Overview of Project Findings
The first joint reference paper for the Ocean Energy Systems (OES) Task 10 Wave Energy Converter modeling verification and validation group was recently published at the European Wave and Tidal Energy Conference (EWTEC), held at Cork (Ireland) in late August 2017.

The group is established under the Ocean Energy Systems Technology Collaboration Programme (OES) within the International Energy Agency. OES was founded in 2001 and Task 10 was proposed by Bob Thresher (National Renewable Energy Laboratory) in 2015 and approved by the OES Executive Committee in 2016. The kickoff workshop took place in September 2016, wherein the initial baseline task was defined.

A heaving sphere (Figure 1) was chosen as the first test case. The team of project participants simulated different numerical experiments, such as heave decay tests and regular and irregular wave cases. The simulation results are presented and discussed in the EWTEC publication. A brief overview on some of the significant findings is given below.

Figure 1 Illustration of the heaving sphere, modeled during the project.

Overview of Project Findings
Good agreement for all investigated load cases was found among linear and weakly non-linear models respectively. Differences between linear and weakly nonlinear codes were evident in terms of motion amplitude and power performance predictions. This underlines the importance of including weak nonlinearities in the numerical analysis of surface piercing wave energy systems. Especially for situations with larger waves and/or motion amplitudes.
 

Figure 2 Free-decay response in heave for 5.0 m initial displacement and breaking radiated waves during large amplitude heave motion (from NREL SNL CFD simulation)

Interesting simulation results were observed for the heave free decay simulation of the surface piercing sphere. For the case with 5.0-m initial displacement, there is a clear separation between linear codes and codes with weak nonlinearities. The group that is leading in phase consists of: DSA NLIN, DTU NLIN, EDRMedeso NLINS, IST, MARIN NLINS, NREL SNL NLIN, Navatek NLINFK, WavEC NLINS and Glosten. All these weakly nonlinear codes consider the instantaneous body position for calculating the hydrostatic restoring force. The influence of this effect is most prominent for large amplitude motions. Because the water plane area of the sphere will change with its position relative to the mean sea level, this geometric nonlinearity will have the largest influence during the first, large oscillations of the sphere. From 0 s to about 20 s, differences in motion amplitude can be observed between purely linear codes and the codes with weak nonlinearities.

The third group that is evident in the heave response for the relatively large initial displacement of 5.0 m are the codes with strong nonlinearities: NREL SNL CFD, PU, KTH, and Chalmers. The phase of the solution from these three models is close to the phase of the codes with weak nonlinearities. However, these three codes predict a larger motion amplitude than the rest of the group. In the three codes with strong nonlinearities, instead of using a linear radiation assumption like the weakly nonlinear codes, they can capture higher order wave radiation effects, which are largely influenced by the instantaneous sphere cross section area at the water surface, particularly at the first oscillation of the free-decay case with the 5.0-m initial displacement. In addition, during the first oscillation, the NREL SNL CFD solution predicts breaking of the radiated wave around the sphere (Figure 2) an effect that can only be captured by CFD models. It is also worth mentioning that the relatively good agreement between the time-domain potential flow code from Chalmers and the CFD solutions (NREL SNL CFD and PU) suggests that the effect of fluid viscosity and wave breaking on the body response plays a relatively small role in the analyzed scenario.

This analysis outlines the value of comparing numerical solutions from models with different levels of fidelity. It allows for the assessment of the underlying model assumptions and simplifications on the predicted response of the wave energy system. It helps to characterize shortcomings and advantages of different modelling techniques which enables the industry to select fit-for-purpose numerical modelling approaches for a given simulation task.

Project Outlook
The project is about to launch into its second phase, which will incorporate a validation aspect by including experimental test data from a heaving, surface piercing wave energy system that was tested at the US Navy MASK basin in 2016, during a test campaign led by Sandia National Laboratories. With this new project phase the group is hoping to further the confidence and accuracy of numerical models and to identify future research needs for computational tools and validation