On the Pressure and Force Field on a Circular Cylinder Oscillating in the Lock-In Region at Sub-Critical Reynolds Number

The vortex induced vibration of a rigid cylinder has been studied in the subcritical Reynolds range in terms of motion parameters and also in terms of instantaneous pressure distribution on the cylinder surface. The resulting force field has been analyzed as a function of the fundamental parameters z* (non-dimensional amplitude) and U_n (critical velocity ratio) showing a possible systematic modelization of the force component synchronous with the oscillation frequency, responsible for the power input in the lock-in region. The magnitude and the phase of the synchronous force component have been studied analyzing build-up events as well as steady state constant amplitude oscillation events. A very close correspondence has been highlighted among the two different analyzed cases, confirming that a quasi-steady model of the force field is a robust and reliable representation of the flow-cylinder interaction force field. This interaction is responsible for the typical transient build-up oscillations of technical interest. The pressure distribution monitored at different locations along the oscillating cylinder axial coordinate allowed finally to show a direct link between the incoming flow velocity distribution and the correlation characteristics of the vortex shedding force distribution along the cylinder axis.