No description available.

In this research effort, we study three exterior penalty function approaches for solving linear programming problems. These methods are an active set l2 penalty approach (ASL2), an inequality-equality based l2 penalty approach (IEL2), and an augmented Lagrangian approach (ALAG). Particular effective variants are presented for each method, along with comments and experience on alternative algorithmic strategies that were empirically investigated. Our motivation is to examine the relative performance of these different approaches based on the basic l2 penalty function in order to provide insights into the viability of these methods for solving linear programs. To test the performance of these algorithms, a set of randomly generated problems as well as a set of NETLIB test problems from the public domain are used. By way of providing a benchmark for comparisons, we also solve the test problems using CPLEX 6.0, an advanced simplex implementation. While a particular variant (ALAG2) of ALAG performed the best for randomly generated test problems, ASL2 performed the best for the NETLIB test problems. Moreover, for test problems having only equality constraints, IEL2, and ASL2 (which is a finer-tuned version of IEL2 in this case) were comparable and yielded a second-best performance in comparison with ALAG2. Furthermore, a set of problems with relatively higher density parameter values, as well as a set of low-density problems were used to determine the effect of density on the relative performances of these methods. This experiment revealed that for linear programs with a high density parameter, ASL2 is the best alternative among the tested algorithms; whereas, for low-density problems ALAG2 is the fastest method. Moreover, although our implementation was rudimentary in comparison with CPLEX, all of the tested methods attained a final solution faster than CPLEX for the set of large-scale low-density problems, sometimes as fast as requiring only 16-23% of the effort consumed by CPLEX. Average rank tests based on the computational results obtained are performed using two different statistics, that assess the speed of convergence and the quality or accuracy of the solution, in order to determine the relative effectiveness of the algorithms and to validate our conclusions. Overall, the results provide insights into selecting algorithmic strategies based on problem structure and indicate that while this class of methods is viable for computing near optimal solutions, more research is needed to design robust and competitive exterior point methods for solving linear programming problems. However, the use of the proposed variant of the augmented Lagrangian method to solve large-scale low-density linear programs is promising and should be explored more extensively. / Master of Science

Lors de cette thèse nous avons étudié le phénomène de convection thermique turbulente. La compréhension des mécanismes du flux thermique induit est encore un défi. Dans cette optique, deux approches innovantes ont été utilisées. La première a consisté en une approche Lagrangienne. Dans un premier temps, nous avons immergé une particule instrumentée munie de capteurs de température. Sa vitesse a été mesurée simultanément afin de calculer le flux thermique local transporté. En comparant les mesures effectuées à des simulations numériques et à des mesures Eulériennes, la pertinence de cette méthode a été révélée. Nous avons ensuite ensemencé l'écoulement avec des particules de diamètre inférieur à l'échelle dissipative du système. Nous avons alors pu effectuer les suivre en 3D et nous intéresser aux statistiques turbulentes, ainsi qu'à la dispersion de paires. Nous avons ensuite étudié une configuration où la plaque du bas comporte des rugosités contrôlées. Il est reconnu que cela amène une augmentation du transfert thermique supérieure à celle reliée à l'augmentation de surface. Nous avons mis en évidence des mécanismes possibles qui expliquent ce phénomène. Des mesures thermométriques dans une cellule remplie d'eau ainsi que des mesures vélocimétriques dans une cellule six fois plus grande remplie d'air ont permis d'observer un changement radical de la structure des couches limites au voisinage des rugosités, notamment l'apparition d'une couche limite turbulente. Ces mesures ont été accompagnées de mesures de vitesse de l'écoulement global qui ont révélé une augmentation drastique des fluctuations de vitesse ainsi que l'apparition d'un nouveau régime de turbulence. / In this work, turbulent thermal convection is studied. Undestanding the mechanisms of induced thermal flux is still a challenge. In this context we used two innovative approaches.The first one consists in a Lagrangian approach. An instrumented particle, designed for temperature measurements, is immersed in the flow. The simultaneous measurement of the temperature view from the particle and its velocity allowsto obtain local Lagrangian heat flux. By comparing these measurements with numerical simulation and Eulerian investigations we can assert the relevance of this method. Then we seeded the flow with particles whose diameter is smaller than the dissipative scale of the system. We performed 3D tracking to access to turbulent statistics and pair dipersion.On the other hand we study a configuration with controlled roughness on the bottom plate. It is well-known that it leads to thermal flux enhancement higher than the one linked to the surface increasing. We bring out possible mechanisms to explain this phenomenon. Thermometric measurements in water-filled cell and anemometric measurements in a six-time larger air-filled cell point out a dramatic change of the boundary layers structure close to roughness. Particularly a turbulent viscous boundary layer appears. Theses investigations are supplemented by velocity measurements of the global flow and reveal a large increase of velocity fluctuations and the appearance of a new turbulence regime.

