Transition mechanisms in a boundary layer controlled by rotating wall-normal cylindrical roughness elements

The transition to turbulence induced by counter-rotating wall-normal rotating cylindrical roughness pairs immersed within a laminar boundary layer on flat plate is investigated with direct numerical simulations, dynamic mode decomposition (DMD) and perturbation kinetic energy (PKE) analysis. As long as the cylinder stub rotating, wake contains steady dominating inner vortex (DIV) surrounded secondary vortex. Its circumferential velocity accelerates fluid one side of decelerates it other side. With low rotation speed, initiated combination elliptical centrifugal instabilities in near wake. At medium speeds, Taylor–Couette-like streamwise vortices are generated decelerated side, resulting protruding reverse-flow zone. Results from DMD analysis corresponding PKE reveal unstable nature deceleration region largest speed investigated, onset perturbations directly located stubs, where mechanism feeds instability. In wake, gradually changes pure instability when increases. far both fade away, streaky flow featuring vigorous DIV then only subject inviscid inflectional