• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 296
  • 78
  • 49
  • 46
  • 29
  • 26
  • 8
  • 8
  • 7
  • 5
  • 3
  • 2
  • 1
  • 1
  • 1
  • Tagged with
  • 709
  • 136
  • 123
  • 121
  • 107
  • 73
  • 69
  • 55
  • 55
  • 53
  • 52
  • 48
  • 48
  • 44
  • 44
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
481

The calibration and sensitivity analysis of a storm surge model for the seas around Taiwan

Pai, Kai-chung 10 August 2009 (has links)
The topographical variations of the seas around Taiwan are great, which make the tides complicated. Taiwan is located in the juncture of the tropical and subtropical area. Geographically, it is located within the region of northwestern Pacific typhoon path. These seasonal and geographical situations causing Taiwan frequently threaten by typhoons during summer and autumn. In addition to natural disasters, the coastal area is over developed for the last few decades, which destroys the balance between nature and man. Storms and floods constantly threaten the lowland areas along the coast. An accurate and efficient storm surge model can be used to predict tides and storm surges. The model can be calibrated and verified with the field observations. Data measured by instruments at the tidal station constituting daily tidal variations and storm surge influences during typhoons. The model can offer both predictions to the management institutions and to the general public as pre-warning system and thus taking disaster-prevention measures. This study implements the numerical model, developed by Yu (1993) and Yu et al. (1994) to calculate the hydrodynamic in the seas around Taiwan. The main purpose of this study is to make a calibration and sensitivity analysis of the model parameters. Tidal gauge data around Taiwan coastal stations collected from June to October 2005 are used for the analysis and the comparison between the modeled data and the observations. Two steps have been taken for the model calibration and sensitivity analysis. First step is to calibrate the model for accurate prediction of the astronomical tide, and then the compound tide with meteorological influences. For the calibration of the astronomical tides, sensitivity analysis has been carried out by adjusting the horizontal diffusion coefficient and the bottom friction coefficients used in the model. The sensitivity of the time-step size used in the model and model grids fitted to coastlines are also checked. A depth dependent Chézy numbers are used in the model to describe bottom friction. The model has a better result when the Chézy value varied within 65 to 85. Modifying grids fitted to the coastline has improved the model results significantly. By improving the dynamic phenomenon brought about by the land features, the model calculation fits the real tidal phenomenon better. The analysis has shown that the model is less sensitive to the horizontal diffusion coefficient. Data from 22 tidal stations around Taiwan have been used for the comparisons. The maximum RMSE (root-mean-square error) is about 10 cm at WAi-Pu, whereas the minimum RMSE is about 1 cm for the stations along eastern coast. The calibration of the compound tide is divided into three cases. The first case is to calibrate the forecasted wind field. This has been done by comparing the forecasted wind field from the Central Weather Bureau with the satellite data obtained from QuikSCAT¡XLevel 3. The satellite wind speed has been applied to adjust the forecasted wind speed. The adjusted forecast wind field has shown improvement to the model predictions in the tidal stations south of Taichung, slightly improved in the eastern coast. The second case is tuning the drag coefficient on sea surface used by the hydrodynamic model. Several empirical formulas to describe the sea surface drag have been tested. The model result has shown little influence using various drag formulations. The third case is to single the influences by the meteo-inputs, i.e. the wind field and the atmospheric pressure. The tidal level is more sensitive to the variation of the atmospheric pressure through out the tests carried out during typhoon periods. The model simulation for 2006 using the best selected parameters has shown that the model is consisted with good stability and accuracy for both stormy and calm weather conditions.
482

