The turbulent wake of two side-by-side circular cylinders

Abstract

This work is an experimental study of the turbulent vortex structures, and heat and momentum transport in the wake of two side-by-side circular cylinders. The spacing T between the cylinder axes was varied from 1.5d to 3d (d is the cylinder diameter). Both cylinders were slightly heated. A movable three-wire probe measured the velocity and temperature fluctuations, and an X-wire provided a phase reference. Measurements were conducted at x/d = 10, 20 and 40 at a Reynolds number of 5800 (based on d and the free-stream velocity U∞). At T/d = 1.5, the phase-averaged velocity and temperature fields display a single vortex street. The two rows of vortices exhibit a significant difference in the maximum vorticity, size and lateral distance from the flow centreline. As T/d is increased to 3.0, the flow is totally different. Two antiphase streets occur initially. They are less stable, with vortices weakening faster, than the street at T/d = 1.5. By x/d = 40, one street only is identifiable. Effective vorticity flux density indicates that, while the outer vortex nearer to the free stream interacts largely with the adjacent oppositely signed inner vortices located near the flow centreline, the inner vortex interacts with the cross-stream inner vortices as well as with adjacent outer vortices. As a result, vorticity associated with the inner vortex is annihilated quicker than that associated with the outer vortex, leading to the early disappearance of inner vortices and formation of a single street. The contribution of the coherent motion of various Reynolds-averaged quantities such as the momentum and heat fluxes has also been quantified and discussed in conjunction with the vortex structures of the flow and temperature fields.