34 research outputs found
A Simple Analytical Model of Evaporation in the Presence of Roots
Root systems can influence the dynamics of evapotranspiration of water out of
a porous medium. The coupling of evapotranspiration remains a key aspect
affecting overall root behavior. Predicting the evapotranspiration curve in the
presence of roots helps keep track of the amount of water that remains in the
porous medium. Using a controlled visual set-up of a 2D model soil system
consisting of monodisperse glass beads, we first perform experiments on actual
roots grown in partially saturated systems under different relative humidity
conditions. We record parameters such as the total mass loss in the medium and
the resulting position of the receding fronts and use these experimental
results to develop a simple analytical model that predicts the position of the
evaporating front as a function of time as well as the total amount of water
that is lost from the medium due to the combined effects of evaporation and
transpiration. The model is based on fundamental principles of evaporation flux
and includes empirical assumptions on the quantity of stoma in the leaves and
the transition time between regime 1 and regime 2. The model also underscores
the importance of a much prolonged root life as long as the root is exposed to
a partially saturated zone composed of a mixture of air and water. Comparison
between the model and experimental results shows good prediction of the
position of the evaporating front as well as the total mass loss from
evapotranspiration in the presence of real root systems. These results provide
additional understanding of both complex evaporation phenomenon and its
influence on root mechanisms.Comment: 10 pages, 6 figure
Spatially heterogeneous dynamics in a thermosensitive soft suspension before and after the glass transition
The microscopic dynamics and aging of a soft thermosensitive suspension was
investigated by looking at the thermal fluctuations of tracers in the
suspension. Below and above the glass transition, the dense microgel particles
suspension was found to develop an heterogeneous dynamics, featured by a non
Gaussian Probability Distribution Function (PDF) of the probes' displacements,
with an exponential tail. We show that non Gaussian shapes are a characteristic
of the ensemble-averaged PDF, while local PDF remain Gaussian. This shows that
the scenario behind the non Gaussian van Hove functions is a spatially
heterogeneous dynamics, characterized by a spatial distribution of locally
homogeneous dynamical environments through the sample, on the considered time
scales. We characterize these statistical distributions of dynamical
environments, in the liquid, supercooled, and glass states, and show that it
can explain the observed exponential tail of the van Hove functions observed in
the concentrated states. The intensity of spatial heterogeneities was found to
amplify with increasing volume fraction. In the aging regime, it tends to
increase as the glass gets more arrested.Comment: 19 pages, 10 figures, Soft Matter accepte
Convection in a vertical channel
International audienceThe flow generated by heat convection in a long, vertical channel is studied by means of particle imagery velocimetry techniques, with the help of the thermal measurements from a previous paper (Gibert et al 2009 Phys. Fluids 21 035109). We analyse the mean velocity profiles and the Reynolds stresses, and compare the present results with the previous ones obtained in a larger cell and at a larger Reynolds number.We calculate the horizontal temperature profile and the related horizontal heat flux. The pertinence of effective turbulent diffusivity and viscosity is confirmed by the low value of the associated mixing length. We study the one-point and two-point statistics of both velocity components. We show how the concept of turbulent viscosity explains the relations between the local probability density functions (pdf) of fluctuations for temperature, vertical and horizontal velocity components. Despite the low Reynolds number values explored, some conclusions can be drawn about the small scale velocity differences and the related energy cascade
Stability and Adsorption Properties of Electrostatic Complexes : Design of Hybrid Nanostructures for Coating Applications
We report the presence of a correlation between the bulk and interfacial
properties of electrostatic coacervate complexes. Complexes were obtained by
co-assembly between cationic-neutral diblocks and oppositely charged surfactant
micelles or 7 nm cerium oxide nanoparticles. Light scattering and reflectometry
measurements revealed that the hybrid nanoparticle aggregates were more stable
both through dilution and rinsing (from either a polystyrene or a silica
surfaces) than their surfactant counterparts. These findings were attributed to
a marked difference in critical association concentration between the two
systems and to the frozen state of the hybrid structures.Comment: 3 pages, 3 figures, 27 references, to appear in Langmuir Letter
Heat convection in a vertical channel : Plumes versus turbulent diffusion
11 pagesInternational audienceFollowing a previous study [Gibert , Phys. Rev. Lett. 96, 084501 (2006)], convective heat transfer in a vertical channel of moderate dimensions follows purely inertial laws. It would be therefore a good model for convective flows of stars and ocean. Here we report new measurements on this system. We use an intrinsic length in the definition of the characteristic Rayleigh and Reynolds numbers. We explicit the relation between this intrinsic length and the thermal correlation length. Using particle imaging velocimetry, we show that the flow undergoes irregular reversals. We measure the average velocity profiles and the Reynolds stress tensor components. The momentum flux toward the vertical walls seems negligible compared to the shear turbulent stress. A mixing length theory seems adequate to describe the horizontal turbulent heat and momentum fluxes, but fails for the vertical ones. We propose a naive model for vertical heat transport inspired by the Knudsen regime in gases
Redispersible Hybrid Nanopowders: Cerium Oxide Nanoparticle Complexes with Phosphonated-PEG Oligomers
Rare earth cerium oxide (ceria) nanoparticles are stabilized using
end-functional phosphonated-PEG oligomers. The complexation process and
structure of the resulting hybrid core-shell singlet nanocolloids are
described, characterized and modeled using light and neutron scattering data.
