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LES analysis of a supersonic air inlet experiencing buzz phenomenon
Get PDFThis paper investigates the flow in a supersonic mixed
compression air intake using numerical simulation. The configuration has been experimentally tested for different backpressures imposed by a moving plug set at the exit of the internal intake (throttled) or without plug (unthrottled) for which data are available in the literature and to which the simulations can be compared. The unthrottled and three different throttled positions are investigated using large-eddy simulation (LES). The purpose of the study is to analyze the shock structures and their interaction with the boundary layers developing on the surfaces of the air intake for these different cases. Also, because the experiments reported potential unsteady positions of the shock structures for large throttling ratios and, more generally, to analyze the unsteady phenomena, the spectral proper orthogonal decomposition (SPOD) algorithm has been used
Regularized Infill Criteria for Multi-objective Bayesian Optimization with Application to Aircraft Design
Get PDFBayesian optimization is an advanced tool to perform efficient global optimization. It consists on enriching iteratively surrogate Kriging models of the objective and the constraints (both supposed to be computationally expensive) of the targeted optimization problem. Nowadays, efficient extensions of Bayesian optimization to solve expensive multi-objective problems are of high interest. The proposed method, in this paper, extends the super efficient global optimization with mixture of experts (SEGOMOE) to solve constrained multi-objective problems. To cope with the ill-posedness of the multi-objective infill criteria, different enrichment procedures using regularization techniques are proposed. The merit of the proposed approaches are shown on known multi-objective benchmark problems with and without constraints. The proposed methods are then used to solve a bi-objective application related to conceptual aircraft design with five unknown design variables and three nonlinear inequality constraints. The prelimilnary results show a reduction of the total cost in terms of function evaluations by a factor of 20 compared to the evolutionary algorithm NSGA-II
Sweep effects on canonical shock wave/turbulent boundary layer interaction
Get PDFThis study focuses on the effect of a sweep angle between the upstream mean flow direction and the shock plane normal in a canonical oblique shock wave/boundary layer interaction. We study a configuration based on the experiments of Bo et al.(2012): an incident shock impinges on a turbulent boundary layer developing at free stream Mach number M = 2:7 and a Reynolds number Re_theta = 3200. The sweep angle is implemented by adding a spanwise velocity component (crossflow) at the inflow and enforcing periodicity conditions on the side boundaries. Several sweep angles are investigated and compared to a canonical unswept case. A viscosity is selected for each case in an effort to keep a constant Req across the whole range of sweep angles. The mean flow structure is analyzed, showing a strongly skewed flow around the separation zone that increases in size compared to a reference unswept case. Pressure loads on the wall surface are also higher for swept cases; and the characteristic low-frequencies of the flow separation also slightly increase
All-At-Once formulation integrating pseudo-spectral optimal control for launch vehicle design and uncertainty quantification
Get PDFLaunch vehicle design is a multidisciplinary process involving different disciplines such as aerodynamics, structure, propulsion and trajectory. This latter is a key discipline in the design process as it is used to assess the overall performance of the vehicle by solving of an optimal control problem involving a system of nonlinear ordinary differential equations. Usual Multidisciplinary Design Optimization methods for launch vehicle design consider the trajectory design discipline as an auxiliary embedded optimization problem to find the optimal trajectory control. In this paper, a multidisciplinary approach is proposed relying on an All-At-Once formulation by using Gauss–Lobatto collocation technique to solve the optimal control problem. The problem is solved using a gradient-based optimizer. Furthermore, based on this framework, an uncertainty quantification technique using post-optimality analysis is derived to perform sensitivity analysis. This allows to analyze the influence of modeling uncertainty on the optimal launch vehicle performance in the early design phases. The efficiency of the proposed approach is illustrated on the optimization and uncertainty quantification of a representative Two-Stage-To-Orbit launch vehicle design problem
Learning Path Constraints for UAV Autonomous Navigation under Uncertain GNSS Availability
Get PDFThis paper addresses a safe path planning problem for UAV
urban navigation, under uncertain GNSS availability. The
problem can be modeled as a POMDP and solved with sampling-based algorithms. However, such a complex domain suffers from high computational cost and achieves poor results under real-time constraints. Recent research seeks to integrate offline learning in order to efficiently guide online planning. Inspired by the state-of-the-art CAMP (Context-
