Active control of streamwise vortices and streaks in boundary layers

Abstract

Coherent structures play an important role in the dynamics of turbulent shear flows. The ability to control coherent structures could have significant technological benefits with respect to flow phenomena such as skin friction drag, transition, mixing, and separation. This paper describes the development of an actuator concept that could be used in large arrays for actively controlling transitional and turbulent boundary layers. The actuator consists of a piezoelectrically driven cantilever mounted flush with the flow wall. When driven, the resulting flow disturbance over the actuator is a quasi-steady pair of counter-rotating streamwise vortices with common-flow away from the wall. The vortices decay rapidly downstream of the actuator; however, they produce a set of high- and low-speed streaks that persist far downstream (well over 40 displacement thicknesses). The amplitude of the actuator drive signal controls the strength of the generated vortices. The actuator is fast, compact, and generates a substantial disturbance in the flow. Its performance has been demonstrated using a small array of sensors and actuators in low-speed water laminar boundary layers with imposed steady and unsteady disturbances. Experiments are reported in which transition from a large disturbance was delayed by 40 displacement thicknesses, and in which the mean and spanwise variation of wall shear under an array of high- and low-speed streaks was substantially reduced downstream of a single transverse array of actuators.