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-dimensional elastic cylinder in a cross flow are not well understood. Experimental data pertaining to the interaction behavior is rather scarce, especially that related to the phenomenon of synchronization where the vortex shedding frequency is approximately equal to the natural frequency of the fluid-structure system. The present investigation attempts to examine this problem experimentally using a laser vibrometer to assess the bending displacements and a laser Doppler anemometer to measure the velocities in the wake. Experiments were carried out over a range of reduced velocities. The reduced velocity was first varied by using cylinders of different materials and then by changing the free stream velocity, while maintaining the cylinder diameter constant. A proper choice of materials and reduced velocity allowed the synchronization phenomenon to be investigated. For the range of reduced velocity investigated, the vibration amplitude of the cylinder is finite, even at synchronization, and increases with reduced velocity. The results further show that more than one mode of vibration is excited away from synchronization; however, only the first mode is evident at synchronization. In addition, 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 field. However, cylinder vibrations enhance turbulent mixing, thus resulting in a substantial increase in the turbulent intensities. This implies that the large-scale vortical motion is also affected. Nevertheless, turbulence structure in the inertial sub-range is not affected by cylinder vibrations. The slopes of the velocity spectra in this range is still measured to be −5/3 for the freely vibrating cylinders investigated. Received: 20 December 1998/Accepted: 20 September 1999