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La thèse traite de la simulation numérique de jets d’air lobés pour l’optimisation des Unités Terminales de Diffusion de l’Air pour le bâtiment. Il est présenté tout d’abord une analyse bibliographique exhaustive sur les jets d’air et sur les moyens passifs de leur contrôle. Vient ensuite un exposé des principes de la modélisation des écoulements turbulents. Sur la base des précédents éléments, nous abordons la phase de simulation et d’analyse de jets d’air lobés en trois parties. La première partie traite de la simulation d’un jet d’air libre à très faible nombre de Reynolds (Reynolds 800) en régime instationnaire. Dans cette étude, nous avons présenté plusieurs cas tests de simulation d’un jet d’orifice en forme de croix que nous avons confrontés à des résultats expérimentaux. Du point de vue quantitatif, la simulation du quart du jet a permis d’obtenir des caractéristiques globales assez satisfaisantes. Pour détecter d’une façon significative l’instationnarité du jet, il a été nécessaire de simuler la totalité du jet, mais ceci a été fait au détriment du nombre de mailles dans le domaine de calcul, ce qui conduit par conséquent à des résultats quantitativement inappropriés. La deuxième partie simule le même jet d’air mais en régime turbulent (Reynolds 3000). Sept modèles de turbulence ont été confrontés à des résultats expérimentaux. Nous montrons la pertinence du modèle RSM (Reynolds Stress Model) pour la prédiction de l’écoulement. La dernière partie est dédiée à la simulation de jets d’air turbulents en interaction. Cette partie se décline en trois études successives et complémentaires. La première a pour objet de rechercher le modèle de turbulence le plus pertinent capable de reproduire les phénomènes et les grandeurs utiles à l’application visée. La conclusion est qu’aucun des modèles de turbulence évalués n’est capable de prédire l’ensemble des caractéristiques dynamiques de l’écoulement de jets lobés en interaction. Cependant, parmi ces modèles, SST apparait nettement supérieur dans la prédiction de l’interaction des jets, de l’expansion dynamique globale et de l’entrainement de l’air ambiant lorsque l’écoulement est résolu à travers le diffuseur lobé. Sur la base des grandeurs dynamiques pertinentes identifiées et dont la prédiction par le modèle SST est jugée satisfaisante, nous entreprenons dans la suite une analyse de l’influence de la géométrie du lobe sur la capacité d’induction de l’écoulement. Un lobe de forme arrondie est alors trouvé plus avantageux qu’un lobe à angles droits. Enfin, la dernière étude s’intéresse à l’effet de la disposition et de l’espacement des orifices lobés sur les grandeurs globales de l’écoulement. Une solution géométrique permettant l’augmentation de la transparence de l’Unité Terminale de Diffusion de l’Air lobée est alors proposée. Elle devra être testée expérimentalement en conditions réelles, dans la cellule test échelle 1 thermiquement gardée nouvellement construite au LEPTIAB. / The thesis deals with the numerical simulation of lobed air jets for the optimization of Air Diffusion Terminal Units for buildings. First is presented a comprehensive literature review of the air jets and of the passive means of flow control. The manuscript continues with the principles of modeling of turbulent flows. Based on the previous considerations, we discuss the simulation and analysis of the air lobed jets following three parts. The first part deals with the simulation of an air jet at very low Reynolds number (Reynolds 800) in the unsteady regime. In this study, we presented several test cases of simulation for a cross-shaped orifice jet that we have confronted with experimental results. From the quantitative point of view, the simulation of a quarter of the jet’s section has yielded reasonably good results for the global characteristics of the flow. In order to detect in a significant way the unsteadiness of the jet, it was necessary to simulate the entire jet, but this was done at the expense of the number of cells in the computational domain, which consequently leaded to inappropriate quantitative results. In the second part we simulated the same air flow but in the turbulent regime (Reynolds 3000). Seven turbulence models have been confronted with experimental results. We show the relevance of the RSM turbulence model (Reynolds Stress Model) for predicting the flow.The last part is dedicated to the simulation of turbulent air jet in interaction. This part is divided into three successive and complementary studies. The first one was dedicated to find which turbulence model can reproduce most relevant phenomena and variables to the concerned application. The conclusion is that none of the turbulence models was able to predict all the dynamic features of the lobed jet flows in interaction. However, of these models, SST appears to be significantly better for the prediction of the interaction of the jets, the global dynamical expansion and the entrainment of the ambient air when the flow is numerically resolved through the lobed diffuser. On the basis of relevant dynamic quantities well predicted by the SST turbulence model, we undertake in the following part an analysis of the influence of the geometry of the lobe on the ability of the flow to entrain. A rounded lobe is then found to be more advantageous than a lobe with right angles. The final study examines the effect of the arrangement and spacing of the lobed orifices on the global quantities of the flow. A geometric solution allowing to increase the transparency of the lobed Air Diffusion Terminal Unit is then proposed. This solution should be tested experimentally in real conditions, in the new full scale model room constructed at LEPTIAB.

