Arrows have been added to indicate directions. The vortex structure is the reason behind the distortion seen in the density contours. This advection, a result of the buoyancy , is the key to enhanced mixing. Buoyantly driven flow after 800 iterations. Part a is a density contour plot.
Dark areas indicate large gradients.
Figures 3a and 3b are, respectively, density and vorticity contour plots for the buoyantly driven flow after 2800 time steps. They appear lower on the grid because, in the interim, the structure flowed out the top of the grid and back in the bottom. This is a result of the periodic boundary conditions employed. Notice in 3a that the lowest density areas are now located out on the sides of the structure, as opposed to in the center. This is due to the enhanced mixing occurring in the center area.