Three-dimensional flow past two stationary side-by-side circular cylinders

Abstract

We conducted three-dimensional direct numerical simulations of flow past two stationary side-by-side circular cylinders. Based on the cylinder diameter, the Reynolds number was set as Re = 500 and the center-to-center spacing ratio was s/D = 1.0–5.0. In the studied parametric space, we identified five distinct flow regimes, namely, single bluff body flow at s/D = 1.1, deflected flow at s/D = 1.2–1.8, flip-flopping flow at s/D = 2.0–2.4, hybrid flow at s/D = 2.5, and antiphase flow at s/D = 2.7–5.0. The intrinsic features of each flow were elucidated using instantaneous vorticity contours, time-averaged flow fields, three-dimensional vortical structures, time histories and spectral frequencies of the drag and lift coefficients. Furthermore, the statistics of hydrodynamic forces, vortex shedding frequency, and wake three-dimensionality within each flow regime were examined thoroughly. Finally, we discussed two critical issues, namely (a) the effects of the three-dimensional vortical structures on the hydrodynamic forces and flow behaviors where both two- and three-dimensional results are compared, and (b) cylinder length effects and underlying mechanism for the switching behavior of the gap flow. We found that (1) the existence of the three-dimensional vortical structures significantly undermines the vortex interactions by absorbing energy from the spanwise vortices, and therefore, both the hydrodynamic and wake behaviors are considerably altered. (2) An increase of the cylinder length leads to the longer switching period of the gap flow in FF regime, while the transform of the gap flow direction is strongly dependent on the phase of the gap side vortices of the two cylinders.