Laboratory study of wind- and mechanically-generated water waves.

摘要

Evolution of the wind-wave spectrum and their down-shifting mechanism were studied experimentally, in both the small and the large wind-wave facilities at the Ocean Engineering Laboratory. We made observations in order to investigate: (1) the generation and evolution of the short wind waves (small facility); (2) the infuence of changes in the wind speed on the evolution of wind waves (small facility); (3) the long-time evolution of a non-linear wave train, breaking in groups, including the influence of wind pumping (large facility).; We statistically analyzed the first observational results under the assumption of a stationary and Gaussian random process. We revealed that within the short fetch (0-7 m), there exist three stages: (1) initial generation of waves due to a coupled viscous shear instability (initial wavelet); (2) sudden down-shifting of energy to form a rhombic wave pattern, created possibly by the large scale turbulence; (3) growth and down-shifting of the wind-wave spectrum as a result of exchange of energy among energetic wave components in a deterministic fashion.; We revealed from the second observational results that wind waves responded to the sudden changes in the wind speed with two distinct time scales, {dollar}Delta tsb1{dollar} and {dollar}Delta tsb2{dollar}. The former is associated with the adjustment of wind waves to achieve a quasi-balance with the new wind speed and the latter is associated with the energy convection in a limited fetch. Some additional findings are: (1) there exists a proportionality between wind pumping and energy dissipation, which leads to a global law between wave height and wave period such as Toba's 3/2 law, satisfied by waves in a quasi-balance with wind; (2) wind wave energy convects along a ray.; In the third observational study we successfully conducted an experiment of the sideband wave system with a variety of parameter combinations (ak, {dollar}delta f/f{dollar} and {dollar}asb{lcub}c{rcub}/asb0{dollar}). We found that: (1) the breaking event causes a permanent down-shifting, while without the breaking event the wave train demodulates; (2) the existence of wind did not change the characteristics of the side band growth; (3) the inclusion of higher frequency waves is necessary for describing the long time evolution of the wave system as demonstrated by the comparison of the experimental results with the numerical solution of the weakly non-linear theory.; The wind wave evolution may be governed by deterministic non-linear processes with a strong coupling of the action of wind and the energy dissipation due to wave breaking.