Experimental Investigation of Compliant Surface Deformation under a Turbulent Boundary Layer*

Previous experiments examined the correlations between a turbulent channel flow and deformation of a compliant wall with shear speed much higher than the flow speed, resulting in submicron deformations. Aiming to extend the scope to two-way coupling, where the deformation amplitude is several wall units, theoretical analysis is used for selecting a compliant material (PDMS + silicone gel) with Young’s modulus (0.15 MPa), coating thickness (5mm), and shear speed (~6 m/s) comparable to the flow velocity. Time-resolved (2 kHz) Mach-Zehnder Interferometry (MZI) with a field of view of 70×35mm² is used for measuring the spatial distribution of surface deformation of the transparent compliant wall. The experiments have been performed in a water tunnel extension to the JHU refractive index matched facility in boundary layers having friction velocity Reynolds numbers in the 1000-7000 range. Data analysis shows that the amplitude of the deformations increases from less than 1μm (rms – 0.2 μm) at a freestream velocity of 1 m/s to well above 20 μm (rms - 6 μm) at 6 m/s, the latter corresponding to several (~4) wall units. Wavenumber-frequency spectra show that at all speeds, the typical advection speeds of surface deformation are consistently about 66% of the freestream velocity.