Experimental study on the evolution of mechanically generated waves mixed with space non-uniform wind-driven waves

This study experimentally investigates the evolution of mechanically generated waves mixed with wind-driven waves. The pre-existing wind-driven waves are time stable and space non-uniform during the tests. Empirical formulations for fetch laws of dimensionless wave energy and peak frequency of wind-driven waves at the laboratory scale are given. Experimental results show that the characteristics of mixed waves are affected by the variations in wind speed, fetch, wave period, and wave height. At small fetches, both waves are generally inhabited after mixing. With increasing fetch, the energy of the generated waves is enhanced at a large wave steepness level. When the frequency gap between the two wave types is small, mixed waves tend to have a chaotic wave evolution. However, the changes in the two types of waves are small when the frequency gap between the two waves is large. A frequency lock-in phenomenon of the wind-driven waves at high-order waves of the generated waves is found in the regular wave tests at large wave steepness levels. Such a phenomenon does not exist in present irregular wave tests. With increasing fetch, the spectrum of mixed waves downshifts at the beginning and upshifts at a large fetch.