Dynamics of mixed convective-stably-stratified fluids

International audienceWe study the dynamical regimes of a density-stratified fluid confined between isothermal no-slip top and bottom boundaries (at temperatures T-t and T-b) via direct numerical simulation. The thermal expansion coefficient of the fluid is temperature dependent and chosen such that the fluid density is maximum at the inversion temperature T-b > T-i > T-t. Thus, the lower layer of the fluid is convectively unstable while the upper layer is stably stratified. We show that the characteristics of the convection change significantly depending on the degree of stratification of the stable layer. For strong stable stratification, the convection zone coincides with the fraction of the fluid that is convectively unstable (i.e., whereT > T-i), and convective motions consist of rising and sinking plumes of large density anomaly, as is the case in canonical Rayleigh-Benard convection; internal gravity waves are generated by turbulent fluctuations in the convective layer and propagate in the upper layer. For weak stable stratification, we demonstrate that a large fraction of the stable fluid (i.e., with temperature T < T-i) is instead destabilized and entrained by buoyant plumes emitted from the bottom boundary. The convection thus mixes cold patches of low density-anomaly fluid with hot upward plumes and the end result is that the T-i isotherm sinks within the bottom boundary layer and that the convection is entrainment dominated. We provide a phenomenological description of the transition between the regimes of plume-dominated and entrainment-dominated convection through analysis of the differences in the heat transfer mechanisms, kinetic energy density spectra, and probability density functions for different stratification strengths. Importantly, we find that the effect of the stable layer on the convection decreases only weakly with increasing stratification strength, meaning that the dynamics of the stable layer and convection should be studied self-consistently in a wide range of applications

Free acidity determination in U (VI) solutions: A modern approach through Sequential Injection Analysis

International audienc

Développement d'un Lab-on-chip adapte au dosage volumique de l’acidité libre de solutions chargées

National audienc

Aspect ratio affects iceberg melting

Iceberg meltwater is a critical freshwater flux from the cryosphere to the oceans. Global climate simulations therefore require simple and accurate parametrizations of iceberg melting. Iceberg shape is an important but often neglected aspect of iceberg melting. Icebergs have an enormous range of shapes and sizes, and distinct processes dominate basal and side melting. We show how different iceberg aspect ratios and relative ambient water velocities affect melting using a combined experimental and numerical study. The experimental results show significant variations in melting between different iceberg faces, as well as within each iceberg face. These findings are reproduced and explained with multiphysics numerical simulations. At high relative ambient velocities melting is largest on the side facing the flow, and mixing during vortex generation causes local increases in basal melt rates of over 50%. Double-diffusive buoyancy effects become significant when the relative ambient velocity is low. Existing melting parametrizations do not reproduce our findings. We propose several corrections to capture the influence of aspect ratio on iceberg melting

Photothermal microfluidic sensor based on an integrated Young interferometer made by ion exchange in glass

International audienc

Monoamide and Diamide Nitric Acid Extraction at the Water/Dodecane Monitored by Second Harmonic Generation

International audienc

Microfluidics and integrated optics glass sensor for in-line microprobing of nuclear samples

International audienceWe study the miniaturisation of Thermal Lens Spectrometry (TLS) towards Lab-on-chip integration in order to reduce the volume of fluid assays in nuclear process control. TLS is of great interest in this context since it combines the advantages of optical detection methods with an inherent suitability for small-scale samples. After validating the experimental principle in a classical thermal lens crossed-beam setup, we show the integration of a Young-interferometer with a microcapillary on a glass substrate, reducing the necessary sample size to 400 nl. The interferometer translates the photothermally induced refractive index change in the fluid to a phase shift of the fringe pattern, which can then be detected by a camera. Measurements of Co(II) in ethanol yield a detection limit of c = 5 ×10 -4 M for the crossed-beam setup and c = 6×10 -3 M for the integrated sensor. At an interaction length of 10μm, it detects a minimum absorbance of AU = 6 × 10 -5 in a probed volume of 10 pl