Estimation of Water Column Surface Displacement of a Fixed Oscillating Water Column by Simple Mechanical Model with Determination of Hydrodynamic Parameters via Physical Experimental Model

Abstract

For the first time, hydrodynamic parameters such as damping coefficient, added mass, and the natural and resonant frequency of a bottom-fixed oscillating water column (OWC) are determined for various underwater chamber openings and orifice sizes by performing physical experimental free decay tests and utilizing logarithmic decrement method. The damping coefficient of the system is found to be exponentially and linearly decaying as the orifice size increases and underwater opening decreases, respectively. The determined damping coefficient also includes the viscous losses under the front wall opening of the OWC system. The water column within the chamber is modeled as a single degree of freedom (SDOF) mechanical system, and obtained damping and added mass values are substituted into the model. The surface water column displacements are determined by both solving the developed model and performing physical experiments under the generated monochromatic incident waves. A remarkable agreement is found between the analytical model results and the physical experimental data when the water column surface acted approximately as a rigid body. However, when a significant amount of sloshing occurs in the chamber, model results diverged from the experimental values. It was observed that relatively high power take-off (PTO) damping and small chamber openings substantially restrain the sloshing motion otherwise inherently generated in the chamber. In addition, for all openings and orifice sizes used in this study, the determined resonant frequencies of the OWC matched well with those obtained from the experimental data.