MODELISATION DE LA TURBULENCE DANS UN SYSTEME DE TAYLOR-COUETTE DIFFERENTIELLEMENT CHAUFFE AVEC FLUX AXIAL

International audienceOn considère les écoulements turbulents de Taylor-Couette soumis à un flux axial et à des gradients thermiques. L'approche numérique est basée sur le modèle RSM d'Elena et Schiestel (1996) déjà validé dans le cas de cavités rotor-stator avec flux et gradients thermiques (Poncet et Schiestel 2007). Il est appliqué ici pour une large gamme de nombres de Reynolds et de Prandtl et de coefficient de débit dans une cavité fortement allongée (rapport d'aspect L=0.013) et des écarts de température allant jusqu'à 84 K. Des corrélations pour les coefficients de transferts le long des cylindres sont dégagées en fonction des paramètres de contrôle

3D Direct Numerical Simulation of Magneto-Caloric regenerators

Magnetic refrigeration uses magnetic field changes to provoke a phase transition in solid Magneto-Caloric Materials (MCM). Then, using a Heat Transfer Fluid, a Brayton-like refrigeration cycle can be produced. Developing these kind of refrigerators is usually costly, due to the amount of rare earth materials needed for the permanent magnet and the MCM. In order to aid in better designing such machines, a 3D DNS solver was developed. The solver includes the simulation of four different physical phenomena: the magnetic field, fluid velocity field, temperature field, and the Magneto-Caloric Effect. To produce an efficient solver, collateral coupling mechanisms were studied and deemed negligible, including the temperature dependency of the HTF (water) viscosity, and the influence of the magnetic permeability on the internal magnetic field of Gadolinium. The resolution of the physical phenomena was benchmarked separately and then the performance of the cycle solver was compared to a measured prototype. The solutions given by the new 3D DNS solver provide a more accurate description of the temperature field than the 2D solver available in the literature. The authors argue that this is probably partially due to the detailed simulation of the magnetic field (not accounted in 2D codes). The produced numerical solver is aimed to serve as a tool in the design process of magnetic refrigerators. By simulating conceptual magnetic regenerators, it is expected that the new prototypes experience a qualitative jump

Simulation numérique d'un écoulement de convection mixte en aval d'une marche

International audienceDes expériences numériques d'un écoulement en aval d'une marche horizontale sont menées en régime laminaire instationnaire, afin d'étudier les mécanismes thermodynamiques générés par la convection mixte dans un canal bidimensionnel de géométrie fixe. On se propose de quantifier l'influence de divers écarts de température dT entre la température de la base du canal et la température du fluide entrant. En analysant l'évolution temporelle de la température, on constate que l'écoulement est stationnaire lorsque dT est inférieur à 4 K, puis est périodique aux alentours de 5 K. Au-delà de cette valeur, le signal temporel de la température montre un comportement apériodique

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF TURBULENT AIR FLOW BEHAVIOUR IN A ROTOR-STATOR CAVITY

International audienceThe present work considers the turbulent air flow inside an annular high speed rotor-stator cavity opened to the atmosphere at the periphery. The interdisk-spacing is sufficiently large so that the boundary layers developed on each disk are separated and the flow belongs to the regime IV of Daily and Nece (1960). In such a system, the solid body rotation of the core predicted by Batchelor in case of infinite disks is not always observed: the flow behavior in the whole interdisk-spacing is governed by the level of the pre-swirl velocity of the fluid which is closely linked to the peripheral geometry (Debuchy et al (2007)). In the first part of the paper, experimental results including mean radial and tangential velocity components, as well as three turbulent correlations, are presented for several peripheral boundary conditions leading to the same value of the pre-swirl ratio. Measurements are performed by hot-wire probes introduced through the stator and connected to a constant temperature anemometer. In the second part, comparisons between experiments and numerical results are provided. The numerical approach is based on one-point statistical modeling using a low Reynolds number second-order full stress transport closure derived from the Launder and Tselepidakis model (1994) and sensitized to rotation effects (Elena and Schiestel 1996). The aim is to find what type of boundary conditions imposed in the RSM provides the best agreement for this set of flow control parameters

ETUDE EXPERIMENTALE DE L’INFLUENCE DE LA DEPRESSION MECANIQUE SUR LE TRANSPORT ET LE COMPORTEMENT RHEOLOGIQUE DU MUCUS BRONCHIQUE SYNTHETIQUE DANS UNE TRACHEE ARTIFICIELLE.

