Flow structure, momentum and heat transport in a two-tandem-cylinder wake

Abstract

Flow structure, momentum and heat transport in the wake of two tandem circular cylinders have been experimentally investigated. Measurements were conducted at $x/d\,{=}\, $10, 20 and 30 ($d$ is the cylinder diameter) at a Reynolds number of 7000 using a three-wire (one cross-wire plus a cold wire) probe, in conjunction with a cross-wire. The upstream cylinder was slightly heated. The flow behind two tandem cylinders is conventionally divided into three regimes based on whether the shear layers separated from the upstream cylinder overshoot or reattach on the downstream cylinder before forming a vortex street, or form vortices between the cylinders. The present investigation uncovers two remarkably different flow structures in the reattachment regime, depending on whether the shear layers from the upstream cylinder reattach on the downstream or upstream side of the downstream cylinder. As such, four cylinder centre-to-centre spacing ratios, i.e. $L/d\,{=}\,$1.3, 2.5, 4.0 and 6.0, were examined, each representing one distinct flow structure. The phase-averaged sectional streamlines and vorticity contours display a single vortex street, irrespective of different regimes. However, the detailed flow structure, in particular, the vortex strength, and its downstream development depend upon $L/d$. The cross-stream distributions of the Reynolds stresses and heat fluxes at a given $x/d$ vary from one to another. Such variation is also evident in the coherent contributions to the Reynolds stresses and heat fluxes. The results are connected to different initial conditions for the four flow structures. The momentum and heat transport characteristics are summarized for each flow structure.