Analyse de stabilité de la convection naturelle thermosolutable dans un milieu de Darcy

International audienceCe travail traite l'influence de l'effet de Soret sur la convection naturelle dans un milieu de Darcy cartésien, saturé par un fluide binaire et soumis à un gradient vertical de température appliqué sur les parois horizontales, les autres parois sont adiabatiques. Toutes les frontières sont imperméables à la masse. Ainsi nous abordons l'étude linéaire de la stabilité du mélange. La prévision des nombres de Ra critiques pour la convection naturelle est faite sur la base d'un problème de valeurs propres, déduit d'une analyse linéaire numérique de stabilité 2D marginale et transitoire basée sur la méthode des éléments finis. Nous donnons des diagrammes de stabilité en fonction des paramètres de contrôle du problème

Potential use of matakaolin as a partial replacement of preformulated lime binder to improve durability of hemp concrete under cyclic wetting/ drying aging

International audienceThis study reports the experimental investigation of a commercial formulated lime binder (Tadical PF70) modification effect on the durability of hemp concrete specimens when subjected to continuous wetting and drying cycles. The influence of partial replacement of Tradical PF70 binder by metakaolin has been studied by testing the physico-mechanical properties of samples including mass loss, porosity, compressive and flexural strengths, and corresponding toughness for aging conditions of 15, 25, and 50 wetting/drying cycles. Considered as a critical factor which contributes to the specimen deterioration, a separate investigation of the degradation rate of the embedded hemp particles in specimens at aging cycle intervals has been also performed. The results have shown that the pozzolanic reaction of amorphous silica contained in metakaolin with dissolved calcium hydroxide appears to significantly minimise the degradation of hemp concrete. The corresponding reaction consumes calcium hydroxide to produce hydrated calcium silicates and in turn will lead to the lowest alkalinity of pore solution which is considered as the main responsible for hydrolysis of hemp particles. The examination of the relationship between embedded hemp particle deterioration degree and the loss of mechanical properties of hemp concrete has shown that the use of metakaolin is an effective means to improve durability of specimens mainly through the restraint of hemp particle degradation

Installation of a thermal energy storage site in an abandoned mine in Picardy (France). Part 1 : Selection criteria and equipment of the experimental site

Assessment of spatio-temporal variability in groundwater storage (GWS) is critical for effective water resource planning and management with rational groundwater use. Haean Basin comprising 64 km2 area, mainly depends on groundwater for agricultural, domestic and other water uses. For quantification of GWS variability, sufficient field-based hydrological data of the ongoing decade (2011–2017) were interpolated in ArcGIS, to give reliable results. Despite satellite remote sensing having advantages in the field of hydrology, the limitation of accessibility at larger spatial resolution (~ 200,000 km2) is making it non-applicable for fine-scale observations. From the surface area, saturated thickness and specific yield of aquifer, a decline of 33 million cubic meters in GWS volume was indicated from 230 to 197 during the observed 6 years. An observable drop (4.6 m) in the mean water level due to increased water use (0.49 MCM) during this decade is responsible for the change in the mean groundwater level. A changing water level can change the surface area and saturated thickness of aquifer that have a positive correlation with GWS, and act as main controlling factors for GWS change. An annual recharge of 0.14 m to the total amount of GWS is near negligible, considering the long-term sustainability of groundwater resource. A shift towards more use of surface water, and limited drilling of groundwater wells is recommended for rational water use. The findings of this study will provide basic knowledge for relating studies, specifically which are based on small (local) scale distribution of groundwater resource

Onset of Primary and Secondary Instabilities of Viscoelastic Fluids Saturating a Porous Layer Heated from below by a Constant Flux

We analyze the thermal convection thresholds and linear characteristics of the primary and secondary instabilities for viscoelastic fluids saturating a porous horizontal layer heated from below by a constant flux. The Galerkin method is used to solve the eigenvalue problem by taking into account the elasticity of the fluid, the ratio between the viscosity of the solvent and the total viscosity of the fluid and the lateral confinement of the medium. For the primary instability, we found out that depending on the rheological parameters, two types of convective structures may appear when the basic conductive solution loses its stability: stationary long wavelength instability as for Newtonian fluids and oscillatory convection. The effect of the lateral confinement of the porous medium by adiabatic walls is to stabilize the oblique and longitudinal rolls and therefore selects transverse rolls at the onset of convection. In the range of the rheological parameters where stationary long wave instability develops first, we use a parallel flow approximation to determine analytically the velocity and temperature fields associated with the monocellular convective flow. The linear stability analysis of the monocellular flow is performed, and the critical conditions above which the flow becomes unstable are determined. The combined influence of the viscoelastic parameters and the lateral confinement on the characteristics of the secondary instability is quantified. The major new findings concerning the secondary instabilities may be summarized as follows: (i) For concentrated viscoelastic fluids, computations showed that the most amplified mode of convection corresponds to oscillatory transverse rolls, which appears via a Hopf bifurcation. This pattern selection is independent of both the fluid elasticity and the lateral confinement of the porous medium. (ii) For diluted viscoelastic fluids, the preferred mode of convection is found to be oscillatory transverse rolls for a very laterally-confined medium. Otherwise, stationary or oscillatory longitudinal rolls may develop depending on the fluid elasticity. Results also showed the destabilizing effect of the relaxation fluid elasticity and the stabilizing effect of the viscosity ratio for the onset of both primary and secondary instabilities

