柔性壁面湍流减阻机理与多尺度相干结构控制的实验研究

用热膜测速仪以高于对应最小湍流时间尺度的分辨率,精细测量了水槽中刚性壁面和柔性壁面平板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的对数律平均速度剖面与壁面摩擦速度、流体粘性系数、壁面摩擦切应力等内尺度物理量的关系,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,通过非线性迭代求解壁面摩擦速度以及湍流边界层壁面摩擦切应力.结果表明柔性壁的湍流边界层速度分布在对数律层有所上移,缓冲层增厚,说明柔性壁面具有一定的减阻作用. 利用湍流多尺度局部平均结构函数的概念和多尺度局部平均结构函数的瞬时强度因子、平坦因子检测多尺度相干结构及其间歇性的方法,提取了湍流边界层多尺度相干结构的条件相位平均波形.对比研究了刚性壁面和柔性壁面平板湍流边界层近壁区域多尺度相干结构的条件相位平均波形及其间歇性的统计特征,分析了柔性壁面具有减阻作用的物理机理.     

壁湍流相干结构和减阻控制机理研究

剪切湍流中相干结构的发现是上世纪湍流研究的重大进展之一,这些大尺度的相干运动在湍流的动力学过程中起重要作用,也为湍流的控制指出了新的方向.壁湍流高摩擦阻力的产生与近壁区流动结构密切相关,基于近壁区湍流动力学过程的减阻控制方案可以有效降低湍流的摩擦阻力,但是随着雷诺数的升高,这些控制方案的有效性逐渐降     

壁湍流相干结构的教学实验（八）—用数字滤波法检测壁湍流相干结构

通过用三丝热膜探针测量湍流边界层的三个速度分量对壁端相干结构进行了实验研究,介绍了检测壁湍流相干结构的数字滤波法。     

壁湍流相干结构的教学实验(八)─—用数字滤波法检测壁湍流相干结构

通过用三丝热膜探针测量湍流边界层的三个速度分量对壁湍流相干结构进行了实验研究，介绍了检测壁湍流相干结构的数字滤波法．     

高分子溶液壁湍流减阻机理的TRPIV实验研究

利用高时间分辨率粒子图像测速技术(TRPIV)对回流式水槽中低浓度高分子溶液壁湍流的减阻机理进行实验研究。通过对比分析高分子溶液和纯水平板湍流边界层在相同来流速度下的平均速度剖面、湍流强度和雷诺应力,发现高分子溶液的壁面摩擦阻力减小了21.77%,并且其缓冲层增厚,按对数律外移,雷诺应力减小;高分子聚合物主要在近壁区起到抑制湍流脉动的作用,而在主流区的作用不太明显。用流向局部平均多尺度速度结构函数和相干结构条件采样方法,检测并对比了高分子溶液和水的壁湍流相干结构“喷射”和“扫掠”事件中的脉动速度、展向涡量、雷诺应力等物理量的二维拓扑形态,发现高分子溶液近壁区相干结构在猝发时的脉动速度减小,涡量受到抑制,雷诺应力明显减小,说明高分子溶液湍流近壁区相干结构“喷射”和“扫掠”的强度变弱,猝发频率降低,动量和能量的输运减弱,揭示出高分子溶液减阻的重要机理。     

提取壁湍流相干结构的数字滤波法

以三丝热线探头测得平板湍流边界层的数据为对象，提出用数字滤波技术将湍流信号分解为接近各向同性的小尺度涡和非各向同性的大尺度涡的方法。并用条件采样技术从大涡信号中提取相干结构。     

柔性壁面湍流边界层相干结构控制的实验研究

本文利用热膜测速技术对刚性壁面和柔性壁面湍流边界层的流向速度分量进行了实验测量,首先研究了柔性壁面对平均速度分布和湍流度分布的影响,结果表明:柔性壁面的边界层速度分布在对数律层向上有所平移,缓冲层加厚,具有一般的壁面减阻特征;而柔性壁的湍流度比刚性壁的湍流度要低,分布也更为平坦。然后综合运用自相关法和条件采样技术研究了湍流近壁区的相干结构,结果表明:刚性壁自相关曲线的第二峰值出现的时间比柔性壁的短,柔性壁的猝发频率比刚性壁的低许多。实验结果表明柔性壁面具有一定的减阻作用。     

