Lorsqu’un train circule en situation de vent traversier, il est exposé à un risque de renversement. L’évaluation de ce risque est possible à partir d’un calcul dynamique basé sur les efforts aérodynamiques, plus particulièrement le moment de renversement. Les coefficients aérodynamiques d’un véhicule ferroviaire peuvent être évalués en soufflerie. Lors de ce type d’essais, le nombre de Reynolds ne peut être respecté. L’expérience a montré que dans ces conditions, une mise à l’échelle directe des détails sur les maquettes de train ne conduit pas nécessairement à une représentation réaliste des efforts aérodynamiques.Ces travaux de thèse portent sur l’étude de l’impact de lignes placées en toiture sur les caractéristiques aérodynamiques de véhicules ferroviaires. Le but est de déterminer dans quelle mesure l’échelle de représentation de ces détails impacte les coefficients aérodynamiques.Deux modèles de trains simplifiés sont employés, le premier est un véhicule idéalisé déjà largement étudié auparavant dans la littérature sur les vents traversiers, le second est plus proche d’un train réel. Les moyens d’investigation employés pour mener cette étude sont principalement expérimentaux. Les efforts et moments aérodynamiques sont mesurés au moyen d’une balance aérodynamique et la répartition de pression à la surface des corps est évaluée à l’aide de capteurs de pression pariétale. Des simulations numériques par approche RANS viennent compléter la base de données afin d’affiner la compréhension de l’écoulement autour du train.Nous avons ainsi démontré que la présence des lignes sur le toit des véhicules augmente le risque de renversement des deux modèles de trains étudiés. Nous avons aussi mis en évidence une forte dépendance de cette augmentation à la taille des lignes de toiture. Une configuration s’est avérée particulièrement critique d’un point de vue industriel. En effet, nous avons identifié une taille des lignes de toiture conduisant à de très faibles modifications des efforts sur le modèle régional. Ainsi, une échelle de reproduction des détails non adaptée lors d’essais de certification pourrait conduire à une large sous-estimation du risque de renversement.Afin de comprendre les causes de l’augmentation du risque de renversement, nous avons identifié les modifications de pression ayant un impact sur les efforts globaux. Nous avons ensuite relié ces modifications de pression à la topologie de l’écoulement. Nous avons finalement démontré que l’augmentation du risque de renversement est liée d’une part à des modifications de pression locales très importantes, mais aussi à des modifications de pression plus globales. / When expoecd to strong crosswinds, railway trains experience aerodynamic loads which tend to overturn them. The evaluation of the risk of overturning can be done with a dynamic calculation based on the aerodynamic loads and more precisely on the rolling moment. The aerodynamic characteristics of the vehicles can be evaluated in windtunnels. The Reynolds number of a full scale train is not applicable in these tests. Experimental tests showed that a reproduction of the train surface details at the same scale as the model would not necessarily lead to a correct estimation of the real aerodynamic loads.The topic of this work is to investigate the effect of roof mounted cables on the aerodynamic characteristics of railway vehicles. The aim is to determine the influence of the reproduction scale of these appendices on the aerodynamic coefficients. Two train models are employed. The first one is an extensively-studied simplified train model. The second one is more similar to a real train, modelled after a regional train. Visualisation tools are employed to investigate the flow topology. Global aerodynamic loads (lift, lateral force and rolling moment) are evaluated with a dynamometric force balance and surface pressure distribution is evaluated with pressure sensors. Additional RANS numerical simulations are performed to improve understanding of the flow topology.We demonstrated an increase of the risk of overtum for the two train models when cables are added on the roof. We also showed that this increase strongly depends on the cable's size.To understand this increase, we identified pressure modifications which are involved in the global load modifications. Then, we linked these pressure changes to flow topology. We finally showed that the incrcase of the risk of overtun is both due to local and global pressure modifications.

