MS Plan B Defense: Viscous Damping Determination For Dynamic Modelling Improvement Of A Floating Oscillating Surge Wave Energy Converter (FOSWEC)

Jesse Gray
Master’s Student
Department of Ocean and Resources Engineering
University of Hawai’i at Mānoa

**This defense will be held both in person (Holmes 287) and over Zoom**
Meeting ID: 941 6465 8935
Passcode: JesseMS
https://hawaii.zoom.us/j/94164658935

Wave energy converters (WEC) have not yet converged on an optimal archetype due to varying and unsteady design challenges of the ocean environment. Computational Fluid Dynamics (CFD) can provide necessary characterization of hydrodynamic parameters for WEC devices, but different CFD methods can vary in accuracy and required computational capacity. OrcaFlex, a dynamic analysis software for offshore marine systems, utilizes the Boundary Element Method (BEM) to simulate fluid-body interaction between waves and vessels. The software is computationally less intensive than other CFD models allowing for quick, efficient prototyping. However, OrcaFlex neglects viscosity effects leading to model inaccuracies for WECs dominated by viscous forces.

This report aims to use experimental data obtained through testing at The O.H. Hindsdale Wave Research Laboratory to augment an OrcaFlex model of the The Floating Oscillating Surge Wave Energy Converter (FOSWEC) and evaluate the accuracy of BEM solvers for the development of WECs. The FOSWEC is a device constructed by Sandia National Laboratories and consists of two hinged flaps connected to a floating platform. To generate electricity, waves excite the flaps and transfer momentum about a hinged axis connected to a power take-off. Testing at the Hindsdale Wave Basin consisted of a series of free decay, regular wave and irregular waves. These tests were compared to a corrected OrcaFlex model for validity.

The uncorrected OrcaFlex model is accurate outside the natural frequency of the flap but overestimated the response at the natural frequency (~2.7s). Incorporation of the derived viscous damping coefficients were found to correct the model’s response around the natural frequency and bring the frequency dependent amplitude response in agreement with experimental testing. This result was confirmed with comparisons of spectral irregular waves. Overall, it was found that OrcaFlex can be a useful tool for rapid analysis of WEC systems if the viscous properties are properly characterized.