STUDY ON TURBULENCE STRUCTURE AND ACTIVE CONTROL OF A THREE-DIMENSIONAL SEPARATED FLOW

在沿分离边缘均匀引入的单频和双频正弦扰动作用下，边界层与半无限长钝圆柱前缘分离形成湍流分离泡。扰动由圆筒内的低音扬声器产生，并通过狭缝引入分离的剪切层。单频强迫的主要结果可概括为：(1)在特定强迫频率下，当强迫的平均幅值小于主流速度的10%时，再附着长度达到一个确定的最小值。提出了一个流动模型来成功地解释这个频率（称为最有效频率F）和最小再附着长度，它是强迫幅度的对数函数。(2)在一定的强迫频率范围内，当强迫幅度大于主流速度的14%时，可以消除分离泡。随着强迫振幅的增大，强迫频率的变化范围趋于增大。该结果可用于涡轮机械翼型和叶栅流动分离的主动控制。(3)分离泡是一种由反馈回路维持的自激流动：在分离泡的再附着区，大尺度涡撞击表面产生的扰动以压力波的形式向上游传播，在分离尖锐边缘处被接受。扰动增强了分离剪切层的卷起作用。(4)利用周向分离点速度波动的相互关联以及烟丝和簇状探头的流动显示，获得了在最有效频率下强制分离泡内大尺度涡的结构。双频强迫的主要结果可概括如下：(5)最有效频率F与其高次谐波频率2F的组合在相位差为0时产生最小的再附着长度，在另一个相位差为pi时产生最大的再附着长度。用经典平面混合层的次谐波不稳定性成功地解释了这一结果。(6)当强迫幅值为主流速度的10%数量级时，双频强迫对再附着长度的减少与单频强迫大致相同。强迫分离气泡的离散涡数值模拟再现了再附着长度与强迫频率之间的关系，更重要的是提供了分离气泡反馈控制的有用信息。少

A turbulent separation bubble formed by the boundary-layr separation from the leading edge of a blunt circular cylinder of semi-infinite length was forced by a single-frequency and two-frequency sinusoidal disturbances introduced uniformly along the separation edge. The disturbance was generated by a woofer inside the cylinder and introduced into the separated shear layr through a thin slot. Main results for the single-frequency forcing may be summarized as follows :(1) The reattachment length attains a definite minimum at a particular forcing frequency if the r.m.s.forcing amplitude is less than 10% of the main-flow velocity. A flow model was proposed to successfully interpret this frequency (referred to as the most-effective frequency F) and the minimum reattachment length, which is a logarithmic function of the forcing amplitude.(2) The separation bubble can be eliminated in a range of the forcing frequency for the forcing amplitudes greater than 14% of the main-flow velocity. This … More range of the forcing frequency tends to increase with increasing forcing amplitude. This result is useful in active control of flow separation on aerofoils and cascades of turbomachinery.(3) The separation bubble is a self-excited flow maintained by a feedback loop : The disturbance produced by a large-scale vortex impinging on the surface in the reattachment region of the separation bubble propagates upstream as a pressure wave to be accepted at the sharp separation edge. The disturbance enhances the rolling-up of the separated shear layr.(4) The structure of large-scale vortices in the separation bubble forced at the most effective frequency was obtained in terms of cross correlations of velocity fluctuations at two points separated in the circumferential direction and flow visualization by smoke wires and tuft probes.Main results for the two-frequency forcing may be summarized as follows :(5) The combination of the most effective frequency F and its higher harmonic 2F yields a minimum reattachment length at a phase difference of 0 and a maximum reattachment at another phase difference of pi. This result was successfully interpreted in terms of the subharmonic instability of the classical plane mixing layr.(6) The two-frequency forcing yields approximately the same reduction of the reattachment length as the single-frequency forcing if the forcing amplitude is of the order of 10% of the main-flow velocity.A discrete-vortex numerical simulation of the forced separation bubble reproduced the relation between the reattachment length and the forcing frequency, and more importantly useful pieces of information on a feedback control of the separation bubble. Less