For the purpose of a careful analysis of the modification of flow by the combined effects of the rotation and the spacing between two cylinders on the flow, numerical simulations are performed at a various range of absolute rotational speeds ͉͑␣ ͉ Յ 2͒ for four different gap spacings of 3, 1.5, 0.7, and 0.2 at one Reynolds number of 100 showing the typical two-dimensional vortex shedding. As ͉␣͉ increases, flow becomes stabilized and finally a steady state beyond the critical rotational speed depending on the gap spacing. In the cases of gap spacings of 3 and 0.2, the wake patterns keep the antiphase-synchronized pattern and the single bluff body pattern, respectively, until flow reaches the steady state. However, for the gap spacings of 1.5 and 0.7, the wake patterns change in the unsteady regime. For the gap spacing of 1.5, the wake pattern shows in-phase-synchronized, flip-flop and antiphase-synchronized patterns in turn according to ͉␣͉. For the gap spacing of 0.7, the flip-flopping wake pattern is changed as the in-phase-synchronized pattern behind the cylinder with the one-row vortex street in downstream with increasing ͉␣͉.This brief communication aims at a careful identification of flow patterns, characterized by the behavior of the wake region, around two rotating circular cylinders in a side-byside arrangement.The flow around two stationary circular cylinders of equal diameter arranged side-by-side has been extensively investigated in the past 1-19 because it can be considered that an elementary flow is helpful for understanding the flow patterns and aerodynamic characteristics around multiple bluff bodies in engineering practice. It is now well known that the wake behind the two side-by-side cylinders depends significantly on the normalized gap spacing, g * = g / D, where g and D are the distance between two cylinder surfaces and the cylinder diameter, respectively.Sumner et al. 8 briefly tabulated the measured quantities according to previous experiments in company with the experimental conditions such as Reynolds number, g * and type of measure. They also confirmed the three basic flow patterns of the single bluff body pattern at 0 Յ g * Ͻ 0.2, the biased flow at 0.2Յ g * Յ 1.2, and the symmetric flow with either antiphase or in-phase synchronized vortex shedding at 1.2 Յ g * Յ 3.5 which have been observed by other researchers. Recently, Kang 19 numerically observed the six kinds of wake patterns over the ranges of 40Յ ReՅ 160 and g * Ͻ 5.The flow past a rotating cylinder can be considered as the elementary problem to the present topic. The rotation of a cylinder has been adopted not only to understand the wake dynamics but also to control the vortical structure in the wake by the numerous researches. As a parameter representing this problem, ␣ is the rotational speed at the cylinder surface normalized by the freestream velocity, accordingly ␣ is defined as ␣ = ⍀D / 2U ϱ , where ⍀ is the angular velocity of the cylinder. The other important parameter is the Reynolds number. Recently, Kang et al., 3...
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