Extraction and recognization of large-scale structures in the turbulent near wake of a circular disc

Abstract

Large-scale structures of the turbulent near wake behind a circular disc at Re = 2.2 × 104 were investigated using large eddy simulation (LES) and multi-plane snapshot proper orthogonal decomposition (POD). Instantaneous data of three cross-stream planes were extracted from LES computation domain (− 5 ⩽ x/D ⩽ 15, 0 ⩽ r/D ⩽ 10, 0 ⩽ θ ⩽ 2π) to form the snapshot for the POD. The application of POD with a vector form decomposed the spatiotemporal wake flow into M modes with different amounts of fluctuation energy and turbulent scales contained. For comparison, the same decomposition was also conducted for the low Reynolds number case, Re = 170, where the unsteady flow is featured by a plane of symmetry, significant shedding process and well visually resolved vortex structures. At high Reynolds number, it was found that modes 1 and 2 presented an anti-symmetric structure, which was associated with the first azimuthal mode, being most energetic at all considered locations. A 90° rotation emerged between the symmetry planes of the first two POD modes. This could be a hint of a double helix system. Quadrupole-like patterns were observed for modes 3 and 4, which were definitely related to higher azimuthal modes, especially m = ± 2. The double helix system was in accord with a three-dimensional POD reconstructed streamwise vorticity pattern.