On the early stages of wind-wave generation under accelerated wind conditions

Abstract

a b s t r a c tThe energy and momentum transfer between the atmosphere and the ocean has typically been studied for conditions where the waves have almost or already reached a local equilibrium with a uniform wind. The purpose of this work is to investigate the early stages of the generation of waves under non-stationary wind conditions and to describe the momentum and energy exchange at the airwater interface for non-equilibrium wind conditions. Some experiments with a characteristic wind acceleration were conducted in a large wind-wave facility at the Institut Pythéas (Marseille-France). Momentum fluxes were estimated through hot-wire anemometry and, the free surface displacement was measured along the wave tank by resistance and capacitance wire probes. Wind speed and water elevation measurements were acquired at a high sampling rate. During the experiments, the wind speed was increased with a constant acceleration over time, reaching a constant maximum intensity of 13 ms −1 . Under accelerated wind conditions, the degree of wave field development associated with a certain value of wind speed depends on the wind acceleration. Accordingly, once the rough flow regime is established, the drag coefficient values associated with a certain wind speed also vary depending on wind acceleration. It was observed that higher wind speed is needed to reach a rough flow regime as the wind acceleration increases. Also, the momentum transfer is reduced as wind acceleration increases. Under the rough flow regime, a less developed wave field induces a higher increase of drag coefficient with wind speed.

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

“…That last fact is proof that acceleration magnitude plays a role in limiting wavefield growth under accelerated wind conditions. Wave growth efficiency differences were shown previously in Robles-Diaz et al (2019), where it was observed that under moderate-low and moderate acceleration wind conditions (experiments 2 and 3), the efficiency of the wavefield capturing the available momentum was the highest.…”

“…Apart from the differences in the amount of moment induced by the wind acceleration, the waves showed higher efficiency capturing the air momentum under low, moderate-low, and moderate wind acceleration conditions (experiment 1, 2, and 3), associated with the wavefield characteristics (Robles-Diaz et al, 2019). One characteristic that could induce this high efficiency is the higher wave steepness observed under low, moderate-low, and moderate acceleration wind conditions than higher acceleration wind conditions, as reflected in higher high-frequencies energy levels.…”

“…That also implies more turbulence and more amount of available momentum. In Robles-Diaz et al (2019) it was shown that once the rough flow conditions are established, the wavefield generated under moderate acceleration wind conditions induces the increase of the available momentum under a certain wind speed.…”

“…Under accelerated wind conditions, the surface roughness and wave characteristics evolve rapidly with wind speed, and their relation to wind stress is essential to understand the momentum transfer process better (Robles-Diaz et al, 2019). This section shows the relationship between the wind stress in the form of drag coefficient, and the wave phase speed and wave age.…”

“…A detailed description of how, during the early stages of wind-wave generation, the wind acceleration magnitude affects the air turbulent boundary layer evolution, the momentum availability, and the wind-wave growth was given in Robles-Diaz et al (2019). The present analysis aims to provide a detailed description of the effect of wind acceleration in the momentum transfer through the air-sea interface by analyzing the wind-wave spectral shape.…”