A vortex in cell method has been used to simulate the flow past a two-dimensional section of a vertical circular cylinder under conditions first of a fixed cylinder and second an elastically mounted cylinder. In the second case the cylinder was chosen to represent conditions at a mid section of a compliant cylinder under its lowest mode of vibration. The incident oscillatory flow was similarly chosen in both rigid and compliant cases to represent the horizontal component of the incident wave velocity. Results are presented for a range of Keulegan Carpenter numbers and frequency ratio parameters. The computations are carried out at Reynolds numbers of the order of 103 for which the boundary layers on the cylinder remain laminar. The predicted response of the cylinder is compared with results of experimental measurements.
Response of compliant cylinders to wave induced hydrodynamic loading can be significant in practical cases. Examples are drag induced and transverse (vortex) force induced vibration of riser pipes and some tethered floating structures. In many cases the hydrodynamic loading arises from a combination of wave (particle orbit) velocities, large scale planar oscillatory velocities due to motion of floating structures and frequently also steady currents. Compliant cylinder response in steady incident flow has been established by numerous studies over the last twenty five years. Two main modes of vibration have been observed to be associated with vortex shedding. A maximum in the transverse vibration occurs at reduced velocities in the region of 5 for circular cylinders in subcritical flow, when the vortex shedding locks on to the cylinder motion and the frequency of the transverse force induced by the shedding is close to the natural frequency of the cylinder. For significant amplitudes of motion the vortex shedding may also change from an antisymmetric to a symmetric mode.
