Round Turbulent Air Jet Submitted to a Pulsed Coflow

Mean and turbulent properties of an unsteady round air jet submitted to a pulsed coflowing airstream were studied from laser Doppler Velocimetry measurements. The ejection velocity of the jet is kept at a constant value, whereas the coflowing stream is pulsed. These measurements revealed that the unsteadiness leads to a longitudinal partition of the jet. Near the jet exit, the flow is a quasi-steady jet flow. Farther downstream, the flow is unsteady with the creation of a large and propagative structure in the jet flow. The objective of the study is comprehen- sive understanding of the main physical mechanisms induced by the coflow unsteadiness. Consequences of the entrainment process are also discussed

Analyse de la dynamique des jets à densité variable en écoulement cocourant pulsé

Cette thèse s'inscrit dans le cadre de l'étude des écoulements turbulents fortement instationnaires avec larges variations de densité. Elle s'insère dans une action globale de compréhension des mécanismes physiques régissant le mélange air / carburant gazeux (GNV) dans les moteurs à piston. L'objectif de cette étude est d'analyser la dynamique des jets à densité variable en écoulement cocourant fortement pulsé. L'analyse a été conduite en se basant à la fois sur des données expérimentales obtenues par ánémométrie Laser Doppler bicomposantes, et une approche numérique par résolution des équations de Navier-Stokes moyennées (RANS). Outre le test de la pertinence des modèles de fermeture (k - e, Rij-e), cettes complémentarité permet d'apprécier l'impact de l'instationnarité sur le développement moyen et turbulent du jet. L'instationnarité modifie le flux d'excès de quantité de mouvement initial et impose à l'écoulement un champ d'accélération variable en temps. La réponse du jet à ces deux sollicitations dépend de sa capacité à s'adapter aux changements des conditions d'initial. Cette capacité liée au transport advectif induit une partition du jet en régions quasi statique et instationnaire. Les effets de mémoire observés dans la zone instationnaire peuvent être appréhendés par un modèle hyperbolique. Un front de vitesse de type jet naissant se développe lors de l'accélération et se traduit par un effet important sur le champ turbulent. Le mélange avec le fluide extérieur est réduit à son passage et plus généralement pendant toute l'accélération. En phase de décélération le gradient de pression contrôle la dynamique de l'écoulement. Dans le cas du jet à densité variable, le couplage avec le gradient de densité induit une force de flottabilité. En situation de jet léger, la structure moyenne de l'écoulement passe alors d'un jet à un sillage. Les conséquences sur la turbulence et le mélange sont identifiées et expliquées.TOULOUSE-ENSEEIHT (315552331) / SudocSudocFranceF

Simulation de la dynamique spatiale et temporelle du rayonnement transmis par une strate arborée d'une parcelle agroforestière. Confort thermique des ruminants en sylvopastoralisme

MasterManagement of the microclimate and shading generated by trees in an agroforestry plot is a potential means of adapting sheep farming systems to thermal stress caused by temperature increases. This study aims to investigate the transmission of incident radiation through the tree canopy and to characterize the spatio-temporal dynamics of the generated shading. To do so, a simulation approach using the Heterofor model hosted on the Capsis platform was chosen. The performance of the model was evaluated on two agroforestry plots, one with intra-plot trees (cherry and sycamore) and border trees, and the other with ash trees in a high hedge. The model makes satisfactory estimates of transmitted radiation (NSE: 0.75 to 0.83 and bias: -2 to 6%). The inaccuracies identified come from both the model and the data. From the simulations, hourly or averaged shading indicators (area or duration) were calculated and spatialized in the form of georeferenced maps. The sensitivity of these indicators to tree configuration, slope and exposure was then investigated. The heat load index (HLI), calculated from the transmitted radiation, show that the trees have a localized buffering effect on the thermal environment of the plotsLa gestion du microclimat et de l’ombrage générés par les arbres d’une parcelle agroforestière est un potentiel moyen d’adaptation des systèmes d’élevage ovins au stress thermique provoqué par des hausses de température. Cette étude vise à étudier la transmission du rayonnement incident à travers le couvert arboré et à caractériser la dynamique spatio-temporelle de l’ombrage généré. Pour ce faire, une approche par simulation via l’utilisation du modèle Heterofor hébergé sur la plateforme Capsis a été choisie. La performance du modèle a été évaluée sur deux parcelles agroforestières dont l’une avec des arbres intra-parcellaires (merisiers et érables sycomores) et des arbres debordure, et l’autre avec des frênes en bordure en forme de haie haute. Le modèle fait des estimations satisfaisantes du rayonnement transmis (NSE: 0.75 à 0.83 et biais: -2 à 6%). Les imprécisions identifiées viennent à la fois du modèle et des données. A partir des simulations, des indicateurs d’ombrage (surface ou durée) horaires ou moyennés, ont été calculés et spatialisés sous forme de cartographies géoréférencées. La sensibilité de ces indicateurs à la configuration des arbres, la pente et l’exposition a ensuite été investiguée. Les indices de charge thermique (HLI), calculés à partir du rayonnement transmis, montrent que les arbres ont un effet tampon localisé sur l’ambiance thermique des parcelle

