Wall-based Actuation for Transition Delay and Drag Reduction

Subjecting a boundary layer to sudden or changing strain can have a profound effect on the flow. In turbulent boundary layers, oscillating the wall has been shown in previous studies as a viable mechanism to reduce drag. At lower Reynolds numbers, and in particular in the transition regime, the flow response is complex. On the one hand, the three dimensionality of the base state can lead to new instabilities. On the other hand, the streaks which are often observed in bypass transition can be weakened, akin to observations in fully-turbulent shear flows. This work investigates the influence of spanwise wall oscillation on bypass transition in zero-pressure-gradient boundary layers. Direct numerical simulations are performed in order to examine the impact of the wall forcing on the non-linear transition process. The simulations demonstrate that appropriate choice of the oscillation amplitude and frequency can delay transition. The non-linear computations are complemented by linear analysis of a simple model that explains the influence of the unsteady shear on the penetration of free-stream vortical disturbances into the boundary layer. This effect, and the weaker streaks in the pre-transitional flow, ultimately lead to a delay in the secondary instability of the streaky base flow and a downstream shift in transition onset.