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 Ο 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 af fects 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 .