Etude statistique des chemins de premier retour aux nombres de Knudsen intermédiaires : De la simulation par méthode de Monte Carlo à l'utilisation de l'approximation de diffusion

En présence de diffusions multiples, les algorithmes de Monte-Carlo sont trop coûteux pour être employés dans les algorithmes de reconstruction d'images de géométries tridimensionnelles réalistes. Pour des trajectoires de premiers retours, l'approximation de diffusion est communément employée afin de représenter la statistique des chemins aux nombres de Knudsen tendant vers zéro. En formulant des problèmes équivalents sur des trajectoires de premiers passages, l'usage de l'approximation est étendue en un développement théorique. Cette nouvelle formulation assure un bon niveau de précision, sur une large plage de valeurs du nombre de Knudsen en ce qui concerne l'évaluation des moments de la distribution des longueurs des chemins de premier retour. La résolution numérique du modèle formulé est confrontée aux simulations numériques type Monte- Carlo sur des géométries mono-dimensionnelles et un cas tridimensionel ouvrant des perspectives vers une généralisation aux applications réelles. ABSTRACT : For multiple scattering, Monte-Carlo algorithms are computationally too demanding for use in image reconstruction of 3D realistic geometries. In the study of first return path, the diffusion approximation is commonly used to represent their statistical behaviour when the Knudsen number tends to zero. With the formulation of equivalent problems for first passage path, the use of the approximation is extended in a theoretical development. The new model provides a good level of accuracy, for a wide distribution of Knudsen numbers when evaluating the moments distribution of the first return paths length. Numerical application of the model is confronted to Monte-Carlo simulations on one dimension geometries and a simple three-dimension case opening perspectives for the generalization to practical applications

Solutions of the Aw-Rascle-Zhang system with point constraints

We revisit the entropy formulation and the wave-front tracking construction of physically admissible solutions of the Aw-Rascle and Zhang (ARZ) " second-order " model for vehicular traffic. A Kruzhkov-like family of entropies is introduced to select the admissible shocks. This tool allows to define rigorously the appropriate notion of admissible weak solution and to approximate the solutions of the ARZ model with point constraint. Stability of solutions w.r.t. strong convergence is justified. We propose a finite volumes numerical scheme for the constrained ARZ, and we show that it can correctly locate contact discontinuities and take the constraint into account

Méthode de Monte Carlo et synthèse d'images : application à des milieux diffusants en transfert radiatif

International audienceL'objectif principal de cette étude est la prise en compte de la diffusion multiple pour le calcul de transferts radiatifs dans des géométries complexes. Pour traiter la diffusion, un algorithme basé sur la méthode de Monte Carlo a été développé et implémenté dans un environnement issu d'un code de synthèse d'images existant. A l'aide de propriétés d'invariance de la statistique des chemins de diffusion multiple, nous avons validé cette approche dans le cas de trois géométries tridimensionnelles usuelles : sphère, cylindre et pyramide

Prediction of thermal radiative properties (300–1000 K) of La2NiO4+δ ceramics.

A multiscale numerical model is developed to predict the thermal radiative properties (TRP) of rough La2NiO4+δ coatings. The model integrates intrinsic and extrinsic contributions related to the chemical composition and the texture, respectively. High-temperature infrared reflectivity and thermogravimetric measurements on a La2NiO4+δ single crystal make it possible to understand the role of the excess oxygen in the intrinsic TRP. We show that dense ceramics with thicknesses higher than 4 μm are optically thick, and that one can adjust the surface roughness parameters to predict their TRP

Can We Distinguish Age-Related Frailty from Frailty Related to Diseases? Data from the MAPT Study

Abstract Background No study has tried to distinguish subjects that become frail due to diseases (frailty related to diseases) or in the absence of specific medical events; in this latter case, it is possible that aging process would act as the main frailty driver (age-related frailty). Objectives To classify subjects according to the origin of physical frailty: age-related frailty, frailty related to diseases, frailty of uncertain origin, and to compare their clinical characteristics. Materials and methods We performed a secondary analysis of the Multidomain Alzheimer Preventive Trial (MAPT), including 195 subjects ≥70 years non-frail at baseline who became frail during a 5-year follow-up (mean age 77.8 years ± 4.7; 70% female). Physical frailty was defined as presenting ≥3 of the 5 Fried criteria: weight loss, exhaustion, weakness, slowness, low physical activity. Clinical files were independently reviewed by two different clinicians using a standardized assessment method in order to classify subjects as: "age-related frailty", "frailty related to diseases" or "frailty of uncertain origin". Inconsistencies among the two raters and cases of uncertain frailty were further assessed by two other experienced clinicians. Results From the 195 included subjects, 82 (42%) were classified as age-related frailty, 53 (27%) as frailty related to diseases, and 60 (31%) as frailty of uncertain origin. Patients who became frail due to diseases did not differ from the others groups in terms of functional, cognitive, psychological status and age at baseline, however they presented a higher burden of comorbidity as measured by the Cumulative Illness Rating Scale (CIRS) (8.20 ± 2.69; vs 6.22 ± 2.02 frailty of uncertain origin; vs. 3.25 ± 1.65 age-related frailty). Time to incident frailty (23.4 months ± 12.1 vs. 39.2 ± 19.3 months) and time spent in a pre-frailty condition (17.1 ± 11.4 vs 26.6 ± 16.6 months) were shorter in the group of frailty related to diseases compared to age-related frailty. Orthopedic diseases (n=14, 26%) were the most common pathologies leading to frailty related to diseases, followed by cardiovascular diseases (n=9, 17%) and neurological diseases (n = 8, 15%). Conclusion People classified as age-related frailty and frailty related to diseases presented different frailty-associated indicators. Future research should target the underlying biological cascades leading to these two frailty classifications, since they could ask for distinct strategies of prevention and management

