Control of laminar-to-turbulent transition via external body forces in channel flow

The effects of an external force control strategy on the transition to turbulence in minimal channel flow is investigated by direct numerical simulation. In this study, the control method is represented by the spanwise external force, and various velocity perturbations are applied to an initial laminar state. A friction Reynolds number considered is from 112 to 310. The external force appears to modify the transition dynamics very little for high perturbation magnitudes, but quite considerably for low perturbations where results show that the transition time strongly depends on the forcing frequency when compared to no control. Furthermore, the results within the Reynolds numbers studied show that the critical perturbation to trigger the transition decreases, suggesting faster transition by forcing with low frequencies for higher Reynolds numbers. A possible mechanism behind this control method is associated with its unique roller-like vortical structures formed near the wall. This interesting phenomenon appears to prevent interactions between inner and outer regions, eventually modulating the transition dynamics. Finally, it is shown that the perturbation magnitude and forcing frequency are important parameters to control the transition to turbulence.