Numerical study of flow past two counter rotating cylinders using immersed boundary method

The adoption of a direct forcing immersed boundary nu- \nmerical method on the uniform flow, at a moderate Reynolds \nnumber of 100, past a pair of two rotating circular cylinders \nplaced side-by-side, is the core of the present study. A sim- \nplified yet novel approach is used to impose a virtual force as a \nsource to the full incompressible two-dimensional Navier- \nStokes equations, which are discretized by the finite volume \nmethod. The usage of a Cartesian grid that ensures minimal \ncomputational cost, is the sali \nent feature of the applied im- \nmersed boundary approach. The gap between the two cylin- \nders, and their rotational direction and speed, are the variable \nparameters used in the analysis of the resulting vortex street. \nA range of absolute rotational speeds ( \n \n(g* 3) \n 3) for different \ngap spacings ( \ng \n* \n \n 3), is considered. Whilst the direction of \nrotational motion is found to e \nither accelerate or decelerate the \ngap flow, the rotational speed has a bearing on the dominant \nflow pattern. An observation of the vorticity contours for the \ndecelerating gap flow indicates \nthat when a critical rotational \nspeed ( \n \n≈ \n1.4) is reached, the flow becomes steady regardless \nof the variation of \ng \n*. Five \n \n-dependent flow modes emerge; \nthe anti-phase, in-phase, flip-flop, single vortex shedding and \nsuppressed modes. A statistical scrutiny of the validated ransient data for the lift ( \nL \nC \n) and drag ( \nD \nC \n) coefficients is \nultimately performed. When \ng \n* = 0.2, the general trend of \ndecreasing \nD \nC \n with reduction in gap size is broken.