Dynamics of a hydroelastic cylinder with very low mass and damping

An experimental facility for the study of the forces and response associated with vortex-induced vibration of a rigid cylinder has been constructed with extraordinarily low normalized mass and normalized damping. This facility achieves a level of combined mass and damping which is an order of magnitude less than previous studies of this type. Measurements of the total integrated forces on a static cylinder demonstrate the importance of the end conditions. By varying the end conditions, the case of vortices shed parallel to the cylinder was compared to the oblique shedding case. The results show that the mean drag is consistently higher in the parallel shedding case, throughout our range of Reynolds number, whereas the variation in r.m.s. lift is highly dependent on Reynolds number. At Re=12 000, the parallel case had five times the r.m.s. lift of the oblique case, a ratio which would become larger as the aspect ratio of the cylinder increases. However, despite this dependence upon Reynolds number in the magnitude of the lift force, the dominant nondimensional frequencies of the lift force were independent of Reynolds number, even in the present case of cellular shedding. Our study of the response in the hydroelastic case has brought two previously neglected points to the fore. Our data at very low mass-damping ratio shows that the response has two branches of resonance. The implication at low mass-damping ratios is that there are actually two distinct levels of resonance, rather than a single one as previously assumed. The second observation is that the adjustment of the mass ratio affects the response, even when the combined mass-damping ratio is kept constant. An equation of motion is developed here, with inclusion of the inviscid ''added-mass'' force which will necessarily become important at low mass ratios. (C) 1996 Academic Press limited.