Numerical Investigation of Irregular Breaking Waves for Extreme Wave Spectra Using CFD

Aggarwal, Ankit (Norwegian University of Science and Technology) | Chella, Mayilvahanan Alagan (Norwegian University of Science and Technology) | Bihs, Hans (Norwegian University of Science and Technology) | P'akzodi, Csaba (SINTEF Ocean) | Berthelsen, Petter Andreas (SINTEF Ocean) | Arntsen, Øivind A. (Norwegian University of Science and Technology)

OnePetro 

ABSTRACT

Offshore structures are exposed to irregular sea states. It consists of breaking and non-breaking waves. They experience breaking wave loads perpetually after being installed in the open ocean. Thus, the study of wave breaking is an important factor in the design of offshore structures. In the present study, a numerical investigation is performed to study breaking irregular waves in deep water. The irregular waves are generated using the Torsethaugen spectrum which is a double-peaked spectrum defined for a locally fully developed sea. The Torsethaugen spectrum takes both the sea and swell waves into account. Thus, the generated waves can be very steep. The numerical investigation of such steep breaking waves is quite challenging due to their high wave steepness and wave-wave interaction. The present investigation is performed using the open-source computational fluid dynamics (CFD) model REEF3D. The wave generation and propagation of steep irregular waves in the numerical model is validated by comparing the numerical wave spectrum with the experimental input wave spectrum. The numerical results are in a good agreement with experimental results. The changes in the spectral wave density during the wave propagation are studied. Further, the double-hinged flap wavemaker is also tested and validated by comparing the numerical and experimental free surface elevation over time. The time and the frequency domain analysis is also performed to investigate the changes in the free surface horizontal velocity. Complex flow features during the wave propagation are well captured by the CFD model.

INTRODUCTION

Offshore wind turbines are exposed to the extreme irregular sea states. Extreme waves exert extreme hydrodynamic loads on the substructures. Thus, the study of irregular breaking waves is very important in the design of offshore wind turbines. Several experimental and field investigations have been performed in the past to study extreme waves. Such spectra exhibit two peaks, due the presence of swell and wind waves. Ochi and Hubble (1976) carried out a statistical analysis of 800 measured wave spectra at the North Atlantic Ocean. They derived a six- parameter double-peaked spectrum. The spectrum is composed of two parts: one which primarily includes the low frequency wave components and second which contains the high frequency wave components. Each part of the wave spectrum is represented by three parameters. The six-parameter spectrum represents almost all stages of the sea condition associated with a storm. Guedes and Nolasco (1992) analysed wave data from the North Atlantic and the North Sea and proposed a four-parameter double-peaked spectrum. This double-peaked spectrum was formulated by superimposing individual spectral components of the JONSWAP type single-peaked spectrum. Torsethaugen (1996) used a similar approach of combining two individual JONSWAP spectra for different frequency ranges, but instead of averaging he used other parameters of the JONSWAP spectrum. Violante-Carvalho et al. (2004) studied the influence of swell waves on wind waves by using buoy data measurements in deep water in the South Atlantic sea. Other researchers have also made efforts in this direction to study the double-peaked spectra (Masson (1993); Dobson et al. (1989)). Pakozdi et al. (2015) performed laboratory experiments with breaking irregular waves using the Toresthuagen spectrum to measure the global impact loads on offshore structures. Their study highlighted the importance of double-peaked spectra for a better representation of extreme sea states.