Vague primaire d'une couche de mélange eau-air : une étude numérique

The shear instability occuring at the interface between a slow water layer and a fast air stream is a complex phenomenon driven by momentum and viscosity differences across the interface, velocity gradients, as well as by injector geometries. Simulating such an instability in the conditions of experiments is numerically challenging and few studies exist in the literature. This work aims at filling a part of this gap by presenting a study of the convergence between two-dimensional simulations, linear theory, and experiments, in regimes where the instability is triggered by confinement, i.e., the finite thicknesses of the gas and liquid streams. Very good agreement between the three approaches is obtained. Moreover, using simulations and linear theory, we explore in details the effects of confinement on the stability of the flow and on the transition between absolute and convective instability regimes, which is shown to depend on the lengthscale of confinement as well as on dynamic pressure ratio. In the absolute regime under study, interfacial wave frequency is found to be inversely proportional to the smallest injector size (liquid or gas). We then study the transition between primary and secondary instability through wave acceleration. In additional, we explore the impact of three-dimensional effects on the flow. Finally, we present the development of an open boundary condition for turbulent multiphase flows and surface waves simulations. Initially thought as a way to improve accuracy and lower needed computational ressources of air-water mixing layer simulations, this work leads to improvements in the use of traction boundary conditions. Particularly, this novel boundary treatment couples Lagrangian traction estimation to backflow stabilization which provides stability, accuracy and non-reflectivity of artificial boundaries.L’instabilité de cisaillement se produisant à l’interface entre une couche d’eau lente et un courant d’air rapide est un phénomène complexe induit par des différences de quantité de mouvement et de viscosité à travers l’interface, de forts gradients de vitesse, et par la géométrie des injecteurs. Simuler numériquement une telle instabilité dans les conditions expérimentales est difficile et peu d’études existent dans la littérature. Ce travail a pour objectif de combler une partie de cette lacune en présentant une étude de la convergence entre simulations à deux dimensions, théorie linéaire et expériences, dans des régimes où l’instabilité est déclenchée par le confinement de l’écoulement. Un très bon accord entre les différentes approches est obtenu. De plus, via des simulations et la théorie linaire, nous explorons les effets du confinement sur la stabilité de l’écoulement et sur la transition entre régimes d’instabilité absolus et convectifs. Cette transition est trouvée comme dépendante de la longueur caractéristique du confinement et du ratio de pression dynamique. Dans le régime absolu étudié, la fréquence des vagues interfaciales est trouvée comme étant inversement proportionnelle à la plus petite taille d’injecteur (liquide ou gaz). Nous étudions ensuite la transition entre les instabilités primaires et secondaires à travers l’accélération de la vague. Nous étudions par la suite l’impact des effets tri-dimensionnels sur l’écoulement. Enfin, nous présentons le développement d’une condition de frontière ouverte pour des écoulements turbulents, multiphasiques et des simulations d’ondes de surface. Initialement pensé comme un moyen d’améliorer la précision et de diminuer les ressources informatiques nécessaires aux simulations de couches de mélange eau-air, ce travail mène à des améliorations dans l’utilisation des conditions de traction. Plus particulièrement, cette nouvelle condition aux limites couple une estimation Lagrangienne de la traction à une stabilisation aux écoulements rentrants, ce qui permet la stabilité, la précision et la non-reflectivité des frontières artificielles

Traction open boundary condition for incompressible, turbulent, single- or multi-phase flows, and surface wave simulations

International audienc

Stability of an air–water mixing layer: focus on the confinement effect

International audienceThe shear instability occurring at the interface between a slow water layer and a fast air stream is a complex phenomenon driven by momentum and viscosity differences across the interface, velocity gradients as well as by injector geometries. Simulating such an instability under experimental conditions is numerically challenging and few studies exist in the literature. This work aims at filling a part of this gap by presenting a study of the convergence between two-dimensional simulations, linear theory and experiments, in regimes where the instability is triggered by the confinement, i.e. finite thicknesses of gas and liquid streams. It is found that very good agreement between the three approaches is obtained. Moreover, using simulations and linear theory, we explore in detail the effects of confinement on the stability of the flow and on the transition between absolute and convective instability regimes, which is shown to depend on the length scale of the confinement as well as on the dynamic pressure ratio. In the absolute regime under study, the interfacial wave frequency is found to be inversely proportional to the smallest injector size (liquid or gas)

Modelling the cycles of winter stem pressure in walnut tree

International audienceFrost hardiness is the main factor affecting plant species distribution at high latitudes and altitudes. The main effects of freeze-thaw cycles on trees are damages to living cells, as well as the formation of gas embolism in xylem vessels, thus blocking sap flow in spring. The effect of frost on trees can be quantified through changes in branch diameter.In order to resorb embolism, some species (walnut, maple, birch, etc.) exhibit an increase in the xylem sap pressure during successive freeze-thaw cycles, which leads to the exudation of sap after pruning. The multiplicity of relevant scales (spatial and temporal), the presence of water simultaneously in gaseous, liquid and solid form, as well as the corresponding phase changes, bring complexity into the modelling of these phenomena.In this work, we present a numerical model coupling heat transfer, phase change, water and osmotic fluxes, taking into consideration different cell types within walnut branch tissues. We show how diameter and pressure variations are inter-related, and we validate the model against experimental results from the literature. We eventually show how this work can be adapted to other types of anatomical structures and to other environmental conditions, in order to explore inter-species differences

