Numerical Investigation of Secondary Vortex Structures in a Rotor Wake

Abstract

In hovering rotor computational fluid dynamics simulations, a common phenomenon is the breakdown of the primary vortex system due to secondary vortices, which are characterized by S-shaped vortical structures appearing between the primary blade-tip vortices. Since the presence of the secondary vortices is strongly influenced by the numerical settings, the degree to which secondary vortices physically occur warrants investigation. Therefore, this study investigates how the numerical settings affect the development of secondary vortices in hover simulations using the rotating blade geometry from previous experiments, where the focus is on temporal convergence. The computed near-wake vortical flow field is qualitatively compared to the experimental results. The calculations are then used to elucidate the secondary vortex development process, where it is shown that the secondary vortices develop by the interaction of the vortex sheet with the primary vortex structure. A number of parametric studies are done on the rotating blade geometry to investigate how the numerical settings affect the secondary vortex development process. Although the focus is on understanding the impact of temporal convergence, the effect of the off-body grid structure, grid rotation, mesh resolution, numerical dissipation, and time-step scaling is also studied. In contrast to previous work, the resulting wake breakdown from using the different numerical settings is compared both qualitatively and quantitatively. For relatively high flow solver subiteration convergence, the off-body grid structure and grid rotation have little effect on the development of the secondary vortex structures. However, at low solver subiteration convergence, larger differences occur between the torus and no torus grid structures. A parametric sweep on subiterations shows that as the subiteration residual drop increases the vortex sheet is broken down less and, correspondingly, both the rotor performance data and peak number of secondary vortices converge. The analysis shows that when the subiteration convergence or grid resolution is not high enough, the vortex sheet and primary vortex are broken down, which disrupt the development process and leads to fewer secondary vortices. To better understand the rotating blade case, a separate vortex ring study is done to observe how vortex breakdown is influenced by off-body grid structure and different levels of subiteration convergence. In agreement with the rotating blade case, when all flow solvers are sufficiently converged, vortex breakdown is minimized and the solution to the rotating and stationary torus is identical.