Dynamique tourbillonnaire en milieu peu profond

Les milieux peu profonds font partie de notre environnement proche : les zones littorales, les lacs, les lagunes ou encore les milieux océaniques ou atmosphériques stratifiés. La connaissance des écoulements en milieu peu profonds et de leurs propriétés de transport est primordiale pour comprendre la morphologie des lits de rivières ou des zones côtières et pour l’analyse des paramètres naturels et anthropogéniques (chaleur, solides dissous ou en suspension, espèces biologiques), ainsi que pour les prévisions météorologiques. Les observations réalisées sur ces milieux ont mis en évidence une dynamique particulière, liée au confinement vertical. On peut assister à la formation de gros tourbillons horizontaux dont la taille excède très largement la profondeur. Ce travail de thèse vise à améliorer la compréhension de ces phénomènes physiques dans le cas d’une dynamique tourbillonnaire générée par un jet impulsionnel turbulent. Grâce à deux dispositifs expérimentaux complémentaires, l’un à petite échelle au laboratoire Master et l’autre à grande échelle sur la plaque Coriolis, nous avons caractérisé la transition d’un milieu profond à un milieu peu profond. Cette transition est contrôlée par un paramètre adimensionnel que nous avons appelé le nombre de confinement C. Lorsque C est faible, le comportement observé relève de la turbulence tridimensionnelle, le milieu est profond. Pour des grandes valeurs de C, on assiste au développement d’une dynamique de turbulence quasi-bidimensionnelle, le milieu est qualifié de peu profond. En milieu peu profond, les jets pulsés se structurent en gros tourbillons de type dipôles. Une étude détaillée de ces dipôles, à la fois à petite échelle (dipôles laminaires) et à grande échelle (dipôles turbulents), a montré l’existence d’un rouleau frontal généré par la friction sur le fond. L’analyse de ces processus physiques a permis de construire un modèle théorique de dipôle dont les prédictions sont comparés avec succès aux résultats expérimentaux.The shallow water flows are found in lowland rivers, coastal areas, lakes, and stratified atmospheric and oceanic flows. A proper knowledge of shallow flows and their transporting capacities is of importance for predicting the flow in and morphology of riverbeds and coastal zones, for the analysis of natural and anthropogenic parameters (heat, dissolved and suspended solids, biological species), and for weather forecasting. The observations carried out on shallow flows have revealed a particular dynamics: the vertical confinement can induce the formation of large horizontal vortices, which size greatly exceeds the depth. The aim of the thesis is to improve the knowledge of these physical processes in the case of a vortex dynamics generated by a turbulent impulsive jet. Two experimental campaign, one at small scale in the Master laboratory and the other one on the Coriolis turntable, have been performed to characterize the transition from a deep water layer to a shallow water layer. This transition is controlled by a dimensionless parameter we have called the confinement number C. When C is weak, the behaviour corresponds to three-dimensional turbulence, the water layer is deep. When C is great, the impulsive jet develops in a quasi-two-dimensional turbulence, the water layer is shallow. When the water layer is shallow, impulsive jets generate large horizontal dipolar vortices. A detailed study of these dipoles, laminar at small scale and turbulent at large scale, has shown the presence of a vertical circulation in the dipole front. The experimental results have been successfully compared with an original theoretical model

Experimental study of turbulent Ekman layer

This paper reports on laboratory experiments concerning frictional rotating turbulent boundary layer in spin-up flow over flat horizontal bottom. Stereoscopic Particle Image Velocimetry technique is used to obtain two-dimensional three components fluctuating velocity fields. Velocity profiles measured in laminar regime show a remarkable agreement with the Ekman theoretical predictions. Results obtained in turbulent regime confirm the measurement method validity and allow to plan an extensive analysis of the turbulent Ekman layers

Ecoulement bi-fluide : Application à l’impact d’une vague solitaire

Nous cherchons à analyser le déferlement d’une  vague solitaire sur une plage en présence de macro-rugosités, par simulation numérique sur un code  volumes finis tridimensionnel. Le modèle bi-fluide à  faible Mach, déjà validé par des confrontations expérimentales, repose sur des schémas numériques  optimisés

Mesures in-situ d'impacts de vagues sur une digue composite In-situ measurements of wave impacts on a composite breakwater

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Déferlement de vague : approche multi-pas

