Response of zero-pressure gradient turbulent boundary layers to step changes in riblet surface textures

We computationally study the response of zero-pressure-gradient (ZPG) turbulent boundary layers (TBLs) to a streamwise step change from a smooth wall to riblets (S-R), and vice versa (R-S). We consider triangular riblets with tip angles 90^o (T9) and 60^o (T6), that respectively increase and reduce drag, with viscous-scaled spacings 50 and 15. A novel grid-generation approach is developed for unstructured spectral-element codes, consistent with the size of turbulent scales across the TBL. We generate a ZPG TBL upstream of the step change (with thickness δ₀) with momentum thickness Reynolds number 680. The TBL departure from equilibrium due to the step change, and its subsequent relaxation, recall previous studies on step changes in surface roughness. Downstream of the R-S step change, the internal equilibrium layer thickness δ_IEL reaches 0.4δ₀ within a shorter distance downstream of T6 (5δ₀) compared to T9 (12δ₀), due to the shorter height of the riblets in the T6 case. For a fixed riblet geometry, δ_IEL grows faster during S-R step change compared to R-S. In all cases, δ_IEL does not reach the boundary layer thickness, even up to a distance of 60δ₀ downstream of the step change, owing to persistent history effects within the frozen wake region.