Freeze dehydration vs. supercooling in tree stems: physical and physiological modelling

International audienceAbstract Frost resistance is the major factor affecting the distribution of plant species at high latitude and elevation. The main effects of freeze-thaw cycles are damage to living cells and formation of gas embolism in tree xylem vessels. Lethal intracellular freezing can be prevented in living cells by two mechanisms: dehydration and deep supercooling. We developed a multiphysics numerical model coupling water flow, heat transfer, and phase change, considering different cell types in plant tissues, to study the dynamics and extent of cell dehydration, xylem pressure changes, and stem diameter changes in response to freezing and thawing. Results were validated using experimental data for stem diameter changes of walnut trees. The effect of cell mechanical properties was found to be negligible as long as the intracellular tension developed during dehydration was sufficiently low compared to the ice induced cryostatic suction. The model was finally used to explore the coupled effects of relevant physiological parameters (initial water and sugar content) and environmental conditions (air temperature variations) on the dynamics and extent of dehydration. It revealed configurations where cell dehydration could be sufficient to protect cells from intracellular freezing, and situations where supercooling was necessary. This model, freely available with this paper, could easily be extended to explore different anatomical structures, different species and more complex physical processes

A model for stem diameter variations during freeze/thaw cycles

International audienc

Multi-physics modelling of freeze-thaw cycles effects on tree branches

International audienceFrost hardiness is the main factor affecting plant species distribution at high latitudes and altitudes. The main effects of freeze-thaw cycles on trees are damages to living cells, as well as the formation of gas embolism in xylem vessels. Frost effect on trees is also quantified through changes in branch diameter.In order to resorb embolism, some species (walnut, maple, birch, etc.) exhibit an increase in xylem sap pressure during successive freeze-thaw cycles. The modelling of such phenomenon is very challenging due to its multi-physics and multi-scale nature. In this work, we present a numerical model coupling heat transfer, phase change, water and osmotic fluxes, taking into consideration different cell types within walnut branch tissues. We show how diameter and pressure variations are inter-related, and we validate the model against experimental results from the literature. We eventually show how this work can be adapted in order to explore inter-species differences

Multi-physics modelling of freeze-thaw cycles effects on tree branches

International audienceFrost hardiness is the main factor affecting plant species distribution at high latitudes and altitudes. The main effects of freeze-thaw cycles on trees are damages to living cells, as well as the formation of gas embolism in xylem vessels. Frost effect on trees is also quantified through changes in branch diameter.In order to resorb embolism, some species (walnut, maple, birch, etc.) exhibit an increase in xylem sap pressure during successive freeze-thaw cycles. The modelling of such phenomenon is very challenging due to its multi-physics and multi-scale nature. In this work, we present a numerical model coupling heat transfer, phase change, water and osmotic fluxes, taking into consideration different cell types within walnut branch tissues. We show how diameter and pressure variations are inter-related, and we validate the model against experimental results from the literature. We eventually show how this work can be adapted in order to explore inter-species differences

Walnut winter pressure build-up explained through physical modelling

International audienceXylem embolism is a significant factor in tree mortality. Restoration of hydraulic conductivity after massive embolisation of the vascular system requires the application of positive pressure to the vessels and/or the creation of new conductive elements. Some species generate positive pressure from the root system to propagate pressure in distal, aboveground, organs in spring, whereas other species generate positive pressure locally at the stem level during winter. We provide a mechanistic explanation for winter stem pressure build-up in the walnut tree. We have developed a physical model that accounts for temperature fluctuations and phase transitions. This model is based on the exchange of water and sugars between living cells and vessels. Our computations demonstrate that vessel pressurization can be attributed to the transfer of water between vessels across the parenchyma rays, which is facilitated by a radial imbalance in sugar concentration. The ability to dispose of soluble sugars in living cells, and to transport them between living cells and up to the vessels, are identified as the main drivers of stem pressure build-up in the walnut tree

On the mechanism for winter stem pressure build-up in walnut trees

International audienceXylem embolism is a significant factor in tree mortality. Restoration of hydraulic conductivity after massive embolization of the vascular system requires the application of positive pressure to the vessels and/or the creation of new conductive elements. Some species generate positive pressure from the root system to propagate pressure in distal, aboveground organs in spring, whereas other species generate positive pressure locally at the stem level during winter. We provide a mechanistic explanation for winter stem pressure build-up in the walnut tree. We have developed a physical model that accounts for temperature fluctuations and phase transitions. This model is based on the exchange of water and sugars between living cells and vessels. Our computations demonstrate that vessel pressurization can be attributed to the transfer of water between vessels across the parenchyma rays, which is facilitated by a radial imbalance in sugar concentration. The ability to dispose of soluble sugars in living cells, and to transport them between living cells and up to the vessels, is identified as the main drivers of stem pressure build-up in the walnut tree