Cette thèse porte sur la ventilation par mélange à haute induction pour le bâtiment au moyen de diffuseurs d’air innovants munis de promoteurs de tourbillons. Il s’agit d’inserts lobés, introduits au sein de diffuseurs d’air commerciaux, sans en modifier le processus de fabrication. L’innovation est née de travaux antérieurs et a fait récemment l’objet d’un brevet européen. Sa mise en situation à l’échelle 1 en conditions typiques de chauffage et de climatisation, constitue l’objectif premier des présents travaux. Par une démarche expérimentale, l’impact des inserts lobés sur la topologie de l’écoulement moteur, sur le confort thermique induit en zone d’occupation d’une cellule climatique thermiquement gardée, et sur les pertes de charges et le bruit générés, sont évalués. Les écoulements moteurs sont analysés par PIV 2D2C grand champ, et le confort thermique est évalué en zone d’occupation à l'aide d’un ensemble de capteurs ponctuels de vitesse et de température et du modèle PMV/PPD de Fanger. Les résultats montrent que les inserts lobés introduits dans le diffuseur, favorisent le mélange du jet qui en est issu avec l’air ambiant à traiter. Le confort thermique en zone d’occupation est amélioré de façon significative, par rapport au cas de référence du même diffuseur sans inserts. L’impact des inserts sur l’acoustique et les pertes de charges n’est pas significatif, ce qui valide l’innovation pour son application immédiate dans le bâtiment. En dernière partie, nous avons comparé la performance des diffuseurs innovants munis d’inserts lobés au diffuseur vortex, présenté sur le marché de la diffusion de l’air comme étant doté d’une grande capacité de mélange, suite à la mise en rotation du jet à l’aide d’ailettes profilées. Les résultats ont montré que l’encastrement au plafond du diffuseur vortex préconisé par le fabricant, dont il résulte l’adhérence immédiate du jet par effet Coanda, inhibe le mouvement de rotation escomptée, et sa performance dans ces conditions est inférieure à celle du diffuseur à inserts lobés. Il résulte de ce constat, la nécessité d’une installation du diffuseur vortex sur conduite apparente pour que sa performance soit réelle ; cela est envisageable pour des locaux commerciaux ou industriels de grande hauteur sous plafond. Les diffuseurs lobés encastrés au plafond, dont on a ainsi démontré la performance, sont parfaitement adaptés aux espaces de bureaux ou d’habitations de faibles hauteurs sous-plafond, et répondent par conséquent au réel besoin de la haute induction dans ce type de locaux. / This thesis deals with high-induction mixing ventilation for buildings using innovative air diffusers equipped with vortex promoters. These are lobed inserts, introduced into commercial air diffusers, without modifying the manufacturing process. The innovation was born from previous work and has recently been the subject of an European patent. The first objective of the present work is to conduct full scale experiments under typical heating and air conditioning conditions. By an experimental approach, the impact of the lobed inserts on the airflow and jet pattern, on the thermal comfort induced in the occupied zone of a thermally guarded climate chamber, and on the pressure drop and the noise generation. Airflow pattern is analyzed by large scale PIV 2D2C, and thermal comfort is evaluated in the occupied zone using a set of temperature and speed sensors and Fanger's PMV / PPD model. The results show that the lobed inserts introduced into the diffusers enhance the mixing between the jet and ambient air whose to be treated. The thermal comfort in the occupied zone is significantly improved, compared to the reference case of the same diffuser without inserts. The impact of the inserts on the acoustics and the pressure drop is not significant, which validates the innovation for its immediate application in buildings. Finally, we compared the performance of innovative diffusers with lobed inserts to the vortex diffuser, presented in the air diffusion market as a high mixing diffuser, because of the rotation generated by profiled fins. The results showed that when the diffuser is flush mounted to the ceiling, which is recommended by the manufacturer, the immediate attachment of the jet due to the Coanda effect inhibits the expected rotational movement. Its performance under these conditions is lower to that of the diffuser with lobed inserts. It follows from this finding that the installation of the vortex diffuser on a free pipe is necessary for its best performance ; this can be performed for high ceiling commercial buildings or industrial buildings. Ceiling flush mounted diffusers with lobed inserts, which we have demonstrated their performance, are perfectly suited for office spaces or homes with low ceiling heights, and therefore meet the real need for high induction in this type of buildings.