Cette thèse a pour objectif l’étude comparée de la turbulence isotherme en hélium I (HeI, fluide classique) et en hélium II (HeII, dont une partie du fluide est inviscide) par une analyse lagrangienne. Les analogies et/ou différences du comportement dynamique en écoulement classique ou superfluide devraient ainsi nous renseigner sur les caractéristiques multi-échelles intrinsèques de la turbulence. Ce type de recherche, qui constituait une première mondiale en début de thèse, semble promis à un bel avenir (à titre d'exemple, des mesures similaires de suivi lagrangien en HeI/HeII, en aval d’un barreau oscillant, sont actuellement en cours à Charles Université, Prague).Nous avons opté pour l’écoulement canonique de grille oscillante car il est isotrope, sans vitesse moyenne (autorisant ainsi un temps d’observation long) et homogène par plan. Nous basons nos mesures sur la visualisation de microsphères creuses de verre, dont la densité est voisine de celle de l’hélium liquide. Cette expérience étant nouvelle, il a fallu procéder à sa conception et à son dimensionnement, à sa mise en place ainsi qu'à sa calibration. Le cryostat que l’on a conçu est entièrement en verre, permettant ainsi une observation multi-angles de l’écoulement. Le système de visualisation utilisé est basé sur l'imagerie haute cadence en éclairage "backlight" (diffusion avant). A partir de la reconstruction des trajectoires des particules, nous avons pu réaliser des analyses lagrangiennes à une particule (à un temps et à deux temps), mais aussi étudier la dispersion relative de paires de particules.La résolution spatiale et temporelle de nos mesures nous a permis de pleinement caractériser les échelles inertielles de la turbulence, tandis que les échelles sont plus marginalement résolues. Dans ces conditions, nous trouvons que les caractéristiques de la turbulence en HeI sont en accord avec les mesures de la littérature dans des écoulements similaires en fluide classique. Ce même comportement est également observé en HeII. / This thesis aims at comparing the isothermal turbulence in Helium I (HeI, classical fluid) and in Helium II (HeII, which has one inviscid component) through a Lagrangian analysis. Analogies and/or differences between the classical and superfluid behaviors are expected to shed new light on the intrinsic multi-scale properties of turbulence. This is a pioneering study, as no similar cryogenic experiment had been performed prior to this thesis, which opens a full range of possible future studies (as an example, similar measurements of Lagrangian tracking in the wake of an oscillating cylinder, are currently in progress at Charles University in Prague). We opted for an oscillating grid turbulence generation, which produces isotropic and homogeneous (per plane) turbulence, with no mean velocity (hence allowing longer observation times). As tracers we use hollow glass microspheres, which are almost neutrally buoyant in liquid Helium. This being a new facility, significant effort was dedicated to the design and the tailoring of the experiment, its implementation and the calibration of the flow. The cryostat hosting the experiment is fully transparent (made of glass), allowing visualization at multiple angles. Measurements were performed using high speed imaging in backlight illumination. Particle trajectories were reconstructed using Lagrangian tracking, from which we performed analysis of single particle statistics (single time and two times) as well as relative dispersion of pairs of particles. The spatial and temporal resolution of our measurements give us access to the dynamics of the flow at inertial scales, while dissipative scales are marginally resolved. In these conditions, we find that turbulence in HeI behaves in agreement with previous results obtained in similar flows using classical fluids. Interestingly, the same turbulence properties stands also for superfluid conditions.