DEMOSTHENE : Démonstrateur de stockage saisonnier d’énergie thermique en carrière souterraine partiellement ennoyée

Le projet DEMOSTHENE a pour objectif de valoriser les nombreuses carrières souterraines abandonnées qui sous-minent le territoire picard en y réalisant des stockages saisonniers d’énergie thermique. Le principe est de stocker les calories disponibles et perdues en été afin de les réutiliser en hiver pour le chauffage des locaux ou des habitations : à ce moment là, le système est alors inversé, ce qui permet de stocker des « frigories » qui seront réutilisables en été pour la climatisation. Ce projet fera progresser les connaissances dans les trois axes scientifiques et techniques suivants : 1) la localisation des principales carrières souterraines de Picardie, 2) l’optimisation des énergies renouvelables par stockage (sous forme de chaleur) en période de production excédentaire et 3) la modélisation de propagation d’un panache thermique dans une cavité semi-remplie d’eau avec des parois perméables

Spatio-temporal stability analysis in mixed convection of a viscoelastic fluid in a porous medium

The stability of mixed convection in a Newtonian fluid medium heated from below and cooled from above, also known as the Poiseuille-Rayleigh-Bénard problem, has been extensively investigated in the past decades. Mixed convection in porous media has received much less attention in the published literature, and the reader is referred to [1] for theoretical and numerical investigations concerning this problem. The present paper extends the mixed convection problem in porous media for the case of a viscoelastic fluid flow owing to its numerous environmental and industrial applications such as the extrusion of polymer fluids, solidification of liquid crystals, suspension solutions and petroleum activities. Without a superimposed through-flow, Hirata et al. [2] treated the natural convection problem of a viscoelastic fluid in a saturated porous medium. Their work highlighted the effects of the viscoelastic properties of the fluid on the linear and nonlinear dynamics of the thermoconvective instabilities. It was shown that the elasticity of the fluid could lead either to a Hopf bifurcation, giving rise to oscillatory structures in the strongly elastic regime, or to a stationary bifurcation in the weakly elastic regime. The nonlinear competition between these two types of instabilities was also investigated in [3].  The objective of this work is to examine the influence of the main horizontal flow on the linear and characteristics of these two types of instabilities. Under the Boussinesq approximation and Darcy's law extended to a viscoelastic fluid, a temporal stability approach shows that the conditions for the appearance of longitudinal rolls are identical to those found in the absence of through-flow. For the general three-dimensional (3D) perturbations, a Squire transformation allows the deduction of the complex frequencies associated with the 3D problem using those obtained by solving the two-dimensional one. The numerical resolution of the eigen-value problem concludes that the through-flow has a destabilizing effect and selects a convective configuration organized in purely transversal rolls which oscillate in time and propagate in the direction of the main flow. In addition, by using the mathematical formalism of absolute and convective instabilities we study the nature of unstable three-dimensional disturbances. It is shown that for a non vanishing through-flow, general three-dimensional instabilities are convectively unstable which means that in the absence of a continuous noise source these instabilities are drifted outside the porous medium and no long-term pattern is observed. In contrast, purely transversal rolls may exhibit a transition to absolute instability regime and therefore affect the porous medium everywhere including in the absence of a noise source. The absolute instability threshold, the frequency and the wave number associated with purely transversal rolls are determined as a function of the Péclet number and the viscoelastic parameters. Results are discussed and compared to those obtained from laboratory experiments in the case of Newtonian fluids.   [1] A. Delache , M.N. Ouarzazi et M. Combarnous, Spatio-temporal stability analysis of mixed convection flows in porous media heated from below: comparison with experiments, Int. J. Heat Mass Transfer, 50 (2007) 1485-1499.  [2] S. Hirata, G. Ella et M. N. Ouarzazi, Nonlinear pattern selection and heat transfer in thermal convection of a viscoelastic fluid saturating a porous medium, Int. J. Thermal Sciences, 95 (2015) 136-146.  [3] A. Taleb, H Ben Hamed, , M.N Ouarzazi, and H Beji, Analytical and numerical analysis of bifurcations in thermal convection of viscoelastic fluids saturating a porous square box, Physics of Fluids, 28 (2016) 05310