Experimental Study of Nonlinear States of Oscillating Water Column in Waves

Physical experiments are conducted in a large-scale wave flume to investigate the nonlinear states of an oscillating water column in waves. A four-state classification (sloshing, piston, freak, and trapping states) is proposed to characterize the free-surface behaviors in the cylindrical chamber. The error range of the linearized potential-flow theory for the description of the sloshing- and piston-state internal wave motion is evaluated. This study, for the first time, identifies the phenomenon of freak-state resonance, which features a steep surface hump with wave overturning or water jets in the cylindrical chamber. This special resonance is triggered by the second-order nonlinear component of the incident waves, which cannot be predicted by the analytical solution. The transverse waves observed in the trapping state cannot be analytically predicted, neither. The instantaneous free surface in the cylinder is reconstructed using the Delaunay-refinement algorithm to investigate the volume fluctuation of the water body in the cylinder. A detrending procedure is applied to decompose each volume fluctuation history into trend and oscillatory components. The analytical solution accurately describes the amplitude of the oscillatory component in the sloshing state but overpredicts that in the piston state. The relationship between histories of the volume fluctuation of the internal water body and the measured wave elevation is identified in sloshing and piston states.