Nonlinear Time-Domain Simulation of a Land-Based Oscillating Water Column

Abstract

A two-dimensional time domain, fully nonlinear numerical wave tank (NWT) based on potential theory, the mixed Eulerian-Lagrangian approach, and the boundary element method was developed and applied to land-based oscillating water columns (OWCs) with compressed air inside the chamber. The nonlinear free-surface condition inside the chamber was specially devised to represent both the viscous effect of the water column motion and the pneumatic effect of the time-varying pressure. The latter not only influenced the water column motions, but also generated radiated waves to interfere with incident and reflected wave fields. Both linear and quadratic models are used for the viscous and pneumatic effects. The regular-wave cases are also compared with irregular-wave cases. The developed NWT was verified through comparison with viscous-flow-based numerical and experimental results for both narrow-gap and wide-gap chambers at two different skirt depths, with and without air ducts. All of the NWT simulations correlated well with experimental values for a variety of wave and chamber conditions, so that the developed numerical tool can be practically used for the optimal design of land-based OWCs. The fully nonlinear simulations were also compared with linear simulations. In case that had the same skirt depth and air duct area, the efficiency of the wide-gap chamber was generally higher than that of the narrow-gap chamber over a wider range of wavelengths, due to the larger amount of compressed air and the corresponding higher flow velocities.