Effect of the wind acceleration magnitude during the first stages of the wind-wave generation process

Abstract

A comprehensive analysis of the wavefield evolution under accelerated wind conditions provides an essential contribution to understanding the wind-wave generation process. A set of experiments was carried out in a large wind-wave facility where it is possible to reproduce high wind speed conditions to study the wind acceleration effect in the wind-wave development.The facility was equipped with high-frequency sampling devices that provide accurate air turbulence and water surface displacement measurements. From this deployment, it was possible to describe the evolution of the wave characteristics under different magnitudes of constant wind acceleration in detail. This study analyzes the wind acceleration effect in the early stages of windwave generation and evolution. The increase of spectrum energy saturation level and the downshift of the peak frequency are processes associated with the spectral shape evolution under low acceleration wind conditions. Under high wind acceleration conditions, the spectral shape did not vary with wind speed and fetch. Despite under low acceleration wind conditions, the wavefield is more developed than under high acceleration wind conditions; there was no direct relation between wind acceleration and the wavefield efficiency to grow. Besides, during these early instants, it was observed that a more developed wave field, associated with low acceleration wind conditions, could slow down the increase of drag coefficient with wind speed.