Interactive vortices in the wake behind a pair of circular cylinders

Abstract

Interactive vortex shedding in the multiply connected domain formed by a pair of circular cylinders was analyzed by the FEM-FDM grid blending technique. The efficiency and accuracy of this technique were investigated by calculating the impulsively started circular cylinder flow problem to compare with the earlier experimental and computational results. Vorticitystreamfunction formulation was used to solve the incompressible NavierStokes equations, with the time dependent wall streamfunctions determined from the pressure constraint condition and the far-field streamfunctions from the integral series formula. Standard Galerkin method was used in the relatively small finite element subdomain and the finite difference method based on the general coordinate system in the rest majority of flow domain. Symmetric and antisymmetric vortex shedding patterns in the coupled flow regime and asymmetric one in the biased flow regime were presented confirming the earlier experimental findings. The bistable nature of the asymmetric vortex shedding as well as the intermittent drifting from one status to the other between the symmetric and antisymmetric wake patterns was reported. The mixed convection heat transfer and fluid flow patterns were studied for the double circular cylinders arranged transverse to the vertical air stream. The same numerical technique as the case of the cold flow was applied to the coupled Navier-Stokes and energy equations. It was found that the Karman vortex street breaks down behind the double heated cylinders in a transient manner for cirtain Richardson number range due to the buoyancy effect and vortex interaction, in contrast to the sudden break-down applicable to a single heated cylinder. It was also found that heat transfer from the double cylinders is more vigorous than from a single cylinder owing to the activated buoyant gap flow.