Prévision du bruit d'ondes de choc d'un turboréacteur en régime transsonique par des méthodes analytiques et numériques

During aircraft takeoff and climb, the shock waves (or N-waves) emitted by the fan of a rotor at transonic rotation speeds can be a dominant noise source. The study of N-waves needs to take account of two main processes: 1) the generation of N-waves for a perfect rotor (in which all bladesare identical) and for a real rotor (considering small geometrical blade dispersion), 2) the N-wavepropagation in the inlet duct producing the blade passing harmonics for a perfect rotor, and the multiple pure tones (harmonics of the rotation frequency) for a real rotor. This thesis intends to study these two processes on the basis of analytical and numerical considerations. Firstly, the main propagation theories and the foremost MPT generation methods are discussed. In a second step, the main relevant semi-analytical methods are cross-checked by applying them to representative turbofan configurations.Moreover, a novel model of irregular N-wave generation based on blade geometrical parameters isinvestigated thanks to test data. Then, a second part of the work investigates a numerical strategybased on elsA ONERA code, solving the full Euler equations (CAA approach). Regular and non- regular shock waves are directly injected in a plane close to the fan and propagated in the inlet. This injection method is firstly applied on an infinitely narrow annular duct and validated through comparisons with the McAlpine & Fisher analytical method. Then, the 3D propagation effects are pointed out by increasing the duct height. Finally, the CAA method is applied to actual intake geometries of modern turbofan demonstrators, and propagation of regular and irregular shock waves are simulated. The numerical results are compared to RANS solutions and to available measurements.Durant le décollage et la montée d’un avion, les ondes de choc (ou ondes en N) engendrées par la soufflante d’un turboréacteur en régime transsonique peuvent devenir une source de bruit dominante. L’étude des ondes en N nécessite de se concentrer sur deux processus majeurs : 1) lagénération d’ondes en N par un rotor parfait (dont toutes les aubes sont identiques) et par un rotor réel (en tenant compte des irrégularités géométriques des aubes) ; et 2) la propagation de ces ondes en N dans la nacelle, produisant du bruit dont le spectre se compose des harmoniques de la fréquence de passage des aubes pour un rotor régulier et des harmoniques aux fréquences multiples de la rotation du rotor (FMR) pour un rotor irrégulier. Le but de cette thèse est d’étudier ces deux phénomènes par des méthodes analytiques et numériques. Dans un premier temps, les principales théories de la propagation des ondes de choc ainsi que les modèles majeurs de génération de FMR sont mis en perspective. Les principales méthodes semi-analytiques de génération et de propagation des chocs sont évaluées et comparées en les appliquant à des configurations de turboréacteurs. En outre, un nouveau modèle de génération de FMR fondé sur des paramètres géométriques des aubes est élaboré par l’intermédiaire d’une campagne d’essais. Le deuxième volet du mémoire concerne le développement d’une méthodologie de simulation numérique basée sur l’utilisation du code elsA de l’ONERA en résolvant les équations d’Euler (approche CAA). Des ondes de choc régulières et irrégulières sont directement injectées dans un plan proche de la soufflante et se propagent en remontant l’écoulement. Cette méthode d’injection est tout d’abord appliquée à un conduit annulaire infiniment mince et validée par la méthode de propagation semi-analytique de McAlpine & Fisher.Ensuite, les effets de propagation 3D sont étudiés en augmentant l’épaisseur du conduit. Enfin, laméthode CAA est appliquée à des configurations de turboréacteurs modernes et des ondes de chocrégulières et irrégulières sont propagées numériquement. Les résultats sont comparés aux solutionsRANS ainsi qu’aux mesures disponibles

The cdx Genes and Retinoic Acid Control the Positioning and Segmentation of the Zebrafish Pronephros

