62 research outputs found
Comparative study of rans turbulence models for impinging jet simulations
Impinging plane jets are flow configurations with applications in many fields. The ability of several turbulence models to simulate this kind of flow was assessed. The numerical results were compared with the data of Cooper et al. (1993) for Re=7.0 104 and a jet nozzle aspect ratio H/D=6, and Re=2.3 104 and H/D=2. The SST, RSM-ε and RSM-BSL turbulence models were chosen because of their capability to predict flows with high streamline curvature and their expected good accuracy on near wall flow. The results show that the SST and RSM- ε perform better than the RSM-BSL in the prediction of the mean velocity profiles in the wall-jet and impact zone. The profiles of turbulent energy were particularly difficult to reproduce. All the turbulence models considered here exhibit a similar behavior with respect to the profiles of turbulent energy. The paper shows again the complexity of turbulent impinging jets and the relevant open challenges in CF
Comparative study of rans turbulence models for impinging jet simulations
Impinging plane jets are flow configurations with applications in many fields. The ability of several turbulence models to simulate this kind of flow was assessed. The numerical results were compared with the data of Cooper et al. (1993) for Re=7.0 104 and a jet nozzle aspect ratio H/D=6, and Re=2.3 104 and H/D=2. The SST, RSM-ε and RSM-BSL turbulence models were chosen because of their capability to predict flows with high streamline curvature and their expected good accuracy on near wall flow. The results show that the SST and RSM- ε perform better than the RSM-BSL in the prediction of the mean velocity profiles in the wall-jet and impact zone. The profiles of turbulent energy were particularly difficult to reproduce. All the turbulence models considered here exhibit a similar behavior with respect to the profiles of turbulent energy. The paper shows again the complexity of turbulent impinging jets and the relevant open challenges in CF
Vortex Structure in the Wall Region of an Impinging Plane Jet
The work presented here comes within a research program dealing with vortex detection in the impingement region
of a planar jet. In this study, experiments have been performed for a submerged turbulent water slot jet impinging
normally on a flat plate, and an emphasis was put on the flow field characteristics. For this purpose, particle image
velocimetry (PIV) have been employed. A comprehensive fluid mechanical data includes instantaneous and mean
flow field, variance of normal and cross velocity fluctuations have been presented. The present work is also
concerned with the flow structure in the impingement region where the transfers (heat/mass) occur. An attempt has
been made to understand the flow structure by employing the vortex detection criteria on the instantaneous velocity
vector field. Accordingly, the PIV measurements were carried out for four different Reynolds numbers: 3000, 6000,
11000 and 16000, and at three different planes: a plane parallel to the impingement plate, transverse plane of the jet
and a plane perpendicular to the jet. A method of filtration, based on proper orthogonal decomposition (POD)
technique was applied first to the instantaneous velocity and filtered velocity database is then used for vortex
detection. Further, the results about the size, shape, spatial distribution and energy content of the detected vortices
have been provided
Automatic eduction and statistical analysis of coherent structures in the wall region of a confine plane
This paper describes a vortex detection algorithm used to expose and statistically characterize the
coherent flow patterns observable in the velocity vector fields measured by Particle Image
Velocimetry (PIV) in the impingement region of air curtains. The philosophy and the architecture of
this algorithm are presented. Its strengths and weaknesses are discussed. The results of a
parametrical analysis performed to assess the variability of the response of our algorithm to the 3
user-specified parameters in our eduction scheme are reviewed. The technique is illustrated in the
case of a plane turbulent impinging twin-jet with an opening ratio of 10. The corresponding jet
Reynolds number, based on the initial mean flow velocity U0 and the jet width e, is 14000. The
results of a statistical analysis of the size, shape, spatial distribution and energetic content of the
coherent eddy structures detected in the impingement region of this test flow are provided.
