The vibrations of a circular cylinder in both uniform and shear flows are investigated experimentally. For the experimental investigation, a low speed water tunnel was designed and built to provide either uniform or shear flow in the test section, depending on the upstream flow management. In the test section, a circular tube of various materials can be flexibly mounted for vibration testing. Two accelerators were carefully installed inside the tube so that one accelerator is sensitive to the cylinder vibration in the streamwise direction only, and the other in the cross-stream direction. The vibration amplitudes of the cylinder in the streamwise and cross-stream directions were simultaneously measured by the two accelerators, and recorded by a two-channel data acquisition system. The orbits of the cylinder motion can be drawn from the data. Experiments were conduced at various mass ratios (the ratio of the cylinder mass per unit length to its buoyancy force) and shear parameters (the non-dimensional velocity gradient of the approaching fluid flow to the cylinder). By analyzing the orbits and amplitude diagrams, it is found that both the shear parameter and mass ratio have profound effects on the cylinder vibration. The orbits of the cylinder in uniform flow are symmetric while they are asymmetric in shear flow. Vibration amplitude as a function of reduced velocity illustrates that the cylinder in uniform or shear flow does not vibrate at low reduced velocities but vibrate significantly beginning at the reduced velocity around 5, initiated by vortex-induced instability. At high reduced velocity, the circular cylinder in shear flow still vibrates at significant amplitude, an evidence of fluid elastic vibration. It is also shown by the amplitude diagrams that low mass ratio promotes the cylinder’s vibration while large mass ratio reduces the vibration.

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