Characteristics of coherent vortical structures in turbulent flows over progressive surface waves

Abstract

Vortical structures in turbulence over progressive surface waves are studied using the data from direct numerical simulation of a stress-driven turbulent Couette flow above a waving surface. Instantaneous flow field and its evolution, vorticity statistics, and conditionally averaged flow field with various sampling methods are examined. Unique vortical structures are identified, which are found to be strongly dependent on the wave motion. For a slow wave (with a small value of wave age c/u∗=2; here c is the phase speed of the wave and u∗ is the friction velocity), the vortical structures are characterized by reversed horseshoe vortices and quasistreamwise vortices. The former is concentrated above the wave trough and is associated with sweep events there; the latter has high intensity over the windward face of the wave and is associated with ejection events. Relative to the waveform, the coherent vortical structures propagate in the downstream direction. Vortex turning and vortex stretching play an important role in the vortex transformation and evolution processes. For an intermediate wave (c/u∗=14) and a fast wave (c/u∗=25), the dominant vortical structure is bent quasistreamwise vortices, which are predominantly horizontal but have a distinctive downward bending in their upstream ends near the wave trough. The vortices are found to propagate in the upstream direction with respect to the waveform. The above-wave coherent vortices identified in this study are found to play an important role in the turbulent transport process.