Ground vehicles, both on roads or on rail, are sensitive to crosswinds and the handling, travelling speeds or in some cases, safety can be affected. Full modelling of the crosswind stability of a vehicle is a demanding task as the nature of the disturbance, the wind gust, is complex and the aerodynamics, vehicle dynamics and driver reactions interact with each other.  One of the objectives of this thesis, is to assess the aerodynamic response of simplified ground vehicles under sudden strong crosswind disturbances by using an advanced turbulence model. In the aerodynamic simulations, time-dependant boundary data have been used to introduce a deterministic wind gust model into the computational domain.  This thesis covers the implementation of such gust models into Detached-Eddy Simulations (DES) and assesses the overall accuracy. Different type of grids, numerical setups and refinements are considered. Although the overall use of DES is seen suitable, further investigations can be foreseen on more challenging geometries.  Two families of vehicle models have been studied. The first one, a box-like geometry, has been used to characterize the influence of the radius of curvature and benefited from unsteady experimental data for comparison. The second one, the Windsor model, has been used to understand the impact of the different rear designs. Noticeably, the different geometries tested have exhibited strong transients in the loads that can not be represented in pure steady crosswind conditions. The static coupling between aerodynamics and vehicle dynamics simulations enhances the comparisons of the aerodynamic designs. Also, it shows that the motion of the centre of pressure with respect the locations of the centre of gravity and the neutral steer point, is of prime interest to design vehicles that are less crosswind sensitive. Recommendations on the future work on crosswind sensitivity for ground vehicles are proposed at the end of this thesis. / <p>QC 20111206</p> / crosswind stability and unsteady aerodynamics

Pendant les dernières décennies les constructeurs automobiles ont été confrontés à la tâche difficile de réduire la consommation de carburant et les émissions de CO2. En réponse à cela, des processus d'optimisation ont été appliquées pour générer de nouvelles formes de véhicule pour réduire la traînée aérodynamique. Malheureusement, les formes optimales obtenues n’ont aucun signe distinctif pour se différentier entre les différentes marques. Les stylistes tentent aujourd'hui de redonner une signature de marque en proposant des formes «non-conventionnelles» caractérisés par des montants arrières fortement arrondis.Ce travail de thèse quantifié expérimentalement l'influence des montants arrondis sur l’aérodynamique des véhicules bi-corps (fastback). Rayonner l’arrêt entre le pavillon et la lunette du corps d’Ahmed s’avère efficace pour réduire la trainée. Ajouter un rayon de courbure sur les montants arrière latéraux ne modifie pas davantage la traînée. Cependant, l'effet net nul sur la trainée est dû à des modifications de traînée opposée.L'organisation du sillage proche et le transport de vorticité ont été analysés et relié à l'augmentation locale de la traînée. Pour augmenter encore la complexité et de la fidélité du modèle, un modèle a été réalisé en partenariat avec le département du style de PSA. La sensibilité au vent latéral a également été étudiée. Il a été montré que les montants arrières arrondis détériorent les performances en vent latéral. La compréhension des effets d'arrondi de la partie arrière des modèles de véhicule s’avère être valide malgré la complexité croissante des modèles. Enfin, des tests à l’échelle 1 on confirme les tendances observées sur les modèles à échelle réduite. / For the last decades car manufacturers have been facing the challenging task of reducing fuel consumption and CO2 emissions. In response to that, optimization processes have been applied to generate new vehicle shapes minimizing the aerodynamic drag. Since the obtained optimal shapes have no brand differentiating details, stylists are nowadays trying to give back a brand signature by proposing "non-conventional" shapes characterized by important rear pillars rounding. The present PhD work experimentally quantifies the influence of such afterbody rounding on the aerodynamic loads and on the flow field development over fastback vehicle models.Rounding the roof/backlight of the well-known Ahmed body intersection is shown to reduce drag. Surprisingly, additional rear curvature associated with side pillars rounding does not further modify the drag. However, the zero net effect is found to result from opposite drag effects. To further increase the complexity and fidelity of the model, a model is realized in partnership with the style department of PSA. Crosswind sensitivity is also investigated by means of yaw angle variation. It is shown that the rear ends with rounded edges deteriorate the lateral. The understanding of the afterbody rounding effects is shown to be valid even for increasing complexity of the models. Finally, full scale test confirms the trends observed with the scaled models.

