Joint PDF modelling of turbulent flow and dispersion in an urban street canyon

The joint probability density function (PDF) of turbulent velocity and concentration of a passive scalar in an urban street canyon is computed using a newly developed particle-in-cell Monte Carlo method. Compared to moment closures, the PDF methodology provides the full one-point one-time PDF of the underlying fields containing all higher moments and correlations. The small-scale mixing of the scalar released from a concentrated source at the street level is modelled by the interaction by exchange with the conditional mean (IECM) model, with a micro-mixing time scale designed for geometrically complex settings. The boundary layer along no-slip walls (building sides and tops) is fully resolved using an elliptic relaxation technique, which captures the high anisotropy and inhomogeneity of the Reynolds stress tensor in these regions. A less computationally intensive technique based on wall functions to represent boundary layers and its effect on the solution are also explored. The calculated statistics are compared to experimental data and large-eddy simulation. The present work can be considered as the first example of computation of the full joint PDF of velocity and a transported passive scalar in an urban setting. The methodology proves successful in providing high level statistical information on the turbulence and pollutant concentration fields in complex urban scenarios.Comment: Accepted in Boundary-Layer Meteorology, Feb. 19, 200

Stabilization of polymer-fullerene bulk-heterojuntion morphology

International audienc

Stabilité thermique de la couche active d’une cellule solaire organique par réticulation

Date du colloque : 11/2012</p

Weaning of Moderately Preterm Infants from the Incubator to the Crib: A Randomized Clinical Trial

OBJECTIVE: To assess whether length of hospital stay is decreased among moderately preterm infants weaned from incubator to crib at a lower vs higher weight. STUDY DESIGN: This trial was conducted in the Eunice Kennedy Shriver National Institute of Child Health and Human Development Neonatal Research Network. Infants with gestational ages 29-33 weeks, birthweight <1600 g, and in an incubator were randomly assigned to a weaning weight of 1600 or 1800 g. Within 60 to 100 g of weaning weight, the incubator temperature was decreased by 1.0°C to 1.5°C every 24 hours until 28.0°C. The infants were weaned to the crib following stable temperature at 36.5°C to 37.4°C for 8 to 12 hours. Clothing and bedcoverings were standardized. The primary outcome was length of hospital stay from birth to discharge; secondary outcomes included length of stay and growth velocity from weaning to discharge. Adverse events were monitored. RESULTS: Of 1565 infants screened, 885 were eligible, and 366 enrolled-187 to the 1600-g and 179 to the 1800-g group. Maternal and neonatal characteristics did not differ among weight groups. Length of hospital stay was a median of 43 days in the lower and 41 days in the higher weight group (P = .12). Growth velocity from completion of weaning to discharge was higher in the lower weight group, 13.7 g/kg/day vs 12.8 g/kg/day (P = .005). Groups did not differ in adverse events. CONCLUSIONS: Among moderately preterm neonates, weaning from incubator to crib at a lower weight did not decrease length of stay, but was safe and was accompanied by higher weight gain after weaning

Large-Eddy Simulation of Flow and Pollutant Transport in Urban Street Canyons with Ground Heating

Our study employed large-eddy simulation (LES) based on a one-equation subgrid-scale model to investigate the flow field and pollutant dispersion characteristics inside urban street canyons. Unstable thermal stratification was produced by heating the ground of the street canyon. Using the Boussinesq approximation, thermal buoyancy forces were taken into account in both the Navier–Stokes equations and the transport equation for subgrid-scale turbulent kinetic energy (TKE). The LESs were validated against experimental data obtained in wind-tunnel studies before the model was applied to study the detailed turbulence, temperature, and pollutant dispersion characteristics in the street canyon of aspect ratio 1. The effects of different Richardson numbers (Ri) were investigated. The ground heating significantly enhanced mean flow, turbulence, and pollutant flux inside the street canyon, but weakened the shear at the roof level. The mean flow was observed to be no longer isolated from the free stream and fresh air could be entrained into the street canyon at the roof-level leeward corner. Weighed against higher temperature, the ground heating facilitated pollutant removal from the street canyon.Singapore-MIT Alliance for Research and Technology. Center for Environmental Sensing and Monitorin