壁湍流相干结构的教学实验(七)--用象限法检测壁湍流相干结构

介绍了检测壁湍流相干结构的ｕｖ象限分裂法。     

固壁温度对壁湍流相干结构的影响

用半导体元件对平板进行加热，用ＶＩＴＡ法和ｍｕ－ｌｅｖｅｌ法两种条件采样技术研究了固壁温度对湍流相干结构的影响，改进了Ｔｉｅｄｅｒｍａｎ等人提出的“归组”方法，并且用“归组”的ＶＩ－ＴＡ法和ｍｕ－ｌｅｖｅｌ法研究了固壁加热对湍流猝发平均周期的影响．实验结果表明，固壁加热使得湍流脉动速度均方根值变大，条件采样检测到的事件数量增多，但猝发周期维持不变     

壁面加热湍流相干结构的实验研究

用热线风速仪测量了风洞中壁面加热平板湍流边界层不同法向位置的流向脉动速度,用条件采样方法中的m-u-level法和VITA法研究了壁面加热对壁湍流相干结构猝发条件平均波形的影响,发现壁面另热能够提高猝发强度,缩短喷射时间,使一次猝发中出现多次喷射事件的现象明显增多,增强猝发的间歇性。     

A study on coherent structures and drag-reduction in the wall turbulence with polymer additives by TRPIV

An experimental measurement was performed using time-resolved particle image velocimetry (TRPIV) to investigate the spatial topological character of coherent structures in wall-bounded turbulence of polymer additive solution. The fully developed near-wall turbulent flow fields with and without polymer additives at the same Reynolds number were measured by TRPIV in a water channel. The comparisons of turbulent statistics confirm that due to viscoelastic structure of long-chain polymers, the wall-normal velocity fluctuation and Reynolds shear stress in the near-wall region are suppressed significantly. Furthermore, it is noted that such a behavior of polymers is closely related to the decease of the motion of the second and forth quadrants, i.e., the ejection and sweep events, in the near-wall region. The spatial topological mode of coherent structures during bursts has been extracted by the new mu-level criteria based on locally averaged velocity structure function. Although the general shapes of coherent structures are unchanged by polymer additives, the fluctuating velocity, velocity gradient, velocity strain rate and vorticity of coherent structures during burst events are suppressed in the polymer additive solution compared with that in water. The results show that due to the polymer additives the occurrence and intensity of coherent structures are suppressed, leading to drag reduction.     

Antisymmetric quadrupole mode of coherent structures in wall-bounded turbulence

Experiments were conducted in a water tunnel by tomographic time-resolved particle image velocimetry (Tomo-TRPIV). The Reynolds number Re_θ is 2460 on the base of momentum thickness. According to the physical mechanism of the stretch and compression of multi-scale vortex structures in the wall-bounded turbulence, the topological characteristics of turbulence statistics in logarithmic layer were illustrated by local-averaged velocity structure function. During coherent structures bursting, results reveal that the topological structures of velocity gradients, velocity strain rates and vorticities behave as antisymmetric quadrupole modes. A three-layer antisymmetric quadrupole vortex packet confirms that there is a tight relationship between the outer layer and the near-wall layer.     

Wavelet spectrum analysis on energy transfer of multi-scale structures in wall turbulence

The streamwise velocity components at different vertical heights in wall turbulence were measured. Wavelet transform was used to study the turbulent energy spectra, indicating that the global spectrum results from the weighted average of Fourier spectrum based on wavelet scales. W＇avelet transform with more vanishing moments can express the declining of turbulent spectrum. The local wavelet spectrum shows that the physical phenomena such as deformation position in the boundary layer, and the or breakup of eddies are related to the vertical energy-containing eddies exist in a multi-scale form. Moreover, the size of these eddies increases with the measured points moving out of the wall. In the buffer region, the small scale energy-containing eddies with higher frequency are excited. In the outer region, the maximal energy is concentrated in the low-frequency large-scale eddies, and the frequency domain of energy-containing eddies becomes narrower.     

