Axisymmetric dynamo action produced by differential rotation, with anisotropic electrical conductivity and anisotropic magnetic permeability

The effect on dynamo action of an anisotropic electrical conductivity conjugated to an anisotropic magnetic permeability is considered. Not only is the dynamo fully axisymmetric, but it requires only a simple differential rotation, which twice challenges the well-established dynamo theory. Stability analysis is conducted entirely analytically, leading to an explicit expression of the dynamo threshold. The results show a competition between the anisotropy of electrical conductivity and that of magnetic permeability, the dynamo effect becoming impossible if the two anisotropies are identical. For isotropic electrical conductivity, Cowling's neutral point argument does imply the absence of an azimuthal component of current density, but does not prevent the dynamo effect as long as the magnetic permeability is anisotropic.Comment: 19 pages, 6 figure

Effects of rotor blade modulation on GNSS anti-jamming algorithms

In hostile environment, Global Navigation Satellite System (GNSS) could be disturbed by intentional jamming. Many adaptive algorithms have been developed to deal with these threats, among which use of antenna arrays is one of the most efficient. However, most of them have been designed under stationary hypothesis and their performances in harsher environments are questionable. For instance, when a GNSS receiver is placed near rotating bodies, the signal undergo complex and non-stationary effects called Rotor Blade Modulation (RBM). These variations can degrade significantly anti-jamming performance. This paper investigates the impact of the RBM on three conventional space-time adaptive processing (STAP). First, to simulate the RBM, the signal received by an antenna mounted on a helicopter is computed thanks to electromagnetic (EM) asymptotic methods. Then, to quantify precisely the loss in performance of each algorithm, we compare post correlation carrier to noise ratio (post - C/N0) and covariance matrix estimation with respect of the time. Finally, the simulation results are confirmed by experiments conducted on an EC-120 helicopter with an L-band Continuous Wave (CW) jammer

High-temperature transport properties of complex antimonides with anti-Th3P4 structure

Polycrystalline samples of R4Sb3 (R = La, Ce, Smand Yb) and Yb4-xR¢xSb3 (R¢ = Sm and La) have been quantitatively synthesized by high-temperature reaction. They crystallize in the anti-Th3P4 structure type (I ¯43d, no. 220). Structural and chemical characterizations have been performed by X-ray diffraction and electron microscopy with energy dispersive X-ray analysis. Powders have been densified by spark plasma sintering (SPS) at 1300 ◦C under 50 MPa of pressure. Transport property measurements show that these compounds are n-type with low Seebeck coefficient except for Yb4Sb3 that shows a typical metallic behavior with hole conduction. By partially substituting Yb by a trivalent rare earth we successfully improved the thermoelectric figure of merit of Yb4-xR¢xSb3 up to 0.75 at 1000 ◦C

Transformation de scores SVM en fonctions de croyance

National audienceLa combinaison de plusieurs classifieurs, entrainés à partir de données ou caractéristiques distinctes, présente de nombreux intérêts pour les problèmes d'apprentissage supervisé. L'une des difficultés majeures de la combinaison est de représenter les sorties des différents classifieurs sous une forme commune, dans la plupart des cas sous la forme d'une probabilité à postériori. Cette transformation, appelée calibration, joue un rôle central dans la combinaison. Dans cet article, nous étendons les approches classiques de calibration probabilistes en utilisant la théorie des fonctions de croyance. Nous montrons, notamment, l'importance d'utiliser une borne inférieure et supérieure plutôt qu'une simple mesure probabiliste. Des résultats expérimentaux sur la transformation de scores SVM montrent l'apport des fonctions croyances

Basics for sensorimotor information processing: some implications for learning

International audienceIn sensorimotor activities, learning requires efficient information processing, whether in car driving, sport activities or human machine interactions. Several factors may affect the efficiency of such processing: they may be extrinsic (i.e., task-related) or intrinsic (i.e., subjects-related). The effects of these factors are intimately related to the structure of human information processing. In the present article we will focus on some of them, which are poorly taken into account, even when minimizing errors or their consequences is an essential issue at stake. Among the extrinsic factors, we will discuss, first, the effects of the quantity and quality of information, secondly, the effects of instruction and thirdly motor program learning. Among the intrinsic factors, we will discuss first the influence of prior information, secondly how individual strategies affect performance and, thirdly, we will stress the fact that although the human brain is not structured to function errorless (which is not new) humans are able to detect their errors very quickly and On most of the cases), fast enough to correct them before they result in an overt failure. Extrinsic and intrinsic factors are important to take into account for learning because (1) they strongly affect performance, either in terms of speed or accuracy, which facilitates or impairs learning, (2) the effect of certain extrinsic factors may be strongly modified by learning and (3) certain intrinsic factors might be exploited for learning strategies

Progress in analytical methods to predict and control azimuthal combustion instability modes in annular chambers

Longitudinal low-frequency thermoacoustic unstable modes in combustion chambers have been intensively studied experimentally, numerically, and theoretically, leading to significant progress in both understanding and controlling these acoustic modes. However, modern annular gas turbines may also exhibit azimuthal modes, which are much less studied and feature specific mode structures and dynamic behaviors, leading to more complex situations. Moreover, dealing with 10–20 burners mounted in the same chamber limits the use of high fidelity simulations or annular experiments to investigate these modes because of their complexity and costs. Consequently, for such circumferential acoustic modes, theoretical tools have been developed to uncover underlying phenomena controlling their stability, nature, and dynamics. This review presents recent progress in this field. First, Galerkin and network models are described with their pros and cons in both the temporal and frequency framework. Then, key features of such acoustic modes are unveiled, focusing on their specificities such as symmetry breaking, non-linear modal coupling, forcing by turbulence. Finally, recent works on uncertainty quantifications, guided by theoretical studies and applied to annular combustors, are presented. The objective is to provide a global view of theoretical research on azimuthal modes to highlight their complexities and potential

Mixed acoustic–entropy combustion instabilities in gas turbines

A combustion instability in a combustor terminated by a nozzle is analysed and modelled based on a low-order Helmholtz solver. A large eddy simulation (LES) of the corresponding turbulent, compressible and reacting flow is first performed and analysed based on dynamic mode decomposition (DMD). The mode with the highest amplitude shares the same frequency of oscillation as the experiment (approximately 320 Hz) and shows the presence of large entropy spots generated within the combustion chamber and convected down to the exit nozzle. The lowest purely acoustic mode being in the range 700–750 Hz, it is postulated that the instability observed around 320 Hz stems from a mixed entropy–acoustic mode, where the acoustic generation associated with entropy spots being convected throughout the choked nozzle plays a key role. The DMD analysis allows one to extract from the LES results a low-order model that confirms that the mechanism of the low-frequency combustion instability indeed involves both acoustic and convected entropy waves. The delayed entropy coupled boundary condition (DECBC) (Motheau, Selle & Nicoud, J. Sound Vib., vol. 333, 2014, pp. 246–262) is implemented into a numerical Helmholtz solver where the baseline flow is assumed at rest. When fed with appropriate transfer functions to model the entropy generation and convection from the flame to the exit, the Helmholtz/DECBC solver predicts the presence of an unstable mode around 320 Hz, in agreement with both LES and experiments