Research axis 7 : Sea motions and interactions with marine structures
Coordinators : Aurélien BABARIT (ECN-LHEEA) and Fabrice ARDHUIN (Ifremer-LOS)
LHEEA (Laboratoire de recherche en Hydrodynamique, Energétique et Environnement Atmosphérique), Ecole Centrale de Nantes
LOS (Laboratoire d'Océanographie Spatiale), Ifremer
Unité Recherches et Developpements Technologiques - Hydrodynamique Océanique, Ifremer, Brest
At small scales typically less than a kilometer, the motion of the oceans is mainly accounted by sea states. Their interactions are predominant with any natural system or artificial structure such as vessels, platforms, marine renewable energy converters, dams or piers, etc. The high stakes at play in term of economic and human impacts imply to properly assess and understand the role of processes from small hydrodynamical to geophysical scales.
For now, research fellows in the axis 7 have emphasized five innovative themes at the crossing of their respective domains of competences and interests. All five are directly related to the understanding, measurement and forecast of sea states. For the next three to four years, efforts will mainly be focused on the first three themes, seen as major topics :
(1) dissipation rates :
A better knowledge and understanding of the dissipation rates of energy in various phases of evolution for water waves seem to currently remain a fundamental challenge. Two main distinct questions are raised into this theme concerning :
- swell dissipation
- breaking waves
(2) pressure waves related to wave breaking
Underwater acoustics offer unique monitoring capabilities. Noise at ultra-low frequencies have been linked to the sea surface roughness properties via the double-frequency effect demonstrated by Longuet-Higgins(1950). The possible contribution of breaking waves to this signal may be the source of important errors in the interpretation of bottom pressure signals in terms of directional wave properties.
(3) generation, evolution and climatology of infra-gravity waves
Waves with periods between 25s and 10 minutes have poorly known properties that limit the design of platforms at sea and also the design of the next generation of satellite altimeters. Key issues are the coastal amplification of these waves and their reflection to the open ocean, but also the mechanical response of moored systems to these low-frequency excitations.
Last two long term themes are listed as:
(4) breaking probabilities : their estimation is intrinsically related to the global dissipation rates for sea states, whitecap coverage and fluxes, remote sensing, etc.
(5) short term deterministic prediction : it refers to the capabilities to measure, assimilate and model short term evolution of phase resolved sea states, for the purpose of safety at sea or efficiency of wave energy converters for instance.
Long sought goals refer to the ability to produce reliable numerical simulations for air, sea and solid interactions, for typical length of processes ranging from few centimeters to kilometers. Efforts will be pushed toward the transfer from those small scale simulations to large scale modeling, including oceanic circulation and meteo-oceanic prospects.