Large-eddy simulations of flow past a square cylinder using structured and unstructured grids

Abstract

The flow past a square cylinder at Re=2.2×104 is analyzed by large-eddy simulation (LES) using the fine grids in order to represent details of near-cylinder flows. The accuracy of LES on structured and unstructured grids is assessed from the engineering viewpoint, compared with previous studies. The finite differencing method code with 4th order central scheme for the convective term is used for structured LES, while the open-source finite volume method code (OpenFOAM 2.3.0) with 1st–2nd order schemes is applied for unstructured LES. Typical schemes in OpenFOAM are tested, i.e., “LUST” (blending of linear and linear upwind schemes), “limitedLinear” (TVD) and “linearUpwind” (linear upwind). In this study, three effects are emphasized: numerical schemes for convective terms, meshing strategies, and spanwise resolution and length. On the whole, OpenFOAM can obtain fairly accurate prediction of the time-averaged and r.m.s quantities no matter which numerical scheme is used. “LUST” and “linearUpwind” are suggested. Meshing refinement in wake can be a solution to improve the far-wake velocity distribution and to overcome the earlier energy decay of turbulent motions in the inertial subrange caused by artificial dissipation. Different degrees of instantaneous flow reattachment near the trailing edges are found in the results obtained by OpenFOAM. Flow reattachment is closely associated with the roll-up of shear layer, and the flow topology between the shear layer and the side wall featured by the frontal and leeward secondary vortex. In this regard, the refinement of hexahedra cells near cylinder gives a best solution among all present cases tested compared with the previous DNS result. Generally the Kelvin–Helmholtz instability can be accurately predicted by OpenFOAM under the present numerical conditions. In addition, the spanwise resolution seems no big effect in predicted results when less than 0.05D. By contrast, the increase in spanwise length to 14D from 4D plays an important role in obtaining reasonable spanwise correlation of pressure and consequently the overall fluctuating lifts.