Vortex collision against static and spinning round cylinders: A lattice Boltzmann study

Abstract

In this paper, the flow physics generated by the collision of a vortex dipole that moves against a spinning round cylinder is investigated numerically. Fluid dynamics is predicted by a combined central-moments-based lattice Boltzmann-immersed boundary method. First, the model is validated against well established consolidated benchmark problems, showing very high accuracy properties. Then, results from a comprehensive numerical campaign are presented. A wide set of values of the Reynolds number (Re) is investigated, ranging from 10 to 1000. The cylinder is forced to spin around its centre with different angular velocities, which are obtained by varying the spinning number (Sp) between 0 (corresponding to the static case) and 0.75. The generation of secondary vortices as a consequence of the impact is elucidated and linked to the time evolution of the kinetic energy, enstrophy and hydrodynamic forces. Interestingly, we find that the flow physics changes drastically when Re≥250, independently from the value of Sp. Through a closer look at the vorticity field, we find that the impact creates two primary–secondary structures and a second impingement takes place when Re≥250. Interestingly, the normalised drag force (Cd) is found to constantly fluctuates around a mean value. Oscillations are due to the vorticity created by the rotation of the cylinder and are more emphasised as Sp grows. Specifically, Cd can achieve marked negative values as a consequence of the velocity field created by the cylinder during its rotation.