Le feu est une des préoccupations majeures des avionneurs et des compagnies aériennesétant donnée la grande quantité de combustibles hautement inflammables à bord. De plus, lamenace au feu augmente par l’allègement grâce à la substitution de l’aluminium par dematériaux composites inflammables. La simulation numérique des grandes échelles estappliquée à un feu de nappe de kérosène de grande échelle soumis à un vent traversier avecprise en compte d’un aéronef de type d’aluminium et celui composite. La combustion esttraitée à l’aide d’une réaction globale et une hypothèse de chimie infiniment rapide, choixvalables tant que le feu est dans un régime de sur-ventilation où la combustion est contrôléepar le mélange des réactifs. L’interaction entre la combustion et la turbulence est traitée parune approche de type EDC (Eddy Dissipation Concept). L’équation des transferts radiatifs estrésolue à l’aide de la méthode des volumes finis. Les propriétés radiatives du milieu sontestimées à l’aide d’un modèle à larges bandes donnant une bonne approximation ducoefficient d’absorption par rapport aux calculs spectraux. Les suies sont modélisées par unmodèle semi-empirique de type Smoke-Point tenant compte du processus de formation dessuies par nucléation. Pour l'instant, le modèle de pyrolyse simple, contrôlée par latempérature, est utilisé à travers d’une approche du nombre de transfert de masse. Desanalyses spécifiques ont été menées par nos soins sur la forme de flamme, ainsi que sur lesflux thermiques, les champs de vitesse, de température et des espèces chimiques. Les résultatsobtenus sur les feux à une échelle intermédiaire sont en accord avec les résultatsexpérimentaux. Ils sont par contre difficilement comparables aux résultats expérimentaux surles feux à une échelle plus importante tridimensionnelle car l'instrumentation dans le domaineen aéronautique est très difficile. Quelques soient la taille de foyer et l’intensité de vent, leflux de chaleur maximal venant de nappe de kérosène semble inférieur à 340 kW/m2. / Large fully turbulent fires, which result as a consequence of an aircraft accident, pose asevere hazard to the occupants and cargo. This numerical study focuses on the firephenomenology associated with the presence of an aircraft immersed, at one particularlocation and orientation, within a large aviation-fuel fire in a moving fluid medium.Turbulence is modelled using a standard Smagorinsky sub-grid scale model in a Large EddySimulation An extension of the eddy dissipation concept is incorporated, allowing toinvestigate the roles of the wind speed and its direction on the fire growth, heat fluxdistribution and smoke products, such as carbon monoxide and soot. The outcome of thestudy is interesting, and the interaction model between turbulence and combustion is indeedadequate. A radiative transfer equation is solved by using a discrete expression adapted to afinite volume method. The soot model is based on a Smoke-Point concept, and the mixture ofsoot and gas behaves as a gray medium. The predicted flame shape and heat flux comparewell with the measurements. The prediction indicates that interaction between aircraft and fireenvironment combined with the influence of wind conditions affects dramatically location ofthe continuous flame zone and heat flux distribution. The highest heat flux occurs on thewindward side of the aircraft for the low and medium winds, but on the leeward side of theaircraft for the high wind. A good reproduction of the experimental trends has been achievedfor the temperature field. Preliminary comparisons of the heat flux distribution on the cylinderskin show promise. The peak heat fluxes to the aircraft for various wind conditions rangefrom 200-340 kW/m2, and the contribution of radiation to the aircraft skin is higher than 95%of the total heat flux.