Les écoulements turbulents à phases séparées sont présents dans de très nombreuses applications. Cependant, la simulation de tels écoulements avec une interface déformable constitue l'un des problèmes les plus complexes de la mécanique des fluides numérique. La prise en compte du bilan des contraintes normales est au cœur du problème de déformation de l'interface. Dans le travail présenté ici, nous développons un algorithme permettant de simuler des écoulements diphasiques incompressibles et turbulents en suivant le déplacement de l'interface par une approche lagrangienne. Les équations de Navier-Stokes instationnaires écrites en variables vitesse-pression sont résolues dans les deux phases en utilisant des maillages curvilignes orthogonaux. Dans un premier temps, nous introduisons un schéma de raccordement des vitesses tangentielles et des cisaillements. Ce schéma est appliqué afin de simuler l'interaction de deux écoulements turbulents séparés par une interface plane. La turbulence est traitée par une approche de simulation des grandes échelles utilisant un modèle dynamique. Un algorithme original est ensuite développé dans le but de satisfaire de façon non-itérative à la fois la continuité des vitesses normales et des contraintes normales sur l'interface et l'incompressibilité dans les deux phases. Différentes simulations d'écoulements diphasiques avec interface déformable sont réalisées afin de valider ces développements. / Turbulent incompressible two-phase separated flows are present in many applications. However, simulation of such flows with a moving interface is one of the most challenging problems in todays computational fluid dynamics. Taking properly into account the normal stress budget accross the interface is the main difficulty of moving interface problems. This work deals with the development of a boundary-fitted method for computing turbulent incompressible two-phase flows. The interface displacement is achieved through a Lagrangian approach. The unsteady Navier-Stokes equations written in a velocity- ressure formulation are solved within the two phases using an orthogonal curvilinear grid. In a first step, we introduce a scheme allowing tangential velocities and shear stresses to match across the interface. We apply this technique to compute the countercurrent flow generated by two streams separated by a plane interface. This scheme is then applied to compute various situations involving the interaction between two turbulent flows separated by a flat interface. The turbulence is treated by using the Large Eddy Simulation approach with a dynamic model. An original algorithm is then developed to satisfy without any internal iteration the continuity of normal velocities and stresses across the interface and the incompressibility condition within both phases. Several simulations of two-phase flows with a moving interface are carried out to validate these developments.

L'émergence des transformateurs piézoélectriques coïncident avec le développement dans les années 1950 des céramiques ferroélectriques appartenant à la famille cristalline des pérovskites qui n'ont cessé de s'améliorer depuis. Outre la compacité dont bénéficie ces structures, les transformateurs piézoélectriques offrent des performances remarquables en terme de gain en tension et rendement utiles pour des applications nécessitant une adaptation de tension ou une isolation galvanique, parfaitement dédiés aux applications de faibles puissances à haut rendement. Toutefois, les transformateurs piézoélectriques peuvent être déviés de leurs applications premières. En effet, la dernière décennie a été marquée par l'apparition de générateur de plasma par effet piézoélectrique utilisant principalement des architectures de type transformateur. Pourtant, si quelques applications usuelles illustrent parfaitement cette interaction, la compréhension des phénomènes physiques qui en sont à l'origine reste à approfondir. L'objectif de cette thèse est d'en expliquer les fondements par une approche méthodique. Ce travail s'articule autour de plusieurs étapes comprenant la mise en oeuvre d'une méthode systématique de la modélisation analytique d'un transformateur piézoélectrique, de l'étude de la carte de champ produit par un transformateur ainsi qu'une étude expérimentale vue des bornes en guise de premières investigations. La modélisation analytique est basée sur l'exploitation du Principe de Moindre Action (PMA). A partir de la théorie linéaire de la piézoélectricité, un modèle général applicable à toutes les géométries de transformateur, exploitant des modes de couplage piézoélectrique multiples, est proposé. Son caractère multimodal est par ailleurs mis en exergue. Cette