The adoption of a direct forcing immersed boundary numerical method on the uniform flow, at a moderate Reynolds number of 100, past a pair of two rotating circular cylinders placed side-by-side, is the core of the present study. A simplified yet novel approach is used to impose a virtual force as a source to the full incompressible two-dimensional NavierStokes equations, which are discretized by the finite volume method. The usage of a Cartesian grid that ensures minimal computational cost, is the salient feature of the applied immersed boundary approach. The gap between the two cylinders, and their rotational direction and speed, are the variable parameters used in the analysis of the resulting vortex street. A range of absolute rotational speeds ( |α| ≦ 3) for different gap spacings (g* ≦ 3), is considered. Whilst the direction of rotational motion is found to either accelerate or decelerate the gap flow, the rotational speed has a bearing on the dominant flow pattern. An observation of the vorticity contours for the decelerating gap flow indicates that when a critical rotational speed (α ≈1.4) is reached, the flow becomes steady regardless of the variation of g*. Five α-dependent flow modes emerge; the anti-phase, in-phase, flip-flop, single vortex shedding and suppressed modes. A statistical scrutiny of the validated transient data for the lift (CL ) and drag (CD ) coefficients is ultimately performed. When g* = 0.2, the general trend of decreasing CD with reduction in gap size is broken.

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