Cui, L.Spinks, N.Sergiienko, N.Y.Tothova, D.Leontini, J.S.Cazzolato, B.S.Manasseh, R.2025-10-082025-10-082025Ocean Engineering, 2025; 333:121432-1-121432-110029-80181873-5258https://hdl.handle.net/2440/147696Wave-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.en© 2025 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Helmholtz resonator; Wave energy converter; Oscillating water column; Potential flow theoryJournal article10.1016/j.oceaneng.2025.121432739766Sergiienko, N.Y. [0000-0002-3418-398X]Cazzolato, B.S. [0000-0003-2308-799X]