Validation of the High Order Spectral (HOS) Method for Extreme and Breaking Waves and Coupling of the HOS-Numerical Wave Tank Model with OpenFOAM
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The objective of this study was to investigate the abilities of the open-source potential flow solver HOS-NWT with regard to extreme wave events and deep-water wave breaking. In addition to this, significant effort was focused on coupling the HOS-NWT software and the computational fluid dynamics software (CFD) OpenFOAM, in an attempt to visualize expected breaking events. Model tests of sea-surfaces with very steep irregular waves were performed in a wave tank at the MARINTEK Ocean Laboratory in Trondheim, Norway. The HOS-NWT model was utilized to recreate a fully non-linear simulation of one of these model tests, which was calibrated at the location of the 12th Wave Gauge in a series of 23 gauges. This calibration point, and the area immediately surrounding it, was the location where the analyses in this study were performed. The data from the aforementioned model test was used to validate the HOS-NWT model for extreme waves. The results showed that, on average, there was a 14.42% error between the wave heights that were measured experimentally and those that were simulated in HOS-NWT. After investigating as to why such a level of error was sustained, it was found that for increasing levels of individual wave steepness, the accuracy of the HOS-NWT model decayed. One of the main objectives of this study was to link the HOS-NWT model with an appropriate breaking onset criterion to identify instances of deep-water breaking waves. In total, 8 breaking wave events were known to have been present at different times during experimentation, as a result of high-speed video clips provided by MARINTEK. Four separate criteria were investigated to determine which was most effective in identifying the 8 events captured on video. Ultimately, the Limiting Slope Breaking Criterion [Longuet-Higgins & Smith 1983], which is related to a limiting angle of wave inclination, was selected. This criterion was able to identify the events observed on video at the proper locations and times. The coupling of HOS-NWT and OpenFOAM proved to be successful; the outputs of HOS-NWT were perfectly replicated within different OpenFOAM simulations. However, the visualization of the breaking processes of the identified breaking waves was unsuccessful. It was determined that the absence of the breaking processes was related to different assumptions of the HOS-NWT software, the employed k-ω SST Turbulence Model, among other factors. Future work utilizing an updated version of the HOS-NWT model would likely improve the accuracy of the HOS-NWT model and consequently, the OpenFOAM simulations. Improvements to the HOS-NWT model, which have yet to be publicly released, include appropriate energy dissipation models, which increase the robustness of HOS-NWT in terms of being able to model breaking wave events [Seiffert et al. 2017].
Master's thesis in Offshore Technology