Turbulent boundary layer over a compliant surface tuned to deform by several wall units*

Previous simultaneous time-resolved measurement of the 3D flow structure and deformation of a compliant wall by a turbulent channel flow involved wall stiffness too high to affect the flow, resulting in one-way coupling between flow and deformation. The current experiments focus on cases with deformations extending to several wall units aimed at generating two-way coupling. Guided by theoretical analysis, the required Young's Modulus (0.15Mpa), shear speed (6m/s), and thickness (5mm) of the compliant surface is achieved. Measurements performed in a developing boundary layer using Mach Zehnder Interferometry indeed confirm that the deformation amplitude increase from submicron to four wall units as the free stream velocity increases from 1 to 6 m/s. An initial series of stereo-PIV measurements along a streamwise plane compare the mean velocity profiles in the boundary layer to those developing over a rigid wall. Based on a log fit, the friction velocity remains unchanged compared to a rigid wall at low speeds, when the deformations are smaller than a wall unit. However, they begin to increase as the deformations exceed a wall unit. On-going measurements with increasing resolution are aimed at resolving the near-wall flow and distributions of Reynolds stresses.