modélisation est appliquée à une structure classique de transformateur piézoélectrique de type Rosen et les résultats obtenus sont validés d'une part par une identification numérique, issue d'un logiciel de calcul par éléments finis, et d'autre part par une caractérisation expérimentale. La modélisation analytique précédente ne tient pas compte dans sa mise en oeuvre de l'influence de l'environnement dans lequel évolue le transformateur piézoélectrique. Afin de caractériser le potentiel électrique produit, un modèle numérique 2D du champ électrique environnant est proposé selon la méthode des différences finies. Ce modèle est basé sur une extension du modèle analytique précédemment développé incluant les pertes mécaniques afin de quantifier le potentiel électrique de surface. Même si l'influence du plasma est négligée en première approximation, la modélisation permet de mettre en lumière les zones de fort champ correspondant aux zones de décharges luminescentes observées expérimentalement. Finalement, afin de valider le concept de générateur de plasma piézoélectrique, une caractérisation vue des bornes du transformateur piézoélectrique de type Rosen a été entreprise. Une étude systématique du déclenchement de la décharge plasma en fonction du niveau de tension et de la pression environnante a été menée. Cette part expérimentale de l'étude constitue une approche pionnière pour qualifier le comportement électromécanique du transformateur et a ainsi permis de mettre en évidence des comportements non linéaires issus de ce mode de fonctionnement atypique qu'est la génération de décharges de surface par effet piézoélectrique. / The emergence of piezoelectric transformers coincides with the development in the 1950s of ferroelectric ceramics belonging to the perovskites crystalline family. In addition to providing small size and weight, piezoelectric transformers offer outstanding performances in terms of galvanic insulation, voltage ratio and efficiency. Furthermore, compared with conventional electromagnetic transformers, piezoelectric transformers are free from electromagnetic interference. They are consequently more suitable for low power and high efficiency applications for small embedded systems. However, piezoelectric transformers can be deviated from their initial applications. Indeed, the emergence of plasma generator by piezoelectric effect, using mainly piezoelectric transformers, made its mark on the last decade. Nevertheless, if a few typical applications perfectly exemplify this interaction, the understanding of instigated physical phenomena remains to go into detail. The aim of this thesis is to explain the fundaments by a methodical approach. This work is based on several steps including the implementation of piezoelectric transformer analytical modeling, the study of the electrical field generated by a piezoelectric transformer and a first experimental investigation from piezoelectric transformer terminals. The analytical modeling is based on the utilization of the least action principle. From linear piezoelectric theory, a general model applicable to all transformer geometries, using multiple piezoelectric coupling modes, is put forward. Its multimodal characteristic is in addition underlined. This modeling is applied to a classical Rosen type transformer and the obtained results are confirmed on one hand by a numerical identication, and on the other hand by an experimental characterization. The previous analytical modeling does not take into account in its application the influence of the environment in which the piezoelectric transformer evolves. In order to qualify the produced electrical potential, a 2D numerical model of surrounding electrical field is put forward according to nite difference method. This model is based on the extension of previously developed analytical model including mechanical losses in order to quantify the surfacic electrical potential. Even if plasma influence is ignored in first approximation, the modeling allows to highlight high electrical field areas matching glow discharges areas experimentally observed. Finally, in order to validate the concept of piezoelectric plasma generator, a characterization from Rosen type piezoelectric transformer terminals has been undertaken. A systematic study of plasma discharge ignition in function of the input voltage level and the surrounding pressure has been carried out. This experimental part of the study constitutes a pioneering approach in order to qualify transformer electromechanical behavior. It has consequently allowed to give rise to nonlinear behaviors from untypical operation mode which is the surfacic discharge generation by piezoelectric effect.

