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 vertical bar alpha vertical bar <= 5, and Reynolds number Re = 100 - 200. How Re, S*, alpha 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 approximate to 1%, when alpha > 2) than stationary cylinders (blockage ratio = 5%). The fluid forces are highly sensitive to the cylinder rotation when vertical bar alpha vertical bar > 2. Although both cylinders undergo the same magnitudes of time-mean drag coefficient vertical bar(C) over bard vertical bar or time-mean lift coefficient vertical bar(C) over barl vertical bar at a given lad, the cylinder rotation shifting from co-rotation to counter-rotation reverses the direction of (C) over bard, 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 alpha = 5 results in a 43% drag reduction for either cylinder. The S*, however, has an insignificant effect on (C) over barl (e.g. upto 5.3% at alpha = 5) while Re increases both vortex shedding frequency (e.g. 16.8% at alpha = 1) and heat transfer (e.g. upto 73.3% at alpha = 1). A flow map in a three-dimensional domain of S*, a 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 vertical bar alpha vertical bar causes a modification of the AC flow to a steady flow and then to a SRB or IR flow. (C) 2021 Elsevier Ltd. All rights reserved.