Although many questions remain open, new insights into the way these structures might form,
organize and evolve are given. Relevant results provide an original picture of the plane turbulent
impinging jet
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
Narcissistic self-sorting of n-acene nano-ribbons yielding energy-transfer and electroluminescence at p-n junctions
The 2,3-didecyloxy derivative of an n-type anthracene (n-BG) and a p-type tetracene (p-R) have been synthesized and their self-assembly into nano-ribbons studied. Hyperspectral fluorescence imaging revealed their narcissistic self-sorting, leading to separated nanoribbons emitting with very different colors (blue or green for n-BG, depending on the growth solvent, and red for p-R). It is unique that the usual origins of self-sorting, such as specific H-bonding, different growth kinetics, or incompatible steric hindrance can be ruled out. Hence, the narcissistic behaviour is herein proposed to originate from a sofar unconsidered cause: the discrepancy between the quadrupolar character of n-BG and dipolar character of p-R. At the p-n junctions of these nanoribbons, inter-ribbon FRET and electro-luminescence switch-on were observed by fluorescence/luminescence microscopy.The authors acknowledge the financial support of the European Research Council Marie Curie Actions (FP7-PEOPLE-2012-ITN SMARTNET Grant agreement Nr 316656); the CNRS; the French Ministry of Education and Research; the Region Aquitaine; the ANR-06-JCJC-0030; the Department of Education, Science and Universities of the Basque Country Government (postdoctoral grant and project IT1639-22); the "Arina" informatic cluster of UPV/EHU; the facilities ELORGA of UB; and the ANR-17-CE24-0033-01 RESOLVE. The authors thank Dr A. Mendez-Ardoy for the CV measurements and CESAMO for structural analyses (ISM, Univ. Bordeaux)
Trade-offs in linking adaptation and mitigation in the forests of the Congo Basin
Recent discussions on forests and climate change have highlighted the potential for conservation of tropical forests to contribute synergistically to both mitigation (reducing emissions of greenhouse gases) and adaptation (increasing capacity to cope with changing climate conditions). Key mechanisms through which adaptive advantages might be gained include the potential for forest resources to support livelihoods in the context of climatic strains on agriculture and the protection that intact forest ecosystems might provide against landslides, flash floods and other hazards related to extreme weather. This paper presents findings from field research with forest communities in three areas of the Congo Basin in Central Africa, in which the adaptive role and potential of forests in these respects is critically analysed. The investigation was carried out through a combination of structured and semi-structured qualitative techniques within six villages in Cameroon, Equatorial Guinea and Rwanda. The findings of the research highlight the need to understand both the limits of synergy, and the constraints and trade-offs for rural livelihoods that may be associated with a forest conservation agenda driven by the additional impetus of carbon sequestration. The search for synergy may be conceptually laudable, but if forest management actions do not take account of on-the-ground contexts of constraints and social trade-offs then the result of those actions risks undermining wider livelihood resilience
Stabilization of polymer-fullerene bulk-heterojuntion morphology
International audienc
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
Narcissistic self-sorting of n-acene nano-ribbons yielding energy-transfer and electroluminescence at p–n junctions
The 2,3-didecyloxy derivative of an n-type anthracene (n-BG) and a p-type tetracene (p-R) have been synthesized and their self-assembly into nano-ribbons studied. Hyperspectral fluorescence imaging revealed their narcissistic self-sorting, leading to separated nanoribbons emitting with very different colors (blue or green for n-BG, depending on the growth solvent, and red for p-R). It is unique that the usual origins of self-sorting, such as specific H-bonding, different growth kinetics, or incompatible steric hindrance can be ruled out. Hence, the narcissistic behaviour is herein proposed to originate from a so-far unconsidered cause: the discrepancy between the quadrupolar character of n-BG and dipolar character of p-R. At the p–n junctions of these nanoribbons, inter-ribbon FRET and electro-luminescence switch-on were observed by fluorescence/luminescence microscopy.Fabrication Supramoléculaire de Dispositifs OLED Haute Résolution par voie ImprimableModulation of Organic Optic Information Interface
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