Vers des systèmes de fenêtrage distribués : l'évolution du drag-and-drop

Collomb, Maxime 01 December 2006 (has links) (PDF)
Ce mémoire a pour objectif l'étude des interactions qui interviennent dans les environnements où plusieurs surfaces d'affichage et dispositifs de pointage sont mis à disposition du ou des utilisateurs, que ces environnements soient distribués ou non. Nous nous sommes plus particulièrement intéressés aux techniques de type drag-and-drop dans le contexte du mur-écran.<br /><br />Le drag-and-drop est une technique très répandue qui atteint ses limites dans certaines conditions d'utilisation. Ces limites peuvent être physiques -- avec des écrans interactifs par exemple -- ou dues à l'implémentation. D'une part, ce mémoire propose plusieurs évolutions du drag-and-drop, avec comme objectif de dépasser les limites physiques du drag-and-drop. En particulier, il devient possible de dépasser les limites de la loi de Fitts ainsi que les limites physiques des surfaces d'affichage. Un ensemble de techniques de types drag-and-drop issues de l'état de l'art ou proposées dans ce mémoire font l'objet d'études utilisateurs. A l'issue de ces études, deux techniques ressortent principalement : le push-and-pop et le push-and-throw accéléré.<br /><br />D'autre part, afin de dépasser les limites dues à l'implémentation du drag-and-drop, nous proposons un modèle d'implémentation pour le drag-and-drop et l'ensemble de ses évolutions. Ce modèle d'implémentation s'appuie sur le modèle d'interaction instrumentale. Ce modèle d'implémentation est mis en oeuvre dans l'API PoIP, elle-même utilisée dans l'application Orchis. Grâce à ce modèle, il est possible d'effectuer des opérations de type drag-and-drop distribuées, la technique d'interaction pouvant être personnalisée en fonction des contraintes matérielles ou des préférences de l'utilisateur.
483

Reglerbar kylartäckning - En lösning till isbildningsproblematik i laddluftkylaren

Hemmingsson, Daniel January 2015 (has links)
Denna rapport ingår i ett examensarbete på avancerad nivå inom ämnet produktutveckling.Arbetet innefattar en fallstudie som genomförts vid RTGA, Scania CV AB i Södertälje.Examensarbetet omfattar 30 högskolepoäng och har genomförts av undertecknad,teknologstudent från Mälardalens Högskola vårterminen 2015. Rapporten har upprättats i tvåversioner, en för Scania CV AB respektive en generell version där känslig företagsspecifikinformation har censurerats. I takt med att utvecklingen av motorprestanda fortskrider, med avseende på ökademotoreffekter samt teknikutveckling för emissionsreglering, så ökar också kravet på prestandaför kylsystem som måste klara av att kyla bort högre effekter. Fordonskombinationer medkraftfulla laddluftkylare löper dock stor risk att drabbas av utfälld kondens som fryser till is iladdluftkylaren vid låga omgivningstemperaturer. Isen medför ett tryckfall i laddluftkylarenmed prestandabortfall som följd. Scanias lösning till problematiken är en så kallad kylargardinsom syftar till att reducera kylluftflödet genom laddluftkylaren till den mån att isbildning intesker. Kylargardinen monteras manuellt av föraren framför kylarpaketet dåomgivningstemperaturen understiger 5 °C. Det finns även andra fördelar med att begränsa det yttre kylluftflödet som passerar genomkylarpaketet. Lastbilars kylsystem är i regel överdimensionerade för normala driftfall såsommotorvägskörning vilket betyder att maximal kylning inte krävs vid dessa situationer. Genomatt reducera det yttre kylluftflödet genom kylarpaketet så kan även hela fordonets totalaluftmotstånd reduceras vilket kan utnyttjas i syfte att reducera bränsleförbrukningen. För att erhålla en bra balansgång mellan aerodynamik och kylprestanda samt motverkaproblemet med isbildning i laddluftkylaren fanns därför en önskan om att en