Les turbines à gaz doivent satisfaire des normes d'émission polluantes toujours en baisse. La formation de polluants est directement liée à la qualité du mélange d'air et de carburant en amont du front de flamme. Ainsi, leur réduction implique une meilleure prédiction de la formation du spray et de son interaction avec l'écoulement gazeux. La Simulation aux Grandes Échelles (SGE) semble un outil numérique approprié pour étudier ces mécanismes. Le but de cette thèse est de développer des modèles phénoménologiques décrivant la phase liquide notamment le film et son atomisation en bout de lèvre d'injecteur, dans un contexte SGE. Ces modèles sont validés ou calibrés sur l'expérience académique réalisée par l'Institut für Thermische Strömungsmaschinen (ITS) de l'université technologique de Karlsruhe (KIT), et appliqués dans une configuration réelle de moteur d'hélicoptère. Dans un premier temps, le film liquide mince est décrit par une approche Lagrangienne. Les particules de film représentent un volume élémentaire de liquide adhérent à la paroi. L'équation du mouvement est donnée par l'intégration des équations de Saint-Venant sur l'épaisseur du film. La dynamique du film est donnée par le gradient de pression longitudinal, le cisaillement interfacial du gaz et la gravité. Dans un second temps, l'atomisation du film est caractérisée par la distribution de taille de gouttes du spray généré. Celle-ci est décrite par une distribution de Rosin-Rammler dont les coefficients sont paramétrés par la vitesse du gaz, la tension superficielle du liquide et l'épaisseur de la lèvre de l'injecteur. Les constantes de ce modèle, baptisé PAMELA, sont calibrées sur l'expérience du KIT-ITS. La simulation de l'expérience KIT-ITS permet de valider le modèle de film, de vérifier la robustesse du modèle PAMELA, et de comparer qualitativement l'angle du spray généré. L'application de ces modèles dans une configuration réelle partiellement instrumentée permet de valider PAMELA sans modification de ses constantes, et d'étudier leur impact sur la structure de flamme, comparé aux méthodes traditionnelles d'injection liquide. / Aeronautical gas turbines need to satisfy growingly stringent demands on pollutant emission. Pollutant emissions are directly related to the quality of fuel air mixing prior to combustion. Therefore, their reduction relies on a more accurate prediction of spray formation and interaction of the spray with the gaseous turbulent flow field. Large-Eddy Simulation (LES) seems an adequate numerical tool to predict these mechanisms. The objectives of this thesis is to develop phenomenological models describing the liquid phase, in particular the film and its atomization at the injector atomizing lips, in the context of LES. These models are validated or calibrated on the academic experiment performed at Institut für Thermische Strömungsmaschinen (ITS) from the Karlsruhe Institute of Technology (KIT), and applied to a helicopter engine real configuration. In a first step, the thin liquid film is described by a Lagangian approach. Film particles represent an elementary volume of liquid at the wall surface. The equation of motion is given by integrating the Saint-Venant equations over the film thickness. The film dynamics derives from the pressure gradient, the interfacial shear and gravity. In a second step, the film breakup is characterized by the drop size distribution of the spray. The former one is described by a Rosin-Rammler distribution, whose coefficients depend on the gas velocity, the liquid surface tension and the atomizing edge thickness of the injector. The model, labelled PAMELA, is calibrated from the KIT-ITS experiment. The simulation of the KIT-ITS experiment allows to validate the film model, to check PAMELA robustness, and to compare qualitatively the spray angle. The application of these models in a real configuration allows to check PAMELA robustness without constants modification, and to study their impact on the flame structure, in comparison with usual methods of liquid injection.

