Thèse Transport des Particules de Plastique dans les Écoulements Turbulents Stratifiés H/F - Doctorat.Gouv.Fr
- CDD
- Doctorat.Gouv.Fr
Les missions du poste
Établissement : Institut National Polytechnique de Toulouse École doctorale : SDU2E - Sciences de l'Univers, de l'Environnement et de l'Espace Laboratoire de recherche : IMFT - Institut de Mécanique des Fluides de Toulouse Direction de la thèse : Matthieu MERCIER ORCID 0000000199653316 Début de la thèse : 2027-09-01 Date limite de candidature : 2026-11-23T23:59:59 La pollution plastique est un enjeu sociétal majeur, et son impact est particulièrement néfaste dans les zones côtières et les estuaires, qui sont des régions très productives mais aussi très fragiles, caractérisées par un environnement marin fortement stratifié. L'objectif principal de ce projet est d'améliorer la prise en compte des effets de la stratification fluide dans les modèles de transport Lagrangien de particules de plastiques qui sont utilisés pour prédire la dispersion de cette pollution à grande échelle, et quantifier les zones d'accumulation.
Les premiers résultats seront obtenus par des expériences de transport de particules inertielles dans des écoulements turbulents stratifiés en densité. En s'appuyant sur des dispositifs existants et des méthodes expérimentales de pointes pour le suivi de particules dans les écoulements, il sera possible d'observer et d'analyser des trajectoires de particules aux propriétés variées au sein des différentes couches de fluide d'écoulements turbulents stratifiés contrôlés (turbulence de grille, courant de gravité). Une finalité de ces analyses sera le développement de nouvelles paramétrisations dans les modèles de transport vertical Lagrangien, qui seront testées par des simulations numériques.
Une meilleure compréhension de la diffusion verticale des particules de plastiques dans leur grande diversité de taille/forme/densité, dans un contexte de turbulence stratifiée en densité permettra d'améliorer le suivi de cette pollution dans différents environnements océaniques où la stratification joue un rôle important.
Global plastic pollution is a subject of active research within many geophysical science communities, particularly with the aim of assessing its extent and impact and guiding policy decisions to address it [4]. Among the various environmental compartments, the oceans play a particular role, as the presence of plastic on the surface was observed as early as the 1970s [7], and they are linked to the United Nations Sustainable Development Goal 14 ('Life Below Water'). While surface pollution has been extensively studied, the presence of microplastics throughout the ocean water column is now confirmed [10], and deep-sea sediments serve as accumulation sites [6]. The most recent results [5] from global ocean pollution modeling estimate the accumulated plastic-across the surface, the water column, and the ocean floor-at over 3,200 kilotonnes for the 1980-2020 period.
Plastic particles found in the oceans vary widely in size, density, and shape [9], resulting in vastly different transport properties depending on whether the particles float or sink, or whether they are small or large relative to the characteristic scales of the flows transporting them [13]. Tools for modeling the transport of these particles in estuarine or coastal zones, or at the global ocean scale, rely on a Lagrangian approach in which particles follow mesoscale flows (on the order of kilometers); complex effects (wind, waves, buoyancy, turbulent mixing, etc.) are often modeled using diffusive or stochastic processes [12]. However, numerous parameters are required to parameterize and understand the physical processes associated with plastic pollution, and laboratory experiments are essential for advancing the modeling of global pollution [R2].
Regarding the specific case of the vertical transport of plastic particles, the agitation of particles floating on the ocean surface has been modeled using grid-turbulence experiments [11] or flow-equipped wave channels [3]. Vertical transport in this context is heavily dominated by the depth-dependent inhomogeneity of turbulence; Lagrangian models yield good results in this configuration, although the value of the Schmidt number remains a subject of ongoing study [2].
When modeling the entire ocean water column, one must account for the fluid's density stratification with depth, which arises from salinity and temperature properties that vary across water masses. Very few experimental studies have investigated vertical particle transport driven by turbulent stratified flow. Two studies have examined the dynamics of floating particles in a two-layer fluid agitated in only one of the layers [1, 11]. A recent study [8] demonstrated that particles with densities matching those of intermediate layers exhibit 'anomalous' vertical diffusion compared to Lagrangian model predictions-though consistent with previous numerical studies.
