Turbulent structures and characteristics of flows past a vertical surface-piercing finite circular cylinder

Abstract

The flows past a vertical surface-piercing finite circular cylinder at Re = 2.7 × 105 and Fr = 1.1 are investigated numerically by means of delayed detached-eddy simulation and a geometric volume-of-fluid method based on piecewise-linear interface calculation. Good agreement with experimental data is achieved in various aspects, thereby demonstrating the reliability and accuracy of the present numerical model. On this basis, the characteristics of typical turbulent structures are analyzed thoroughly, as is the spanwise variation of the flow field caused by complex interactions. Because of the effects of the free surface and the free end, the velocity profile, separation angle, vorticity, and turbulent kinematic energy at different spanwise positions exhibit strong three-dimensionality, including the outward-spreading trend at the interface and the fluctuation induced by the upwash flow near the free end. By using the modified Omega-Liutex method ω R, instantaneous and time-averaged primary turbulent structures are identified well with the iso-surfaces of proper thresholds. A complete necklace vortex and a pair of wave-induced vortices are observed below the free surface, while a pair of large-scale arch vortices and two pairs of tip vortices are generated near the free end. The Liutex lines and streamlines are then used to analyze the spatial formations and developments of these structures. Furthermore, by performing spectral analysis at different probes on the cylinder surface and in the wake region, the dominant frequencies for each primary turbulent structure are determined.