Ocean Energy Systems Wave Energy Modeling Task 10.4: Numerical Modeling of a Fixed Oscillating Water Column

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.

Ocean Energy Systems Wave Energy Modeling Task 10.4: Benchmark Study of the DTU OWC Chamber with One-Way Absorption

“…The fundamentals of linear potential flow theory are covered in detail by, for example, the classic textbooks of Newman (2017) and Falnes (2002). The specific application of the theory to OWCtype WECs described here follows closely the presentations found in Bingham et al (2021) and Joensen et al (2023).…”

“…Initial results and conclusions were reported in Wendt et al (2019) for two single degreeof-freedom point absorbers. This was followed by a study of a breakwater-mounted oscillating water column (OWC) device tested at large-scale (1:4) and reported in Bingham et al (2021). In parallel, a set of highly-accurate decay test experiments on a floating sphere were presented by Kramer et al (2021), providing benchmark results with small enough uncertainties to definitively quantify the accuracy of different numerical models.…”

Hydrodynamic analysis of one-way energy capture by an oscillating water column wave energy device

“…We will here briefly summarize the theory, and define the standard approaches used in the calculations. The presentation closely follows that of Bingham et al (2021), which can be consulted for further details.…”

“…A second approach is to treat the interior free surface as a massless element of the boundary, which is predefined to move in a set of generalized modes. This is also implemented in WAMIT, and was applied to an OWC chamber by Bingham et al (2021). The generalized modes approach is more straightforward and is adopted here.…”

“…For the radiation problem, the internal free surfaces of all the internal chambers move together with that of the actual chamber. In order to accurately compute the infinite-frequency added resistance, the method described by Bingham et al (2021) is applied, which is critical to getting accurate results in the (Bingham and Read, 2022). To compare response amplitudes, we perform a harmonic analysis of each time series as described in Section 3.1.1.…”