Influence of thermal buoyancy on vortex shedding behind a circular cylinder in parallel flow

Abstract

In this study, we perform a numerical investigation to better comprehend the domination and the effect of strong thermal buoyancy on hydrodynamic and thermal characteristics, vortex shedding in particular, of a horizontal circular heated cylinder subject to a vertically upward laminar flow stream. The Reynolds number is varied in the range 20 ≤Re≤ 150 while maintaining the Prandtl number constant at Pr = 7.1. The effect of thermal buoyancy on the system is determined by varying the Richardson number up to Ri = 5. The behaviour is determined by solving the unsteady laminar two-dimensional Navier–Stokes and standard energy equations numerically, utilising the spectral-element method with buoyancy considered using the Boussinesq approximation. Representative vorticity, streamline, and thermal patterns are presented, and the average Nusselt numbers are plotted as a function of the Richardson number for a set of Reynolds numbers to explain, in detail, the role of superimposed thermal buoyancy on the wake flow and rates of heat transfer. The predictions demonstrate that the wake flow exhibits unsteady periodic characteristics for the selected moderate Reynolds numbers, and as the buoyancy parameter increases, the fluid flow and temperature fields become less stable. Subsequently, it was observed that heat transfer augmented considerably, the vortex shedding stopped abruptly, the wake converted to steady twin vortices beyond certain critical Richardson numbers, and the Nusselt numbers suddenly reduced to minimum values. These critical values are determined to increase with the Reynolds number. The results also confirm that for increased heating, the recirculating flow in the cylinder near the wake disappears and flow separation occurs only at the backward stagnation point.