壁湍流结构的二维子波分析

2-D WAVELET ANALYSIS OF STRUCTURES IN WALL TURBULENCE

对槽道湍流直接数值模拟的脉动速度场进行了二维子波分析．研究了近壁区低速条带结构的多尺度特性．利用定义的局部雷诺应力测度结合平坦因子的方法对湍流脉动速度分解，得到脉动速度的间歇成份和高斯成份．用局部雷诺应力测度正的峰值为特征，通过条件统计平均，得到近壁区典型的拟序结构．

In the present study, the direct-numerical-simulated channel turbulence is analyzed by 2-D wavelet. The multi-scale character of near wall low-speed-streaks is studied, and the characteristic scales of the streaks are obtained. The average spanwise space λ～+ of the multi-scale low-speed-streaks and their varaition with wall normal distance y～+ are detected. It is shown that λ～+ decreases with scales, but it is constant with y～+ at the same scale. It is observed λ～+ ≈100 at the scale of maximum energy at different y～+. Considering the relation of conherent structure and Reynolds stress, we have defined the Local Reynolds stress Measure (LRM). A new method for extracting intermittent signal from turbulence based on the LRM is developed. Velocity fluctuations are decomposed to intermittent signals and Gaussian signals with this method. It is found that the scaling expontents of intermittent signals have a considerable deviation from q/3, whereas the scaling expontents of Gaussian signals are close to q/3. It is confirmed that intermittent signals are mainly responsibe for the anomalous scalings. Locally characterized by the positive peaks of LRM, the typical structures in near wall are observed by conditional statistical averaging, a socalled 'spike'-event in viscous sublayer, and an ejection-event in buffer sublayer. It is shown that the local character of structures in turbulence can be identified by LRM.