Effects of wake confinement and buoyancy on three-dimensional flow transitions for a square cylinder near a moving wall

Three-dimensional flow transitions for a square cylinder placed near a moving wall are examined in the presence of buoyancy. Computations are performed for the gap ratio G/D = 0.3, 0.5, and 1.0 for -0.5 & LE; Ri & LE; 0.5 at Re = 150. The flow rate near the cylinder, interactions between the shear-layers near the cylinder and the wall, and the baroclinic production of vorticity play key roles in the onset of three-dimensionality. With a decrease in G/D, the onset of three-dimensionality occurs at a lower value of Re compared to an isolated square cylinder. Both positive (Ri > 0) and negative buoyancy (Ri < 0) assist the onset of three-dimensionality. Mode A, mode B, and mode C of different wavelengths are formed for various values of Ri and G/D. The streamwise baroclinic vorticity is in-phase with the streamwise total vorticity and strengthens three-dimensionality. For negative buoyancy, the flow rate near the cylinder decreases, which reduces the strength of three-dimensionality compared to positive buoyancy. Positive buoyancy enhances the tendency of subcritical transition, while the tendency of supercritical transition increases for negative buoyancy. A stable limit cycle for supercritical transitions and an unstable limit cycle for subcritical transitions are observed. The initiation of three-dimensional transition may occur in the far-wake region, where velocity-vorticity interactions play a significant role. With an increase in Ri, buoyancy enhances enstrophy production, dissipation, and diffusion in the near-wake region. The force coefficients are obtained for different values of Ri.