Experimental Investigation into the Air Compressibility Scaling Effect on OWC Performance and Wave Height

Abstract

Wave energy converter arrays have the potential to provide coastal protection in addition to generating power from incoming waves. As part of a wider experimental study to investigate dual-use applications, this paper presents the results of wave flume testing conducted to analyse the performance characteristics of a single, generic Oscillating Water Column (OWC) device, in preparation for the next phase of study that will focus on multi-device arrays. The specific focus of this flume testing was to characterise parameters such as Response Amplitude Operator (RAO), Capture Width Ratio (CWR) and Phase Response, as well as the device’s effect on the local wave field.

A potential limitation when using scaled experimental results for OWCs are the differing scaling factors that should be applied to the device’s submerged volume (λ³) and air volume (λ²) which, together with the Power Take Off (PTO) damping, can greatly affect the air-spring stiffness experienced within the OWC.

A subset of 34 tests (out of a total 134) were conducted under monochromatic wave conditions with wave heights of 20 mm or 40 mm and wave periods ranging from 1.2 s to 2.2 s. In these tests the PTO damping was varied by adjusting the orifice diameter, while the air volume was varied via an adjustable auxiliary air chamber. Results show that for the smaller orifice diameters (i.e., higher damping) tested, air-spring stiffness played a significant role and counterintuitively increased with increased air volume.

Effects of the single OWC device on the wave field within the flume were also investigated. Results revealed that while there is a marked difference when comparing the OWC to an identically-shaped blockage, there was no significant measurable difference in the wave height change observed for all the damping and air volume parameter settings that were modelled, despite a general trend when comparing to the empty flume.