Numerical analysis of flow shedding over an obstacle at low Reynolds number

This is a numerical investigation of the unsteady mixed convective flow over an isothermally heated circular cylinder. The flow is directed vertically downwards with buoyancy forces in the opposite direction. A parametric analysis was conducted at five Reynolds numbers Re=10,20,30,40,100, and for Richardson numbers between Ri=0 and 0.5, to evaluate the influence of thermal buoyancy on the instantaneous flow behaviour, vortex-shedding characteristics, thermal fields, and mean heat-transfer rates. The simulations show that at the highest Reynolds number investigated, Re=100, the normal periodic von Kármán vortex street still characterises the flow as Richardson number is increased; however, the oscillation frequency decreases and the oscillation amplitude increases. For Re≤40, a new vortex street appears, named the “Buoyancy-Opposing Vortex Street”, due to thermal buoyancy modifying the von Kármán street. Interestingly, at very low Re=20 and 10, the von Kármán street completely disappears in the heated wakes, whereas the “Buoyancy-Opposing Vortex Street” remains as the sole vortex street characterising the flow.