Numerical investigation on buoyancy-driven flow over a circular cylinder in a channel with nonparallel walls

Abstract

The flow over a stationary cylinder is destabilized mainly in the separated shear layer (SSL) and wake regions and transverse shedding forms downstream of the cylinder. It has been revealed that aiding/opposing buoyancy or diverging/converging channel could stabilize/destabilize the separated and wake flows, while their joint effect has not been investigated. In this numerical investigation, the low-Re buoyancy-driven flow over a circular cylinder placed in a channel consists of diverging or converging walls is studied. We aim to reveal the effects of two parameters, i.e. Richardson number Ri representing buoyancy and channel angle α, on the flow dynamics and heat transfer. The cases of aiding (Ri > 0) and opposing (Ri < 0) buoyancy, and diverging (α > 0) and converging (α < 0) channels are considered in this work. The effects of Ri and α are presented in terms of the instantaneous flow field, global characteristic quantities, heat transfer between the fluid and cylinder surface, separation and shearing of wake flow, and temperature variations of the channel walls. We found that the flow transits to steady subjected to aiding buoyancy at a lower value of Ri where the fluctuations of force and heat transfer rate reduces to zero; however, the mean drag and Nusselt number do not exhibit monotonic variations with Ri. The convection heat transfer is prone to be notably affected by Ri rather than α on the leeward side of the cylinder, and its fluctuation is significant in the separation zone downstream of the separation point. Depending on the value of Ri, the steady and unsteady wake flow present different velocity profile as well as the shearing; the deficit wake flow recovers more rapidly for the steady flow cases.