This document presents the results of a validation study of numerical tools used for hydrodynamic modelling of a cyclorotor-type wave energy converter in the scope of the LiftWEC project. The numerical tools assessed here consists of a high-fidelity tool, based on the Reynolds-Averaged Navier-Stokes (RANS) equation, and a ‘global’ model, based on linear potential flow theory.

According to their level of fidelity and corresponding computational effort, these tools are used for various purposes within the project, such as foil design and investigation of separation phenomena, as well as structural design and assessment of annual energy production (AEP).

Before these tools are applied to their respective purposes, a validation study is conducted to assess the associated level of uncertainty and potentially propose mitigation strategies. For this purpose a series of experimental model tests has been conducted in a quasi-two-dimensional setting in the wave flume of École Centrale de Nantes (ECN). Several of the model test scenarios are replicated numerically and results compared to experimental measurements.

In the first section of this document, a short introduction into the background of the LiftWEC project is given. The principle of lift-based wave energy conversion is explained and references for further reading are provided. In section 2, the fundamental theories which form the basis of the two employed hydrodynamic solvers are explained. This serves to provide an understanding of the source of potential modelling and discretization errors, which are discussed later in the document.

A description of the experimental model tests is given in section 3. Since more detailed descriptions were provided in corresponding reports written by those project partners conducting the experimental reports, only a fundamental overview on test setup and calibration is provided. References to the mentioned reports can be found in this section.

The global model, owing to simplicity, relies on a set of empirically defined coefficients which approximate the behaviour of the WEC hydrofoils in different inflow conditions. A procedure to derive these coefficients from reference data was specifically designed for this study and is presented, including a validation of the method, in section 4. In the subsequent part of this report, section 5, the model is then applied to replicate the physical tests and numerical results obtained for rotor operation in regular waves is compared to experimental measurements. The results indicate that the model is able to reproduce mean forces with relatively good accuracy for most cases. However, when comparing time series showing force fluctuations on a wave-by-wave basis, significant differences are found which require further investigation.

In section 6, a comparison of RANS-based results and experimental force measurements is presented for single and double-foil rotor operation in calm water and in regular waves. Radial and tangential forces are compared in form of time series, mean forces and fundamental amplitudes. Mean forces and fundamental amplitudes in most wave conditions are generally well met, although several exceptions are found. Due to the occurrence of considerable wave reflections in the experiment, it is unclear whether this reduced reliability is to be assigned to the numerical model or the available reference data. A comparison of force signals time series shows strong and high-frequent fluctuations in both experiment and simulation. Apart from reflections, this is likely due to the formation of vortices in the foil wake due to fluctuation of inflow velocity vector. When passing through or in vicinity of vortices, foil circulation shows strong fluctuation, while tangential forces remain largely unaffected for small angles of attack.

Finally, a set of conclusions regarding further improvements and consideration of global model and high-fidelity model is presented in section 7. These are extended by several observations made and recommendations drafted while analysing the experimental data, which might help to improve the quality of a 3D model test series on the LiftWEC concept, which is currently scheduled for late spring of 2022.