Kidney function depends on the nephron, which comprises a blood filter, a tubule that is subdivided into functionally distinct segments, and a collecting duct. How these regions arise during development is poorly understood. The zebrafish pronephros consists of two linear nephrons that develop from the intermediate mesoderm along the length of the trunk. Here we show that, contrary to current dogma, these nephrons possess multiple proximal and distal tubule domains that resemble the organization of the mammalian nephron. We examined whether pronephric segmentation is mediated by retinoic acid (RA) and the caudal (cdx) transcription factors, which are known regulators of segmental identity during development. Inhibition of RA signaling resulted in a loss of the proximal segments and an expansion of the distal segments, while exogenous RA treatment induced proximal segment fates at the expense of distal fates. Loss of cdx function caused abrogation of distal segments, a posterior shift in the position of the pronephros, and alterations in the expression boundaries of raldh2 and cyp26a1, which encode enzymes that synthesize and degrade RA, respectively. These results suggest that the cdx genes act to localize the activity of RA along the axis, thereby determining where the pronephros forms. Consistent with this, the pronephric-positioning defect and the loss of distal tubule fate were rescued in embryos doubly-deficient for cdx and RA. These findings reveal a novel link between the RA and cdx pathways and provide a model for how pronephric nephrons are segmented and positioned along the embryonic axis

Worker heterogeneity, new monopsony, and training

A worker's output depends not only on his/her own ability but also on that of colleagues, who can facilitate the performance of tasks that each individual cannot accomplish on his/her own. We show that this common-sense observation generates monopsony power and is sufficient to explain why employers might expend resources on training employees even when the training is of use to other firms. We show that training will take place in better-than-average or ‘good’ firms enjoying greater monopsony power, whereas ‘bad’ firms will have low-ability workers unlikely to receive much training

How the rise of teleworking will reshape labor markets and cities

In recent years the land-rent gradient for the city of London has flattened by 17 percentage points. Further, teleworking has increased 24 percentage point for skilled workers, but much less for unskilled workers. To rationalize these stylized facts, we propose a model of the monocentric city with heterogeneous workers and teleworking. Skilled workers, working in final goods production, can telework while unskilled workers, working in either final goods or local services production, cannot. We show that increased teleworking flattens the land-rent gradient, and eventually skilled workers move from the city center to the city’s periphery, fundamentally changing the city structure. The increased teleworking has implications for unskilled workers who move from the local services sector into final goods, leading to greater wage inequality between skilled and unskilled workers. The model is extended to two cities which differ in productivity. Teleworking allows skilled workers of the more productive city to reside in the less productive city where housing is cheaper. This increases housing prices in the less productive city, relative to the more productive city, and has implications for unskilled workers in both cities. We provide empirical evidence from housing prices in England which is consistent with this result

Unexpected Novel Relational Links Uncovered by Extensive Developmental Profiling of Nuclear Receptor Expression

Nuclear receptors (NRs) are transcription factors that are implicated in several biological processes such as embryonic development, homeostasis, and metabolic diseases. To study the role of NRs in development, it is critically important to know when and where individual genes are expressed. Although systematic expression studies using reverse transcriptase PCR and/or DNA microarrays have been performed in classical model systems such as Drosophila and mouse, no systematic atlas describing NR involvement during embryonic development on a global scale has been assembled. Adopting a systems biology approach, we conducted a systematic analysis of the dynamic spatiotemporal expression of all NR genes as well as their main transcriptional coregulators during zebrafish development (101 genes) using whole-mount in situ hybridization. This extensive dataset establishes overlapping expression patterns among NRs and coregulators, indicating hierarchical transcriptional networks. This complete developmental profiling provides an unprecedented examination of expression of NRs during embryogenesis, uncovering their potential function during central nervous system and retina formation. Moreover, our study reveals that tissue specificity of hormone action is conferred more by the receptors than by their coregulators. Finally, further evolutionary analyses of this global resource led us to propose that neofunctionalization of duplicated genes occurs at the levels of both protein sequence and RNA expression patterns. Altogether, this expression database of NRs provides novel routes for leading investigation into the biological function of each individual NR as well as for the study of their combinatorial regulatory circuitry within the superfamily

Parametrically excited surface waves: Two-frequency forcing, normal form symmetries, and pattern selection

Motivated by experimental observations of exotic standing wave patterns in the two-frequency Faraday experiment, we investigate the role of normal form symmetries in the pattern selection problem. With forcing frequency components in ratio m/n, where m and n are co-prime integers, there is the possibility that both harmonic and subharmonic waves may lose stability simultaneously, each with a different wavenumber. We focus on this situation and compare the case where the harmonic waves have a longer wavelength than the subharmonic waves with the case where the harmonic waves have a shorter wavelength. We show that in the former case a normal form transformation can be used to remove all quadratic terms from the amplitude equations governing the relevant resonant triad interactions. Thus the role of resonant triads in the pattern selection problem is greatly diminished in this situation. We verify our general results within the example of one-dimensional surface wave solutions of the Zhang-Vinals model of the two-frequency Faraday problem. In one-dimension, a 1:2 spatial resonance takes the place of a resonant triad in our investigation. We find that when the bifurcating modes are in this spatial resonance, it dramatically effects the bifurcation to subharmonic waves in the case of forcing frequencies are in ratio 1/2; this is consistent with the results of Zhang and Vinals. In sharp contrast, we find that when the forcing frequencies are in ratio 2/3, the bifurcation to (sub)harmonic waves is insensitive to the presence of another spatially-resonant bifurcating mode

