Double diffusive natural convection in binary gases with horizontal thermal and solutal gradients resulting in opposing buoyancy forces

The natural convection of air in a square cavity with horizontal temperature and concentration gradients which give opposing buoyancy forces near the vertical walls was studied numerically. The basic equations solved are the continuity equation, the Navier-Stokes equations, the energy equation and the concentration equation. Computations were carried out for Prandtl number Pr=0.71, aspect ratio A=\, Rayleigh number Ra=\Q*, buoyancy ratio ( = solutal buoyancy / thermal buoyancy) N= 2 and various Lewis numbers Le. For Le=0.5 a strong main flow was formed near the walls and then a stratified layer in both the temperature and the concentration fields was formed between two convection layers. Secondary small roll cells in each convection layer migrated periodically. For Le=\ and 2 the flow gradually became steady. For Le=5 a complicated pattern of small roll cells appeared near the walls.