Chapter 8 - Conventional Multiple Degree-of-Freedom Array Models

Abstract

In general it is possible to extend modelling techniques that are used for modelling single wave energy converters (WECs) to model WEC arrays; it is simply correctly defining the geometric layout and degrees-of-freedom. Indeed, these multiple degree-of-freedom array models are the most common method used for modelling WEC arrays. However, in addition to any issues associated with the particular modelling technique, there are additional issues that need to be considered when modelling WEC arrays. For models based on linear potential flow theory, where the hydrodynamic coefficients are generated using a boundary element method (BEM), it is important to recognize that the computational effort increases approximately with the square of the number of degrees-of-freedom. In addition, in a WEC array model it is also important to model all the degrees-of-freedom of each WEC (for single WECs the nongenerating modes are often legitimately ignored). Another issue with these models is that the number of frequency components required to accurately generate the impulse response function also increases as does the required duration of the impulse response function. Consequently, the models are typically limited to small arrays of up to about 10 devices. In computational fluid dynamics (CFD) models, the computational effort does not increase significantly with the number of WECs, but with the volume of fluid that needs to be modelled. Thus, depending on the spatial extent of the WEC array this could result in a significant increase in the computational requirements. However, perhaps more significantly, internal numerical dissipation could be an issue for some CFD array models because it would become difficult to separate the WEC array interactions from the numerical dissipation.