A significant challenge therefore remains: understanding (and modeling) the vertical diffusion of plastic particles-given their wide range of sizes, shapes, and densities-within a density-stratified turbulent environment. Such models must be applicable to various oceanic settings where stratification plays a key role, such as estuarine or coastal zones. Determine the effects of stratified turbulence on the dispersion of inertial particles with varying properties (density, size, shape) in the context of oceanic plastic pollution.
Observe and analyze-through laboratory experiments-the trajectories of inertial particles within the various fluid layers of controlled stratified turbulent flows (grid turbulence, gravity currents).
Develop new parameterizations for Lagrangian vertical transport models and test them using numerical simulations. The primary objective is to determine the effects of stratified turbulence on the dispersion of inertial particles with varying properties (density, size, shape) in the context of oceanic plastic pollution.
An initial approach will involve conducting experiments on plastic particle transport within 'model' stratified turbulent flows. The properties of the plastic particles (density, size, or shape) influencing their sedimentation or buoyancy will be varied to represent particles observed in the oceans; these parameters will be compared against the properties of the turbulent flows under consideration [R2]. Two generic configurations of stratified turbulent flows will be studied using various existing setups at the IMFT:
1. 'Stationary' agitation without mean flow, achieved by oscillating a grid within a two-layer fluid; turbulence intensity can be varied by controlling the grid's oscillation parameters. This setup has notably been used to study stratified turbulence [R3].
2. 'Stationary' agitation with mean flow, achieved by injecting a dense fluid into a quiescent fluid, creating a propagating gravity current. Preliminary studies have already examined the vertical distribution of particles based on the density difference relative to the dense fluid [R4].
The main challenge of the experiments lies in metrology; time-resolved tracking of particles in stratified flows requires refractive index matching-an area of expertise already available at the laboratory. Furthermore, analyzing particle trajectories over sufficiently long periods presents an additional challenge, as this is necessary to compare observations with various types of Lagrangian diffusive models.
Lagrangian models describing the vertical transport of plastics constitute the project's second objective. Experimental data will serve to validate Lagrangian model parameterizations developed by M. Duran-Matute's team [R5, R6]. The recruited individual will develop a digital twin of the experiments during their stay in the Netherlands, and the adjustment of model parameters will be carried out as part of the collaboration.
Les premiers résultats seront obtenus par des expériences de transport de particules inertielles dans des écoulements turbulents stratifiés en densité. En s'appuyant sur des dispositifs existants et des méthodes expérimentales de pointes pour le suivi de particules dans les écoulements, il sera possible d'observer et d'analyser des trajectoires de particules aux propriétés variées au sein des différentes couches de fluide d'écoulements turbulents stratifiés contrôlés (turbulence de grille, courant de gravité). Une finalité de ces analyses sera le développement de nouvelles paramétrisations dans les modèles de transport vertical Lagrangien, qui seront testées par des simulations numériques.
Une meilleure compréhension de la diffusion verticale des particules de plastiques dans leur grande diversité de taille/forme/densité, dans un contexte de turbulence stratifiée en densité permettra d'améliorer le suivi de cette pollution dans différents environnements océaniques où la stratification joue un rôle important.
Global plastic pollution is a subject of active research within many geophysical science communities, particularly with the aim of assessing its extent and impact and guiding policy decisions to address it [4]. Among the various environmental compartments, the oceans play a particular role, as the presence of plastic on the surface was observed as early as the 1970s [7], and they are linked to the United Nations Sustainable Development Goal 14 ('Life Below Water'). While surface pollution has been extensively studied, the presence of microplastics throughout the ocean water column is now confirmed [10], and deep-sea sediments serve as accumulation sites [6]. The most recent results [5] from global ocean pollution modeling estimate the accumulated plastic-across the surface, the water column, and the ocean floor-at over 3,200 kilotonnes for the 1980-2020 period.
