Effect of a wall on the wake dynamics of an infinite square cylinder

Abstract

The present study investigates the influence of a solid wall on the wake structure of an infinite cylinder of square cross-section when it is placed in proximity to the wall. Three different values of the gap ratio were considered, i.e. g/D=0,0.5and1, in addition to the reference case of a cylinder in a free stream with no wall. The Reynolds number based on the cylinder width and free stream velocity was Re=500. A coarse-grid Large Eddy Simulation was used to predict the resolved-scale velocity and pressure fields, and also generate the input data for the Proper Orthogonal Decomposition (POD) analysis. When the cylinder was in close proximity to the wall, it introduced strong asymmetry into the near-wake, which was manifest in the reorientation of the upper and lower recirculation cells, as well as the development of a secondary recirculation zone on the bottom wall itself. For the case of the cylinder resting directly on the bottom wall, the wake was dominated by a single large, elevated recirculation zone, and vortex shedding was entirely suppressed. For each gap ratio, the coherent structures associated with the first three POD modes were identified and the results compared with the case of an infinite cylinder in a free stream. Both the energy distribution among the POD modes and the flow pattern associated with the coherent structures for each mode were strongly influenced by the presence of the wall for the two smallest gap ratios.