energies Article Ocean Energy Systems Wave Energy Modeling Task 10.4: Numerical Modeling of a Fixed Oscillating Water Column Harry B. Bingham 1, * , Yi-Hsiang Yu 2 , Kim Nielsen 3,4 , Thanh Toan Tran 2 , Kyong-Hwan Kim 5 , Sewan Park 5 , Keyyong Hong 5 , Hafiz Ahsan Said 6 , Thomas Kelly 7 , John V. Ringwood 6 , Robert W. Read 1 , Edward Ransley 8 , Scott Brown 8 and Deborah Greaves 8   Citation: Bingham, H.B.; Yu, Y.-H.; Nielsen, K.; Tran, T.T.; Kim, K.-H.; Park, S.; Hong, K.; Said, H.A.; Kelly,T.; Ringwood, J.V.; et al. Ocean Energy Systems Wave Energy Modeling Task 10.4: Numerical Modeling of a Fixed Oscillating Water Column. Energies 2021, 14, 1718. https://doi.org/10.3390/en14061718 Academic Editor: Madjid Karimirad Received: 31 January 2021 Accepted: 16 March 2021 Published: 19 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Mechanical Engineering, Technical University of Denmark (DTU), DK-2800 Kgs. Lyngby, Denmark; rrea@mek.dtu.dk 2 National Renewable Energy Laboratory (NREL), 15013 Denver West Parkway, Golden, CO 80401, USA; Yi-Hsiang.Yu@nrel.gov (Y.-H.Y.); ThanhToan.Tran@nrel.gov (T.T.T.) 3 Ramboll Group A/S, Hannemanns Allé 53, DK-2300 Copenhagen S, Denmark; KIN@ramboll.com 4 Department of Civil Engineering, Aalborg University (AAU), Thomas Mann Vej 23, 9220 Aalborg Ø, Denmark 5 Korea Research Institute of Ships and Ocean Engineering (KRISO), 1312-32 Yuseong-daero, Yuseong-gu, Daejeon 34103, Korea; kkim@kriso.re.kr (K.-H.K.); sewanpark@kriso.re.kr (S.P.); khong@kriso.re.kr (K.H.) 6 Center for Ocean Energy Research, Maynooth University, W23 F2H6 Co. Kildare, Ireland; hafiz.said.2020@mumail.ie (H.A.S.); john.ringwood@mu.ie (J.V.R.) 7 Center for Renewable Energy, Dundalk Institute of Technology, A91 K584 Dundalk, Ireland; thomas.kelly@dkit.ie 8 School of Engineering, Computing and Mathematics, University of Plymouth, Plymouth PL4 8AA, UK; edward.ransley@plymouth.ac.uk (E.R.); scott.brown@plymouth.ac.uk (S.B.); deborah.greaves@plymouth.ac.uk (D.G.) * Correspondence: hbb@mek.dtu.dk Abstract: This paper reports on an ongoing international effort to establish guidelines for numerical modeling of wave energy converters, initiated by the International Energy Agency Technology Collaboration Program for Ocean Energy Systems. Initial results for point absorbers were presented in previous work, and here we present results for a breakwater-mounted Oscillating Water Column (OWC) device. The experimental model is at scale 1:4 relative to a full-scale installation in a water depth of 12.8 m. The power-extracting air turbine is modeled by an orifice plate of 1–2% of the internal chamber surface area. Measurements of chamber surface elevation, air flow through the orifice, and pressure difference across the orifice are compared with numerical calculations using both weakly-nonlinear potential flow theory and computational fluid dynamics. Both compressible- and incompressible-flow models are considered, and the effects of air compressibility are found to have a significant influence on the motion of the internal chamber surface. Recommendations are made for reducing uncertainties in future experimental campaigns, which are critical to enable firm conclusions to be drawn about the relative accuracy of the numerical models. It is well-known that boundary element method solutions of the linear potential flow problem (e.g., WAMIT) are singular at infinite frequency when panels are placed directly on the free surface. This is problematic for time-domain solutions where the value of the added mass matrix at infinite frequency is critical, especially for OWC chambers, which are modeled by zero-mass elements on the free surface. A straightforward rational procedure is described to replace ad-hoc solutions to this problem that have been proposed in the literature. Keywords: wave energy; experimental measurements; numerical modeling; simulation; boundary element method; computational fluid dynamics 1. Introduction Wave energy converters (WECs) represent a small, but potentially significant, segment of the global renewable energy mix. In order to be competitive with offshore wind or solar Energies 2021, 14, 1718. https://doi.org/10.3390/en14061718 https://www.mdpi.com/journal/energies