Bypass transition through interactions of localized disturbances in wall-bounded flows

A new mechanism bypass in a wall-bounded internal flow is proposed and the proposal is checked by direct numerical simulations of high temporal and spatial resolution. The mechanism is based on the interactions of the localized perturbations, rather than the effect of a single perturbation investigated so far in the classical bypass transition process. It is first shown by theoretical considerations that two pairs of quasistreamwise vortices can interact near the wall in such a manner that the compression (stretching) of the existing wall-normal vorticity induced by one of the pairs can enhance a new streamwise vorticity zone that can lead to new coherent structures and enhance considerably the transition process. Direct numerical simulations confirm this hypothesis.