'Buffeting of single cylinders'

The fluctuating lift and drag induced on a rigid cylinder immersed in a turbulent flow in the range of Reynolds numbers straddling the transition from subcritical to supercritical regime is examined. The buffeting forces are induced in response to turbulence generated by grids. In the present experiments, the integral scales of turbulence, l, for the grids used are less than the cylinder diameter (l/d < 1). Interpretation of these and other published data strongly suggest that the free stream disturbances (turbulence) affect the force response in two ways. In the absence of grid turbulence below a certain threshold level, it appears to simply modulate the spanwise spatial coherence of vortex shedding. In this situation, the spectral character retains the discrete or monochromatic Strouhal nature, while the RMS force levels decrease with increase in the measured free stream disturbance. On the other hand, the cylinder response to broad band excitation by the high intensity grid generated turbulence is no longer discrete although the spectral character still retains a 'humped' shape of a band pass filter. Centre band frequency of this band pass is very nearly, although not quite, the Strouhal frequency in the 'clean' flow. In any event, the RMS amplitudes continue to decline as the turbulent intensity is increased. While the evidence is suggestive, it is not as yet complete. The data on hand strongly suggest the domination of the cylinder buffeting phenomenon by the shed wake, at the lease for l/d < 1. A principle can also be extracted from the data. The persistence of the Strouhal frequency into the buffeting response spectrum suggests that the time scale so represented is inherent to flow/cylinder interactions even when the upstream flow is highly turbulent (and l/d < 1). (author)