| Uniform flow past rotating and oscillating circular cylinders is investigated by two- and three-dimensional numerical simulations. A second order accurate in time fractional-step method and combined finite-difference/spectral approximations are used to solve the incompressible unsteady Navier-Stokes equations in general curvilinear coordinate. The turbulent wake is modeled by an LES technique. Vortex shedding and wake structure are visualized by computational streaklines. The focus is on the effect of cylinder motions on the hydrodynamic forces, vortex shedding and wake structure.;For flow past a rotating cylinder, the Reynolds number, Re, ranges from 200 to 1.4 x 105. As the nondimensional cylinder rotational velocity, alpha, increases, the flow experiences four successive stages. We have captured a second vortex shedding mode for the first time in a three-dimensional study. A physical explanation to the second vortex shedding mode is proposed based on our streakline visualization. During the ranges of alpha where the vortex shedding is suppressed, the flow behaves like potential flow past a circular cylinder with circulation. Our computed mean lift increases as a increases, exceeds the lift limit given by Prandtl, and approaches the theory proposed by Glauert. The drag is predicted to approach zero.;Two- and three-dimensional computations of flow past a rotationally oscillating cylinder are carried out at Re = 150 and 1.5 x 10 4, respectively, to study the effectiveness of drag reduction through cylinder rotary oscillation. Two forcing amplitudes O = 0.5 and 2 with a wide range of forcing frequency fe, are considered. The mean drag has a sharp increases during the lock-on range, and then decreases continuously to a minimum value. Up to 57% of drag reduction is achieved at Re = 1.5 x 104. As the forcing frequency further increases, the effect of cylinder oscillation diminishes and the mean drag approaches the value of flow past a stationary cylinder. Several different wake patterns, due to the variation of forcing frequency fe, are observed by computational streaklines. |