Contribution à l'étude de la durée d'humectation au sein d'un couvert de pommier

La pomme, fruit le plus cultivé sur le sol français, est principalement menacée par le pathogène Venturia inaequalis, responsable de la maladie de la tavelure qui génère des pertes considérables si elle n'est pas traitée. La politique actuelle de gestions des risques phytopathologiques en France incite à une forte réduction des traitements phytosanitaires tout en maximisant le rendement et la qualité des productions. Dans ce contexte, il apparaît indispensable de mieux comprendre les interactions entre l'arbre, son pathogène, et leur environnement, qui s'articulent pour le cas de la tavelure du pommier autour de la durée d'humectation des feuilles. Au cours de ce travail nous nous sommes intéressés à ce paramètre pour essayer de mieux comprendre les interactions entre microclimat de l'arbre et durée d'humectation. L'étude s'est déroulée en trois étapes majeures : la modélisation de l'évaporation d'une goutte sur un support végétal, l'étude expérimentale de la mouillabilité des feuilles de pommier, et l'étude expérimentale de la variabilité spatiale de la durée d'humectation sous un couvert de pommiers. Ce travail a permis d expliciter la forte variabilité intra-couronne de la durée d'humectation via la prise en compte de la structure de l arbre et de la dynamique horaire du microclimat. Le modèle développé, au delà des liens déjà connus entre l intensité du flux évaporatif et les variables climatiques, a montré la sensibilité importante du temps d évaporation à la mouillabilité du support via la forme de la goutte d eau, mettant en avant la nécessité de quantifier au mieux cette interaction goutte support via l estimation des angles de contact statiques et dynamiques.Apples, which are the most cultivated fruit in France, are mainly endangered by the fungal pathogen Venturia inaequalis that cause apple scab disease on apple. This disease can be responsible of major products loss unless orchards are treated against apple scab. Nowadays in France, the phytopathological diseases management policies are encouraging growers to reduce considerably the use of pesticides, while keeping a high quality and yield level. In this context, one must understand better how the plant, the pathogen and their environment, interact with each other: for apple scab, the most important environmental parameter is leaf wetness duration. During this work, we studied leaf wetness duration to understand the interactions that occur between the tree microclimate and the wetness duration. To do that we divided our work in three major steps : the modeling of evaporation of a droplet at rest on a leaf, the experimental study of apple leaves wettability, and the experimental study of wetness duration spatial variability within an apple trees orchard. This study led us to clarify the strong intra-crown variability of leaf wetness duration through the consideration of tree structure and hourly dynamics of microclimate. The model we developed, beyond the known links between the evaporative flux intensity and the climatic parameters, showed a strong sensibility of the evaporation duration to the substrate wettability, highlighting the necessity to quantify at best this interaction, through the estimation of static and dynamic contact angles.CLERMONT FD-Bib.électronique (631139902) / SudocSudocFranceF

Modelling spatial and temporal leaf temperature dynamics at leaf and canopy scales

Poster avec acteabsen

Experimental and numerical study of a confined variable density turbulent jet submitted to a strongly pulsed coflow

An experimental and numerical work dedicated to the study of a confined variable density jet subjected to a time-varying coflow with high acceleration/deceleration levels is presented in this paper. A global analysis and a hyperbolic model based on momentum equation written for the centerline excess velocity are first developped to obtain physical understanding and to estimate relevant effects governing such a complex unsteady flow. In order to observe the jet as it interacts with the main flow and to quantify those effects, two-component laser Doppler velocimetry and optical concentration measurements based on Mie scattering are used and adapted to unsteady conditions. Light, heavy and air jets are successively studied. During acceleration phases, convective phenomena govern part of the flow and a front formation appears which strength increases as initial jet density is lower. This front propagates at a constant velocity and deeply modifies mixing processes and mass transfer in comparison with steady coflowing jet. Comparisons with RANS numerical simulations using k-eps model are presented. Agreement with measurements of longitudinal velocity field is acceptable

