This paper describes the results of measurements of the lift and drag forces acting on a circular cylinder undergoing amplitude modulated motion. Experiments were conducted in the Towing Tank Facility at the" Massachusetts Institute of Technology, involving a circular Aluminum cylinder towed through still water and forced to oscillate with dual frequency beating motion transverse to the" direction of towing. Lift and drag forces, were measured and" analyzed for mean, rms, and spectral amplitude and phase information. The results, are presented with comparison to those from pure-sinusoidal excitations using the" same apparatus. The principal· conclusion is that the modulation appears to· reduce the mean drag coefficient and increase the oscillating drag coefficient. Hypotheses on the origins; of this unexpected phenomenon are presented.


A classic "open-flow" problem in fluid mechanics. is the flow around a circular cylinder. This, is· because of both. the rich physical phenomena that occurring the wake; as well as the cylinder's importance as an engineering element in man-made structures. At all but the lowest Reynolds numbers, the wake behind the cylinder is unstable:. the boundary layers on either side of the cylinder separate and rollup into discrete. vortices that form the celebrated Karman vortex street. This "vortex shedding"; in turn, induces oscillating lift and drag forces on the cylinder,. which may cause it to vibrate. From a practical perspective, the study of these vortex-induced forces are very important because one would like to predict the· responses of real world structures exposed to fluid flow. The traditional method of investigating the problem has been through laboratory experiments, with important contributions including those of Bishop and Hassan (1964), Protos et al(1968), Sarpkaya (1977), and Staubli (1983a., 1983b). From the efforts of all these researchers over the years.

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