Vecteur de Poynting | EPFL Graph Search

En physique, le vecteur de Poynting est la densité de flux liée à la propagation de l'onde électromagnétique. Sa direction est la direction de propagation. On le note \vec{\Pi}, \vec S, \vec{R} ou \vec{N}. Le flux du vecteur de Poynting à travers une surface (fermée ou non) est égal à la puissance véhiculée par l'onde à travers cette surface. Le module de ce vecteur est donc une puissance par unité de surface, c'est-à-dire une densité de flux d'énergie ; il est homogène à un éclairement énergétique et à une exitance énergétique ; et, dans le Système international (SI) d'unités, il s'exprime en watts par mètre carré. Histoire L'éponyme du vecteur de Poynting est le physicien anglais John Henry Poynting (-) qui l'a introduit en . Oliver Heaviside (-) l'a découvert quelques mois plus tard et de manière indépendante. Définition Le Vocabulaire électrotechnique international (VEI) définit le vecteur de Poynting

The Alfvenic nature of chromospheric swirls

Andrea Francesco Battaglia, Aleksi Antoine Bossart

Context. Observations show that small-scale vortical plasma motions are ubiquitous in the quiet solar atmosphere. They have received increasing attention in recent years because they are a viable candidate mechanism for the heating of the outer solar atmospheric layers. However, the true nature and the origin of these swirls, and their effective role in the energy transport, are still unclear.Aims. We investigate the evolution and origin of chromospheric swirls by analyzing numerical simulations of the quiet solar atmosphere. In particular, we are interested in finding their relation with magnetic field perturbations and in the processes driving their evolution.Methods. The radiative magnetohydrodynamic code CO5BOLD is used to perform realistic numerical simulations of a small portion of the solar atmosphere, ranging from the top layers of the convection zone to the middle chromosphere. For the analysis, the swirling strength criterion and its evolution equation are applied in order to identify vortical motions and to study their dynamics. As a new criterion, we introduce the magnetic swirling strength, which allows us to recognize torsional perturbations in the magnetic field.Results. We find a strong correlation between swirling strength and magnetic swirling strength, in particular in intense magnetic flux concentrations, which suggests a tight relation between vortical motions and torsional magnetic field perturbations. Furthermore, we find that swirls propagate upward with the local Alfven speed as unidirectional swirls driven by magnetic tension forces alone. In the photosphere and low chromosphere, the rotation of the plasma co-occurs with a twist in the upwardly directed magnetic field that is in the opposite direction of the plasma flow. All together, these are clear characteristics of torsional Alfven waves. Yet, the Alfven wave is not oscillatory but takes the form of a unidirectional pulse. The novelty of the present work is that these Alfven pulses naturally emerge from realistic numerical simulations of the solar atmosphere. We also find indications of an imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects being at the origin of the swirls. At the base of the chromosphere, we find a mean net upwardly directed Poynting flux of 12.86.5 kW m(-2), which is mainly due to swirling motions. This energy flux is mostly associated with large and complex swirling structures, which we interpret as the superposition of various small-scale vortices.Conclusions. We conclude that the ubiquitous swirling events observed in numerical simulations are tightly correlated with perturbations of the magnetic field. At photospheric and chromospheric levels, they form Alfven pulses that propagate upward and may contribute to chromospheric heating.

EDP SCIENCES S A2021

Assessment of ammonia as energy carrier in the use with reversible solid oxide cells

Jan Van Herle, Michele Zendrini

Ammonia represents one of the most promising potential solutions as energy vector and hydrogen carrier, having a higher potential to transport energy than hydrogen itself in a pressurized form. Furthermore, solid oxide fuel cells (SOFCs) can directly be fed with ammonia, thus allowing for immediate electrical power and heat generation. This paper deals with the analysis of the dynamic behavior of commercial SOFCs when fueled with ammonia. Several measurements at different temperatures have been performed and performances are compared with hydrogen and a stoichiometrically equivalent mixture of H2 and N2 (3:1 M ratio). Higher temperature led to smaller drops in voltage for both fuels, thus providing higher efficiencies. Ammonia resulted slightly more performant (48% at 760 °C) than hydrogen (45% at 760 °C), in short stack tests. Moreover, different ammonia-to-air ratios have been investigated and the stack area-specific resistance has been studied in detail by comparing numerical modeling predictions and experimental values.

2021

On design strategies for binary dielectric metasurfaces based on the Fourier modal method

Kévin Müller

Binary dielectric metasurfaces are arrays of sub-wavelength structures that act as a thin layer of artificial material. They are generally lossless and relatively simple to fabricate since only a single structuring step is required. By carefully designing the metasurface, the phase, amplitude and the polarization of the incident light can be controlled at will. In practice, fabrication constraints and the limited choice of materials reduce what can be done with metasurfaces. But a wide range of functionalities can be implemented with the proper design techniques and knowledge. This thesis contributes to both. The modes are key to understand the phenomena occurring inside a metasurface. To facilitate the analysis of the modes, the Poynting operation, which is related to the Poynting vector, is introduced. We describe how this operation can be used to reformulate the boundary condition in order to estimate the reflection and transmission coefficients with reduced knowledge on the modes involved, and to orthonormalized the modes. The Fourier modal method, which is the method used for the rigorous simulation of metasurfaces in this work, is improved in order to facilitate the access to valuable information that can be used to better understand the phenomena occurring inside a metasurface, and to speed up the design and optimization process. This method computes the eigen-modes present in the metasurface. To better analyze them, the eigen-modes are orthonormalized using the Poynting operation. We show that most of the modes can be filter out in order to simulate a metasurface with different thicknesses in milliseconds. From the analysis of the modes propagating in metasurfaces, two types of metasurface are identified: single-mode metasurfaces and multi-mode metasurfaces. For single-mode metasurfaces, we provide design techniques that translates the desired response into internal properties of the metasurfaces. For multi-mode metasurfaces, the concept of self-coupling mode is developed. We show that, based on this concept, the angular and spectral response of a metasurface can be interpolated safely with a few simulations even if high-Q resonances are present. For both types of metasurfaces, examples of design are provided. Gradient-based optimization methods allow to obtain the optimal metasurface in a few iterations, but the derivative of the merit function is necessary. The adjoint method computes the functional derivative of the merit function with respect to the permettivity and permeability. We provide the equations of the adjoint method and apply them to diffractive optical elements such that they can be used in conjunction with the Fourier modal method. This thesis contributes to design challenges for complex electromagnetic problems,and it does not only provides concepts, but also the tools to put them into operation.