reglerbarkylartäckning skulle utvecklas för att kunna reglera kylluftflödet genom fordonets kylarpaketvid behov. Detta arbete innefattar en fallstudie där olika produktutvecklingsverktyg använts för att ta framolika konceptlösningar, anpassade för en specifik lastbil. Datorstödda flödessimuleringarutnyttjas för att utvärdera respektive verifiera konceptens funktion jämfört med olika referensmodeller. Resultatet visar bland annat att det slutgiltiga konceptets funktionsduglighet överensstämmermed den lösning som används idag samt att kylluftflödet inte påverkas nämnvärt då maximalkylkapacitet eftersträvas. Resultatet från den aerodynamiska flödessimuleringen indikerar ävenatt fordonets totala luftmotstånd kan reduceras med - Drag Counts (DC) vid hastigheten90 km/h och 0° vindriktning. Detta medför att bränsleförbrukningen i det specifika driftfallet,enligt tumregel, antas reduceras med - % för det specifika fordonet. / This report is part of a Master thesis project in the subject of Product Development. The workincludes a case study which was carried out at RTGA, Scania CV AB in Södertälje. The workcomprises 30 credits and was conducted by a student from Mälardalen University during thespring semester 2015. The report has been prepared in two versions; one for Scania CV AB anda generalized version where sensitive company specific information has been censored. The development of vehicle engine performance progresses in a rapid pace. This progressinclude increased engine power and improved technical features for emission control. This alsogoes for the requirement on performance of cooling systems in heavy trucks, which has tohandle dissipation of the increased power. Vehicles with powerful charge air coolers however,runs a high risk of being affected by iced condensation in the charge air cooler at low ambienttemperatures. The ice build-up results in a pressure drop in the charge air cooler with a loss ofengine performance as a consequence. Scania's solution to the problem is a so called radiatorblind designed to reduce the cooling air flow through the charge air cooler to the extent thatice build-up does not occur. The blind is mounted manually in front of the cooler package whenthe ambient temperature is expected to be below 5 °C. There are also other advantages to limit the outer cooling air flow passing through the coolingpackage. The cooling systems in heavy trucks are usually oversized for normal operatingconditions such as highway cruising, which means that maximum cooling is not required inthese situations. By reducing the airflow through the cooler package, the vehicle's total airresistance is reduced, which can be utilized in order to reduce fuel consumption. To obtain a good balance between aerodynamics and cooling performance, and to counteractthe problem of ice build-up in the charge air cooler, there was a desire for an adjustable radiatorcoverage which would be able to regulate the airflow through the vehicle's cooling packagewhen needed. This work includes a case study in which various product development tools are used toinvestigate different concept solutions, designated for a specific truck model. Computationalfluid dynamics (CFD) are used to evaluate and validate the concept’s functionality andcomparing them to different reference models. The results reveal that the final concept functionality is consistent with the one used today, andthat the cooling air flow is not significantly affected when maximum cooling capacity is strivedfor. The results of the aerodynamic flow simulation also indicate that the vehicle's total airresistance can be reduced by - Drag Counts (DC) at the speed of 90 km/h and 0° yaw angle.This means that fuel consumption in the specific operating case, as rule of thumb, is assumedto be reduced by - % for the specific vehicle.
484