Rapid mixing and spreading of supersonic jets are two important characteristics in supersonic ejectors, noise reduction in jets and fuel mixing in supersonic combustion. It helps in changing the acoustic and thermal signature in supersonic exhaust. The supersonic nozzles in most cases result in compressible mixing layers. The subsonic nozzles form incompressible mixing layers but at high Mach numbers even they form compressible mixing layers. Compressible mixing layers have been found to have much lower mixing and spreading rates than incompressible mixing layer Birch & Eggers (1972). In order to enhance the spreading and mixing of mixing layers from supersonic nozzles various active and passive methods have been deviced. Active methods include fluid injection, fluid lobes and plasma actuation. Passive methods are mostly based on modifying the nozzle geometry such that the fluid expansion is ideal or the shock cell is broken. Many nozzles with exotic shapes have been developed to obtain mixing enhancements in supersonic jets Gutmark et al. (1995). To achieve enhanced mixing an innovative nozzle named as the Elliptic Sharp Tipped Shallow (ESTS) lobed nozzle has been developed in L.H.S.R., I.I.Sc., India Rao & Jagadeesh (2014). This nozzle has a unique geometry involving elliptical lobes and sharp tips. These lobes are generated using a simple manufacturing process from the throat to the exit. This lobed and sharp tipped structure introduces stream wise vortices and azimuthal velocity components which must help in enhanced mixing and spreading. The ESTS lobed nozzle has shown mixing enhancement with 4 lobes. The spreading rate was found to be double of the reference conical nozzle. This thesis is motivated by the need to investigate the flow physics involved in the ESTS lobed nozzle. The effect of varying the number of lobes and the design Mach number of the nozzle on the mixing and spreading characteristics will be further discussed. Visualisation studies have been performed. The schlieren and planar LASER Mie scattering techniques have been used to probe the flow. Instantaneous images were taken at axial planes with the reference conical and ESTS nozzles with three, four, five and six lobes. The nozzles are for design Mach number 2.0 and 2.5. The stagnation chamber pressure was maintained to obtain over expanded, ideally expanded and under expanded flows. LASER scattering was obtained by seeding the flow with water to observe the behaviour of the primary flow. The condensation of moisture due to the cold primary flow mixing with the ambient air was exploited to scatter laser and observe the flow structures in the mixing layer. A comparison of the images of the reference conical nozzle and the ESTS lobed nozzles shows changes in the mixing layers due to the ESTS lobed nozzles. The image of the reference conical nozzle shows a distinct potential core and mixing layers all along the length of the image. For the ESTS lobed nozzles this distinction becomes unclear shortly after the nozzle exit. Thus mixing of the primary flow and ambient air is seen to be enhanced in the case of all the ESTS lobed nozzles. The flow in the case of the ESTS lobed nozzles if found to be highly non axis symmetric. The starting process of the nozzles has been visualised using time resolved schlieren. Image processing was performed on the nozzles to quantify the spread rate. The shock structure of the nozzles has been studied and found to be modified due to the lobed geometry. The level of convolution of the mixing layer due to the lobed structure has been studied using fractal analysis. The four lobed nozzle was found to have the highest spread rate and th most convoluted shear layer. Hence this nozzle was further studied using background oriented schlieren and particle image velocimetry to quantify the flow field. These experimental results have been compared with CFD simulations using the commercial software CFX5. The computations and experiments don’t match accurately but the trends match. This allows for simulations to be used as a good first approximation. The acoustic properties of a jet are dependent on the flow structure behaviour. The ESTS lobes have been found to change the flow structure. Hence the ESTS lobed nozzle was predicted to change the acoustic signature of the flow. The acoustic measurements of the flow were carried out at National Aerospace Laboratories, Bengaluru. The screech of the overexpanded flow was seen to be eliminated and the overall sound levels were found to have been reduced in all cases. Thus the lobed nozzle was found to have acoustic benefits over the reference conical nozzle. Thus the ESTS lobed nozzle has been studied and compared with the conical nozzle using several methods. The changes due to the lobed structure have been studied quantitatively. Future studies would focus on the change in thrust due to the lobed structure. Also new geometries have been proposed inspired by the current design but with possible thrust benefits or manufacturing benefits.

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