Abstract
<p>A lid-driven cavity flow facility has been used to experimentally study circulation and mixing in two layered stratified and unstratified flows. The flow has been visualized using liquid crystal tracer particles. The stratification has been created by dissolving salt and local density gradients determined by using a custom designed electrical conductivity meter with specially designed miniature probes. The development of the flow with time was observed and analyzed.Unstratified cases were run for aspect ratios of 1, 2 and 3 for Reynolds numbers (Re) ranging from 5000 to 30000. Flow structures for aspect ratios 1 and 2 were found to be similar in nature but differ considerably with those for aspect ratio 3. The flows were found to be three-dimensional and unsteady as no steady state was observed. Taylor-Gortler like 3-D instabilities were observed in regions of high streamline curvature and shear instabilities, probably of the Kelvin-Helmholtz type were observed in the eddy cores. The flow showed indications of turbulence for Re > 10000.An order of magnitude analysis of the governing equations for the observed flow structures has indicated the relevant convective, energy transport and diffusive time scales for the unstratified cases. The main flow structures develop over a period comparable to the energy transport time scales, whereas minor adjustments in the flow occur over a longer period comparable to diffusive time scales.Destratification experiments were run using an initial two layer stratified fluid for Richardson Numbers (Ri) of 0.4 to 1.6 based on the belt speed (V), initial density difference and the initial depth of the upper layer. Flow visualization indicated that most of the mixing occurs over a small region near the downstream end of the cavity where the primary circulation cell, in the upper layer, deflects the interface. Mixing mechanisms have been identified as wave breaking, secondary eddies, scouring, Taylor-Gortler like 3-D vortices and convective transport. The first three mechanisms are dominant for Ri 1.Entrainment rate (Ue), defined as the rate of deepening of the interface, remains constant for each experiment and the following empirical entrainment law was found: Ue/V = 1.9 x 10('-4) Ri('-1.4).An analysis of the results and comparison with literature indicates dependence of entrainment rates on the cavity spanwise aspect ratio.</p>