Experimental in-plane behavior of a generic scale model drag embedment anchor in Kaolinite test beds

McCarthy, Katelyn Barbara 07 July 2011 (has links)
The trajectory and capacity are key components of the design of drag embedment anchor and drag-in vertically loaded anchors. This experimental testing program quantifies two factors that describe the anchor trajectory and capacity: the equilibrium bearing factor (Ne) and the tangential bearing factor (Ne). These factors can aid in the development of a numerical model of anchor behavior. A magnetometer device is used to track the orientation and location of the anchor during drag embedment. The results of the experimental testing program were compared with the results from a predictive model. The experimental program consisted of drag embedment tests with various testing conditions including different anchor line diameters and different initial pitch orientations. The results with the different anchor lines indicated that thinner anchor lines cause the anchor to dive deeper in the soil. The different initial pitch results indicate that regardless of the initial pitch of the anchor, the anchor rotates to a unique pitch trajectory within 2 fluke lengths. / text
485

The Hydrodynamic Effects of Long-line Mussel Farms

Plew, David Russell January 2005 (has links)
The hydrodynamic effects of long-line mussel farms are studied through a two-pronged approach. Large-scale hydrodynamic effects are investigated through the use of field measurements, primarily at a large mussel farm in Golden Bay, New Zealand (230 long-lines, covering an area of 2.45 km by 0.65 km). The research focuses on three areas: the effect of the farm on currents, mixing and stratification, and the dissipation of wave energy. Measurements are also made of the forces on long-line anchor ropes, and a limited investigation is made of phytoplankton depletion. The second approach is the use of laboratory drag measurements and Particle Tracking Velocimetry (PTV) to study the effect of mussel dropper (vertical lengths of mussel-encrusted crop rope) roughness and spacing on flow at small scales. These experiments provide data on very rough cylinders, and on cylinder arrays. The field measurements show that the local effects of mussel farms on currents are significant, but that magnitudes of the effects depend on dropper density, mussel sizes, orientation of the long-lines to the flow, and other parameters that are necessary to characterise the complex interactions between a farm and the flow. The drag on the submerged structures reduces water velocities within the Golden Bay farm by between 47% and 67%. Mussel farms present a porous obstacle to the flow, and flow that does not pass through the farm must be directed around or beneath it. The field measurements indicate that at the study site, most of the flow is diverted around the farm despite its large horizontal dimensions. The droppers at the study site extend over most of the water column (average dropper length ~ 8 m, average water depth ~ 11 m), providing a restriction to the flow beneath the farm. The strength of the density stratification may also favour a horizontal diversion. The flow around the farm is essentially two-dimensional. This suggests that two-dimensional numerical models should be sufficient to obtain reasonable predictions of the velocity drop within, and the diversion around, mussel farms. A simple two-dimensional pipe-network model gives reasonable estimates of the velocity within the farm, demonstrating that the drag of the farm may be adequately parameterised through local increases of bed friction. A wake in the form of reduced velocities extends downstream of the farm, and a mixing layer analogy suggests that this wake spreads slowly. The downstream extent of the wake cannot be determined, although it is likely to be limited by the tidal excursion. The degree of vertical mixing caused by the flow through a mussel farm cannot be quantified, although there are clear interactions between the stratification and the farm. Two mixing mechanisms are considered. A shear layer is generated beneath the farm due to the difference in velocities between the retarded flow within the farm and the flow beneath. Shear layers beneath mussel farms are likely to be weak unless the ambient currents are strong. It will be necessary for stratification to be weak or non-existent for this mechanism to generate significant mixing. The second mechanism is smaller-scale turbulence generated by the mussel droppers. Although the efficiency of this form of mixing is likely to be low, the large number of mussel droppers suggests that there will be some enhancement of vertical mixing. Frequency-dependent wave attenuation is recorded, and is predicted with some success by an analytical model. Both the model and the field data show that wave dissipation increases as the wave period decreases. Wave energy dissipation at the study site averages approximately 10%, although the measurements are made during a period of low wave heights (Hs < 0.25 m). Measurements of long-line anchor rope tension at two study sites indicate that the loadings are induced by the tide, currents, and waves. Dynamic wave loadings may be significant, and higher wave forces are measured at the offshore end of a long-line. The issue of seston or phytoplankton depletion is considered briefly through the examination of fluorescence, turbidity, and acoustic backscatter data. Although the results are consistent with a reduction of seston within the farm, differences between the inside and outside of the farm are not statistically significant. Mussel droppers resemble extremely rough circular cylinders, with the mussel shells forming the surface roughness elements. Drag measurements and PTV flow visualisation are used to investigate the importance of the large surface roughness, and the influence of dropper spacing and long-line orientation on flow. Drag measurements conducted with smooth and rough cylinders show that high surface roughness (ks/D ~ 0.092) has little effect on the drag coefficient of single cylinders in the range 4,000 < Re < 13,000, yet increases the drag coefficient of a row of cylinders normal to the flow. High surface roughness on single cylinders has the effect of shortening the near-wake region, increasing the peak turbulent kinetic energy (TKE) behind the cylinder, and decreasing the Strouhal number (St = 0.21, 0.19, 0.17 for ks/D = 0, 0.048, and 0.094 respectively). Arrays of rough cylinders (ks/D = 0.094) demonstrate similar flow characteristics to those of smooth cylinders. At cylinder spacings of S/D < 2.2, the surface roughness acts to favour the formation of a particular metastable wake pattern, whereas different metastable wake patterns are formed each run behind the smooth cylinders. The experiments show that the drag on single row arrays of cylinders are related to the cylinder spacing (increasing drag with decreasing spacing), and the drag also varies with the sine of the angle to the flow, except where the array is at low angles to the flow. The PTV measurements provide new data regarding the two-dimensional distributions of velocity, TKE, and turbulence statistics behind the cylinder arrays.
486

Aerodynamic Validation of Emerging Projectile Configurations

Sor, Wei Lun 01 November 2012
Approved for public release; distribution is unlimited. / Ever-increasing demands for accuracy and range in modern warfare have expedited the optimization of projectile design. The crux of projectile design lies in the understanding of its aerodynamic properties early in the design phase. This research first investigated the aerodynamic properties of a standard M549, 155mm projectile. The transonic speed region was the focus of the research as significant aerodynamic variation occurs within this particular region. Aerodynamic data from wind tunnel and range testing was benchmarked against modern aerodynamic prediction programs like ANSYS CFX and Aero-Prediction 09 (AP09). Next, a comparison was made between two types of angle of attack generation methods in ANSYS CFX. The research then focused on controlled tilting of the projectile’s nose to investigate the resulting aerodynamic effects. ANSYS CFX was found to provide better agreement with the experimental data than AP09.
487

