| Flow over a fixed circular cylinder is a fundamental fluid mechanics problem that involves a simple geometry; yet, as the Reynolds number is incremented, increasingly complex flow patterns emerge. Most notably, when the Reynolds number exceeds a value of about 46, a Benard-von Karman vortex street pattern appears. It is associated with a characteristic natural vortex shedding frequency. The objectives of this study are to numerically investigate the effects of an incoming free-stream velocity pulsation with a mean Reynolds number of 100 on the dynamic forces acting on a fixed circular cylinder, on the flow regimes behind the cylinder, and on heat transfer between the cylinder and surrounding fluid. Sinusoidal free-stream pulsations with velocity amplitude Av values of 25%, 50% and 75% that of the mean free-stream velocity, and selected frequencies in the range of one fourth the natural shedding frequency fs to five times fs were considered. Of particular interest, is the interaction between forced pulsating frequencies and the natural vortex shedding frequency. At frequencies close to the natural frequency, and to twice the natural frequency, sudden reductions in the mean value of the drag coefficient are observed, as the imposed frequency increases. These sudden reductions are accompanied by complex changes in flow regimes and vortex shedding patterns. The complexity of flow regimes as a function of the imposed pulsation variables was compiled into a frequency-amplitude map. This original map enables one to correlate changes in flow regimes to both dynamic forces and heat transfer characteristics. No net mean drag reduction was observed in the range of dimensionless amplitudes and frequencies considered, at a Reynolds number of 100. Local heat transfer enhancements of up to 86% were achieved for a specific frequency-amplitude pair. A particular finding is the appearance, for a narrow range of frequencies, of a non-zero mean lift coefficient, despite the overall symmetry of the problem. This phenomenon, which was observed in experiments, is reminiscent of the bifurcation responsible for the emergence of the Benard-von Karman vortex street. |
