Allumage des moteurs fuseés cryotechniques

Les lanceurs spatiaux ont aujourd'hui besoin de moteurs-fusées cryotechniques capables de s'allumer plusieurs fois au cours du même vol. L'allumage étant un mécanisme extrêmement délicat dans les conditions en vol, il est nécessaire de développer et d'utiliser des outils précis et fiables pour aider au développement de cette technologie. Cette thèse développe la simulation des grandes échelles (SGE) pour traiter les écoulements transitoires supersoniques réactifs. Différents aspects sont abordés : cinétique chimique de l'auto-allumage et diffusion différentielle, traitement numérique des écoulements supersoniques, combustion. Des comparaisons avec des données expérimentales sur des configurations académiques permettent de valider les développements réalisés et de comprendre en détail le mécanisme d'auto-allumage. Sur la base de ces résultats, des simulations SGE de configurations industrielles sont maintenant envisageables, afin d'étudier les différents régimes transitoires d'allumage. ABSTRACT : Today, space launchers require cryotechnic rocket engines able to reignite during flight. The ignition phases in flight conditions are particularly critical and the development of restartable engines needs accurate and reliable tools. The present thesis develops a Large Eddy Simulation approach (LES) for the study of unsteady supersonic reactive flows. Several aspects are treated : chemical kinetics, auto-ignition and differential diffusion, numerical methods suited to supersonic flows and their discontinuities, combustion. Comparisons with experimental data on academic test cases validate the models, and give detailed insights into the auto-ignition process. Based on these achievements, LES of industrial configurations may be now envisaged, allowing the study of unsteady ignition regimes and the optimization of devices

Simulation of a supersonic hydrogen-air autoignition-stabilized flame using reduced chemistry

A three-step mechanism for H2-air combustion (Boivin et al., Proc. Comb. Inst. 33, 2010) was recently designed to reproduce both autoignition and flame propagation, essential in lifted flame stabilization. To study the implications of the use of this reduced chemistry in the context of a turbulent flame simulation, this mechanism has been implemented in a compressible explicit code and applied to the simulation of a supersonic lifted co-flowing hydrogen-air flame. Results are compared with experimental measurements (Cheng et al. C&F 1994) and simulations using detailed chemistry, showing that the reduced chemistry is very accurate. A new explicit diagnostic to readily identify autoignition regions in the post-processing of a turbulent hydrogen flame simulation is also proposed, based on variables introduced in the development of the reduced chemical mechanism.This work was supported by the UE Marie Curie ITN MYPLANET, by the Spanish MCINN through projects # ENE2008-06515 and # CSD2010-00010 and by the Comunidad de Madrid through project # S2009/ENE-1597. We acknowledge fruitful discussions on hydrogen chemistrywith Prof. A.L. S´anchez and Prof. F.A. Williams. We also wish to thank Prof. T. S. Cheng and Prof. R. W. Pitz for providing experimental data in electronic form.European Community's Seventh Framework ProgramPublicad

A Thickened-Hole Model for Large Eddy Simulations over Multiperforated Liners

International audienceIn aero-engines, mutiperforation cooling systems are often used to shield the combustor wall and ensure durability of the engine. Fresh air coming from the casing goes through thousands of angled perforations and forms a film which protects the liner. When performing Large Eddy Simulations (LES) of a real engine, the number of sub-millimetric holes is far too large to allow a complete and accurate description of each aperture. Homogeneous models allow to simulate multiperforated plates with a mesh size bigger than the hole but fail in representing the jet penetration and mixing. A heterogeneous approach is proposed in this study, where the apertures are thickened if necessary so that the jet-crossflow interaction is properly represented. Simulations using homogeneous and thickened-hole models are compared to a fully resolved computation for various grid resolutions in order to illustrate the potential of the method

Directional fluid transport along artificial ciliary surfaces with base-layer actuation of counter-rotating orbital beating patterns

The generation of metachronal waves of beating cilia is a complex mechanism when reproduced in laboratory experiments. In addition, local manipulation of cilia in larger arrays becomes non-trivial when the beating patterns are not unidirectional. Herein, a more complex pattern of beating cilia is studied, where the cilia perform an orbital tip motion and rows of these cilia are counter-rotating in a traveling wave-like actuation. Interest in this type of potential fluid transport results from technical applications where localized streaks of directional flow need to be produced and the speed and direction of the fluid transport is open to be manipulated. A simple solution is found to generate the corresponding traveling wave and beating patterns using a base-layer actuation of a membrane covered with artificial cilia. A device is built where such a wave is generated mechanically using a ball chain positioned below the membrane. When the ball chain is moved, the elevation leads to an orbital beating pattern of the tips of the cilia on top of the membrane. This beating shows characteristics of non-symmetric motion in terms of fast and slow motion phases in the orbital cycle. When the ball chain is positioned centered between parallel rows of the cilia and moved parallel to the rows, a well-defined directional fluid transport is generated in the gap. Micro-PIV revealed that successive traveling waves generate a steady streaming transport along the rows of the cilia as a combination of the fluid squeezing and streaming components. Changing the direction and speed of the ball chain is possible and allows the localized fluid transport in speed and direction to be altered. 2D Arrays of piston-like actuators in the form of a grid of balls may give further full control of position and direction of transport pathways along the ciliary surface. © 2012 The Royal Society of Chemistry

Simulation aux grandes échelles: instabilités thermo-acoustiques, combustion diphasique et couplages multi-physiques

