Physical scale model testing of a flexible membrane wave energy converter: Videogrammetric analysis of membrane operation

In small-scale testing of wave energy converters (WECs), a key focus is on characterising the interdependent relationship between the primary converter and simulated power take-off system. If primary conversion is via the deformation of a flexible material, this task often requires non-contact measurement. In this paper, we introduce the development of an underwater non-contact measurement technique called videogrammetry, and its novel application to characterise the primary converter operation of a flexible membrane WEC. The work was part of Bombora Wave Power's concept validation wave tank tests at 1:15 scale. Details of the WEC and how it works is followed by an in depth description on applying underwater videogrammetry. A qualitative and quantitative analysis of membrane operation in a regular wave case is provided and discussed in terms of absorbed energy and power production. Two data sets are compared in this analysis. One data set is from videogrammetry and the other is airflow measurement data (airflow induced in the system due to membrane deformation converts wave energy to mechanical energy). This comparison quantifies the accuracy of videogrammetry, and also serves to verify airflow measurements that were used to determine performance indicators of the WEC throughout the entire test campaign. The results compare reasonably well. Sources of uncertainty for videogrammetry are discussed and improvements suggested. Preliminary best practices for applying videogrammetry in wave energy experiments are provided.