Estudo em túnel de vento do arrasto aerodinâmico sobre torres treliçadas de linhas de transmissão / Wind tunnel study of the aerodynamic drag forces on transmission lines lattice towers

Rippel, Leandro Inácio January 2005 (has links)
Estruturas treliçadas de linhas de transmissão submetidas à ação do vento têm sido utilizadas de forma rotineira em aplicações da engenharia estrutural há mais de um século. Entretanto, o conhecimento dos efeitos do vento sobre este tipo de estrutura é, ainda hoje, imperfeito e as prescrições das normas a respeito destes efeitos são, em muitos casos, mutuamente inconsistentes e em desacordo com os dados experimentais. Partindo deste contexto, no caso particular de estruturas para linhas de transmissão, podemos acrescentar que as normas existentes não são, em princípio, aplicáveis à maior parte das geometrias utilizadas nos projetos das torres. Além disso, maiores níveis de segurança e confiabilidade são atingidos quando a consideração criteriosa dos efeitos do vento é feita a partir da etapa de concepção sendo, geralmente, este o processo de menor custo e de maior eficiência. Sabe-se também que o modelo físico adotado para consideração dos carregamentos difere do real, seja pela forma de atuação ou pela complexidade para estimativa da interação entre fluido-estrutura. Nesse sentido, através do presente trabalho procurou-se: desenvolver e aperfeiçoar instrumentos e técnicas de medição de coeficientes de arrasto; verificar a aplicabilidade dos critérios de estimativa do carregamento devido ao vento sobre trechos de estruturas treliçadas; propor coeficientes de arrasto compatíveis com geometrias tradicionalmente utilizadas em torres de linhas de transmissão determinados a partir da aplicação de dois critérios de área de referência, a saber, área calculada e área projetada; além de comparar os resultados experimentais com outras referências como normas técnicas de projeto. Este trabalho de pesquisa contempla o estudo de duas torres de suspensão autoportantes em túnel de vento. Para a realização dos ensaios, através de modelos reduzidos, cada torre foi dividida em módulos. Os módulos foram ensaiados para diferentes ângulos de incidência do vento e também para diferentes níveis de velocidade do vento. As forças de arrasto sobre os modelos foram determinadas através de uma balança de forças unidirecional instalada na base da câmara de ensaios. / Transmission lines lattice structures submitted to wind action have been widely used in structural engineering applications for more than a century. However, the knowledge of the wind effect on this type of structure is yet imperfect, being the codes recommendations, in several cases, inconsistent and in disagreement with the experimental data. From this point of view, in the particular case of transmission lines structures, we can add that the existing codes are not, in principle, applicable to a large amount of geometries used in the towers design. Furthermore, largest safety and reliability levels are reached when careful consideration of the wind effects is made from the conception stage, being this process, in general, the less costly and the most efficient. It is also known that the physical model adopted for the consideration of the loads differs from the real, either for the way in which the load is really applied or the complexity for an accurate estimate of the fluid-structure interaction. In this sense, the present work had the aim of: developing and improving instrumentation and measurement techniques for drag coefficients; verifying the applicability of the criteria for estimating wind loads on modules of lattice structures; proposing drag coefficients applicable to geometries traditionally adopted in transmission lines towers, obtained from the application of the two reference areas criteria, calculated area and projected area; as well as comparing experimental results with other references such as design codes. The research contemplates wind tunnel studies of two self-supported suspension towers. For the accomplishment of the tests, through scaled models, each tower was divided in modules. The modules had been tested for different angles of incidence of the wind and also for different wind speeds. The drag forces on the models were measured through a unidirectional force balance installed in the base of the models.
488

Delinquency among adolescents with psychopathic traits in high-risk neighborhoods. / Brottslighet bland ungdomar med psykopatiska drag i högriskområden.

