Free vibrations of an elastic cylinder in a cross flow and their effects on the near wake

Fluid-structure interactions resulting from the free vibrations of a two-di mensional elastic cylinder in a cross flow are not well understood. Experim ental data pertaining to the interaction behavior is rather scarce, especia lly that related to the phenomenon of synchronization where the vortex shed ding frequency is approximately equal to the natural frequency of the fluid -structure system. The present investigation attempts to examine this probl em experimentally using a laser vibrometer to assess the bending displaceme nts and a laser Doppler anemometer to measure the velocities in the wake. E xperiments were carried out over a range of reduced velocities. The reduced velocity was first varied by using cylinders of different materials and th en by changing the free stream velocity, while maintaining the cylinder dia meter constant. A proper choice of materials and reduced velocity allowed t he synchronization phenomenon to be investigated. For the range of reduced velocity investigated, the vibration amplitude of the cylinder is finite, e ven at synchronization, and increases with reduced velocity. The results fu rther show that more than one mode of vibration is excited away from synchr onization; however, only the first mode is evident at synchronization. In a ddition, the near-wake flow behind the elastic cylinder. at three different Reynolds numbers in the sub-critical range, was measured in detail and the data was used to analyse the vibration effects on the mean and turbulence field compared to those measured behind a relatively rigid cylinder at the same Reynolds numbers. It is found that cylinder vibrations have little or no effect on the mean drag and the normalized mean held. However, cylinder vibrations enhance turbulent mixing, thus resulting in a substantial increa se in the turbulent intensities. This implies that the large-scale vortical motion is also affected. Nevertheless, turbulence structure in the inertia l sub-range is not affected by cylinder vibrations. The slopes of the veloc ity spectra in this range is still measured to be -5/3 for the freely vibra ting cylinders investigated.