Effects of Periodic Modulation of Compliant Wall Stiffness on the Wall Deformation and Boundary Layer Structure

Previous investigations have demonstrated that compliant coatings modify the profiles of mean flow and turbulence in turbulent boundary layers well into the log layer even when the rms of wall‐ deformation remains well below a wall unit. In an ongoing study, we attempt to modulate the deformation and boundary layer by periodically inserting 3 mm high, 'hard' inserts with storage modulus of 930 kPa inside a softer (158 kPa), 5 mm thick, compliant wall. The softer material matches the stiffness previously employed in homogeneous‐coating experiments. The streamwise length of the inserts and space between them exceed the characteristic wavelength – three times the wall thickness. Both media are made of transparent PDMS allowing precision measurements of the wall deformation using Mach–Zehnder interferometry. The water tunnel experiments show that the surface‐wave speed is 49% of the freestream velocity over both regions, but the rms deformation over the hard inserts are 40% lower than those above the completely soft regions. Spectral wavenumber-frequency analysis reveals that the inserts cause generation of standing waves, and disperse the deformation energy to a broader spectral range, especially above the stiff inserts. The wave structure becomes three dimensional and extend to shorter wavelengths and higher frequencies. The presentation will also include results of stereo-PIV measurements showing the impact of the modulated wall deformation on the velocity profiles and Reynolds stresses.