Convection granulaire

Résumé

droite|vignette|Les noix du Brésil se retrouvent en haut du mélange de noix. La convection granulaire est un phénomène où un matériau granulaire soumis à des secousses ou à des vibrations montre un mouvement interne analogue à la convection dans les fluides. Ce phénomène contre-intuitif est surnommé l'effet noix du Brésil (voir ségrégation en milieu granulaire) lorsque les particules les plus grosses se retrouvent en surface d'un matériau granulaire contenant un mélange d'objets de tailles différentes : cela vient de l'exemple typique d'un conteneur de noix mélangées, où les plus grandes sont les noix du Brésil. Le phénomène est également connu sous le nom d'effet muesli, puisqu'on peut le voir dans les paquets de céréales de petit-déjeuner contenant des particules de différentes tailles mais de densité similaire, comme le mélange de muesli. Dans des conditions expérimentales, on a observé que la convection granulaire de particules de taille différente forme des cellules de convecti

Source officielle

https://fr.wikipedia.org/wiki/Convection_granulaire

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Model for the erosion onset of a granular bed sheared by a viscous fluid

Matthieu Wyart

We study theoretically the erosion threshold of a granular bed forced by a viscous fluid. We first introduce a model of interacting particles driven on a rough substrate. It predicts a continuous transition at some threshold forcing theta(c), beyond which the particle current grows linearly J similar to theta - theta(c). The stationary state is reached after a transient time t(conv) which diverges near the transition as t(conv) similar to vertical bar theta - theta(c)|(-z) with z approximate to 2.5. Both features are consistent with experiments. The model also makes quantitative testable predictions for the drainage pattern: The distribution P(sigma) of local current is found to be extremely broad with P(sigma) similar to J/sigma, and spatial correlations for the current are negligible in the direction transverse to forcing, but long-range parallel to it. We explain some of these features using a scaling argument and a mean-field approximation that builds an analogy with q models. We discuss the relationship between our erosion model and models for the plastic depinning transition of vortex lattices in dirty superconductors, where our results may also apply.

Amer Physical Soc2016

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Calculations of the particle flux caused by neutrals originating from the edge are performed for TCV and JET using the one-dimensional kinetic transport code Kn1D. The analysis, confirmed by experimental evidence, shows that for TCV and JET edge fuelling as well as neutral beam fuelling cannot be responsible for the density gradient in the plasma bulk, confirming the presence of inward particle convection. The results corroborate the expectation of a peaked density profile in ITER, despite the lack of bulk particle fuelling.

2006

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Observation of inward and outward particle convection in the core of electron cyclotron heated and current driven plasmas in the Tokamak a Configuration Variable

Ivo Furno, Henri Weisen

In the Tokamak a Configuration Variable [F. Hofmann, J.B. Lister, M. Anton et al., Plasma Phys. Controlled Fusion 36, B277 (1994)], inward or outward convection in the core of electron cyclotron heated and current driven plasmas is observed, depending on discharge conditions. In sawtoothing discharges with central electron cyclotron heating, outward convection is observed when a quasicontinuous m=1 kink mode is present, resulting in inverted sawteeth on the central electron density, while in the absence thereof, inward convection between successive sawtooth crashes leads to "normal" sawteeth. The occurrence of a kink mode depends sensitively on plasma triangularity. When sawteeth are stabilized with central co- or counterelectron cyclotron current drive, stationary hollow electron density profiles are observed in the presence of m=1 modes, while peaked or flat profiles are observed in magnetohydrodynamic quiescent discharges. The observation of peaked density profiles in fully electron cyclotron driven plasmas demonstrates that pinch processes other than the Ware pinch must be responsible for these phenomena. (C) 2003 American Institute of Physics.

2003

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