Modélisation déterministe des états de mer - Application à la rétrodiffusion d'ondes radar

Researches conducted in this thesis deal with the realistic evolution of wave fields in oceanic conditions. The interest is more particularly focused on various aspects including specificities of a deterministic fully non-linear approach, the wind wave coupling and energy dissipation through white capping, and non-linear simplified modeling for operational use. This work aims to provide efficient and realistic modeling of wave fields so as to enable a better understanding of radar backscattering over oceanic surfaces. Direct applications include short-term forecasts of sea states, or detection of marine targets thanks to remote sensing. A numerical tridimensional model based on a High Order Spectral (HOS) method enables to fully account for non-linear processes in sea states. Some specificities are enlighten and provide a better framework for the efficient use of the method. Among them, the need for reference conditions at early stages of a modeled sea state is underlined. A non-linear method is proposed and its performances evaluated. Momentum transfers associated with wind forcing and dissipation through whitecapping are then proposed for the HOS deterministic approach. Two way coupling between atmospheric and oceanic layers, and dissipation of energy due to wave breaking are achieved thanks to a formulation at use in common stochastic wave models. Forcing and dissipation intervene in the HOS formulation by the mean of pressure fields related to the sea state. The recent set of parametered proposed bay Bidot et al.(2005) is tested and comparatively validated with the WaveWatch3 stochastic wave model. The HOS deterministic approach is finally able to properly take into account the evolution of oceanic wave field under wind forcing and dissipation. The problem of simplified modeling of sea state for the specific purpose of operational use in remote sensing applications is finally studied. A numeric method able to take into account non-linear specificities of the wave field up to a second order in wave elevation and third order in term of phase velocity is proposed and implemented. Its performances are comparatively evaluated with fully non-linear evolutions as references.Les études et développement menés dans ce travail de thèse ont porté sur divers aspects du traitement de l'évolution réaliste des états de mer, notamment sur les spécificités d'une approche déterministe non-linéaire, sur la prise en compte d'un forçage par le vent et de la dissipation dans une approche à phase résolue, et sur un développement à but opérationnel des techniques de modélisations réalistes. En parvenant à une meilleure modélisation des états de mer, ils visent à permettre une meilleure interprétation des images radar rétrodiffusées par la surface de la mer. Les applications concernent entre autres la caractérisation et la prévision court terme en temps réel des conditions de mer, ainsi que la détection de cibles marines. Un modèle numérique tridimensionnel basé sur une méthode High Order Spectral sert de support à l'étude des processus non-linéaires à l'œuvre en conditions océaniques. Une première partie de cette étude permet de mettre en lumière un ensemble de critères nécessaires à la bonne mise en œuvre de cette méthode. On met notamment plus particulièrement en évidence une problématique d'initialisation liée à un besoin de simulations de référence dès les premiers instants de modélisation. Une méthode d'initialisation non-linéaire est proposée et évaluée. La prise en compte d'un forçage par le vent et de dissipation par moutonnement est ensuite proposée. La paramétrisation du couplage rétroactif océan-atmosphère et de la dissipation, en pression, à l'œuvre dans les modèles stochastiques de prévision d'état de mer est adaptée à la formulation déterministe. Une formulation récente du couplage (Bidlot et al. (2005)) est testée et validée comparativement avec la modélisation stochastique usuelle issue du modèle WAVEWATCH3. L'approche déterministe est finalement capable de prendre efficacement en compte les termes de forçage et dissipation et permet des évolutions d'états de mer réalistes. Le problème de modélisation opérationnelle des états de mer, à moindre coût calcul mais avec une prise en compte suffisamment réaliste des spécificités océaniques est abordé. Une méthode numérique est proposée et testée comparativement à l'approche complètement non-linéaire de référence

A Dynamic Large-Scale Driving-Force to Control the Targeted Wind Speed in Large Eddy Simulations above Ocean Waves

We performed large eddy simulations to study micro-scale wind–wave interactions under undisturbed freestream conditions. We identified that standard approaches lead to wave-related disturbances at the top boundary. Therefore, we developed a numerical strategy to maintain an undisturbed wind speed at the top, while considering arbitrary waves at the bottom. In a broader context, the method is capable of controlling the wind speed at any height in the domain, and may also be used to enhance atmospheric simulations over land. The method comprises an evolution equation that controls the dynamic evolution of the large-scale driving force, representing the geostrophic forcing from the meso- to the micro-scales. In flat-bottom applications, this guided the reference freestream velocities towards a certain target; convergence to a steady state regime was favored and self-similarity was ensured. In wavy bottom applications considering the prescription of a monochromatic wave, we were able to maintain a quasi-steady wind speed close to the target on the freestream. The wave-induced disturbances were then investigated as functions of varying wave age conditions. We performed a systematic wave age variation study by varying the reference wind speed, and evaluated wave-induced disturbances in the velocity, normal, and shear stress profiles

Advanced measurement and analysis of waves and turbulence using 5-, 7- or 8-beam ADCPs

International audienc

Infragravity waves and moored floating structures

Up to this day, design recommendations or practices for floating structures, such as DNVs RP-C205 and F205 for example, do not account for the existence of low frequency free Infragravity (IG) waves. These are low-frequency water waves which represent an additional excitation for (moored) floating structures. In this study, the influence of free IG waves on moored floating structures response is investigated and compared to second-order low-frequency wave forces, as the standard low frequency excitation, for two selected important free IG events in 2013 and 2014 at the SEM-REV (Site d'Experimentation En Mer - Récupération de l'Energie des Vagues) test site near Le Croisic, Pays-de-la-Loire, France. The wave forces and motion response for two example floater-mooringsystems are calculated and compared for the different frequency ranges separately and combined. Using the latest developments in the wind-wave framework WAVEWATCHIII®, e.g. the parameterized implementation of free IG sources at the reflecting boundaries, directional wave spectra including the low-frequency IG wave band are used. This study has been conducted between November 2014 and October 2015 in the scope of the axis 7 «Sea motions and interaction with marine structures» of the cluster of Excellence LabexMER "A changing ocean"

An LES characterization of turbulent and wave induced flows in the atmospheric boundary layer above regular non-linear swells.