Ibrahim, Laura, Kaur, Adarshprit January 2018 (has links)
The study examine if there is a direct link between adolescents with psychopathic traits and delinquency, secondly if there is a direct link between neighborhood and delinquency. Gender differences were also examined as a control variable. The third aim is to look at the interaction between psychopathic traits and neighborhood on delinquency. We hypothesized that adolescents with psychopathic traits are more likely to engage in delinquent behaviour. We also hypothesized that adolescents who lives in high-risk areas are more prone to engage in delinquent acts. Regarding gender, we believe that boys have higher risk of becoming delinquent. We hypothesized that the interaction between psychopathic traits and high-risk neighborhood would increase the likelihood to engage in delinquent behaviour. The study is conducted by self-report of 1.485 adolescents (10–18 years of age) in a medium-sized city in Sweden. Analyses show that adolescents with high levels of psychopathic traits are more engaged in delinquency. Regarding neighborhood the analyses showed that high-risk neighborhood increases the risk for adolescents to engage in delinquent behaviour. The findings suggest that boys in high-risk areas is more likely to engage in delinquent behaviour compared to girls. Last, the moderation analysis shows that neighborhood accentuated the differences in delinquency between adolescents who has low, medium or high levels of psychopatic traits. In low-risk neighborhoods there are few differences in delinquency between adolescents who has low, medium or high levels of psychopathic triats. In high-risk neighborhoods there are substantial differences between adolescents who has low, medium or high levels of psychopathic traits. This suggest that the effect personality risks have on delinquency is accentuated by contextual risk factors. / Denna studie undersöker om det finns ett samband mellan ungdomar med psykopatiska drag och brottslighet, det andra syftet med studien är att undersöka om det finns ett samband mellan bostadsområde och brottslighet. Könsskillnader undersöktes också som en kontrollvariabel. Det tredje och sista syftet är att undersöka interaktionen mellan psykopatiska drag och bostadsområde på brottslighet. Vår hypotes var att ungdomar med psykopatiska drag är mer benägna att engagera sig i brottsligt beteende. Vår hypotes var även att ungdomar som bor i högriskområden är mer benägna att delta i brottsliga handlingar. När det gäller kön tror vi att pojkar har högre risk att bli brottslingar. Vår tredje hypotes är att samspelet mellan psykopatiska drag och högriskområden skulle öka sannolikheten för att delta i brottsligt beteende. Studien genomförs av självuppskattning av 1.485 ungdomar (10-18 år) i en medelstor stad i Sverige. Analyserna visar att ungdomar med höga nivåer av psykopatiska drag är mer engagerade i brottslighet. Gällande bostadsområde visar analysen att högriskområden ökar risken för ungdomar att engagera sig i brottsligt beteende. Resultaten tyder på att pojkar i högriskområden har en större sannolikhet att engagera sig i brottsligt beteende jämfört med tjejer. Slutligen visar moderationsanalysen att bostadsområde framhäver skillnaden i brottslighet hos ungdomar som har låg, mellan eller höga nivåer av psykopatiska drag. I lågriskområden är det få skillnader i brottslighet hos ungdomar som har låg, mellan eller höga nivåer av psykopatiska drag. I högriskområden är det väsentliga skillnader hos ungdomar som har låg, mellan eller höga nivåer av psykopatiska drag. Detta tyder på att den effekt som personlighetsrisker har på brottslighet accentueras av kontextuella riskfaktorer.
489

Development of subgrid models for a periodic circulating fluidized bed of binary mixture of particles / Développement de modèle de sous-maille pour la simulation numérique d'un écoulement polydisperse réactif

