Flow-induced in-line oscillation of a circular cylinder

Flow-induced in-line oscillation of a circular cylinder either spring-mounted (two-dimensional rigid cylinder) or cantilevered was experimentally studied in free-oscillation tests in a water tunnel. In the free-oscillation tests of a two-dimensional cylinder, two different types of excitations appear at approximately half of the resonance flow velocity. The response amplitudes are sensitive to the reduced mass-damping parameter for the in-line oscillation in the first excitation region with a symmetric vortex street, while the alternate vortices are locked-in with the vibration of the cylinder in the second excitation region. For a cantilevered circular cylinder with a finite span-length, it is found that cylinders with aspect ratios of 5 and 10 have a single excitation region, and cylinders with aspect ratios of 14 and 21 have two excitation regions. The mechanism of vibration was also investigated using a splitter plate in the wake to prevent alternate vortices. It is noted that the amplitude of oscillation with the splitter plate is greater than that without the splitter plate, especially at Vr=2.3 to 3.0, where a cylinder with an end plate shows the second excitation region. In other words, the alternate vortices suppress the excitation amplitude in this range. The results of this study are providing important supporting data for the recent Standard JSME S012-1998, 'Guideline for Evaluation of Flow-Induced Vibration of a Cylindrical Structure in a Pipe', by the Japan Society of Mechanical Engineers. (authors)