Large eddy simulation of the flow around single and two side-by-side cylinders at subcritical Reynolds numbers

The flow around single and two side-by-side infinite cylinders is numerically modelled using dynamic Smagorinsky large eddy simulation (LES). For the single cylinder, the Reynolds number based on the diameter and the free stream velocity is 3900. A complete sensitivity study was conducted based on the extrusion length in the span-wise direction, the grid refinement at the wall, the convection scheme, and the sub-grid scale (SGS) model. It was found that the mean solution is not influenced by the extrusion length beyond 4 diameters or by 1% up-winding. However, coarsening the mesh in the wall normal direction or switching off the sub-grid scale model led to drastic effects on the recirculation length and on the underlying velocity field. The two side-by-side cylinders were tested for a range of pitch to diameter ratios (T/D=1.0,1.25 <= T/D <= 5.0) at a Reynolds number of 3000. For the intermediate pitch to diameter ratios (1.25 <= T/D <= 1.75), multiple shedding frequencies were detected with a biased wake flow deflection. Furthermore, this biased flow deflection was found to be bistable, i.e., it changes the direction (flipping over) intermittently from one side to the other. This behavior was found to be consistent with reported experimental measurements. During the flip-over from one stable mode to the other, the intermittent gap vortex shedding was found to be stronger than for a stable mode, with in-phase vortex shedding. However, for the higher pitch ratio cases (T/D >= 2), a symmetrical wake behavior with anti-phase vortex shedding was observed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3596267]