Active Control of Turbulent Separated Flows

(1)沿分离边缘引入单频正弦扰动，迫使前缘切割为方形的钝圆柱（钝圆柱）前缘分离泡形成；系统地改变了强迫的频率和水平，得到了分离剪切层中涡结构对强迫的响应。在特定的强迫频率f_<min>时，分离泡的再附着长度达到最小。如果按自由流速度和圆柱体半径归一化，该频率在不同的作用力水平下保持不变。最小再附着长度随强制频率的增加而减小。(2) f_<min>的频率是分离泡再附着区大尺度涡脱落频率的2^n （n：整数）倍。这一结果是在假设分离泡是一个自激系统的情况下得到的，并且在分离泡形成后，旋涡合并使大尺度旋涡立即从分离泡的再附着区脱落时，最小再附着长度更大。(3)通过引入f_1和f_2两个频率的正弦扰动，选取扰动各分量的强度相同，研究了双峰强迫。频率f_1等于上述频率f_<min>，而第二分量f_2的频率为第一分量的高次谐波和次谐波。两个分量之间的相位从0变为2。当f_2=2f_1，即第一次高次谐波时，重附长度对两组分相位差PHI的依赖性最大。(4)在分离缘下游较短的位置形成较大的旋涡，使再附着长度减小。对于足够高的强迫水平，可以绕过剪切层的自然合并过程来产生大涡。这是再附着长度减小的机理。少

(1) The leading-edge separation bubble of a blunt circular cylinder with the square-cut leading edge (the blunt circular cylinder) was forced by single-frequency sinusoidal disturbances introduced along the separation edge; the frequency and level of the forcing were systematically changed to obtain the response of the vortex structure in the separated shear layer to the forcing. The reattachment length of the separation bubble attained a minimum at a particular forcing frequency f_<min>. This frequency, if normalized by the free-stream velocity and the radius of the cylinder, remained constant at different forcing levels. The minimum reattachment length decreased with increasing forcing frequency. (2) The frequency f_<min> is 2^n (n:integer) times the frequency of shedding of large-scale vortices from the reattachment region of the separation bubble. This result was obtained by assuming that the separation bubble is a self-excited system and that the minimum reattachment length is rea … More lized when the large-scale vortices are shed from the reattachment region immediately after its formation by the vortex amalgamation. (3) The bimodal forcing was studied by introducing sinusoidal disturbances with two frequencies f_1 and f_2, the strength of each component of the disturbances being chosen to be the same. The frequency f_l was equal to the said frequency f_<min> while the frequency of the second component f_2 was higher harmonics and subharmonics of the first component. The phase between the two components was changed from 0 to 2pi. The most significant dependence of the reattachment length on the phase difference PHI between the two components was observed for f_2=2f_1, i.e. the first higher harmonics. (4) The decrease in the reattachment length by the forcing is produced by the formation of larger vortices at shorter positions downstream of the separation edge. For sufficiently high level of forcing, the natural amalgamation process of the shear layer is bypassed to produce the large vortices. This is the mechanism of the decrease of the reattachment length. Less