A bloodâ based nutritional risk index explains cognitive enhancement and decline in the multidomain Alzheimer prevention trial

IntroductionMultinutrient approaches may produce more robust effects on brain health through interactive qualities. We hypothesized that a bloodâ based nutritional risk index (NRI) including three biomarkers of diet quality can explain cognitive trajectories in the multidomain Alzheimer prevention trial (MAPT) over 3â years.MethodsThe NRI included erythrocyte nâ 3 polyunsaturated fatty acids (nâ 3 PUFA 22:6nâ 3 and 20:5nâ 3), serum 25â hydroxyvitamin D, and plasma homocysteine. The NRI scores reflect the number of nutritional risk factors (0â 3). The primary outcome in MAPT was a cognitive composite Z score within each participant that was fit with linear mixedâ effects models.ResultsEighty percent had at lease one nutritional risk factor for cognitive decline (NRI â ¥1: 573 of 712). Participants presenting without nutritional risk factors (NRI=0) exhibited cognitive enhancement (β = 0.03 standard units [SU]/y), whereas each NRI point increase corresponded to an incremental acceleration in rates of cognitive decline (NRIâ 1: β = â 0.04 SU/y, P = .03; NRIâ 2: β = â 0.08 SU/y, P < .0001; and NRIâ 3: β = â 0.11 SU/y, P = .0008).DiscussionIdentifying and addressing these wellâ established nutritional risk factors may reduce ageâ related cognitive decline in older adults; an observation that warrants further study.Highlightsâ ¢Multiâ nutrient approaches may produce more robust effects through interactive propertiesâ ¢Nutritional risk index can objectively quantify nutritionâ related cognitive changesâ ¢Optimum nutritional status associated with cognitive enhancement over 3â yearsâ ¢Suboptimum nutritional status associated with cognitive decline over 3â yearsâ ¢Optimizing this nutritional risk index may promote cognitive health in older adultsPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152935/1/trc2jtrci201911004.pd

Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.

RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Statistical study of first return paths for intermediate Knudsen numbers : from Monte-Carlo simulations to the diffusion approximation use

En présence de diffusions multiples, les algorithmes de Monte-Carlo sont trop coûteux pour être employés dans les algorithmes de reconstruction d'images de géométries tridimensionnelles réalistes. Pour des trajectoires de premiers retours, l'approximation de diffusion est communément employée afin de représenter la statistique des chemins aux nombres de Knudsen tendant vers zéro. En formulant des problèmes équivalents sur des trajectoires de premiers passages, l'usage de l'approximation est étendue en un développement théorique. Cette nouvelle formulation assure un bon niveau de précision, sur une large plage de valeurs du nombre de Knudsen en ce qui concerne l'évaluation des moments de la distribution des longueurs des chemins de premier retour. La résolution numérique du modèle formulé est confrontée aux simulations numériques type Monte- Carlo sur des géométries mono-dimensionnelles et un cas tridimensionel ouvrant des perspectives vers une généralisation aux applications réelles.For multiple scattering, Monte-Carlo algorithms are computationally too demanding for use in image reconstruction of 3D realistic geometries. In the study of first return path, the diffusion approximation is commonly used to represent their statistical behaviour when the Knudsen number tends to zero. With the formulation of equivalent problems for first passage path, the use of the approximation is extended in a theoretical development. The new model provides a good level of accuracy, for a wide distribution of Knudsen numbers when evaluating the moments distribution of the first return paths length. Numerical application of the model is confronted to Monte-Carlo simulations on one dimension geometries and a simple three-dimension case opening perspectives for the generalization to practical applications

Etude statistique des chemins de premier retour aux nombres de Knudsen intermédiaires (de la simulation par méthode de Monte Carlo à l'utilisation de l'approximation de diffusion)

En présence de diffusions multiples, les algorithmes de Monte-Carlo sont trop coûteux pour être employés dans les algorithmes de reconstruction d'images de géométries tridimensionnelles réalistes. Pour des trajectoires de premiers retours, l'approximation de diffusion est communément employée afin de représenter la statistique des chemins aux nombres de Knudsen tendant vers zéro. En formulant des problèmes équivalents sur des trajectoires de premiers passages, l'usage de l'approximation est étendue en un développement théorique. Cette nouvelle formulation assure un bon niveau de précision, sur une large plage de valeurs du nombre de Knudsen en ce qui concerne l'évaluation des moments de la distribution des longueurs des chemins de premier retour. La résolution numérique du modèle formulé est confrontée aux simulations numériques type Monte- Carlo sur des géométries mono-dimensionnelles et un cas tridimensionel ouvrant des perspectives vers une généralisation aux applications réelles.For multiple scattering, Monte-Carlo algorithms are computationally too demanding for use in image reconstruction of 3D realistic geometries. In the study of first return path, the diffusion approximation is commonly used to represent their statistical behaviour when the Knudsen number tends to zero. With the formulation of equivalent problems for first passage path, the use of the approximation is extended in a theoretical development. The new model provides a good level of accuracy, for a wide distribution of Knudsen numbers when evaluating the moments distribution of the first return paths length. Numerical application of the model is confronted to Monte-Carlo simulations on one dimension geometries and a simple three-dimension case opening perspectives for the generalization to practical applications.TOULOUSE-INP (315552154) / SudocSudocFranceF

Photon path statistics in turbid media

International audienc