Nous simulons numériquement le déferlement de vagues par un modèle d'écoulement multi fluide à faible Mach grâce à une formulation explicite efficacement parallélisable. Le modèle repose sur un schéma par volumes finis de type Godunov du second ordre en temps et en espace et éventuellement un raidissement de l'interface. Nous introduisons une approche multi-pas qui autorise de conséquents gains en temps de calcul. Cette approche est validée par des confrontations expérience /simulation sur le déferlement de vague solitaire 2D sur plan incliné et la rupture de barrage 3D avec obstacle

Field evidence of swash groundwater circulation in the microtidal rousty beach, France

International audienceThis manuscript reports on a novel field experiment carried out on a microtidal beach in Camargue, France. For the first time in the field, a comprehensive description of the groundwater dynamics under sandy beach swash zone is presented. A cross-shore network of 15 buried pressure sensors is combined with terrestrial LiDAR measurements to study the swash-groundwater dynamics. The presented data focus on the decay of a moderate storm which allows to monitor the evolution of the groundwater pressure field in response to the retreat of the swash zone. Both horizontal and vertical head gradients are measured within the porous sand soil to estimate the groundwater flow field using Darcy’s law. Time-averaged analysis demonstrates the presence of a rather consistent groundwater circulation pattern under the swash zone, shifting offshore with the swash zone. The main tendency is an offshore directed flow, with infiltration/exfiltration in the upper/lower parts of the swash zone. Time-resolved analysis highlights the typical groundwater response to swash events which consists mainly of an overall infiltration flow during the bed inundation by the swash tongue, a seaward flow during the swash retreat and, for long backwash events, a localized exfiltration flow under the next incoming uprush

Infragravity waves: From driving mechanisms to impacts

Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “short waves” (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the short-wave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1–10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the flood phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's “hum” (background free oscillations of the solid earth)

Field Measurements of a High-Energy Headland Deflection Rip Current: Tidal Modulation, Very Low Frequency Pulsation and Vertical Structure

Headland rips, sometimes referred to as boundary rips, are rip currents flowing against natural or artificial obstructions extending seaward from the beach, such as headland or groynes. They can be driven either by the deflection of the longshore current against the obstacle or by alongshore variation in breaking wave height due to wave shadowing in the lee of the obstacle. The driving mechanism therefore essentially depends on the angle of wave incidence with respect to the natural or artificial obstruction. We analyze 42 days of velocity profile measurements against a natural headland at the high-energy meso-macrotidal beach of Anglet, southwest France. Measurements were collected in 6.5–10.5-m depth as tide elevation varied, during the autumn–winter period with offshore significant wave height and period ranging 0.9–6 m and 8–16 s, respectively, and the angle of wave incidence ranging from −20 ∘ to 20 ∘ . Here we analyze deflection rip configurations, corresponding to approximately 24 days of measurements, for which the current meter was alternatively located in the rip neck, rip head or away from the rip as wave and tide conditions changed. Deflection rips were associated with large offshore-directed velocities (up to 0.6 m/s depth-averaged velocities) and tide modulation for low- to moderate-energy waves. The vertical profile of deflection rips was found to vary from depth-uniform in the rip neck to strongly depth-varying further offshore in the rip head with maximum velocities near the surface. Very low frequency motions of the rip were dramatic, ranging 10–60 min with a dominant peak period of approximately 40 min, i.e., with longer periods than commonly reported. The strong offshore-directed velocities measured well beyond the surf zone edge provide new insight into deflection rips as a dominant mechanism for water and sediment exchanges between embayed (or structurally-controlled) beaches and the inner-shelf and/or the adjacent embayments

A Numerical Assessment of Artificial Reef Pass Wave-Induced Currents as a Renewable Energy Source

International audienceThe present study aims to estimate the potential of artificial reef pass as a renewable source of energy. The overall idea is to mimic the functioning of natural reef-lagoon systems in which the cross-reef pressure gradient induced by wave breaking is able to drive an outward flow through the pass. The objective is to estimate the feasibility of a positive energy breakwater, combining the usual wave-sheltering function of immersed breakwater together with the production of renewable energy by turbines. A series of numerical simulations is performed using a depth-averaged model to understand the effects of each geometrical reef parameter on the reef-lagoon hydrodynamics. A synthetic wave and tide climate is then imposed to estimate the potential power production. An annual production between 50 and 70 MWh is estimated