|Wind wave field adjustment to wind gusts |
Auteur(s): Caulliez G.
Conference: Waves In Shallow Environments 19th International Conference (Barcelona, ES, 2012-04-17)
Over the last decades, significant progress has been made in modelling wave field development by wind observed at sea, based on more elaborated numerical schemes and refined parametrizations of wind energy input and wave dissipation. In such models, the wind wave growth in space or time is generally governed by the average wind speed evaluated at one reference level and the natural wind speed variability is neglected. However, the impact of this assumption is not really known, mainly because of the lack of appropriate observations. The analysis made by Abdalla and Cavaleri (2002) gives an estimate of the statistical wind variability effect on wave growth but does not shed light on the effect of one individual gust. To revisit this question, we report a detailed laboratory investigation aimed at describing the wave field evolution resulting from an isolated gust simulated by an abrupt local wind speed increase. The experiments were conducted in the large Marseille-Luminy wind wave tank for moderate to high wind conditions. At 23 m fetch, a contraction of the wind tunnel section by a convergent profile created a spatial wind speed acceleration over a distance of about 2 m. Downwind, the wind speed, increased by a factor 1.4, was kept constant up to the end of the water tank. The wind wave field development induced by such a "wind gust" was investigated at successive fetches by capacitance wave probes moving along a rail and compared to those observed at similar fetches for homogeneous wind conditions. These observations first revealed that the wind increase induces a significant enhancement of short ripples at the water surface. The growth of the wave energy in the corresponding wave spectral equilibrium range follows strikingly the change in wind speed, while the ratio of the new equilibrium energy level to the initial one depends slightly on wind speed. Compared to small-scale wave roughness, the dominant wind wave field adjusts to wind increase very differently: the processes involved are of more complex nature. At first, the dominant wave steepness which decreases slowly for constant wind conditions, follows the wind speed in a slower way and with a significant space lag. Consistently, the respective dominant wave rms amplitude exhibits a similar evolution with fetch. It is noteworthy that for well-established gravity wave fields, the space relaxation scales which describe this evolution do not depend noticeably on wind, all the curves collapse into a single one when wave quantities are normalized by their value observed just upstream the convergent profile. The wave growth rate observed for the new equilibrium state can be described by the Hasselman et al. (1973) relationship but with an “equivalent’’ shorter fetch since, in such conditions, the dominant wave age decreases drastically. The evolution of the dominant frequency as well as the dominant wavenumber have two distinct stages. First, there is a marked slowdown of the downshift immediately after the wind increase. This stage is then followed by a fast adjustment to the uniform wind regime corresponding to the new wind speed. The spectral wave features related to this downshifting phenomenon are examined extensively and compared with the model results obtained by Annenkov and Shrira (2009).