用子波系数概率密度函数研究湍流多尺度结构的间歇性

用热线风速仪在充分发展平板湍流边界层中测量流向速度信号的时间序列并进行湍流多尺度结构统计特性的实验研究。用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风洞中平板湍流边界层不同法向位置的流向速度分量的时间序列信号,用子波分析对湍流脉动速度信号进行多尺度分解,将推广的自相似性(ESS)应用于子波分析。用子波系数的概率密度函数验证了多尺度相干结构对间歇性的影响,并证明与距壁面距离有关。对比研究了提取湍流边界层多尺度相干结构前后湍流多尺度结构函数间歇性的变化规律及概率密度函数的变化规律,验证了多尺度相干结构是湍流边界层产生奇异标度律的原因。当这些相干结构被分离出后,由子波系数计算的ESS标度律就回到K olm ogorov线性标度律,这一现象在整个湍流边界层都是很明显的。

Experimental Investigation of Multi-Scale Eddy Structures' Intermittency in Turbulent Flow Using Probability Density Function of Wavelet Coefficients

The longitudinal velocity component in a fully developed turbulent boundary layer was measured by IFA300 hot wire anemometer and the statistical properties were studied experimentally.The velocity time sequence at different vertical locations in the turbulent boundary layer was obtained with resolution higher than the frequency that corresponds to Kolmogorov dissipative scale.The velocity signals were analyzed by wavelet transform,which described the local relative strain distortions for multi-scale eddy structures in turbulent flows.The Extended Self-Similarity(ESS) was applied to the statistical analysis of the multi-scale wavelet coefficients.The intermittency features,studied through the probability density function of the wavelet coefficients,were found to be influenced by multi-scale coherent eddy structures and were confirmed to be dependent on the distance from the wall.The intermittency features and the probability density function were compared between before and after excluding multi-scale coherent eddy structures.It has been observed that multi-scale coherent eddy structures were responsible for the anomalous scaling law in turbulent boundary layer.When such coherent eddy structures are eliminated,intermittency anomalies are no longer observed and the Kolmogorov linear scaling prediction is achieved.This phenomenon is obvious in the entire turbulent boundary layer.