Flow around a rotating circular cylinder at a Reynolds number of 500 is investigated numerically. The aim of this study is to investigate the effect of high rotation rate on the wake flow past a circular cylinder. Simulations are performed at a constant Reynolds number of 500 and a wide range of rotation rate from 1.6 to 6. Rotation rate is the ratio of the rotational speed of the cylinder surface to the incoming fluid velocity. It is found that increasing rotation rate beyond a critical value results in transition from no vortex shedding regime to secondary instability regime where the oscillation of the lift force on the rotating cylinder increases drastically. This secondary instability regime was also observed at low Reynolds number where the flow is laminar.
Vortex shedding flow in the wake of circular cylinders has been investigated extensively mainly due to its increasing applications in but not limited to offshore oil and gas engineering. Many experimental and numerical studies have been conducted on the transition of the wake flow to turbulence (Roshko 1954, Williamson 1988, Hammache and Gharib 1991, Karniadakis and Triantafyllou 1992, Barkley and Henderson 1996, Thompson, Hourigan et al. 1996). It was concluded that the transition of wake flow from two-dimensional to threedimensional flow occurs when the Reynolds number is around Re=140 to 190. The Reynolds number is defined as Re=UD/v where U is the incoming velocity, D is the diameter of the cylinder and ν is the kinematic viscosity of the fluid. In the numerical study by Zhao et al. (2013), the critical Reynolds number was found to be 200. The critical Reynolds number varies in different experimental studies mainly because of the end effects in the experimental condition.
When a circular cylinder rotates in a fluid flow, the speed of the rotation affects the symmetry of the wake flow and thus influences the corresponding forces on the cylinder. In addition to the Reynolds number, the rotation rate a has significant influence on flow past a rotating cylinder (Chang and Chern 1991). The rotation rate α is the non-dimensional rotational speed of the cylinder relative to the fluid velocity, which is defined as α = ωD/(2U) where ω is the angular rotation speed of the cylinder. Early studies on flow past a rotating cylinder were focused on very small Reynolds numbers of Re ≤ 100 (Tang and Ingham 1991). Vortex shedding flow around a rotating circular cylinder was found to be fully suppressed when a is greater than 2 (Diaz, Gavalda et al. 1983, Badr, Coutanceau et al. 1990, Chew, Cheng et al. 1995, Chou 2000). Kang, Choi et al. (1999) investigated the laminar flow past a rotating circular cylinder through numerical simulations and found that the critical α above which there was no vortex shedding increased logarithmically with increasing Reynolds number. As a result, he observed the critical rotation rates of about 1.4, 1.8 and 1.9 at Re = 60, 100 and 160 respectively. Moreover, for rotation rates lower than the critical rotation rate, flow pattern changes but the Strouhal number did not change much with increasing rotation rate.