Plastic particles found in the oceans vary widely in size, density, and shape [9], resulting in vastly different transport properties depending on whether the particles float or sink, or whether they are small or large relative to the characteristic scales of the flows transporting them [13]. Tools for modeling the transport of these particles in estuarine or coastal zones, or at the global ocean scale, rely on a Lagrangian approach in which particles follow mesoscale flows (on the order of kilometers); complex effects (wind, waves, buoyancy, turbulent mixing, etc.) are often modeled using diffusive or stochastic processes [12]. However, numerous parameters are required to parameterize and understand the physical processes associated with plastic pollution, and laboratory experiments are essential for advancing the modeling of global pollution [R2].
Regarding the specific case of the vertical transport of plastic particles, the agitation of particles floating on the ocean surface has been modeled using grid-turbulence experiments [11] or flow-equipped wave channels [3]. Vertical transport in this context is heavily dominated by the depth-dependent inhomogeneity of turbulence; Lagrangian models yield good results in this configuration, although the value of the Schmidt number remains a subject of ongoing study [2].
When modeling the entire ocean water column, one must account for the fluid's density stratification with depth, which arises from salinity and temperature properties that vary across water masses. Very few experimental studies have investigated vertical particle transport driven by turbulent stratified flow. Two studies have examined the dynamics of floating particles in a two-layer fluid agitated in only one of the layers [1, 11]. A recent study [8] demonstrated that particles with densities matching those of intermediate layers exhibit 'anomalous' vertical diffusion compared to Lagrangian model predictions-though consistent with previous numerical studies.
A significant challenge therefore remains: understanding (and modeling) the vertical diffusion of plastic particles-given their wide range of sizes, shapes, and densities-within a density-stratified turbulent environment. Such models must be applicable to various oceanic settings where stratification plays a key role, such as estuarine or coastal zones. Determine the effects of stratified turbulence on the dispersion of inertial particles with varying properties (density, size, shape) in the context of oceanic plastic pollution.
Observe and analyze-through laboratory experiments-the trajectories of inertial particles within the various fluid layers of controlled stratified turbulent flows (grid turbulence, gravity currents).
Develop new parameterizations for Lagrangian vertical transport models and test them using numerical simulations. The primary objective is to determine the effects of stratified turbulence on the dispersion of inertial particles with varying properties (density, size, shape) in the context of oceanic plastic pollution.
An initial approach will involve conducting experiments on plastic particle transport within 'model' stratified turbulent flows. The properties of the plastic particles (density, size, or shape) influencing their sedimentation or buoyancy will be varied to represent particles observed in the oceans; these parameters will be compared against the properties of the turbulent flows under consideration [R2]. Two generic configurations of stratified turbulent flows will be studied using various existing setups at the IMFT:
1. 'Stationary' agitation without mean flow, achieved by oscillating a grid within a two-layer fluid; turbulence intensity can be varied by controlling the grid's oscillation parameters. This setup has notably been used to study stratified turbulence [R3].
2. 'Stationary' agitation with mean flow, achieved by injecting a dense fluid into a quiescent fluid, creating a propagating gravity current. Preliminary studies have already examined the vertical distribution of particles based on the density difference relative to the dense fluid [R4].
The main challenge of the experiments lies in metrology; time-resolved tracking of particles in stratified flows requires refractive index matching-an area of expertise already available at the laboratory. Furthermore, analyzing particle trajectories over sufficiently long periods presents an additional challenge, as this is necessary to compare observations with various types of Lagrangian diffusive models.
Lagrangian models describing the vertical transport of plastics constitute the project's second objective. Experimental data will serve to validate Lagrangian model parameterizations developed by M. Duran-Matute's team [R5, R6]. The recruited individual will develop a digital twin of the experiments during their stay in the Netherlands, and the adjustment of model parameters will be carried out as part of the collaboration.
Le profil recherché
Master 2 - Plusieurs profils peuvent convenir à ce projet ; la personne pourra avoir une expérience avancée en mécanique des fluides, physique, géophysique, mathématiques appliquées, etc. Cependant, un intérêt marqué pour les approches expérimentales en laboratoire et pour les analyses de données associées, ainsi qu'un intérêt marqué pour la compréhension des mécanismes physiques, seront très importants.
Application link: https://edd-projets.utoulouse.fr/
Application link: https://edd-projets.utoulouse.fr/
Compétences requises
- Analyse de données
- Mécanique des fluides
- Mathématiques