Investigations of skin friction drag mitigation over viscoelastic surfaces in supersonic flows

Drag mitigation by passive surfaces with engineered material properties has gained significant attention in low -speed flows. The present work investigates the skin friction drag mitigation caused by flow interactions with a nonrigid surface at supersonic speeds. The nonrigid surface was formed by implanting viscoelastic rubber materials of different elastic moduli over a rigid surface. Direct measurements of the skin friction coefficient ( C f ) were made along the length of the viscoelastic surfaces and compared against the corresponding values over a rigid surface. The C f over the implants employed in this work demonstrated measurable decrease compared to a rigid surface along their entire length. A maximum local decrease in C f of 30% was measured at a station located 30 mm downstream of the implant leading edge, demonstrating that the boundary layer responds to the presence of the compliant surface within a short distance after encountering the surface. The mechanisms that drive the reduction in C f was investigated through a combination of turbulent statistics measurements and theoretical analysis. The results suggest that the unsteady deformations of the viscoelastic surface can affect VLSMs (very large-scale motions) as well as the near -wall burst cycles, which in turn could drive the observed C f mitigation.