壁面加热湍流相干结构的子波分析实验研究

通过在平板湍流边界层沿流向固壁表面平行放置若干条通电加热的金属细丝,在平板表面形成沿展向周期性分布的温度场,利用该温度场引起的空气热对流,在湍流边界层近壁区域产生一组沿湍流边界层展向周期分布的大尺度流向涡结构,改变了平板湍流边界层中不同尺度结构及其能量分布。采用对壁湍流多尺度结构的子波分析表明,在湍流边界层近壁区域产生规则的流向涡结构将壁湍流各种尺度湍涡结构不规则的脉动有序地组织起来,抑制了壁湍流各种尺度湍涡结构脉动,特别抑制了能量最大尺度湍涡结构的脉动,减小由于湍流脉动引起的在湍流边界层法向和展向的动量和能量损耗,从而减小了湍流的阻力。     

Coherent structures and riblets

This work deals with the effect of the riblets on the coherent structures near the wall. The emphasis is put on the genesis of the quasi-streamwise vortices in the presence of the riblets. The quasi-streamwise vortices regenerate by the tilting of wall normal vorticity induced by prevailing structures. This requires a mechanism which leads to a temporal streamwise dependence near the elongated flow structures and to a subsequent formation of new wall normal vorticity. It is suggested here that the action of existing quasi-streamwise vortices on the sidewalls of wall normal vorticity may create a local, streamwise dependent spanwise velocity and therefore, a secondary wall normal vorticity field. A preliminary analysis of the set-up and the time and space development of this secondary three-dimensional flow associated with the regeneration mechanism, is given. An attempt is made, in order to explain the drag reduction performed by the riblets through an intermittent model, based on the protrusion height. Logical estimates of the amount of drag reduction are obtained. The differences between the mechanism suggested here and those based on forced control experiments are also discussed.     

Vortex quadrupoles and two-dimensional oscillating grid turbulence

The planar motions induced by a localized forcing of the type of a point force-dipole is reproduced using a vertical rod oscillating horizontally in a thin layer of fluid. This forcing gives rise to the formation of a vortex quadrupole. To model a chaotic vortical flow, a system of quadrupoles is generated using a linear grid of oscillating rods. As a result of quadrupole interactions, a turbulent layer with two sharp boundaries forms and propagates from the grid.     

槽道湍流近壁结构的DPIV观测实验

采用DPIV系统(由两台CCD相机组成)对槽道湍流进行速度场时间历程的观测实验,通 过对大量测量结果的综合分析,取得了槽道湍流近壁结构的空间结构及其时间演化过程特征 的结果,可以揭示上扫下掠、湍流瞬时速度型等现象与大尺度涡演化的物理关系,解释若干 湍流大尺度结构的特征机理,还表明DPIV系统提供了一种定量观测湍流的时空结构特征的手 段.     

EXPERIMENTAL STUDY OF MEASUREMENT FOR DISSIPATION RATE SCALING EXPONENT IN HEATED WALL TURBULENCE

Experimental investigations have been devoted to the study of scaling law of coarse-grained dissipation rate structure function for velocity and temperature fluctuation of non-isotropic and inhomogeneous turbulent flows at moderate Reynolds number. Much attention has been paid to the case of turbulent boundary layer, which is typically the non-istropic and inhomogeneous trubulence because of the dynamically important existence of organized coherent structure burst process in the near wall region . Longitudinal velocity and temperature have been measured at different vertical positions in turbulent boundary layer over a heated and unheated flat plate in a wind tunnel using hot wire anemometer. The influence of non-isotropy and inhomogeneity and heating the wall on the scaling law of the dissipation rate structure function is studied because of the existence of organized coherent structure burst process in the near wall region . The scaling law of coarse-grained dissipation rate structure function is foun