Laminar boundary layer forcing with active surface deformations

We experimentally evaluate the use of active wall-normal surface deformations as a method of actuation for the control of wall-bounded flows. Circular surface deformations are generated locally beneath a laminar boundary layer at a Reynolds number of Re delta* = 340, where delta* is the displacement thickness, and high-speed particle image velocimetry is used to investigate how the resulting motions vary as a function of the frequency and amplitude of actuation. We consider frequencies ranging from St = 0.1 to 1.0 where St is the Strouhal number based on the actuator diameter and freestream velocity, and amplitudes ranging from 0.005 to 0.020 times the actuator diameter (0.12 to 0.49 times the local boundary-layer thickness). We find that the actuation strategy can effectively produce both high-and low-speed motions with similar magnitudes which reach up to roughly one-third of the freestream velocity in some cases. The frequency of actuation dictates the spatial structure of these motions, while the amplitude of actuation dictates their strength. The motions are found to vary considerably, with St = 0.1, 0.2, 0.6, and 0.7 producing single flow structures concentrated along the centerline of the actuator (type-1 modes) while the remaining St produce structures with double extrema displaced in the spanwise directions (type-2 modes). Overall, the results indicate that surface deformations at the lowest frequencies (St 0.2) are the most promising for flow control because the resulting motions are stronger, more stable, and are concentrated along the centerline of the actuator. Finally, we show that a simple linear model adequately captures the input-output dynamics of the actuated flow-a promising result for future implementation in active control systems.