La combustion turbulente, que ce soit dans des configurations de laboratoire ou dans des configurations réelles industrielles, met en oeuvre un nombre important de physiques fortement couplées: chimie, turbulence, multi-phasique, thermique, etc. Pour répondre aux demandes de plus en plus exigeantes des concepteurs, qui doivent proposer des solutions concurrentielles tout en respectant les contraintes environnementales de bruit et d'émission de polluants, la simulation numérique est devenue incontournable. Plus précisément, la simulation maintenant utilisée comme outil de conception, doit être fiable et précise. Dans le domaine de la combustion turbulente, à fort caractère instationnaire, la Simulation aux Grandes Echelles (SGE) s'est récemment imposée. Cette technique s'est en effet avérée capable de prédire finement le comportement des brûleurs dans des environnements complexes, et permet aujourd'hui d'aborder des problématiques encore mal maîtrisées telles que les instabilités thermo-acoustiques ou la combustion diphasique. On donne ici quelques exemples de problèmes encore ouverts dans ce domaine

The PELskin project: part II—investigating the physical coupling between flexible filaments in an oscillating flow

The fluid-structure interaction mechanisms of a coating composed of flexible flaps immersed in a periodically oscillating channel flow is here studied by means of numerical simulation, employing the Euler-Bernoulli equations to account for the flexibility of the structures. A set of passively actuated flaps have previously been demonstrated to deliver favourable aerodynamic impact when attached to a bluff body undergoing periodic vortex shedding. As such, the present configuration is identified to provide a useful test-bed to better understand this mechanism, thought to be linked to experimentally observed travelling waves. Having previously validated and elucidated the flow mechanism in Paper 1 of this series, we hereby undertake a more detailed analysis of spectra obtained for different natural frequency of structures and different configurations, in order to better characterize the mechanisms involved in the organized motion of the structures. Herein, this wave-like behaviour, observed at the tips of flexible structures via interaction with the fluid flow, is characterized by examining the time history of the filaments motion and the corresponding effects on the fluid flow, in terms of dynamics and frequency of the fluid velocity. Results indicate that the wave motion behaviour is associated with the formation of vortices in the gaps between the flaps, which itself are a function of the structural resistance to the cross flow. In addition, formation of vortices upstream of the leading and downstream of the trailing flap is seen, which interact with the formation of the shear-layer on top of the row. This leads to a phase shift in the wave-type motion along the row that resembles the observation in the cylinder case

MHD dissipative flow and heat transfer of casson fluids due to metachronal wave propulsion of beating cilia with thermal and velocity slip effects under an oblique magnetic field

A theoretical investigation of magnetohydrodynamic (MHD) flow and heat transfer of electrically-conducting viscoplastic fluids through a channel is conducted. The robust Casson model is implemented to simulate viscoplastic behavior of fluids. The external magnetic field is oblique to the fluid flow direction. Viscous dissipation effects are included. The flow is controlled by the metachronal wave propagation generated by cilia beating on the inner walls of the channel. The mathematical formulation is based on deformation in longitudinal and transverse velocity components induced by the ciliary beating phenomenon with cilia assumed to follow elliptic trajectories. The model also features velocity and thermal slip boundary conditions. Closed-form solutions to the non-dimensional boundary value problem are obtained under physiological limitations of low Reynolds number and large wavelength. The influence of key hydrodynamic and thermo-physical parameters i.e. Hartmann (magnetic) number, Casson (viscoplastic) fluid parameter, thermal slip parameter and velocity slip parameter on flow characteristics are investigated. A comparative study is also made with Newtonian fluids (corresponding to massive values of plastic viscosity). Stream lines are plotted to visualize trapping phenomenon. The computations reveal that velocity increases with increasing the magnitude of Hartmann number near the channel walls whereas in the core flow region (centre of the channel) significant deceleration is observed. Temperature is elevated with greater Casson parameter, Hartmann number, velocity slip, eccentricity parameter, thermal slip and also Brinkmann (dissipation) number. Furthermore greater Casson parameter is found to elevate the quantity and size of the trapped bolus. In the pumping region, the pressure rise is reduced with greater Hartmann number, velocity slip, and wave number whereas it is enhanced with greater cilia length

A Lattice Boltzmann-Immersed Boundary method to simulate the fluid interaction with moving and slender flexible objects

A numerical approach based on the Lattice Boltzmann and Immersed Boundary methods is proposed to tackle the problem of the interaction of moving and/or deformable slender solids with an incompressible fluid flow. The method makes use of a Cartesian uniform lattice that encompasses both the fluid and the solid domains. The deforming/moving elements are tracked through a series of Lagrangian markers that are embedded in the computational domain. Differently from classical projection methods applied to advance in time the incompressible Navier–Stokes equations, the baseline Lattice Boltzmann fluid solver is free from pressure corrector step, which is known to affect the accuracy of the boundary conditions. Also, in contrast to other immersed boundary methods proposed in the literature, the proposed algorithm does not require the introduction of any empirical parameter. In the case of rigid bodies, the position of the markers delimiting the surface of an object is updated by tracking both the position of the centre of mass of the object and its rotation using Newtonʼs laws and the conservation of angular momentum. The dynamics of a flexible slender structure is determined as a function of the forces exerted by the fluid, its flexural rigidity and the tension necessary to enforce the filament inextensibility. For both rigid and deformable bodies, the instantaneous no-slip and impermeability conditions on the solid boundary are imposed via external and localised body forces which are consistently introduced into the Lattice Boltzmann equation. The validation test-cases for rigid bodies include the case of an impulsively started plate and the sedimentation of particles under gravity in a fluid initially at rest. For the case of deformable slender structures we consider the beating of both a single filament and a pair filaments induced by the interaction with an incoming uniformly streaming flow