Helmholtz-type resonator with increased tunability and reduced viscous loss with application to wave energy converters

• Reduced size compared to a conventional device. • Reduced viscous loss compared to existing Helmholtz-resonator-type devices. • Easy to tune by changing the position of the insert. • Suitable for protecting the coast against long period swells. • Semi-analytical model verified by experiments and computational methods. Wave-energy converters such as Oscillating Water Columns (OWCs) are designed to resonate at ocean-wave frequencies to extract renewable energy, but the low frequencies of the energetic waves demand large, expensive devices. To resolve this predicament, smaller-sized Helmholtz OWCs (H-OWCs) were designed. Their small sizes are achieved by reducing the cross-sectional area of their entrance sections. However, this leads to viscous losses at the transition between an H-OWC’s entrance and main sections. Moreover, an H-OWC is difficult to tune due to its fixed geometry. This paper proposes a modified H-OWC, called an I-OWC. Its main innovation is to use an “insert” placed in a chamber to reduce the cross-sectional area of the entrance section, avoiding the transition in an H-OWC to reduce vortex damping. Another benefit of the I-OWC is an increased tunability via changing the length of the insert in the chamber. A model of I-OWC is designed semi-analytically; the method is validated by wave flume tests. A conceptual design of a full-scale I-OWC is then proposed. In a broader sense, the design provides an alternative approach to tuning the frequency of a general Helmholtz-type resonating cavity.