Forces on Submerged Cylinders Oscillating near a Free Surface

Abstract

Forces on submerged, two-dimensional bodies of constant circular or square cross section oscillating in water of infinite depth have been measured using a forced oscillation test. The bodies were forced to sway and heave sinusoidally with small amplitudes, for several submergences below a free surface. The added-mass and wavedamping coefficients are shown to be influenced strongly by the free-surface effect and are presented as a function of frequency and direction of oscillation and of depth of submergence from the free surface. There appears to be a critical frequency that was measured consistently for heave oscillations near the free surface and was not predicted by existing theories. The measured coefficient values for the sway and heave oscillations of the circular cross section near the free surface were shifted in frequency, whereas the corresponding theoretical coefficients are identical. Comparisons of the experimental results with computations by a potential theory show reasonably good agreement. Use of the coefficients in equations of motion for floating ocean structures is described.