Abstract
We present wall-resolved large-eddy simulation (LES) of flow with free-stream velocity U-infinity over a cylinder of diameter D rotating at constant angular velocity Omega, with the focus on the lift crisis, which takes place at relatively high Reynolds number Re-D = U-infinity D/nu, where nu is the kinematic viscosity of the fluid. Two sets of LES are performed within the (Re-D, alpha)-plane with alpha = Omega D/(2U(infinity)) the dimensionless cylinder rotation speed. One set, at Re-D D 5000, is used as a reference flow and does not exhibit a lift crisis. Our main LES varies alpha in 0 <= alpha <= 2.0 at fixed Re-D = 6 x 10(4). For alpha in the range alpha = 0.48-0.6 we find a lift crisis. This range is in agreement with experiment although the LES shows a deeper local minimum in the lift coefficient than the measured value. Diagnostics that include instantaneous surface portraits of the surface skin-friction vector field C-f, spanwise-averaged flow-streamline plots, and a statistical analysis of local, near-surface flow reversal show that, on the leeward-bottom cylinder surface, the flow experiences large-scale reorganization as alpha increases through the lift crisis. At alpha = 0.48 the primary-flow features comprise a shear layer separating from that side of the cylinder that moves with the free stream and a pattern of oscillatory but largely attached flow zones surrounded by scattered patches of local flow separation/reattachment on the lee and underside of the cylinder surface. Large-scale, unsteady vortex shedding is observed. At alpha = 0.6 the flow has transitioned to a more ordered state where the small-scale separation/reattachment cells concentrate into a relatively narrow zone with largely attached flow elsewhere. This induces a low-pressure region which produces a sudden decrease in lift and hence the lift crisis. Through this process, the boundary layer does not show classical turbulence behaviour. As alpha is further increased at constant Re-D, the localized separation zone dissipates with corresponding attached flow on most of the cylinder surface. The lift coefficient then resumes its increasing trend. A logarithmic region is found within the boundary layer at alpha = 1.0.