Wake transitions of the flow around two side-by-side elliptic cylinders

Abstract

The wake transitions of the flow past two side-by-side elliptical cylinders were numerically investigated using the lattice Boltzmann method at Reynolds numbers (Re) of 40, 100, and 150, with various spacing ratios (L/D, where L represents the distance between the cylinder centers and D is their diameter) and aspect ratios (AR). This study elucidated the effects of Re, AR, and L/D on the flow characteristics, including wake structures, hydrodynamic coefficients, Strouhal number (St), and the spectral energy of the flow exerted on both cylinders. Six distinct flow patterns are observed in the AR−L/D space, such as steady shear layers, anti-phase synchronized streets, in-phase synchronized streets, single bluff body, flip-flopping, and chaotic flow. These patterns were characterized through detailed analyses, including vorticity contour plots, time histories of drag and lift coefficients, power spectral density, and proper orthogonal decomposition of vorticity fluctuations into deterministic spatial structures. Additionally, flow pattern maps and diagrams of the time-averaged pressure coefficients on the surface of the cylinders were provided to assess the influence of Re, AR, and L/D on the flow behavior. The hydrodynamic coefficients of both cylinders showed near-identical trends with significant variations depending on L/D and AR. When AR is small, the time-averaged drag coefficient and the root-mean-squared lift coefficient of both cylinders are found to be substantially higher than those of an isolated elliptical cylinder. Furthermore, a notable increase in the time-averaged lift coefficient was observed when L/D was small, attributed to the repulsive forces between the cylinders. At higher Reynolds numbers (Re=100 & 150), substantial differences in St emerge, particularly for smaller AR values, despite the cylinders being in a side-by-side configuration.