Generation method of wind waves under long-fetch conditions over a broad range of wind speeds

Abstract

It is important to develop a wave generation method for extending the fetch in laboratory experiments, because current laboratory studies are limited to fetch shorter than 100 m. Two wave generation methods are proposed for generating wind waves under long-fetch conditions in a wind-wave tank using a programmable irregular-wave generator. The first method is the spectral-model-based wave-generation method (SBWGM), which is appropriate at normal wind speeds for extending the fetch. The SBWGM also can be used at extremely high wind speeds if we know the spectral shape. In SBWGM, a conventional model of the wind-wave spectrum is used for the movement of the programmable irregular-wave generator. The second method is the loop-type wave-generation method (LTWGM), which can be used at wide range of wind speeds and is especially appropriate to be used at extremely high wind speeds, where the spectral shape is unknown. In LTWGM, the waves whose characteristics are most similar to the wind waves measured at the end of the tank are reproduced at the entrance of the tank by the programmable irregular-wave generator to extend the fetch. Water-level fluctuations are measured at both normal and extremely high wind speeds using resistance-type wave gauges. The results show that SBWGM can produces wind waves with a fetch over 500 m, but only at normal wind speeds. However, LTWGM can produce wind waves with long fetches exceeding the length of the wind-wave tank across a broad range of wind speeds, but considerable time is required to produce wind waves at long-fetch conditions, i.e. fetch over 500 m. It is observed that the wind-wave spectrum with a long fetch reproduced by SBWGM is consistent with that of the modelled wind-wave spectrum, although the generated wind waves are different from those in the open ocean because of the finite width of the tank. In addition, the fetch laws with significant wave height and period are confirmed for wind waves under long-fetch conditions. This implies that the ideal wind waves under long-fetch conditions can be reproduced using SBWGM with the programmable irregular-wave generator.