Protecting Apricot Orchards with Rain Shelters Reduces Twig Blight Damage Caused by Monilinia spp. and Makes It Possible to Reduce Fungicide Use

International audienceBlossom and twig blight, caused by Monilinia spp., is the main disease in apricot trees. In this study, we installed transparent rain shelters in apricot orchards to study their influence on the modification of the microclimate at the level of the tree canopy and on the reduction in moniliosis damage in twigs. Rain shelters significantly reduced the leaf wetness time measured within the foliage compared to the unsheltered trees (a reduction of between 43% and 67%). However, very few differences were observed in the daily averaged air temperature (up to 6%) and daily averaged air relative humidity (up to 1%). In the first experiment, on the apricot variety Bergarouge® (CEP Innovation, Lyon, France), moniliosis damage on twigs in the absence of phytosanitary protection was reduced by up to 62% for the trees provided with rain protection compared to the trees that did not receive rain shelters. A second experiment, involving five apricot tree varieties, made it possible to verify that fungicide protection could be reduced for the trees protected by rain covers, reducing moniliosis damage on twigs compared to full fungicide protection combined without rain protection. Finally, a third experiment comprising two apricot tree varieties has shown that in organic orchards, rain protection provides protection against moniliosis (twig blight) that is equivalent to an organic farming fungicide protection programme based on the use of copper sulphate and calcium polysulphide

Freeze dehydration vs. supercooling in tree stems: physical and physiological modelling

International audienceAbstract Frost resistance is the major factor affecting the distribution of plant species at high latitude and elevation. The main effects of freeze-thaw cycles are damage to living cells and formation of gas embolism in tree xylem vessels. Lethal intracellular freezing can be prevented in living cells by two mechanisms: dehydration and deep supercooling. We developed a multiphysics numerical model coupling water flow, heat transfer, and phase change, considering different cell types in plant tissues, to study the dynamics and extent of cell dehydration, xylem pressure changes, and stem diameter changes in response to freezing and thawing. Results were validated using experimental data for stem diameter changes of walnut trees. The effect of cell mechanical properties was found to be negligible as long as the intracellular tension developed during dehydration was sufficiently low compared to the ice induced cryostatic suction. The model was finally used to explore the coupled effects of relevant physiological parameters (initial water and sugar content) and environmental conditions (air temperature variations) on the dynamics and extent of dehydration. It revealed configurations where cell dehydration could be sufficient to protect cells from intracellular freezing, and situations where supercooling was necessary. This model, freely available with this paper, could easily be extended to explore different anatomical structures, different species and more complex physical processes

Implications of Urban Land Management on the Cooling Properties of Urban Trees: Citizen Science and Laboratory Analysis

International audienceTrees participate in mitigating the urban heat island phenomenon thanks to their transpiration and shading. This cooling potential is highly dependent on leaf area. Nevertheless, leaf traits potentially vary across different land management practices in urban settings, thereby challenging the models used to estimate thermal budgets. The present study aims to investigate the variability of leaf area traits of linden (Tilia spp.) urban trees, and their effect on simulated tree transpiration. Reconstruction of the leaf area was undertaken at the tree scale at three different urban land management sites from three cities: London and Birmingham (UK) and Chantilly (France). The reconstruction combined allometric measurements at shoot and leaf scales, and a tree-scale 3D digitization with laboratory analysis using field data collected by citizen scientists. The management practices had a significant impact on leaf area, and on tree allometric relationships, which were propagated through the reconstruction process. Relative differences between the management practices ranged between 12% and 48% according to the city where the variable was considered (e.g., leaf area index, total leaf area, or tree transpiration). Trees in managed sites (i.e., individualized leaf crowns, frequent leaf litter removal, and standard thinning/pruning operations) develop a higher leaf area, thus promoting cooling potential. This study shows that the variability of leaf traits, and their responses to different land management, can be studied by comprehensive data collection through citizen science and lab-based modelling. It also highlights the importance of appropriate, well-designed urban planning, where landscaping using urban trees can play an even better role in climate proofing cities