Vortex-induced vibration and heat dissipation of multiple cylinders under opposed thermal buoyancy

The vortex-induced vibration of three cylinders in an equilateral triangular arrangement with opposed thermal buoyancy are studied by numerical method. The cylinders are uniformly elastic and can freely vibrate in the transverse flow direction. The temperature of cylinders is higher than free stream. The incoming flow is in the same direction as gravity and opposite to thermal buoyancy. The ranges of Richardson number and reduced velocity considered in this work are - 1 <= Ri <= 0 and 3 <= U*<= 12. The maximum amplitude of each cylinder rises significantly under opposed thermal buoyancy, and the maximum amplitude of cylinder 1, cylinder 2, and cylinder 3 increased by 206%, 174%, and 172%, respectively. When U*>8, the vibrations of cylinder 2 and cylinder 3 are close to the same phase. The opposed thermal buoyancy hinders the separation of flow boundary layer, resulting in thickening of thermal boundary layer and deterioration of heat transfer. The deterioration of heat transfer can be alleviated at high reduced velocity (U*>= 10). The increase of Richardson number will lead to the weakening of heat transfer at up-flow surface of cylinder and enhancement of heat dissipation near the back stagnation point. The local Nusselt number distribution around cylinder is affected by vortex-induced vibration, vortex shedding, and the interaction of shear layers from each cylinder.