The adsorption mechanism is non-stoichiometric, yielding the number of adsorbed
chains per particle Nads = 270 at saturation. Adsorption isotherms show a high
affinity of the phosphonate head for the ceria surface (adsorption energy ~ -16
kT) suggesting an electrostatic driving force for the complexation. The ease,
efficiency and integrity of the complexation is highlighted by the formation of
nanometric sized cerium oxide particles covered with a well anchored PEG layer,
maintaining the characteristics of the original sol. This solvating brush-like
layer is sufficient to solubilize the particles and greatly expand the
stability range of the original sol up to pH = 9. We underscore two key
attributes of the tailored sol: i) strong UV absorption capability after
functionalization and ii) ability to re-disperse after freeze-drying as powder
in aqueous or organic solvents in varying concentrations as singlet
nanocolloids. This robust platform enables translation of intrinsic properties
of mineral oxide nanoparticles to critical end use.Comment: 10 figures, 12 pages, accepted at ACSNano March 0
Comparison between rough and smooth plates within the same Rayleigh-Bénard cell
International audienceIn a Rayleigh-Bénard cell at high Rayleigh number, the bulk temperature is nearly uniform. The mean temperature gradient differs from zero only in the thin boundary layers close to the plates. Measuring this bulk temperature allows to separately determine the thermal impedance of each plate. In this work, the bottom plate is rough and the top plate is smooth; both interact with the same bulk flow. We compare them and address in particular the question whether the influence of roughness goes through a modification of the bulk flow
Thermal boundary layer near roughnesses in turbulent Rayleigh-Bénard convection: flow structure and multistability
We present global heat-transfer and local temperature measurements, in an asymmetric parallelepiped Rayleigh-B ́enard cell, in which controlled square-studs roughnesses have been added. A global heat transfer enhancement arises when the thickness of the boundary layer matches the height of the roughnesses. The enhanced regime exhibits an increase of the heat transfer scaling. Local temperature measurements have been carried out in the range of parameters where the enhancement of the global heat transfer is observed. They show that the boundary layer at the top of the square-stub roughness is thinner than the boundary layer of a smooth plate, which accounts for most of the heat-transfer enhancement. We also report multistability at long time scales between two enhanced heat-transfer regimes. The flow structure of both regimes is imaged with background-oriented synthetic Schlieren and reveals intermittent bursts of coherent plumes
Modelling and dynamic simulation of solar heat integration into a manufacturing process in Réunion Island
International audienceAccording to the International Energy Agency, solar thermal energy is rarely used in industrial sector, despite its capacity to generate heat with the existing technologies. Moreover, a large part of the firms of Réunion Island are in the food-processing industry, which is the most heat consuming sector according to these studies. Among them, the Sorélait-Danone company (yoghurt production) has decided to collaborate so as to analyze the potential impact of solar heat integration into its actual heat generation system. The company has a fuel boiler which generates 7 bar saturated vapor (corresponding to a temperature of 165°C) at a nominal flow rate of 2,5 tons per hour. This boiler is supplied by a tank filled with water. In the latter, condensate coming from the process unit is mixed with the district's cold water, in order to compensate the vapor lost in the process. In this work, possibilities in terms of solar heat integration into the system have been investigated. 4 different cases have been simulated (direct or indirect integration, into the mixing tank or the boiler tank) and the corresponding saved fuel quantities have been estimated. The results of simulation show that in the best case, a fuel saving of 25 % can be expected with a total collector area of 600 m² for this company