Turbulent patches in a stratified shear flow

The behavior of an isolated turbulent patch generated within the sheared interface between two counter-flowing fluid layers of different densities was investigated experimentally. This flow configuration permitted investigations of stratified shear flows with Rig⩾1, where Rig is the background gradient Richardson number. The turbulent patch was generated within the interface by an oscillating grid. The measurements in the presence and absence of shear under otherwise identical forcing and stratification conditions showed that the background shear significantly increases the patch size when Rig→1. This observation is consistent with the theoretical results that turbulence within a patch can interact with mean shear and enhance turbulence production when Rig<Ric, where Ric=O(1) is a critical Richardson number, and that below Rig∼1 shear across the patch boundaries can enhance local mixing. Both of these mechanisms can increase the size of a sheared turbulent patch beyond its mean shear-free counterpart. The opposite is observed at Rig>Ric, where gravitational collapse compounded with deformation by background shear causes a decrease of the patch size.