Investigations Of Turbulent Structure And Turbulent Counter-Gradient Transport Characteristics In Stratified Flows

Stably density stratified flows are ubiquitous in geophysical field and engineering application. When the effects of mean shear and stratification both exist in turbulence, the competition between Inertial force and buoyancy force make the turbulent features more complex, especially the issues on the growth and the evolution of turbulence structure, turbulent scalar and momentum counter-gradient transport characteristics still need further detailed study. Based on the previous study of turbulent mixing and transport, the main work of this paper is focused on the collective effects of mean shear and stratification on turbulence structure and turbulent scalar and momentum counter-gradient transport features by the experimental, numerical and theoretical investigations. The main results are given as follows:First, the system of experimental facilities on turbulent stratified flow were constructed in our laboratory, and a set of experiments were conducted in two kinds of water flows, neutral and stratified flow, using laser induced fluorescence (LIF) and particle imagine velocimetry (PIV). The effects of the mean shear and stratification were investigated qualitatively. It shows that, turbulent mixing is improved by mean shear but impressed by stratification; the large scale structures are restricted within the mixing layer, particularly in stratified case. In addition, very complicated phenomena of stretching, uniting and distorting between vortexes are found in the mixing layer and nearby the interface, remarkably the revolving and anti-revolving vortex pairs and the finger-like structure are discovered in the flow visualization.Second, Large Eddy Simulation was applied to study stably stratified turbulence with a sharp interface, turbulent mixing process, turbulence structure, turbulent statistics and turbulent counter-gradient transport characteristics were analyzed in details. It can be inferred from the simulation that: (1) the streamwise fluctuating intensities are larger than the vertical fluctuating intensities; turbulent fluctuations decay more rapidly at the present of stratification; turbulent potential energy can be enhanced by shear effects in the same stratified condition; the offset of the mixing layer to the side with small inlet velocity is calculated quantitatively; the buoyancy length scale and the turbulent length scale are strong affected by the stratification and mean shear; there are much differences of the statistics between the fluctuating temperature and the fluctuation velocities, it reveals that the diffusion of temperature is not only influenced by the velocity field, but also attributed to itself. (2) turbulent mixing layer is made up of two vortex belts with contrary revolving direction; stratification results in the anisotropy of turbulent field; mixing layer develops with subsections, the small-scale scalar structure decreases with the decrease of stratification. (3) turbulent counter-gradient scalar transport is found at the central area of the flow with the range of x/M <10 in streamwise direction, but the turbulent counter-gradient momentum transport is found at x/M=48 in the area depart from the central line with the range of about -2