The effect of flow perturbations on the near wake characteristics of a circular cylinder

The mean and fluctuating velocity fields in the near wake of a circular cylinder subjected to an incident mean flow with periodic velocity perturbations superimposed upon it were examined using laser Doppler anemometry. From these measurements the wake was characterized in terms of the recirculation bubble length, vortex formation length, maximum intensity of the velocity fluctuations and the wavelength of the vortex street. The well-known ‘lock-on’ phenomenon was observed for perturbation frequencies around two times the natural vortex shedding frequency. It is shown that the wake structure is modified in a systematic manner within the lock-on range. The forced wake shares many basic characteristics as that of a cylinder oscillating either transversely or in line, relative to the flow direction. These include the shortening of the recirculation bubble and the vortex formation region as well as the variation of the longitudinal vortex spacing with perturbation frequency. Differences but also similarities between forced wakes at low (less than 350) and relatively higher Reynolds numbers (greater than 350) are discussed.