Directional Energy Transfers in a Deep-Water, Extreme Ocean Wave

Seminar | September 19 | 12-1 p.m. | 3110 Etcheverry Hall

 Dylan Barratt, DPhil Student, Department of Engineering Science; University of Oxford, UK

 Department of Mechanical Engineering (ME)

Abstract: Steep, focusing waves can experience fast and local nonlinear evolution of the spectrum due to wave-wave interactions resulting in energy transfer to both higher and lower wavenumber components. The shape and kinematics of a steep wave may, thus, differ substantially from the predictions of linear theory. We have investigated the role of nonlinear interactions on group shape for a steep, narrow-banded, directionally-spread wave group focusing in deep water using the fully-nonlinear potential flow solver, OceanWave3D. The initially narrow-banded amplitude spectrum exhibits the formation of sidelobes at angles of approximately 35 degrees to the spectral peak during the simulated extreme wave event, occurring in approximately 10 wave periods, with a preferential energy transfer to high-wavenumber components. The directional energy transfer is attributed to resonant third-order interactions with a qualitative resemblance to the resonant interactions of a degenerate quartet. The rate of growth of resonant components has also been calculated in terms of wave action density and found to agree well with the faster ‘dynamical’ time-scale associated with nearly-resonant interactions and the Zakharov equation rather than the slower ‘kinetic’ time-scale associated with exactly-resonant interactions and the kinetic wave equation. The engineering implications of energy transfer to oblique high-wavenumber components during an extreme wave event are also discussed.

Biography: Dylan Barratt is currently completing a DPhil degree (10/2017-present) in the Department of Engineering Science at the University of Oxford, UK, with a EPSRC Studentship from the UK government. His thesis focuses on the local and global features of resonant third-order interactions amongst surface gravity waves and relies upon fully-nonlinear potential flow simulations as a means of analysis. Before arriving at Oxford, he was employed as a scientific assistant at ETH Zurich, Switzerland, in the Institute for Energy Technology (07/2015-09/2017) where he designed and built a model wind turbine test facility to measure the unsteady aerodynamic loads of a floating offshore wind turbine in collaboration with Hitachi Ltd. He obtained an MSc in mechanical engineering from Seoul National University, Korea, where he completed a thesis on the internal cooling of turbine blades as a member of the Turbomachinery Laboratory (09/2013-07/2015) with sponsorship from the Oppenheimer Memorial Trust of South Africa. He also holds a BSc in mechanical engineering (01/2009-12/2012) from the University of the Witwatersrand, South Africa, receiving the William John Walker Gold Medal as the top graduate in the School of Mechanical, Industrial and Aeronautical Engineering., 510-643-2591