<p>Ground vehicles are subjected to crosswind from various origins such as weather, topography of the ambient environment (land, forest, tunnels, high bushes...) or surrounding traffic. The trend of lowering the weight of vehicles imposes a stronger need for understanding the coupling between crosswind stability, the vehicle external shape and the dynamic properties. Means for reducing fuel consumption of ground vehicles can also conflict with the handling and dynamic characteristics of the vehicle. Streamlined design of vehicle shapes to lower the drag can be a good example of this dilemma. If care is not taken, the streamlined shape can lead to an increase in yaw moment under crosswind conditions which results in a poor handling.</p><p>The development of numerical methods provides efficient tools to investigate these complex phenomena that are difficult to reproduce experimentally. Time accurate and scale resolving methods, like Detached-Eddy Simulations (DES), are particularly of interest, since they allow a better description of unsteady flows than standard Reynolds Average Navier-Stokes (RANS) models. Moreover, due to the constant increase in computational resources, this type of simulations complies more and more with industrial interests and design cycles.</p><p>In this thesis, the possibilities offered by DES to simulate unsteady crosswind aerodynamics of simple vehicle models in an industrial framework are explored. A large part of the work is devoted to the grid design, which is especially crucial for truthful results from DES. Additional concerns in simulations of unsteady crosswind aerodynamics are highlighted, especially for the resolution of the wind-gust boundary layer profiles. Finally, the transient behaviour of the aerodynamic loads and the flow structures are analyzed for several types of vehicles. The results simulated with DES are promising and the overall agreement with the experimental data available is good, which illustrates a certain reliability in the simulations. In addition, the simulations show that the force coefficients exhibit highly transient behaviour under gusty conditions.</p> / ECO2 Crosswind Stability and Unsteady Aerodynamics for Ground Vehicles

Ground vehicles are subjected to crosswind from various origins such as weather, topography of the ambient environment (land, forest, tunnels, high bushes...) or surrounding traffic. The trend of lowering the weight of vehicles imposes a stronger need for understanding the coupling between crosswind stability, the vehicle external shape and the dynamic properties. Means for reducing fuel consumption of ground vehicles can also conflict with the handling and dynamic characteristics of the vehicle. Streamlined design of vehicle shapes to lower the drag can be a good example of this dilemma. If care is not taken, the streamlined shape can lead to an increase in yaw moment under crosswind conditions which results in a poor handling. The development of numerical methods provides efficient tools to investigate these complex phenomena that are difficult to reproduce experimentally. Time accurate and scale resolving methods, like Detached-Eddy Simulations (DES), are particularly of interest, since they allow a better description of unsteady flows than standard Reynolds Average Navier-Stokes (RANS) models. Moreover, due to the constant increase in computational resources, this type of simulations complies more and more with industrial interests and design cycles. In this thesis, the possibilities offered by DES to simulate unsteady crosswind aerodynamics of simple vehicle models in an industrial framework are explored. A large part of the work is devoted to the grid design, which is especially crucial for truthful results from DES. Additional concerns in simulations of unsteady crosswind aerodynamics are highlighted, especially for the resolution of the wind-gust boundary layer profiles. Finally, the transient behaviour of the aerodynamic loads and the flow structures are analyzed for several types of vehicles. The results simulated with DES are promising and the overall agreement with the experimental data available is good, which illustrates a certain reliability in the simulations. In addition, the simulations show that the force coefficients exhibit highly transient behaviour under gusty conditions. / ECO2 Crosswind Stability and Unsteady Aerodynamics for Ground Vehicles

