LiftWEC Deliverable D3.2: Preliminary assessment of computational capability report
Creators: name: Olbert, Gerrit
name: Scharf, Martin
Publisher: name: Zenodo
Year: 2020
Subjects: LiftWEC
Wave Energy
Wave Energy Converter
Lift-Based Wave Energy Converter
H2020 Deliverable

description: This deliverable shows a preliminary assessment of the performance of the employed numerical tools for the purpose of hydrodynamic modelling within the LiftWEC project. Two modelling approaches and their respective implementation are presented and their theoretical background is discussed. This background is required in order to assess the impact of certain modelling assumptions on the results of the numerical simulations. Subsequently, the numerical tools are applied to three different scenarios of hydrofoils interacting with one or multiple fluids. As the hydrodynamic performance of a lift-based wave energy converter (WEC) largely depends on its ability to capture wave energy and transform it into a lift force accelerating it in tangential direction of its rotary path while maintaining a low drag, the investigation of the foil near-flow field as well as its direction interaction with the free surface was deemed an appropriate starting point for a preliminary assessment. Due to the limited number of research groups which have hitherto worked on the concept of lift-based wave energy converters, only few sources of data for validation purposes are available. Therefore, two of three validation studies presented in this document are based on experimental reference cases for foils in straight flight. This approach was chosen to separate numerical modelling uncertainties due to based foil hydrodynamics from modelling uncertainties due to the circular motion of the foils. The third validation study is based on an experimental investigation in which waves radiated by a cyclorotor operating in still water were measured. Except for the first study, for which measurement data of lift and drag forces are available, the foil induced wave field at the free surface interface was used as a metric to compare numerical and experimental results. For most cases investigated, the RANS-based numerical method could produce good agreement with the absolute values recorded during the experiments. Uncertainties related to laminar-turbulent transition modelling and prediction of flow separation are highlighted and possible measures for reducing the uncertainty in future simulations are discussed. The results show that the panel method <em>pan</em>MARE is also capable of reproducing the relative changes of radiated wave height of a cyclorotor rotating in still water. In terms of absolute values, the results of the first two studies show that the drag force is underpredicted. As this is assigned to the pressure-induced drag, a correction is implemented based on boundary layer theory. The results of the third study however, obtained while applying this correction, indicate that the high turbulence levels in the foil wake reduce the impact of the boundary layer. Based on the assumptions on which the potential flow theory method is based, separation effects are not captured as can be seen in several results. Empirical corrections for this are discussed for future improvement of the method. For all three validation scenarios, short setup studies are conducted. The learning derived from these studies is summarized in the respective sections. Differences between numerical results and experimental measurements are discussed regarding their possible origin and regarding their impact on an accurate simulation of lift-based WEC hydrodynamics.
description: This work was produced as part of the LiftWEC Project. This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 851885. This output reflects the views only of the author(s), and the European Union cannot be held responsible for any use which may be made of the information contained therein.