Numerical investigation of a rotationally oscillating cylinder in mean flow

Vortex shedding and the development of a wake behind a rotationally oscilla ting circular cylinder was investigated using a hybrid vortex method at a R eynolds 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-shedd ing frequency from a stationary cylinder varied from 0 to 10. The time-depe ndent pressure, lift and drag forces exerted on the cylinder were studied t ogether with the flow patterns in the wake, Some behaviours of vortex shedd ing 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 f orm of vortex shedding and lock-on exhibit a particularly strong resonance between the flow perturbations and the vortex wake, and the mean value of t he 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 th an a certain value beyond the lock-on range, small-scale vortices are shed at the forcing frequency in the near wake. Subsequently, these vortices coa lesce and result in a large-scale antisymmetrical structure in the far wake similar to the Karman vortex street past a stationary cylinder. The mean v alue of the drag coefficients decreases in the post lock-on frequency range . The larger the amplitude, the more distinct is the drag coefficient decre ase, and the minimum value is lower than that for flow past a stationary cy linder. 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.