Dans cette thèse, l’étude des interactions entre des particules ultrafines émises par les pots d’échappement et l’écoulement de sillage créé par le véhicule émetteur a été réalisée principalement selon une approche numérique. Une campagne expérimentale a été conduite à des fins de validation. L’objet de la thèse vise à comprendre l’impact des particules issues des pots d’échappement sur l’environnement proche tant du côté piéton que du côté des passagers des véhicules suiveurs. Pour cela, l’écoulement du fluide a été traité avec une approche eulérienne type URANS (Unsteady Reynolds Average Navier-Stokes) combinée à un suivi lagrangien pour les nanoparticules. En effet, cette thèse est conduite en parallèle d’un projet collaboratif financé par l’ADEME (CAPTIHV) dont le but est d’évaluer la qualité de l’air des habitacles des véhicules automobiles, et en particulier de l’infiltration des particules ultrafines issues du trafic environnant. L’étude de la dispersion des particules fines en écoulements turbulents nécessite une analyse fine des structures turbulentes qui s’y développent. Notre étude numérique a donc consisté, en premier lieu, à analyser cette dispersion dans le cas d’un écoulement de sillage classique à l’aval d’un cylindre. Cela nous a permis de caractériser la dynamique d’interactions de nanoparticules solides de carbone avec les structures tourbillonnaires en considérant l’impact de la turbulence et de la diffusion brownienne. Cela a permis d’évaluer l’influence des principaux mécanismes influençant la dispersion. Les résultats de ces simulations nous ont permis de sélectionner les mécanismes/forces importants pouvant influencer la dispersion de telles particules dans le sillage d’un véhicule automobile ; Cela nous a facilité la mise en place et l’analyse des simulations relativement plus complexes de l’aérodynamique du corps d’Ahmed à culot droit en présence des nanoparticules simulant les suies des gaz d’échappement. Les interactions des particules ultrafines avec les structures tourbillonnaires se créant dans le sillage des véhicules ont été évaluées à partir de profils de concentrations et les coefficients de dispersions transversales. La dernière étape a consisté en une campagne d’essais en soufflerie qui nous a permis de caractériser les champs de vitesses moyens et turbulents ainsi que les champs de concentrations particulaires à l’aval du véhicule pour valider les résultats numériques / In this thesis, the study of the interactions between ultrafine particles emitted by the exhaust pipes and the wake flow generated by the emitting vehicle was carried out mainly using a numerical approach. An experimental campaign was conducted for validation purpose. The goal of the thesis is to understand the impact of exhaust particles on the surrounding environment on both the pedestrian and the passengers of the following vehicles. For this purpose, the fluid flow was resolved with an Eulerian type URANS model (Unsteady Reynolds Average Navier-Stokes) combined to the Lagrangian approach for the nanoparticles trajectories calculation. This thesis is conducted simultaneously with a collaborative project funded by ADEME (CAPTIHV) whose purpose is to assess the air quality of automotive car cabins, and particulate infiltration from the surrounding traffic in particular of ultrafine particles. The study of the dispersion of fine particles in turbulent flows requires a fine analysis of the turbulent structures that develop in such flows. Our numerical study therefore consisted, first, in analyzing this dispersion in the case of a classic wake flow downstream of a cylinder. This enabled us to characterize the interaction of solid carbon nanoparticles with vortical structures evaluating at the same time the impact of turbulence and Brownian diffusion. This allowed determining the influence of the main mechanisms influencing nanoparticles dispersion. In a second step, we replaced the cylinder configuration by a simplified geometry of a motor vehicle, Ahmed body configuration. Therefore, simulations with and without of particles presence have been conducted and have allowed to highlight the swirls structures and to characterize the particles dispersion through particle concentration profiles and the particles dispersion coefficients. The results of these simulations allowed us determining the important mechanisms / forces that can influence the dispersion of such particles in the wake of a ground vehicle; this facilitated the implementation and analysis of relatively more complex simulations of the aerodynamics of the square back Ahmed body in the presence of nanoparticles simulating soot from the exhaust gases. The interactions of ultrafine particles with vortical structures appearing in the wake of vehicles were evaluated from concentration profiles and transverse dispersion coefficients. The final step was a wind tunnel experimental campaign that allowed us to characterize the average and turbulent velocity fields as well as the particle concentration fields downstream of the vehicle to validate the numerical results

