Experimental and numerical studies of piston-mode resonance in a three-dimensional circular moonpool

We develop a modified potential flow model (MPFM) for a circular moonpool by quantifying the viscous damping. The flow-separation-induced damping coefficients of MPFM do not need to be tuned by the users as they are derived from the conduit problem studied by Idelchik, where the variation of such coefficients with geometry configurations is given based on experiments. To verify the MPFM, we conducted laboratory experiments in heave radiation (forced harmonic heave motion) and wave diffraction (fixed in regular wave) problems with a broad range of frequencies and compared the results with those of MPFM and a boundary element method without viscosity modeling (BEM). The MPFM achieves fairly good agreement with the experimental results, including those around the resonance frequency where BEM over-predicts. A simple time-domain model is developed to analyze the sources of the different harmonics in the free-surface elevation observed in the experiment.