shear stress. Analysis of the video images provided information about the main characteristics of coherent flow structures associated with the occurrence of low-speed streak ejections near the bed. These structures consisted mainly of oscillating shear layers that were converted in the downstream direction and lifted away from the bed. A visual detection criterion was developed to obtain ensemble averaged profiles of the velocity and shear stress data during ejection events, allowing for the characterization of the associated flow field during the occurrence of coherent structures. Conditional averaging suggests that the occurrence of such coherent patterns affects mainly the turbulence structure in the wall region, and that the observed events reveal a plausible mechanism by which energy is extracted from the mean flow by large scale turbulent fluctuations, and then further transferred towards smaller eddies, while the structures lose their coherence. The intermittent nature of production and dissipation of turbulent energy becomes noticeable, taking place about 21% of the time. The results obtained also provide evidence that seems to link the structures responsible for the turbulent vertical transport of momentum, and for the maintenance of the turbulent state, with the mechanism that triggers the entrainment of sediment into suspension. Comparison of present results with other experiments conducted in different types of flows strongly confirms a universal structure of coherent events in wall bounded flows.The support of the Fluid, Hydraulic, and Paniculate System Program of the National Science Foundation (Grant CTS-9210211) and the donors of the Petroleum Research Fund of the American Chemical Society (Grant PRF 24328-G2) is gratefully acknowledged.     

小波变换在湍流数值研究中的应用

小波变换具有时空双局部性特点,恰好适应了湍流特性。本文主要阐述了小波在湍流数值计算中的两大研究进展:一个是利用连续小波(高斯小波)可使L ap lace算子降阶的特性来求解N-S方程;另一个是正交小波与有限元法相结合的方法——相干涡模拟。指出它们的优缺点及其存在的问题,并对小波在湍流计算中的应用前景作了展望。     

Assessment of the organization of a turbulent separated and reattaching flow by measuring wall pressure fluctuations

The effect of local forcing on the organization of a turbulent separated and reattaching flow was assessed by measuring wall pressure fluctuations. Multi-arrayed microphones were installed on the surface to measure the simultaneous spatial and temporal wall pressure fluctuations. Local forcing at the separation edge was applied to the separated flow over a backward-facing step through a thin slit. The organization of the separated and reattaching flow was found to be greatest at the effective forcing frequency. The flow structure was diagnosed by analyzing several characteristics of the wall pressure fluctuations: the wall pressure fluctuation coefficients, wall pressure spectrum, wavenumber-frequency spectrum, coherence, cross-correlation, and multi-resolution autocorrelations of pressure fluctuations using the maximum overlap discrete wavelet transform and continuous wavelet transform. Features indicative of the amalgamation of vortices under the local forcing were observed; this amalgamation process accounted for the observed reduction of the reattachment length. Examination of the wall pressure fluctuations revealed that introduction of local forcing enhanced flapping motion as well as the streamwise and spanwise dispersions of vortical structures.