Flow and heat transfer across two inline rotating cylinders: Effects of blockage, gap spacing, Reynolds number, and rotation direction

The manipulation of the heat transfer and flow field using cylinder rotation is of a classical issue. It is of fundamental importance to study the dependence of wake and thermal topologies of two rotating cylinders in tandem arrangements. This study numerically investigates the time-resolved laminar flow, fluid forces, Strouhal number, and convective heat transfer over two isothermal co-rotating and counter-rotating circular cylinders in tandem arrangements for scaled cylinder center-to-center spacing S * = 2.5 - 6, non-dimensional rotational speed | α | ≤ 5, and Reynolds number Re  = 100 - 200. How Re, S *, α and computational domain size influence the wake dynamics and thermal attributes is the focus of this study. The numerical procedure is validated against the available data in the literature for a single rotating cylinder. The influence of the blockage ratio on the single- and two-cylinder flow is determined first to decide the appropriate computational domain. It is found that rotating cylinders require a larger computational domain (blockage ratio ≈ 1%, when α > 2) than stationary cylinders (blockage ratio = 5%). The fluid forces are highly sensitive to the cylinder rotation when | α | > 2. Although both cylinders undergo the same magnitudes of time-mean drag coefficient | C¯d | or time-mean lift coefficient | C¯l | at a given | α |, the cylinder rotation shifting from co-rotation to counter-rotation reverses the direction of C¯d, i.e. repulsive for the co-rotation and attractive for the counter-rotation. On the other hand, an increase in S * from 2.5 to 6 with α = 5 results in a 43% drag reduction for either cylinder. The S *, however, has an insignificant effect on C¯l (e.g. upto 5.3% at α  = 5) while Re increases both vortex shedding frequency (e.g. 16.8% at α  = 1) and heat transfer (e.g. upto 73.3% at α  = 1). A flow map in a three-dimensional domain of S *, α and Re is provided, distinguishing steady and unsteady flows. Four distinct flow regimes are labeled, namely steady flow, alternate coshedding (AC) flow, single rotating bluff-body (SRB) flow, and inverted-rotation (IR) flow. An increase in | α | causes a modification of the AC flow to a steady flow and then to a SRB or IR flow.