<p>The present dissertation focuses on two themes: (i) elastic instability of flow and (ii) elastic instability of microscopic filaments.</p> <p><br></p> <p>(i) The presence of macromolecules often leads to the viscoelastic nature of industrial and biological fluids. The flow of viscoelastic fluids in porous media is important in many industrial, geophysical, and biological applications such as enhanced oil recovery, groundwater remediation, biofilm formation, and drug delivery. The stretching of polymeric chains as the viscoelastic fluid passes through the microstructure of the porous media induces large elastic stresses, which leads to viscoelastic instability at the Weissenberg number greater than a critical value, where the Weissenberg number quantifies the ratio of elastic to viscous forces. Viscoelastic instability can lead to a time-dependent chaotic flow even at negligible inertia, which is sometimes also known as elastic turbulence due to its analogous features to traditional inertial turbulence. In the present thesis, we investigate the pore-scale viscoelastic instabilities and the flow states induced by the instabilities in symmetric and asymmetric geometries. We found that the topology of the polymeric stress field regulates the formation of different flow states during viscoelastic instabilities. Viscoelastic instability-induced flow states exhibit hysteresis due to the requirement of a finite time for the transformation of polymeric stress topology. Further, we study viscoelastic flows through ordered and disordered porous geometries and explore the effect of viscoelastic instability on sample-scale transport properties. Viscoelastic instability enhances transverse transport in ordered porous media and longitudinal transport in disordered porous media. We also derive a relationship between the polymeric stress field and the Lagrangian stretching field. The Lagrangian stretching field helps to predict the feature of flow states and transport in complex flows. The experimental measurement of the polymeric stress field is extremely challenging. The framework established here can be used to obtain the topology of the polymeric stress field directly from the easily measured velocity field.  </p> <p><br></p> <p><br></p> <p>(ii) The interaction between flow and elastic filaments plays an important role in sperm and bacterial motility and cell division. The sperm cells of many organisms use long elastic flagellum to propel themselves and also face complex flows and boundaries during their search for egg cells. Strong flows have the potential to mechanically inhibit flagellar motility through elastohydrodynamic interactions. We explore the effects of an extensional flow on the buckling dynamics of sperm flagella through detailed numerical simulations and microfluidic experiments. Compressional fluid forces lead to rich buckling dynamics of the sperm flagellum beyond a critical dimensionless sperm number, which represents the ratio of viscous force to elastic force. Shear flows navigate the sperm cells in complex geometries and flows. We have also studied the effect of flow strength and flagellar elastic deformation on the sperm trajectory in simple shear and Poiseuille flows.</p>

In power plants using large utility coal-fired boilers for generation of electricity the coal is pulverised in coal mills and then it has to be pneumatically transported and distributed to a larger number of burners (e.g. 30-40) circumferentially arranged in several rows around the burning chamber of the boiler. Besides the large pipework flow splitting devices are necessary for distribution of an equal amount of pulverised fuel (PF) to each of the burners. So called trifurcators (without inner fittings or guiding vanes) and ''riffle'' type bifurcators are commonly used to split the gas-coal particle flow into two or three pipes/channels with an equal amount of PF mass flow rate in each outflow cross section of the flow splitting device. These PF flow splitting devices are subject of a number of problems. First of all an uneven distribution of PF over the burners of a large utility boiler leads to operational and maintenance problems, increased level of unburned carbon and higher rates of NOX emissions. Maldistribution of fuel between burners caused by non uniform concentration of the PF (particle roping) in pipe and channel bends prior to flow splitting devices leads to uncontrolled differences in the fuel to air ratio between burners. This results in localised regions in the furnace which are fuel rich, where insufficient air causes incomplete combustion of the fuel. Other regions in the furnace become fuel lean, forming high local concentrations of NOX due to the high local concentrations of O2. Otherwise PF maldistribution can impact on power plant maintenance in terms of