specific Abstract Markov decision Process) formalization, this paper proposes an offline process which learns the path
constraint to impose for online POMDP solving. More precisely, the offline learnt constraint selector returns the best path constraint according to the GNSS availability in the environment. This constraint is then imposed during online planning to reduce the policy search space. Conclusions of experiments, carried out for different environments, show that using the proposed approach allows to improve the quality of a solution reached by an online planner, within a fixed decision-making timeframe, particularly when GNSS availability probability is low
Advances in fine line-of-sight control for large space flexible structures
Get PDFThe increased need in pointing performance for Earth observation and science Space missions together with the use of lighter and flexible structures directly come with the need of a robust pointing performance budget from the very beginning of the mission design. An extensive understanding of the system physics and its uncertainties is then necessary in order to push control design to the limits of performance and constrains the choice of the set of sensors and actuators. A multi-body framework, the Two Input Two Output Ports approach, is used to build all the elementary flexible bodies and mechanisms involved in a fine pointing mission. This framework allows the authors to easily include all system dynamics with an analytical dependency on varying and uncertain mechanical parameters in a unique Linear Fractional Transformation (LFT) model. This approach opens the doors to modern robust control techniques that robustly guarantee the expected fine pointing requirements. In particular, a novel control architecture is proposed to reduce the microvibrations induced both by reaction wheel imbalances and Solar Array Drive Mechanism driving signal, by letting them work during the imaging phase. Thanks to a set of accelerometers placed at the isolated base of the payload and in correspondence of the mirrors with the largest size in a Space telescope (typically the primary and secondary ones), it is possible to estimate the line-of-sight error at the payload level by hybridizing them with the low-frequency measurements of the camera. While a classical Fast Steering Mirror in front of the camera can compensate for a large amount of microvibration, an innovative architecture with a set of six Proof-Mass Actuators installed at the payload isolator level can further improve the pointing performance. In particular, it is shown how the proposed architecture is able to robustly guarantee an absolute performance error of 10 arcsec in face of system parametric uncertainties at low frequency (≈ 1 rad/s) with a progressive reduction of the jitter down to 40 marcsec for higher frequencies where micro-vibration sources act
From atom scale to casting : A contemporary monograph on silicon cast irons microstructure
Get PDFThe origin of this monograph lies in a simple fact: there is a paradigm with cast irons, namely that these alloys have been produced and cast for thousands of years, yet they are among the most complicated alloys if we consider the formation of their microstructure by solidification and transformations in the solid state. The positive side of this complexity is that it offers a wide range of possibilities for manipulating their microstructure. With the evolution of melting furnaces in the 19th century, the silicon content of cast irons increased, leading to the development of the silicon cast irons that are the subject of this monograph. The essential step, however, was the discovery in the middle of the 20th century that it is possible to change the shape of graphite by transforming the interconnected lamellae into discrete spheroids. In this way, cast iron became a material for safety parts and was no longer limited to construction. This historical development and the research effort during the first part of the 20th century have been described in reviews published in the 1960s. Since then, developments in metallographic analysis have led to a wealth of research aimed at describing and understanding graphite formation and the microstructure of cast irons during solidification and heat treatment. This monograph is not intended to be an exhaustive review of the literature of the last 50 years, but aims to propose a coherent vision of the formation of the microstructure of cast irons based on the work carried out jointly or in parallel by its authors. Current controversies are sometimes indicated but are not discussed in order to emphasize open questions. Finally, we should mention that our work has greatly benefited from the dynamism of the European Cast Iron group (ECI), an informal forum open to all (European academics and industrialists) for more than 10 years. It is in this same spirit of discussion that this monograph is freely available. \\//
