ACTIVE CONTROL OF TURBULENT SEPARATED FLOWS

摘要

An open-loop control and a closed-loop control were attempted for a turbulent separation bubble formed along the side of a blunt circular cylinder with a square leading edge. For the open-loop control, a single-frequency sinusoidal disturbance is introduced into the separated shear layer through a thin slot along the separation edge. Disturbances with different frequencies and amplitudes were produced by a woofer installed inside the cylinder. The reattachment length attains a minimum at a particular forcing frequency Fm, which scales with the frequency of shedding of vortices from the reattachment region of the shear layer. This optimum forcing frequency was employed for the closed-loop control as the central frequency of a band-pass filter through which the velocity fluctuation detected at a representative position in the separated shear layer was fed to the woofer to generate the disturbance with phase lag q. The reattachment length under the closed-loop control was found to decrease with increasing phase lag in the range of 0* < q < 160*. Beyond this range of q the closed-loop control generated intermittent, extremely high-amplitude oscillations of the instantaneous velocity and thus the input voltage of the woofer, causing a structural vibration of the blunt circular cylinder.