Free-Surface Turbulent Shear Flows

Abstract

The structure and dynamics of turbulent wakes and shear layers in the presence of a clean free surface have been investigated experimentally using digital particle image velocimetry (DPIV). The purpose of this study was to determine the extent and characteristics of the influence, if any, of the free surface on these underlying turbulent shear flows. The free surface was found to affect the dynamics of turbulence within a surface layer on the order of one half-width of the submerged wake and one half of the local vorticity thickness of the submerged shear layer. Within this layer, the vertical velocity fluctuations are inhibited and the turbulence kinetic energy is redistributed to the horizontal components. The self-induced motion of surface-parallel vortical structures under the influence of their images was shown to lead to large-scale mean streamwise secondary flows and associated outward surface currents-symmetric for the wake and asymmetric for the shear layer. This motion was the origin of the significantly higher lateral spreading rates of these surface shear flows compared to the spreading rates of their fully-submerged counterparts — 20% and 25% for the wake and shear layer respectively. In addition, the evolution of the streamwise and surface-normal enstrophy components within the surface layer was consistent with the normal connection of vortical structures required at a free surface. The influence of the secondary flows was tracked back to the splitter plate's turbulent boundary layers where they were hence deduced to originate. A simple analysis of the mixed-boundary corner flows of the splitter plate made using the mean streamwise vorticity equation coupled with the evolution of the values of the transverse velocity confirmed the latter. In this picture of the mean flow, the secondary flows present in the near-surface edges of these shear flows were related to the pair of outer secondary vortices generated thereby. Furthermore, using a simplified equation for the surface-normal Reynolds stress, it was shown that the mutual interaction of the surface-parallel vortical structures with their images yielded a decrease in vertical velocity fluctuations as the free surface was approached. This equation shed further light on the redistribution of the vertical kinetic energy of turbulence into the other two Reynolds normal stresses. The resulting free-surface Reynolds-stress anisotropy in turn gave birth to the two streamwise secondary flows.