Numerical investigation of a rotationally oscillating cylinder in mean flow

Vortex shedding and the development of a wake behind a rotationally oscillating circular cylinder was investigated using a hybrid vortex method at a Reynolds number of 1000 over a wide range of forcing frequency and amplitude. The normalised peripheral velocity-oscillation amplitude of the cylinder ranged from 0 to 3 while the ratio of forcing frequency to the vortex-shedding frequency from a stationary cylinder varied from 0 to 10. The time-dependent pressure, lift and drag forces exerted on the cylinder were studied together with the flow patterns in the wake, Some behaviours of vortex shedding are revealed and the lock-on range for vortex shedding is obtained. It is found that, in the case of a very low frequency ratio, vortices are shed at a frequency close to that from a stationary cylinder when the amplitude is small; however, the vortices are shed at cylinder-oscillation frequency when the amplitude is large. When the frequency ratio is close to 1, the form of vortex shedding and lock-on exhibit a particularly strong resonance between the flow perturbations and the vortex wake, and the mean value of the drag coefficients increases remarkably. Its maximum value increases with increasing amplitude within the lock-on range and shifts towards the lower frequency end of the lock-on range. When the frequency ratio is greater than a certain value beyond the lock-on range, small-scale vortices are shed at the forcing frequency in the near wake. Subsequently, these vortices coalesce and result in a large-scale antisymmetrical structure in the far wake similar to the Karman vortex street past a stationary cylinder. The mean value of the drag coefficients decreases in the post lock-on frequency range. The larger the amplitude, the more distinct is the drag coefficient decrease, and the minimum value is lower than that for flow past a stationary cylinder. After the minimum is reached, the drag coefficient increases again with further increase in cylinder-oscillation frequency and approaches the value for the stationary cylinder. (C) 2001 Academic Press.