Wake dynamics of finite-aspect-ratio rotating circular cylinders at low Reynolds number

We perform direct numerical simulations of flows over finite-aspect-ratio rotating circular cylinders at a Reynolds number of 150 over a range of aspect ratios (AR=2-12) and rotation rates (=0-5), aiming to reveal the free-end effects on wake dynamics and aerodynamic performance. As a direct consequence of lift generation, a pair of counter-rotating tip vortices form at the free ends. At low rotation rates, the finite rotating cylinder behaves like a typical bluff body that generates unsteady vortex shedding with three-dimensional modal structures. Such unsteady flows can be stabilised not only by increasing the rotation rate, which weakens the free shear layer, but also by decreasing the aspect ratio which enhances the tip-vortex-induced downwash. A further increase of triggers the onset of unsteadiness in the tip vortices. At still higher rotation rates, C-shaped Taylor-like vortices bounded on the cylinder surface emerge from the free ends and migrate towards the midspan due to self-induced velocity from vortex–wall interaction. With increasing, the free-end effects penetrate to the inboard span, leading to reduced lift and elevated drag compared with two-dimensional flows. The three-dimensional end effects can be effectively suppressed by adding end plates, which position the tip vortices away from the cylinder, thereby significantly improving aerodynamic performance. This study reveals the mechanisms for three-dimensional wake formation under the influence of the free ends of finite rotating cylinders. The insights obtained here serve as a stepping stone for understanding complex high- Re flows relevant to industrial applications.