Chevrier, Solène 11 July 2017 (has links)
Des études numériques ont montré que la taille de la cellule de maillage peut avoir un effet drastique sur la modélisation du lit fluidisé circulant avec des petites particules. En effet, la taille de la cellule doit être de l’ordre de quelques diamètres de particules pour prédire avec précision le comportement dynamique d’un lit fluidisé. En conséquence, les simulations numériques d’ Euler-Euler des processus industriels sont généralement effectuées avec des grilles trop grossières pour permettre la prédiction des effets de ségrégation locale. La modélisation appropriée, qui prend en compte l’influence des structures non résolues, a déjà été proposée pour les simulations monodispersés. Dans ce travail, l’influence des structures non résolues sur un mélange binaire de particules est analysée et on propose des modèles pour tenir compte de cet effet dans des simulations de lit fluidisé polydispersés. Pour atteindre cet objectif, des simulations Euler-Euler de références sont réalisées avec un raffinement du maillage aboutissant à une solution indépendante de la taille de la cellule. Ce type de simulation numérique est très coûteux et se limite à des configurations très simples. Dans ce travail, la configuration se consiste en un lit circulant périodique 3D, qui représente la région établie d'un lit circulant. Parallèlement, une approche filtrée est développée où les termes inconnus, appelés contributions de sous-maille, doivent être modélisés. Les filtres spatiaux peuvent être appliqués aux résultats de simulation de référence afin de mesurer chaque contribution de sous-maille apparaissant dans l’approche théorique filtrée. Une analyse est réalisée afin de comprendre et de modéliser l’effet de la contribution des termes de sous-maille. L’opération de filtrage fait apparaître de nouveaux termes, les termes de sous-maille. Un terme filtré est la somme d’un terme résolu, obtenus à partir des champs filtrés, et d’ un terme de sous-maille. L’analyse de l’équation filtrée de quantité de mouvement montre que les contributions résolues de la traînée des particules fluides et la collision entre particules surestiment les effets de transferts de quantité de mouvement filtrés. L’analyse de l’équation filtrée de l’énergie cinétique des particules montre que la production résolue par le cisaillement moyen et par le mouvement relatif moyen des particules sous-estime contribution filtrée. Des modèles fonctionnels sont proposés pour les contributions de sous-maille de la traînée et des collisions inter-particule. / Detailed sensitivity numerical studies have shown that the mesh cell-size may have a drastic effect on the modelling of circulating fluidized bed with small particles. Typically, the cell-size must be of the order of few particle diameters to predict accurately the dynamical behaviour of a fluidized bed. Hence, the Euler-Euler numerical simulations of industrial processes are generally performed with grids too coarse to allow the prediction of the local segregation effects. Appropriate modelling, which takes into account the influence of unresolved structures, have been already proposed for monodisperse simulations. In this work, the influence of unresolved structures on a binary mixture of particles is investigated and models are proposed to account for those effect on bidisperse simulations of bidisperse gas-solid fluidized bed. To achieve this goal, Euler-Euler reference simulations are performed with grid refinement up to reach a mesh independent solution. Such kind of numerical simulation is very expensive and is restricted to very simple configurations. In this work, the configuration consists of a 3D periodical circulating fluidized bed, that could represent the established zone of an industrial circulating fluidized bed. In parallel, a filtered approach is developed where the unknown terms, called sub-grid contributions, appear. They correspond to the difference between filtered terms, which are calculated with the reference results then filtered, and resolved contributions, calculated with the filtered fields. Then spatial filters can be applied to reference simulation results to measure each sub-grid contribution appearing in the theoretical filtered approach. A budget analysis is carried out to understand and model the sub-grid term. The analysis of the filtered momentum equation shows that the resolved fluid-particle drag and inter-particle collision are overestimating the momentum transfer effects. The analysis of the budget of the filtered random kinetic energy shows that the resolved production by the mean shear and by the mean particle relative motion are underestimating the filtered ones. Functional models are proposed for the subgrid contributions of the drag and the inter-particle collision.
490

Design methodology for wing trailing edge device mechanisms

Martins Pires, Rui Miguel January 2007 (has links)
Over the last few decades the design of high lift devices has become a very important part of the total aircraft design process. Reviews of the design process are performed on a regular basis, with the intent to improve and optimize the design process. This thesis describes a new and innovative methodology for the design and evaluation of mechanisms for Trailing Edge High-Lift devices. The initial research reviewed existing High-Lift device design methodologies and current flap systems used on existing commercial transport aircraft. This revealed the need for a design methodology that could improve the design process of High-Lift devices, moving away from the conventional "trial and error" design approach, and cover a wider range of design attributes. This new methodology includes the use of the innovative design tool called SYNAMEC. This is a state-of-the-art engineering design tool for the synthesis and optimizations of aeronautical mechanisms. The new multidisciplinary design methodology also looks into issues not usually associated with the initial stages of the design process, such as Maintainability, Reliability, Weight and Cost. The availability of the SYNAMEC design tool and its ability to perform Synthesis and Optimization of mechanisms led to it being used as an important module in the development of the new design methodology. The SYNAMEC tool allows designers to assess more mechanisms in a given time than the traditional design methodologies. A validation of the new methodology was performed and showed that creditable results were achieved. A case study was performed on the ATRA - Advance Transport Regional Aircraft, a Cranfield University design project, to apply the design methodology and select from within a group of viable solutions the most suitable type of mechanism for the Variable Camber Wing concept initially defined for the aircraft. The results show that the most appropriate mechanism type for the ATRA Variable Camber Wing is the Link /Track Mechanism. It also demonstrated how a wide range of design attributes can now be considered at a much earlier stage of the design.

Page generated in 0.0667 seconds