uneven wear on PF pipework, flow splitters as well as the effects on boiler panels (PF deposition, corrosion, slagging). In order to address these problems in establishing uniform PF distribution over the outlet cross sections of flow splitting devices in the pipework of coal-fired power plants the present paper deals with numerical prediction and analysis of the complex gas and coal particle (PF) flow through trifurcators and ''riffle'' type bifurcators. The numerical investigation is based on a 3-dimensional Eulerian- Lagrangian approach (MISTRAL/PartFlow-3D) developed by Frank et al. The numerical method is capable to predict isothermal, incompressible, steady gas- particle flows in 3-dimensional, geometrically complex flow geometries using boundary fitted, block-structured, numerical grids. Due to the very high numerical effort of the investigated gas-particle flows the numerical approach has been developed with special emphasis on efficient parallel computing on clusters of workstations or other high performance computing architectures. Besides the aerodynamically interaction between the carrier fluid phase and the PF particles the gas-particle flow is mainly influenced by particle-wall interactions with the outer wall boundaries and the inner fittings and guiding vanes of the investigated flow splitting devices. In order to allow accurate quantitative prediction of the motion of the disperse phase the numerical model requires detailed information about the particle-wall collision process. In commonly used physical models of the particle-wall interaction this is the knowledge or experimental prediction of the restitution coefficients (dynamic friction coefficient, coefficient of restitution) for the used combination of particle and wall material, e.g. PF particles on steel. In the present investigation these parameters of the particle-wall interaction model have been obtained from special experiments in two test facilities. Basic experiments to clarify the details of the particle-wall interaction process were made in a test facility with a spherical disk accelerator. This test facility furthermore provides the opportunity to investigate the bouncing process under normal pressure as well as under vacuum conditions, thus excluding aerodynamically influences on the motion of small particles in the near vicinity of solid wall surfaces (especially under small angles of attack). In this experiments spherical glass beads were used as particle material. In a second test facility we have investigated the real impact of non-spherical pulverised fuel particles on a steel/ceramic target. In this experiments PF particles were accelerated by an injector using inert gas like e.g. CO2 or N2 as the carrier phase in order to avoid dust explosion hazards. The obtained data for the particle-wall collision models were compared to those obtained for glass spheres, where bouncing models are proofed to be valid. Furthermore the second test facility was used to obtain particle erosion rates for PF particles on steel targets as a function of impact angles and velocities. The results of experimental investigations has been incorporated into the numerical model. Hereafter the numerical approach MISTRAL/PartFlow-3D has been applied to the PF flow through a ''riffle'' type bifurcator. Using ICEM/CFD-Hexa as grid generator a numerical mesh with approximately 4 million grid cells has been designed for approximation of the complex geometry of the flow splitting device with all its interior fittings and guiding vanes. Based on a predicted gas flow field a large number of PF particles are tracked throughout the flow geometry of the flow-splitter. Besides mean quantities of the particle flow field like e.g. local particle concentrations, mean particle velocities, distribution of mean particle diameter, etc. it is now possible to obtain information about particle erosion on riffle plates and guiding vanes of the flow splitting device. Furthermore the influence of different roping patterns in front of the flow splitter on the uniformness of PF mass flow rate splitting after the bifurcator has been investigated numerically. Results show the efficient operation of the investigated bifurcator in absence of particle roping, this means under conditions of an uniform PF particle concentration distribution in the inflow cross section of the bifurcator. If particle roping occurs and particle concentration differs over the pipe cross section in front of the bifurcator the equal PF particle mass flow rate splitting can be strongly deteriorated in dependence on the location and intensity of the particle rope or particle concentration irregularities. The presented results show the importance of further development of efficient rope splitting devices for applications in coal-fired power plants. Numerical analysis can be used as an efficient tool for their investigation and further optimisation under various operating and flow conditions.