Turbulence and transport in density-stratified shear flow

Results from an experimental study of turbulent, density stratified, sheared flow are presented. The flow field is a combination of decaying grid-generated turbulence, stratification effects, and shear production. The evolution of velocity and scalar variances, in addition to several lengthscales and lengthscale ratios, are used to characterize the dynamical balances of the turbulent flow. Transport of vertical momentum and vertical buoyancy flux are quantified by dimensionless shear stress and buoyancy flux coefficients. The mechanisms of their transport are examined through a detailed analysis of time series resolving individual mixing events.; Shear effects are found to increase the vertical transport of buoyancy; therefore, shear production must be accounted for explicitly in the flux Richardson number, a new definition for which is proposed. Values of the flux Richardson number for strongly sheared flow are about 0.5, which is an order of magnitude greater than values for shear-free flow. The analysis of the small scales of the flow is accomplished via examination of the velocity and scalar derivatives as well as structure functions. There is considerable evidence that the large scales play some role in determining characteristics of the small scales. This suggests that the turbulent cascade process may not be as universal as is usually accepted.; The dissipation rates of the turbulent kinetic energy and of the scalar are both underestimated in the flow field with current experimental techniques. A correction method based on velocity structure functions and a mixed-moment scalar equation is proposed, and resulting corrections are of $O$(1) for the velocity field and $O$(100) for the scalar field due to a lack of resolution of the Batchelor scales required for a high Schmidt number flow. The interactions between a non-vertical velocity gradient and vertical density stratification are also investigated. Due to differing initial conditions, many results are not comparable with previous numerical simulations, but beyond this difference, the angle of shear manifests itself in the results in only a subtle manner and produces no difference in values of the flux Richardson number.