Flow Near Self-Excited and Forced Vibrating Circular Cylinders

Many studies of flow-bluff body interactions have been undertaken because of the practical need to know the fluid forces acting on a body immersed in a flowing stream. Experiments were performed to investigate the interaction process between the vibrating cylinder and the wake flow for free-stream Reynolds numbers between 550 and 900 in the irregular range. Two circular cylinders of equal diameter were used for experimentation—one was tuned to self-excite under the influence of lift forces and another was a rigid cylinder mounted in a shaker and forced to vibrate sinusoidally perpendicular to the mean flow direction under conditions that duplicated the Reynolds number, Strouhal number, motion amplitude, and relative frequency of the tuned cylinder. Experimental results for the two means of vibration and the mechanics of the wake formation are discussed in terms of the fluid dynamical parameters that influence the vibrations. One finds the wake formation process to be similar for both cylinders under the same conditions of flow, and the amplitude and distribution of near wake velocity fluctuations also to be the same. A phase shift of ninety deg is observed between eighty and one hundred and ten percent of the peak self-excited resonance condition, with the phase angle between the velocity fluctuation and cylinder motion signals being equal for both means of excitation. The critical relative frequency that corresponds to peak resonant conditions is less than unity. The independent parameters that govern the vibrations are not the same for the two means of excitation, and the bounds for the forced and self-excited motions are discussed in terms of the present results and those reported in the literature.