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This paper presents the viscous flow predictions for fixed, slender and cylindrical Oscillating Water Columns (OWCs). Comparisons with linear potential solutions indicate nonlinearities in the scattered wave and the internal water surface elevations. The predictions also confirm high-frequency contents and asymmetrical shapes in the forces acting on the water columns. To identify the physical causes of the nonlinearities, various geometries, excitation conditions and mechanisms experienced by the OWCs are considered. This study confirms that a nonlinear wave scattering by the cylindrical structures occurs yet has little implication for the nonlinearities in the internal water surface elevations and the forces. In contrast, the vortex shedding at the lower edges of the OWCs and the nonlinear inertia forcing instigate the nonlinear characteristics of the elevations. The vortex shedding also provokes the nonlinearities in the acting forces. While the influence of the vortex shedding on the second-harmonic air force diminishes, the role of the water column oscillation on the harmonics becomes increasingly important at large amplitudes of internal surface elevation. Given the improved physical understanding, practical implications concerning the guidance on developing an appropriate time-domain modelling and improving the hydrodynamic efficiency of an OWC arise in this study. • Fixed OWCs undergoing forced oscillations and wave excitations are studied. • Viscous flow and linear potential solutions are compared to investigate the OWCs. • Nonlinear inertia forces and vortex shedding cause nonlinear internal elevations. • Vortex shedding generates zeroth and second-harmonics in inline pressures. • Vortex shedding and water column oscillation drive nonlinearities in air forces. [ABSTRACT FROM AUTHOR]

This paper is concerned with the radiated wave and the rotational flow fields generated by the forced oscillations of the water columns within surface-piercing hollow cylinders. The present study utilised Open Foam as the viscous flow solver to numerically model the forced oscillations. An investigation of the radiated waves in the fields near the water columns confirms two distinct nonlinear characteristics. These are progressively reduced amplitude and progressively reduced phase difference. An evaluation of the rotational flow fields around the mouths of the water columns helps explain their origins. This study confirms that the nonlinear characteristics in the near-field are physically caused by the formation and motion of vortex ring at the bottom edge of the cylinder. Interestingly, the progressively reduced amplitude is not evident in the far-field. All these new findings support the confirmation regarding the appropriateness of linear radiation theory and the implication of the vortex motion on the effective draft of a fixed cylindrical Oscillating Water Column (OWC). • Wave radiations generated by forced oscillations of water columns are studied. • Near-field radiated waves have progressive reductions in amplitude and phase difference. • Vortex formation and motion cause nonlinear characteristics in near field. • Far-field radiated waves exhibit insignificant progressive amplitude reductions. • Location of vortex closest to the mouth affects the effective draft of water column. [ABSTRACT FROM AUTHOR]