Numerical and Experimental Analysis of Three-Dimensional Boundary-Layer Transition Induced by Isolated Cylindrical Roughness Elements

Extensive research to establish an index of roughness that causes abrupt turbulent transition downstream of the roughness and affect natural laminar flow is performed. This study investigates turbulent transitions dominated by the roughness-induced crossflow instability on a swept wing and a swept flat plate. Direct numerical simulations (DNS) are performed considering the swept wing to determine the critical roughness height that induces abrupt turbulent transition. Compared to its no-roughness counterpart, the roughness increases both the pressure drag and skin-friction drag and decreases the lift owing to the onset of upstream turbulence. The wind tunnel tests are performed considering a swept flat-plate boundary-layer model. The development of turbulent regions due to the roughness is investigated. The results reveal the occurrence of a laminar-to-turbulent transition and relaminarization, along the boundary between laminar and localized turbulent regions. Moreover, the DNS results reveal the generation of traveling crossflow vortices from the localized turbulent region.