Path to turbulence in a transitional asymmetric planar wake

Abstract

We report on a transitional, high-resolution direct numerical simulation of a temporally developing planar asymmetric wake at Re = 4000 based on the mass flux deficit. The asymmetric wake is formed by a Blasius and a fully turbulent boundary layer on either side of an infinitely thin splitter plate. Such a setup has direct relevance in low-Reynolds number aeronautics where pressure gradients on an airfoil can relaminarize transitional wall-bounded flows, thus generating a half-laminar/half-turbulent wake. The spreading and normalized turbulence intensity of the asymmetric wake are lower than the initially fully laminar wake but greater than the initially turbulent wake. In the far-field, the flow reaches a fully symmetric and nearly self-similar state with a high level of structural organization, originating from the transition of the laminar side. The structures are generated by the mutual interaction of the turbulent/laminar half-wakes. A forcing from the turbulent side accelerates the development of spanwise-organized structures on the laminar side, which evolve and develop a high-level of spanwise coherence. Unlike the classical transitioning wakes, the pairing of the roller is bypassed. Instead, the spanwise-aligned bulges appear from the initially turbulent half-wake. Under the local shear of the Blasius boundary layer, these bulges undergo a “kinking-and-stretching” mechanism similar to that of the mixing layer. The spanwise organization of the structures is maintained far downstream.