Active Control of Turbulent Separated Flows

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(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