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

半无限长钝圆柱体前缘边界层分离形成的湍流分离泡，受到沿分离边缘均匀引入的单频和双频正弦扰动的作用。沿着分离边缘均匀引入单频和双频正弦扰动。扰动由圆柱体内部的低音扬声器产生，并通过细槽引入分离的剪切层。单频强迫的主要结果如下：（1）当强迫振幅小于主流速度的10%时，在特定的强迫频率下，再附着长度达到一个确定的最小值。提出了一个流动模型，成功地解释了这个频率（称为最有效的频率F）和最小再附着长度，这是一个对数函数的强迫振幅。(2)当强迫振幅大于主流速度的14%时，在强迫频率范围内，分离泡可以被消除。这 关于我们 强迫频率的变化范围随强迫振幅的增大而增大。这一结果对主动控制涡轮叶栅和翼型的流动分离有一定的参考价值。(3)分离泡是一种由反馈回路维持的自激流动：大尺度涡流撞击分离泡再附着区表面产生的扰动作为压力波向上游传播，在尖锐的分离边缘处被接受。扰动增强了分离剪切层的卷起。(4)通过对分离泡周向两点速度脉动的互相关和烟丝及簇状探针的流动显示，得到了在最有效频率下分离泡内大尺度涡的结构，主要结果如下：（五）最有效频率F及其高次谐波2F的组合在相位差为0时产生最小再附着长度，并且在相位差为0时产生最大再附着长度。在另一个相位差π处重新附着。这一结果成功地解释了经典平面混合层的亚谐不稳定性。(6)如果强迫振幅为主流速度的10%量级，则双频率强迫产生的再附着长度减少量与单频率强迫大致相同。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，关于分离气泡的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