EXPERIMENTS ON THE INHIBITION OF MIXING IN STABLY STRATIFIED DECAYING TURBULENCE USING LASER DOPPLER ANEMOMETRY AND LASER-INDUCED FLUORESCENCE

The decay of velocity and density fluctuations in buoyancy influenced turbulence generated by a biplanar grid, towed horizontally through a stably stratified fluid with uniform optical index of refraction was measured for three different constant density gradients. Experiments were performed in a closed rectangular test section. Fluid velocities were measured with a laser Doppler anemometer. Density was measured by adding a fluorescent dye (Rhodamine 6G) to the working fluid in such a way that the light fluorsced by the dye when excited by a laser was proportional to the local density of the solution. Simultaneous single-point measurements of the vertical velocity, the horizontal velocity in the direction of grid motion, and the density were obtained at a fixed point in the center of the test section. Statistical moments representing the state of the turbulent flow field at different periods of the decay were computed by ensemble averaging data from 100 tows with similar initial conditions. Density profiles of the quiescent fluid, before and after the grid was towed through the test section, were measured, and estimates of the total mixing accomplished by a single sweep of the grid through the fluid were calculated. The measured internal Richardson number Ri(i) = {-1/2[g-rho'-2/(partial-rhoBAR/partial-z)]} x (1/1/2-rhoBAR<w2>) indicates that, immediately behind the grid, the turbulence was unaffected by the stable stratifications. At a time depending on the strength of the mean density gradient, the buoyancy forces acting on the turbulent eddies become as large as the inertial forces and have significant effects on the final stages of the decay of the turbulent flow field. Measurements of the convective mass flux indicate that the vertical transport of mass due to turbulence was significantly reduced by buoyancy forces. This was confirmed by integral estimates of the mixing, which indicate that the ability of the turbulent eddies to homogenize the stratified fluid decreases significantly with increasing density gradients. These results are compared with earlier measurements obtained in both stratified air and water facilities.