Two-phase flow simulations of fixed 3D oscillating water columns using OpenFOAM: A comparison of two methods for modeling quadratic power takeoff

This study examines the performance of a numerical method that introduces an artificial sink term to the Reynolds-averaged Navier Stokes equations to simulate the flow through an orifice used as a quadratic Power Takeoff (PTO) for Oscillating Water Columns (OWCs). The method replaces the quadratic PTO by an artificial Forchheimer-flow region (referred to as the artificial Forchheimer-flow method). The performance of this method was evaluated by making comparisons with the existing experimental results for two OWCs and the numerical results obtained by using air-water two-phase flow simulations of the air flow through an orifice (referred to as the orifice-flow method). The surface elevations, velocity fields and pressure fields obtained by the orifice-flow and artificial Forchheimer-flow methods are compared. To use the artificial Forchheimer-flow method, an equation for specifying the Forchheimer coefficient is provided and the sensitivity of the pneumatic efficiency to the Forchheimer coefficient is discussed. It can be concluded that the artificial Forchheimer-flow method can satisfactorily reproduce the measured pneumatic efficiency, pressure field in the air, the velocity field in the water and the cross-sectional average velocity of the air. Compared to the orifice-flow method, the artificial Forchheimer-flow method can speed up the simulation by at least 25 times.