International audienc

Influence of resource definition on defining a WEC optimal size

International audienceThis work is a follow up from two previous studies which have been investigating the difference in wave resource between sites and the impact this has on the optimal Wave Energy Converter (WEC) size using scatter diagrams of the sites only. This study expands these works by using omni-directional spectra time series to describe the wave resource instead of scatter diagrams. Two well known wave energy test sites are considered: EMEC (Billia Crew) in the North of Scotland and the SEM-REV on the West coast of France. The sloped IPS buoy is used as a case study, and a succinct description is provided. As in previous work, only the hydrodynamic power capture is considered, and no power-takeoff efficiency or cap are introduced. For both sites, around one full year of data is available. Using both the scatter diagrams and the spectra directly, WEC performance metrics are computed for each site and compared. The results show that using spectral time series instead of the scatter diagrams yield lower annual energy productions, and that the highest average capture width ratio is obtained for larger scale devices. Spectral time series allows also the establishment of a simple O&M model. The effect on device availability of annual planned downtime days, of annual failure rates of 1, 3 and 5 and of operability threshold of 2m and 2.5m are investigated. The results show that larger scales might indeed have higher availability, but are exposed to higher risks

Near-optimal use of a MRE export cable considering thermal and techno-economic aspects

International audienceDue to the ever-increasing electricity demand, along with the need to reduce the dependence on fossil or nuclear resources, a growing amount of renewable energy is integrated in the energy mix of many countries. However, the cost-effective integration of wave energy remains difficult as its cost is still not competitive compared with other energy sources. This paper deals with the energy production management of a simulated wave energy converter farm based on point absorbers that could be installed in the vicinity of the SEM-REV site. The approach considers the electrothermal behaviour of the export cable in combination with techno-economic aspects. The method can be used to extend the power export capability of a test site at no cost, which could be very interesting to install more marine renewable energy (MRE) converters and/or of greater rated power. Our study shows that exploiting the thermal inertia of a wave farm submarine export cable, while also considering techno-economic aspects, may lead to an increase of 18% in the annual energy production without modifying this cable

An improved Lagrangian model for the time evolution of nonlinear surface waves

Accurate real-time simulations and forecasting of phase-revolved ocean surface waves require nonlinear effects, both geometrical and kinematic, to be accurately represented. For this purpose, wave models based on a Lagrangian steepness expansion have proved particularly efficient, as compared to those based on Eulerian expansions, as they feature higher-order nonlinearities at a reduced numerical cost. However, while they can accurately model the instantaneous nonlinear wave shape, Lagrangian models developed to date cannot accurately predict the time evolution of even simple periodic waves. Here, we propose a novel and simple method to perform a Lagrangian expansion of surface waves to second order in wave steepness, based on the dynamical system relating particle locations and the Eulerian velocity field. We show that a simple redefinition of reference particles allows us to correct the time evolution of surface waves, through a modified nonlinear dispersion relationship. The resulting expressions of free surface particle locations can then be made numerically efficient by only retaining the most significant contributions to second-order terms, i.e. Stokes drift and mean vertical level. This results in a hybrid model, referred to as the \u27improved choppy wave model\u27 (ICWM) (with respect to Nouguier et al.\u27s J. Geophys. Res., vol. 114, 2009, p. C09012), whose performance is numerically assessed for long-crested waves, both periodic and irregular. To do so, ICWM results are compared to those of models based on a high-order spectral method and classical second-order Lagrangian expansions. For irregular waves, two generic types of narrow- A nd broad-banded wave spectra are considered, for which ICWM is shown to significantly improve wave forecast accuracy as compared to other Lagrangian models; hence, ICWM is well suited to providing accurate and efficient short-term ocean wave forecast (e.g. over a few peak periods). This aspect will be the object of future work

Reconstruction et prédiction déterministe à résolution de phase de champs de vagues non-linéaires par télédétection optique

International audienceLes performances d'une méthode de prédiction de champs de vagues non-linéaires à partir de mesures optiques spatio-temporelles d'élévation de surface sont étudiées. Afin de quantifier la qualité des prédictions, des indicateurs d'erreur sont estimés dans le cas d'une houle unidi-rectionnelle en moyennant les résultats sur un grand nombre de données synthétiques. Pour des quelques cambrures caractéristiques, nous comparons les résultats issus d'approches linéaire et faiblement non-linéaire. Il est montré que le modèle non-linéaire utilisé, basé sur une description Lagrangienne de la dynamique de surface libre, permet de capter des effets non-linéaires jouant un rôle important et grandissant avec la cambrure, i.e., asymmétrie de forme et déphasage. Des données expérimentales sont également utilisées pour illustrer les performances de la méthode appliquées à des mesures réelles.We investigate the performance of a phase-resolved algorithm for the prediction of non-linear ocean wave fields based on spatio-temporal remote optical measurements of free surface elevations. To quantify the prediction accuracy, error estimates are calculated for the case of a unidirectional wave field by ensemble averaging a large number of synthetic data sets. For several characteristic wave steepnesses, we compare results based on linear and weakly nonlinear approaches. It is shown that our nonlinear wave model, based on a Lagrangian description of the free-surface dynamics, allows to catch relevant nonlinear effects that play an increasingly large role as the waves get steep, i.e., wave shape asymmetry and phase shift. Experimental data are also used to illustrate the performance of the method applied on real field measurements