Hydrodynamic damping of a large scale surface piercing circular cylinder in planar oscillatory motion

Measurements of the hydrodynamic damping acting on a vertical, 0.5 m diameter cylinder in planar oscillatory motion at Stokes parameter, β, up to 1.4 × 10⁵ are presented. The results are also shown as a variation of drag coefficient, C_d, with Keulegan-Carpenter number, KC, where the range of KC numbers studied is from 1 × 10⁻³ to 1. The experiments were carried out in the Delta Flume at Delft Hydraulics Laboratories in Holland and the cylinder was mounted from a pendulum suspension system. The hydrodynamic damping is the sum of radiation damping, due to gravity waves generated by the cylinder piercing the water surface, and viscous damping. A frequency-domain solution from Dalrymple & Dean (1972) is used to predict the radiation damping. An estimate of the viscous damping is then obtained by subtracting the predicted radiation damping from the measured hydrodynamic damping. Results for the viscous damping derived in this way are found to be close to those expected from experimental studies carried out by Bearman & Russell (1996) and Chaplin & Subbiah (1996) to measure viscous damping on a submerged cylinder.