Les bronches sont recouvertes d’un film de mucus qui s’écoule en permanence depuis les voies respiratoires inférieures vers la trachée, afin d’empêcher les corps étrangers d’entrer en contact avec les parois bronchiques et de contaminer l’organisme. Ce phénomène naturel porte le nom de clairance mucociliaire. Les pathologies de stagnation, d’accumulation et d’infections respiratoires chroniques conduisent à une modification rhéologique du mucus. L’objectif de cette étude est de caractériser le comportement rhéologique d’une solution synthétique simulant le mucus bronchique avant et après passage dans un appareil d’aide à la respiration et de voir l’effet des dépressions mécaniques de l’appareil sur le transport du mucus synthétique tout au long d’une trachée artificielle. Les résultats obtenus ont mis en évidence des baisses de viscosité. Les dépressions mécaniques influent sur le transport du mucus synthétique dans une trachée modèle

Rheological characterization of macromolecular colloidal gels as simulant of bronchial mucus

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Rheological characterization of macromolecular colloidal gels as simulant of bronchial mucus

International audienc

Inertial Flow of Viscoelastic Second-Grade Fluid in a Ciliated Channel under a Magnetic Field and Darcy’s Resistance

This paper proposes a mathematical analysis of the inertial flow of an MHD second-grade non-Newtonian fluid in a ciliated channel. The two-dimensional flow is modelled under the effect of inertial forces, magnetic field and Darcy’s resistance, which make the system of partial differential equations highly non-linear. To solve the complex system of partial differential equations, the Homotopy Perturbation Method (HPM) is preferred. The HPM solutions for the velocity profile, stream function and pressure gradient are obtained using the software MATHEMATICA. The significances of the Reynolds number (due to inertial forces), Hartmann number (due to magnetic field), porosity parameter (due to Darcy’s resistance) and fluid parameters (related to the second-grade fluid) on the pressure gradient, stream function and velocity profile are discussed in detail. The pertinent parameters show that the horizontal velocity decays in the presence of a magnetic field, whereas it rises under the effect of inertial forces, Darcy’s resistance and fluid viscosity in the centre of the channel. This research indicates that, for the ciliary flow of a second-grade fluid, a favourable pressure gradient (negative pressure gradient) in the horizontal direction increases when applying a magnetic field, whereas it decreases due to the porous medium. This mathematical model can be helpful to observe ciliary activity under magnetic resonance imaging, when ciliary activity is abnormal

Thermo-physical properties of synthetic mucus for the study of airway clearance

International audienceIn this article, dynamic viscosity, surface tension, density, heat capacity and thermal conductivity, of a bronchial mucus simulant proposed by Zahm et al., Eur Respir J 1991; 4: 311–315 were experiementally determined. This simulant is mainly composed of a galactomannan gum and a scleroglucan. It was shown that thermophysical properties of synthetic mucus are dependant of scleroglucan concentrations. More importantly and for some scleroglucan concentrations, the syntetic mucus, exhibits, somehow, comparable thermophysical properties to real bronchial mucus. An insight on the microstructure of this simulant is proposed and the different properties enounced previously have been measured for various scleroglucan concentrations and over a certain range of operating temperatures. This synthetic mucus is found to mimic well the rheological behavior and the surface tension of real mucus for different pathologies. Density and thermal properties have been measured for the first time

Modelling wave-structure interactions including air compressibility: A case study of breaking wave impacts on a vertical wall along the Saint-Lawrence Bay

[EN] Climate change and the associated rise in sea levels impose a challenge for current methods used to predict the processes involved in wave-coastal structure interactions: stochastic nature, coexistence of various phenomena with very different time and length scales, wave transformations ... The main objective of this study is to extend the range of numerical tools available to coastal engineers by presenting a two-phase Favre-Averaged Navier- Stokes model to study wave dynamic interactions with impermeable structures. The model takes into account air compressibility as well as turbulence effects. The model is first validated with two experimental test cases: a sloshing tank with impacts, and wave impacts on a vertical wall with an overhanging slab. After validation, the model is confidently used for the analysis of wave interactions with vertical walls along the Saint-Lawrence Bay, Quebec, Canada. Four incident wave cases representing frequent and extreme storm conditions are selected using the statistical data of a buoy. Results are discussed primarily in terms of the hydrodynamic loads and overtopping on the structure for two breaking and two non-breaking wave cases.The authors would like to thank the financial support of Min-istere des Transports du Quebec through the project R828.1. All cal-culations have been performed using the HPC facilities of Compute Canada/Calcul Quebec, which are here also gratefully acknowledged.Croquer, S.; Díaz-Carrasco, P.; Tamimi, V.; Poncet, S.; Lacey, J.; Nistor, I. (2023). Modelling wave-structure interactions including air compressibility: A case study of breaking wave impacts on a vertical wall along the Saint-Lawrence Bay. Ocean Engineering. 273. https://doi.org/10.1016/j.oceaneng.2023.11397127