Abstract
This dissertation focuses on overcoming known challenges and uncertainties in the model testing of wave energy converters. Specifically, it presents novel quantified findings on the effects of degrees of freedom (DOF) on wave energy converter (WEC) performance, and presents methods for quantifying and propagating uncertainty in physical model testing. To provide a holistic perspective, both fundamental numerical theory and practical experimental influences, including friction, nonlinearities, and experimental limitations, are presented and discussed. In the first chapter, regular and irregular waves are run in both numerical and experimental modeling to evaluate the effects of 1) increasing the number of hydrodynamically active bodies from one to two, and 2) increasing the degrees of freedom from heave-only to the full six. It was shown that the one-body heave-only configuration captures more power over a larger range of wave periods than the two-body heave-only configuration, which is contrary to previous works that neglect friction, stiction, and mooring effects. This chapter also shows that increasing the DOF from two-body heave-only to six-DOF results in 40% more normalized power capture below the resonant period and up to 176% less power capture above the resonant period, shifting the whole power curve. This work provides new considerations for interpreting prior research on WECs with a single DOF when their realistic deployment is moored with six DOFs. In the second chapter, a detailed uncertainty analysis is developed to quantify the uncertainty of inputs, propagate them through the Monte Carlo Method, and present expanded uncertainty of WEC performance metrics in regular waves. A novel phase-domain method is presented for WEC power take-off measurement uncertainty. The capture width expanded uncertainty is shown to be up to +/-18%, primarily driven by wave power uncertainty from wave gauge calibrations. This chapter also demonstrates how nonlinearities are significant at WEC resonance, showing the importance of wave height selection for device characterization. In the third chapter, a detailed uncertainty analysis is used to quantify the expanded uncertainty of WEC performance metrics compared to the standard deviation in irregular waves. In the literature, there is a lack of consensus on error metrics and unclear definitions of uncertainty and standard deviation which are discussed and clarified in this chapter. The standard deviation was 2-10 times larger than the expanded uncertainty, a significant difference when presenting error bars and confidence in data. A methodology for time-to-frequency domain Monte Carlo uncertainty propagation is presented. Recommendations are given to inform international standards, as guidelines for uncertainty analysis of model testing in irregular waves are not presently specified and are unique compared to regular waves. The numerical and experimental models, computer-aided drafting files, data, control and processing codes, and videos and photos developed throughout the dissertation are released publicly to support open-access knowledge sharing. These resources have already had a significant impact on the marine energy community, as internal and external collaborators have used them for fundamental research in WEC design, power take-off controls, hydrodynamics, numerical modeling, motion tracking, underwater robot docking, mooring studies, optimization, educational courses and workshops, and more. LUPA has been designated as a U.S. Department of Energy Water Power Technologies Office Signature Project. Their website hosts the suite of open-source data and publications related to LUPA.