Skin friction control using tailored surface roughness

Elevated skin friction in turbulent boundary layers (TBLs) constrains many engineering designs, motivating the need for efficient drag control strategies. We explore the efficacy of strategically placed tailored roughness modelled as a staggered array of cuboidal elements, to mitigate turbulent drag in a zero-pressure gradient TBL. The roughness Reynolds number (k⁺) varies from 19 to 47, switching from a hydrodynamically smooth to a fully rough regime. Spectral analysis of velocity fluctuations in the baseline TBL identifies a dominant spanwise wavelength of λ_z⁺≈100 corresponding to buffer layer streaks. Surface texturing modulates the near-wall features, inducing significant energy deposition in specific length scales, resulting in an altered log layer. Formation of two major vortex structures is evident in the scales imposed on the spanwise and streamwise velocity components. Streamwise fluctuations display peaks at two wavelengths (λ_z⁺≈100 and λ_z⁺≈200). The former results from the hairpin vortices shed from the staggered arrangement, while the latter can be linked to the wake behind each element, interspersed between the hairpin legs. In the spanwise fluctuations, energization of the roughness wavelength results from the stagnation region formed on the forward-facing side of each element, which develops into a streamwise vortex on the lateral sides. Effects of flow control on Reynolds stresses and skin friction drag are also studied to gain a deeper understanding of flow modulation.