Hydroelastic instabilities of square cylinders

Abstract

Experiments were conducted to investigate the vortex resonance and galloping instabilities of a square cylinder, mounted elastically and with oscillations restricted to a plane normal to the incident water flow. The cylinders tested comprised sharp-edged sections and sections with corner radius ratios of 0·164 and 0·318. The still water added mass of a cylinder was found to be independent of the oscillation amplitudes, while the still fluid damping was observed to be viscous for amplitudes up to 30% of the cylinder width. In the dynamic tests of sharp-edged cylinders, the vibrations began at a reduced flow velocity slightly less than the vortex resonance speed. In the amplitude domain, depending on the magnitude of the response parameter K s = β (nU r 2) (β = reduced damping, n = mass parameter and U r = vortex resonance speed), three different patterns were identified. In the first pattern, associated with the tests with K s < 0·69, the vortex resonance was found to be indistinguishable from galloping, while in the second pattern, belonging to the tests with 0·73 < K s < 1·4, at reduced flow velocities close to the vortex resonance speed, a build up-drop off behaviour in oscillation amplitudes was observed. This behaviour appeared to have been replaced by a complete separation of vortex resonance from galloping in the third pattern, associated with the tests with K s > 1.64. The system response frequency in a streaming flow was always found to be less than the system natural frequency. Both the vortex resonance and galloping excitations were found to be sensitive to the value of the corner radius ratio. The well known quasi-steady theory was modified to take into account the forces induced by the vortex shedding process.