Experimental Characterization of Effects of Wall Inhomogeneity on Surface Deformation in a Turbulent Boundary Layer over a Compliant Wall*

Recent investigations have revealed that deformations of compliant surfaces have significant influence on the flow and turbulence in the inner part of the boundary layer, all the way to the logarithmic region. The two-way coupling between flow and deformation are affected by the Reynolds number, visco-elastic wall properties, and the wall thickness, with the wall longitudinal strain being proportional to the rms pressure divided by the storage modulus. Consistent with theoretical predictions, in homogeneous materials, the wavelength of wall deformation is equal to three times the compliant layer thickness (l₀). This study attempts to modulate the surface wave characteristics by placing alternating strips of ‘high’ and ‘low’ stiffness, having a storage moduli of 930 kPa and 158 kPa, respectively (referred to ‘hard’ and ‘soft’ material) in the streamwise direction. The strip widths are 18 mm for a thickness of 5 mm, i.e. slightly larger than 3l₀. Time-resolved Mach-Zehnder interferometry, which has a submicron resolution, is employed to map the surface deformation in a 65 mm diameter circular field of view, covering at least 1.5 times of the cycle of hard and soft strips. Applications of spectral and statistical data analysis tools, which are still in progress, show a series of effects, including attenuation, changes to the wavelength, as well as reflections and formation of standing waves at the interfaces.