When driving a vehicle on the road, the driver has to compensate continuously for small directional deviations from the desired course due to disturbances such as crosswinds, road irregularities and unintended driver inputs. These types of deviations have a tiring effect on the driver and should therefore be minimised. When the magnitude of these disturbances increases, especially with crosswind, the directional deviation might become so large that the driver will have difficulties in compensating for it, and will thereby affect the traffic safety. The objective of this research work is to increase the understanding of the crosswind sensitivity of buses and to find solutions to the problem of improving the safety of buses with respect to crosswind performance. The work presented in this thesis contributes to increased knowledge about the directional stability of buses under the influence of crosswind gusts through parameter studies using detailed vehicle simulation models, through full-scale experiments and through studies of the effect of steering feel on the subjective and objective evaluation of crosswind performance. A natural crosswind gust model has been derived from wind tunnel measurements and implemented in a multi-body dynamics simulation tool. The aerodynamic loads of the crosswind gust model have been applied on a detailed vehicle model and the behaviour of the vehicle model has been studied for various vehicle configurations in both open- and closed-loop manoeuvres. The vehicle model, with parameters corresponding to real vehicle data, has been validated and the agreement with measurements is good. A method for estimating the aerodynamic loads on a bus due to crosswind on a road section is also presented. Aerodynamic loads under real conditions were estimated using this method and these data were thereafter used in a study where the effect of steering feel on the subjective and objective evaluation of crosswind performance was investigated using a moving-base driving simulator, with the aim of finding a relationship between steering feel and crosswind sensitivity. The thesis covers the influence of changing chassis-related parameters and aerodynamics-related parameters on the crosswind sensitivity, as well as the influence of the setting of the steering system on the crosswind performance of the driver-vehicle system. The results identify areas of high potential for improving the crosswind sensitivity of buses, such as the centre of gravity location and the yaw moment overshoot at gust entry. Furthermore, the study shows the importance of having a vehicle that facilitates prompt driver corrections for reducing the lateral deviation under crosswind excitation; i.e. it is shown that a steering system with the possibility of changing the yaw rate gradient referencing the steering-wheel input when the vehicle is subjected to a sudden crosswind has a good potential for improving the crosswind performance of the driver-vehicle system. / QC 20100722

This thesis deals with the investigation of unsteady effects on vehicle that has been exposed to a crosswind gust. First crosswind velocity function was created, which is a function of both time and space. A comparison of continual and trapeze gust model was carried out. Next step was to compare unsteady and quasi-steady evolution of the aerodynamic loads using accumulated forces, surface pressures and flow field around the car. The penultimate part deals with investigation of unsteady behaviour of drag and in the last part the influence of rear geometry of the car on unsteady phenomena was investigated.

The diploma thesis focuses on CFD simulation of a vehicle with various geometric modifications and non-zero angle flow. The goal of the work is to evaluate the effect of crosswinds for individual vehicle modifications together with the validation of these data sets with measured data from the wind tunnel. The last part of the thesis contains analysis of the flow field.

This thesis discusses methods to measure several atmospheric parameters related to turbulence. Techniques used by two different scintillometers based on weak turbulence theory are discussed along with a method to estimate the inner scale developed by Hill. The theory and minimization algorithm used to infer the atmospheric parameters are discussed. The main focus is on the analysis and collection of experimental data with a three-aperture receiver system. Intensity fluctuations from a CW laser source are collected over a 1km path with three different receiving apertures. The scintillation index is found for each receiving aperture and recently developed theory for all regimes of optical turbulence is used to infer three atmospheric parameters, Cn2, l0, and L0. The transverse wind speed is also calculated from the experimental data using a cross-correlation technique. Parallel to the three-aperture data collection is a commercial scintillometer unit which reports Cn2 and crosswind speed. There is also a weather station positioned at the receiver side which provides point measurements for temperature and wind speed. The Cn2 measurement obtained from the commercial scintillometer is used to infer l0, L0, and the scintillation index. Those values are then compared to the inferred atmospheric parameters from the experimental data. Hill's method is used as an estimate to l0 based upon path-averaged wind speed and is compared to the inferred l0 values. The optimal aperture sizes required for three-aperture data collection are presented. In closing, the technique for measuring crosswind speed is discussed along with the ideal aperture size and separation distance for data collection. Suggestions are offered for future experimentation.