W. Wu, L. Laguarda, D. Modesti, S. Hickel (2024)
Flow, Turbulence and Combustion (in press) doi: 10.1007/s10494-024-00580-0
A novel passive flow-control method for shock-wave/turbulent boundary-layer interactions (STBLI) is investigated. The method relies on a structured roughness pattern constituted by streamwise-aligned ridges. Its effectiveness is assessed with wall-resolved large-eddy simulations of the interaction of a Mach 2 turbulent boundary layer flow with an oblique impinging shock with shock angle 40°. A parametric study is performed to investigate the effect of the spacing between the ridges. We find that ridges with small spacing effectively mitigate the low-frequency unsteadiness of STBLI and slightly reduce total-pressure loss.
The structured roughness pattern is constituted by streamwise-aligned ridges, which cover the entire wall and are fully resolved by a cut-cell based immersed boundary method. The ridges induce secondary flow of Prandtl’s second kind. The size and intensity of the induced streamwise vortices increase with increasing non-dimensional ridge spacing and reach a maximum at D/δ0 = 1.0. We found that a large ridge spacing (D/δ0 = 2.0, 1.0) reduces the separation area by 15% but leads to stronger wall pressure fluctuation. A ridge spacing of D/δ0 = 0.25 increases the separation area and decreases the peak value of the pressure fluctuation by 12% compared with the smooth wall case. Based on the analysis of wall-pressure spectra and a dynamic mode decomposition of 2D pressure fields, the reduction in pressure fluctuation is demonstrated to be associated with the attenuated low-frequency unsteadiness of the separation shock. To the best of our knowledge, this study presents the first observation that ridge-type roughness can reduce the wall pressure fluctuation peak near the separation point. The increase in total pressure recovery further under-lines the potential of ridge-type roughness for engineering applications, such as in supersonic engine inlets.
The present study provides a proof-of-concept for a relatively low Reynolds number (Reτ ≈ 355). We expect the control effect of the turbulence induced secondary flow will persist for high-Reynolds STBLI with a sharper separation shock and more pronounced low-frequency unsteadiness; however, this hypothesis needs to be corroborated with future studies.