Cette monographie trouve son origine dans un fait simple : il existe un paradigme avec les fontes, à savoir que ces alliages sont produits et coulés depuis des milliers d'années, mais qu'ils comptent pourtant parmi les alliages métalliques les plus compliqués si l'on considère la formation de leur microstructure par solidification et transformations à l'état solide. Le côté positif de cette complexité est qu'elle offre un large éventail de possibilités pour la manipulation de leur microstructure. Avec l'évolution des fours de fusion au 19ème siècle, la teneur en silicium des fontes a augmenté conduisant au développement des fontes au silicium qui font l'objet de cette monographie. L'étape essentielle, cependant, a été la découverte au milieu du 20ème siècle qu'il est possible de changer la forme du graphite en transformant les lamelles interconnectées en sphéroïdes disjoints. Les fontes sont ainsi devenues un matériau pour pièces de sécurité et n'ont plus été limitées à la construction. Cette évolution historique et l'effort de recherche durant la première partie du 20ème siècle ont été décrits dans les revues publiées dans les années 1960. Depuis cette époque, l'évolution des analyses métallographiques a généré nombre de recherches destinées à décrire et comprendre la formation du graphite et de la microstructure des fontes lors de leur solidification et des traitements thermiques. Cette monographie ne se veut pas une revue exhaustive de la littérature de ces 50 dernières années, mais vise à proposer une vision cohérente de la formation de la microstructure des fontes graphitiques basée sur les travaux conduits en commun ou en parallèle par ses auteurs. Les controverses actuelles sont quelquefois indiquées mais ne sont pas discutées afin de mettre l'accent sur les questions ouvertes. Enfin, nous avons plaisir à mentionner que notre travail a largement profité du dynamisme du groupe européen de la fonte (ECI), forum informel ouvert à tous (universitaires et industriels européens) depuis plus de 10 ans. C'est dans ce même esprit de discussion que cette monographie est en accès libre
Aeroelastic scaling of flying demonstrators: mode tracking technique
Get PDFAeroelastic scaling theory shows that the design problem of aeroelastically equivalent scaled aircraft can be treated as a structural-only design problem if the aerodynamic shape and airflow properties of the full scale aircraft are preserved. In that case, the theory shows that it is sufficient to match the scaled natural mode shapes, frequencies and mass of the reference aircraft. In this paper, we present a new method for the dynamic scaling of flexible structures where the objective function is based on the modal assurance criterion (MAC). This criterion is used for a mode tracking strategy during the optimization process. Finally, we apply this method to a scaled version (1:5) of the uCRM wing, achieving an agreement greater than 99% on the average MAC value of the first 5 modes
Multi-inception patterns of emitter array/collector systems in DC corona discharge
Get PDFMultiple emitters systems have been previously used so as to increase charge density in the drift region, many times without producing sensible increment neither in total current nor ionic wind. This contribution focuses on analyzing the detailed physics behind this failure, that is named "multiple emitters un-scalability". It is established that multiple emitters un-scalability is related to the inability of multiple corona discharge inceptions when increasing the emitter number and/or density. This confirms recent findings that corona discharge inception is shielded by electro-static interactions between emitters. This contribution demonstrates that this shielding can be balanced by emitter/collector electrostatic interactions depending on the considered configuration. For sufficiently close collector-emitter distances, ignition starts at the array center, whereas, on the contrary, when the collector is distant, the ignition not only starts at the array's periphery but might also be limited there.It is also demonstrated that emitter/emitter electrostatic interactions can be balanced by emitter/collector ones, depending of their chosen configuration. This lead to a variety of multi-inception patterns, the condition of which are analyzed. Intermediate configurations for which the collector is neither sufficiently close nor distant from the emitter array center provide a variety of multi-inception patterns that are hereby analyzed. Combining finite element computations of multi-inception drift-diffusion modeling with experimental measurements, provides a coherent picture explaining why multiple emitters sources systems do not lead to full ignition, and also exhibit conditions for which it does, leading to multiple emitters scalable systems
Lie Group Modelling for an EKF-Based Monocular SLAM Algorithm
Get PDFThis paper addresses the problem of monocular Simultaneous Localization And Mapping on Lie groups using fiducial patterns. For that purpose, we propose a reformulation of the classical camera model as a model on matrix Lie groups. Thus, we define an original-state vector containing the camera pose and the set of transformations from the world frame to each pattern, which constitutes the map’s state. Each element of the map’s state, as well as the camera pose, are intrinsically constrained to evolve on the matrix Lie group SE(3). Filtering is then performed by an extended Kalman filter dedicated to matrix Lie groups to solve the visual SLAM process (LG-EKF-VSLAM). This algorithm has been evaluated in different scenarios based on simulated data as well as real data. The results show that the LG-EKF-VSLAM can improve the absolute position and orientation accuracy, compared to a classical EKF visual SLAM (EKF-VSLAM)