Passive Attenuation of Tollmien-Schlichting Waves by Phononic Subsurfaces

The drag reduction through the delay of laminar-to-turbulent transition in wall-bounded flows has been a focus of the flow control community for decades. One path previously investigated for achieving these benefits is through active opposition control of Tollmien-Schlichting (TS) waves using a piezoelectrically driven oscillating surface (PDOS, Amitay et al. 2016). However, the dynamic interaction of the fluid and the solid surface can also be exploited through passive coupling. Hussein et al. (2015) computationally demonstrated a solid phononic subsurface (PSub) flow stabilization technique to attenuate or amplify the growth of TS waves. In this method, the interaction between the fluid and the PSub passively generates a spatio-temporal elastic deformation profile at the surface of the solid which counters the growth of the TS-waves. In the current work, this technique is explored experimentally within the University of Colorado Boulder low-speed wind tunnel. TS waves are seeded into the flow by an upstream PDOS within the laminar boundary layer of a 3.5 m long flat plate assembly at a freestream speed of 15 m/s. Time-resolved particle image velocimetry measurements are used to assess the growth and mitigation of the TS waves for both the baseline flow and the case with a PSub installed downstream for comparison with the prior computational results of Hussein et al. (2015).