Active control of streak structures in wall turbulence using an actuator array producing inclined wavy disturbances

Abstract

By using a piezo-ceramic actuator array, turbulent flow control in a two-dimensional water channel was attempted at the flow Reynolds number of 7500. Eight piezo-ceramic actuators were installed spanwise in the channel wall. The actuator array could produce artificial disturbances by the wall-normal oscillation of each piezo-actuator element. In a series of experiments, it was found that the near-wall turbulent structure could be modified at a certain mode of oscillations of the actuator array. In the wavy mode of oscillation, eight actuators oscillate from f _a  = 0 to 250 Hz with the phase difference of 45° to each other, where f _a is the frequency of the wavy-mode oscillation. The wavy-mode oscillation can produce inclined wavy disturbances in the vicinity of the wall. Measurements with a particle image velocimeter revealed that the inclined wavy disturbances interacted with the low-speed streaks near the wall and the regularity of the streak structures tended to disappear at y ⁺ = 50 for higher frequencies of Γ = f _a /f _B  > 10, where f _B is the averaged frequency of the burst event. In the uncontrolled case, these coherent structures were arranged in the spanwise direction at intervals of about 100 wall units. Thus it is considered that the critical frequency at which coherent structures begin to break into smaller ones does not depend on the typical frequency of the burst interval; instead it is supposed to depend on the frequency introduced by the timescale of the burst duration. In the wavy-mode oscillation, low-speed structures generated by the actuator array may interact with the longitudinal vortices and inhibit to construct the coherent structures in the vicinity of the wall.

This article was chosen from selected Proceedings of the Second International Symposium on Turbulence and Shear Flow Phenomena (KTH-Stockholm, 27-29 June 2001) ed E Lindborg, A Johansson, J Eaton, J Humphrey, N Kasagi, M Leschziner and M Sommerfeld.