3D effects on measured results using a 2D model in a narrow wave channel

The forced oscillation test in heave using a box-shaped floating body was conducted in a narrow wave channel to obtain the added-mass and damping coefficients. When the gap between the sidewalls of the model and the wave channel was 5 mm, measured results showed an unnatural variation at higher frequencies, and there was a large discrepancy from computed results by a 2D BEM based on the potential-flow theory. Measured results after lessening the gap from 5 mm to lmm became reasonable, but we could still observe a noticeable discrepancy and dependency on the oscillation amplitude in the damping coefficient. To understand the hydrodynamic reasons in these observations, numerical computations were performed using a 3D BEM for potential flows and an in-house 2D computer code for nonlinear viscous fluids. We confirmed through comparisons that the unnatural variation observed at higher frequencies for the gap equal to 5 mm was associated with trapped waves generated in the gap, and that the discrepancy in the damping coefficient between the experiment and the computation by BEM was associated with the effect of vortex shedding.