Session: 06-16-01 Wave Mechanics, Modeling and Wave Effects - I

Submission Number: 175496

Using Relaxation Zones to Force High-Order Spectral (HOS) Method for Modelling Extreme Waves 

HOS method remains one of the most efficient methods for simulating fully nonlinear extreme ocean waves. Typically, these models are run in the so-called ‘ocean’ mode that starts from the prescribed linear surface elevation and velocity potential at a given time instance, and is subsequently time marched by solving the pair of free surface boundary conditions. To avoid the introduction of error waves due to the mismatch between the initially prescribed linear solution and the fully nonlinear free surface boundary conditions, the nonlinear terms of the boundary conditions are ramped gently to a full strength allowing the nonlinear corrections to the free surface elevation and potential to develop gradually. While this method works extremely well, for some wave simulation problems such as waves over a variable bathymetry and waves interacting with non-uniform currents, the Ocean mode does not work due to the imposed spatial periodicity of the wave field. This is typically overcome by running the HOS models in a so-called ‘Numerical Wave Tank (NWT)’ mode by incorporating a relaxation zone where a known analytical solution is spatially ramped from quiescent conditions while a numerical beach is utilised at the downstream end of the domain to absorb the waves to quiescent conditions. This maintains the spatial periodicity required for the HOS models but re-introduces the problem of mismatch between fully nonlinear boundary conditions and the limited nonlinear analytical solutions imposed in the relaxation zone. Although a second order analytical solution is adequate for most sea states, this is no longer sufficient when highly nonlinear fully random sea states are simulated where multiple near breaking waves can be present in the domain; this includes the relaxation zone as well. In the present work the ‘ocean’ mode and the ‘NWT’ mode have been combined to solve this issue; the fully nonlinear solution from the ‘ocean’ domain is imposed directly at the relaxation zone of the ‘NWT’ domain. Through comparison of higher order statistical moments across the domain in random sea states, the superiority of the present method is demonstrated.

Presenting Author: Mohamed Latheef Universiti Teknologi Petronas

Presenting Author Biography: Dr Mohamed Latheef is a Senior Lecturer at Universiti Teknologi Petronas. Prior to this he was a Post-doctoral Research Associate at Imperial College London, where he obtained his PhD.

Authors:

Mohamed Latheef Universiti Teknologi Petronas
Marios Christou Imperial College London
Constantin Craciunescu Imperial College London