Unsteady mixed convection in a square enclosure with an inner cylinder rotating in a bi-directional and time-periodic mode

This work first numerically investigated the mixed convection in a cold square enclosure with an inner hot circular cylinder at Ra = 10(6). The cylinder rotates about its center in a time-periodic and bidirectional mode, thus the fluid is driven by both the viscous shear stress from the cylinder surface and buoyancy from density variation. Our objective is to explore the effect of cylinder rotation on the unsteady thermal and flow characteristics, and to determine the propagation of thermal and flow perturbations within the enclosure. Numerical results reveal that the cylinder rotation weakens the mean heat transfer but imposes notable perturbation on the local heat transfer on both solid walls. At large alpha and P, the perturbation is the most significant as reflected by the pronounced temporal fluctuation of heat transfer rate on the solid walls and remarkable distortion of circulating vortices. The perturbation propagates from the cylinder into the whole fluid domain and results in phase lag between the variation histories of characteristic quantities, and the fluctuating amplitude of Nusselt number on the enclosure surface can be much larger than that of the cylinder. The thermal field is the most perturbed in the region close to the cylinder at small P, while it also exhibits a large fluctuation close to the top wall of the enclosure at large P due to the unsteady circulating flow. (C) 2019 Elsevier Ltd. All rights reserved.