Phosphine emission measurements from a tobacco factory using cryogenic sampling and GC-ICP-MS analysis

cited By 7International audienceA cryosampling system together with a low temperature GC-ICP-MS method have been developed for the determination of phosphine for industrial hygiene purposes. The effect of sampling temperature on the collection efficiency of PH3 was investigated. Two trapping mechanisms were differentiated for temperature ranges from -190 to -160°C and from -110 to -35°C. A cartridge filled with NaOH was used during the analytical desorption step to separate CO2 from phosphine. Phosphine recovery was found to be better than 98% with peak area and retention times RSD values better than 4% and 2%, respectively. The use of a NafionB drying membrane before cryogenic trapping induced losses of PH3 of about 15%. Sample conservation in a cryocontainer at -190°C over a period of 15 days did not lead to significant losses of PH3. For field experiments, air samples were collected in the fumigation room of a tobacco factory after fumigation, and outside the fumigation room during fumigation, using both cryogenic sampling and standard filters impregnated with silver nitrate. The results showed that phosphine concentrations in the tobacco factory were below the limit values for occupational exposure to phosphine (VME (France) and TLV-TWA (USA)) and also below the detection limit of the silver impregnated filter-ICP-AES method. Cryogenic trapping in combination with GC-ICP-MS allowed us to determine phosphine concentrations. The concentrations in ambient air are estimated to be about 1 ng m-3 and the detected concentrations in the vicinity of the fumigation room during the fumigation process are lower than 10 ng m -3. © The Royal Society of Chemistry 2003

Metrology: Cryogenic sampler for analyzing VOC [Métrologie: Echantillonneur cryogénique pour l'analyse des COV]

cited By 0International audienceCryogenic trapping is an efficient technique to sample VOC and very volatile inorganic compounds such as hydrides in ultra traces. The cryogenic trap is an alternative to sampling systems on solid supports. The cryogenic sampler for hydrides attached to a powerful detector coupling, a GC to MS with induced plasma, ICP/MS. A sampling method based on cryogenics was chosen to conserve intact, up to analysis, all VOC species and notably those with very low boiling point the problems with capturing VOC on absorbents is the selectivity of absorbents, making quantification difficult. In cryogenic capture, VOC are condensed by the vaporization of liquid N2 in a cryostat permitting attainment of - 180° to - 20°C, with sampling tubes covered with glass wool. Collected samples are analyzed by flash thermal desorption developed by Gerstel, connected to GC with flame ionization detector. VOC studied: methanol, acetone, ethyl acetate, p-xylene, dichloromethane, methylethyl ketone, dichloroethane, methylisobutyl ketone, and toluene

Biogeochemical cycle and speciation of As and Cr in an acid mine environment: The case of Carnoulès Creek, France

cited By 4International audienceThe aim of the work was to understand the biogeochemical cycles of two pollutants of interest found in the Carnoulès Creek: arsenic and chromium. The data for dissolved chromium speciation showed that this element is rather homogeneously distributed along the seepage water with 70-80% of Cr (VI); in particles collected from the same samples, chromium is totally present under the trivalent form. These results are consistent with the fact that hexavalent chromium is much more mobile than Cr (III). According to the results of dissolved arsenic speciation, arsenite was the only species found. In the particles, As (V) and As (III) were detected and arsenate was the predominant form. A spatial evolution was evidenced with a global increase of As (V) percentage from the tailing basis to the river. This evolution can be explained by an oxidation phenomenon. Speciation analyses were also performed in solid samples: sediments and bacteria films taken in the seepage water. In sediments, more than 80% of As was under the pentavalent form whereas only 65% of arsenate is found in bacterial films