Prdm1- and Sox6-mediated transcriptional repression specifies muscle fibre type in the zebrafish embryo

The zebrafish u-boot (ubo) gene encodes the transcription factor Prdm1, which is essential for the specification of the primary slow-twitch muscle fibres that derive from adaxial cells. Here, we show that Prdm1 functions by acting as a transcriptional repressor and that slow-twitch-specific muscle gene expression is activated by Prdm1-mediated repression of the transcriptional repressor Sox6. Genes encoding fast-specific isoforms of sarcomeric proteins are ectopically expressed in the adaxial cells of ubotp39 mutant embryos. By using chromatin immunoprecipitation, we show that these are direct targets of Prdm1. Thus, Prdm1 promotes slow-twitch fibre differentiation by acting as a global repressor of fast-fibre-specific genes, as well as by abrogating the repression of slow-fibre-specific genes

The road less travelled: oligopoly and competition policy in general equilibrium. In:

Abstract I review previous approaches to modelling oligopoly in general equilibrium, and propose a new view which in principle overcomes their deficiencies: modelling firms as large in their own market but small in the economy as a whole. Implementing this approach requires a tractable specification of preferences. Dixit-Stiglitz preferences (which imply iso-elastic perceived demand functions) could be used, but "continuum-quadratic" preferences (which imply linear perceived demand functions) are more convenient. To illustrate their usefulness, I construct a simple closed-economy model of oligopoly in general equilibrium and derive some surprising implications for competition policy. JEL: D50, L13, L4

Morphogenetic mechanisms forming the notochord rod: The turgor pressure‐sheath strength model

The notochord is a defining feature of chordates. During notochord formation in vertebrates and tunicates, notochord cells display dynamic morphogenetic movement, called convergent extension, in which cells intercalate and align at the dorsal midline. However, in cephalochordates, the most basal group of chordates, the notochord is formed without convergent extension. It is simply developed from mesodermal cells at the dorsal midline. This suggests that convergent extension movement of notochord cells is a secondarily acquired developmental attribute in the common ancestor of olfactores (vertebrates + tunicates), and that the chordate ancestor innovated the notochord upon a foundation of morphogenetic mechanisms independent of cell movement. Therefore, this review focuses on biological features specific to notochord cells, which have been well studied using clawed frogs, zebrafish, and tunicates. Attributes of notochord cells, such as vacuolation, membrane trafficking, extracellular matrix formation, and apoptosis, can be understood in terms of two properties: turgor pressure of vacuoles and strength of the notochord sheath. To maintain the straight rod-like structure of the notochord, these parameters must be counterbalanced. In the future, the turgor pressure-sheath strength model, proposed in this review, will be examined in light of quantitative molecular data and mathematical simulations, illuminating the evolutionary origin of the notochord

Neural and Synaptic Defects in slytherin, a Zebrafish Model for Human Congenital Disorders of Glycosylation

Congenital disorder of glycosylation type IIc (CDG IIc) is characterized by mental retardation, slowed growth and severe immunodeficiency, attributed to the lack of fucosylated glycoproteins. While impaired Notch signaling has been implicated in some aspects of CDG IIc pathogenesis, the molecular and cellular mechanisms remain poorly understood. We have identified a zebrafish mutant slytherin (srn), which harbors a missense point mutation in GDP-mannose 4,6 dehydratase (GMDS), the rate-limiting enzyme in protein fucosylation, including that of Notch. Here we report that some of the mechanisms underlying the neural phenotypes in srn and in CGD IIc are Notch-dependent, while others are Notch-independent. We show, for the first time in a vertebrate in vivo, that defects in protein fucosylation leads to defects in neuronal differentiation, maintenance, axon branching, and synapse formation. Srn is thus a useful and important vertebrate model for human CDG IIc that has provided new insights into the neural phenotypes that are hallmarks of the human disorder and has also highlighted the role of protein fucosylation in neural development