Numerical investigation of flow across three co-rotating cylinders in side-by-side arrangement

Flow across three side-by-side co-rotating cylinders is investigated at a Reynolds number of 100 and non-dimensional rotation rates varied from 0 to 8 , for spacing ratios of L / D = 1.5 , 2 , and 4 through two-dimensional numerical simulations, where D and L are cylinder diameter and the center-to-center spacing between the cylinders, respectively. For L / D = 1.5 and 2 , the wakes are classified into regime FF (flip-flopping) at smaller rotation rates and regime SB (single-body) at higher rotation rates. Each regime can be further divided into sub-regimes based on the wake patterns. Regime FF is a regime where the flow switches between two patterns intermittently. The three sub-regimes of SB at L / D = 1.5 : vortex shedding (SB-VS), steady state (SB-SS), and secondary instability (SB-SI) are the same as those of a single rotating cylinder as the flow through the gap is too weak to have effect on global wake. A new sub-regime single-body quasi-steady (SB-QS) is found for L / D = 2 , where the two shear layers in the wake of the three cylinders interact weakly with each other but do not form strong vortices. For L / D = 4 , two new regimes are found: regime 3V-to-3S (transition from three vortex shedding wake to three steady wake), where the vortex shedding from the three cylinders are suppressed consecutively one by one with the increase in the rotation rate, and regime TB (two-body) where two of the three cylinders behave as a single body. Regime TB for L / D = 4 has two sub-regimes: steady state (TB-SS) and secondary instability (TB-SI). The effects of the flow regimes on the force coefficients are quantified. For all the spacing ratios L / D = 1.5 , 2 , and 4 , the standard derivation drag and lift coefficients are significantly greater than that of a single cylinder when vortex shedding occurs.