AN EXPERIMENTAL STUDY ON 1/f^<tx> STRUCTRE AND ENTRAINMENT IN TURBULENT BOUNDARY LAYER

本文根据分形学中定义的1/f^&lt；tx&gt；间歇性，研究了湍流边界层中与大尺度涡相关的卷吸机制。实验在平板边界层和自由流幂函数变化下形成的自保边界层中进行。对于自保边界层，自由流是减速的，因此边界层受到轻微的负压梯度。本文研制了一种新的直接测量局部壁面剪应力的装置，并应用于实验研究。该测量装置具有20 mm的圆形摩擦面和零位移电路，使浮动元件保持在初始位置。将该装置应用于平板边界层，实验得到的壁面剪应力具有足够高的精度和可靠性，在两种边界层中，壁面附近的平均速度分布均可用对数平均速度分布表示，卡曼常数为0.41，附加常数为5.0。逆压梯度使外层平均速度剖面中尾迹分量的强度明显增大。三重速度积，即雷诺切应力和湍流强度的湍流扩散，绝对值变大。用条件采样技术研究了外层的间歇性，并将其应用于湍流和非湍流区的识别。分别计算了湍流区和非湍流区的两点双速关联函数。湍流凸起中预计会有大尺度的顺流涡旋运动，涡旋运动的展向大小受到不利压力梯度的挤压。湍流的流向间隔概率可以用幂函数成功地解释。

In the present study, entrainment mechanisms associated with large scale eddies in turbulent boundary layers has been investigated in terms of 1/f^<tx> intermittent nature defined in fractals. The experiment has been made in a flat plate boundary layer and a self-preserving boundary layer developed under power-law variation of the free-stream. The free-stream is decelerating with x^<-0.188> for the self-preserving boundary layer, thus the boundary layer is subjected to a mild adverse pressure gradient.A new direct measurement device for the local wall shear stress has been developed and applied in the present experimental study. The measurement device has 20mm circular friction surface and zero-displacement circuit that keeps the floating element at initial position. Application of the new device to the flat plate boundary layer proves that the wall shear stress can be obtained experimentally with sufficiently high accuracy and reliability.In both the boundary layers, the mean velocity profile near the wall can be reasonably expressed in the logarithmic mean velocity profile with Karman constant of 0.41 and additive constant of 5.0. Strength of wake component in the outer layer mean velocity profile is strongly enlarged by the adverse pressure gradient. The triple velocity products, which mean turbulence diffusion of Reynolds shear stresses and turbulent intensity, become greater in magnitude of absolute values.The outer layer intermittent nature has been investigated by conditional sampling technique that is applied to recognize turbulent and non-turbulent zones. Two-point double velocity correlation functions were calculated for turbulent zone and non-turbulent zone, respectively. A large scale streamwise vortical motion is expected in turbulent bulge and spanwise size of the vortical motion is squeezed by the adverse pressure gradient. The probability of streamwise interval of turbulent can be successfully explained by power-law functions.