U. Enfin and . Etude-approfondie-de-la, transition entre les régimes de chute libre et d'ondes de densité serait riche d'enseignement. Une analyse préliminaire de stabilité linéaire du régime de chute libre a montré que l'´ ecoulement de grains devient instable pour des débits d'air et de grains critiques et présente alors une structuration spatiale: une longueur d'ondeparticulì ere est sélectionnée

. Enfin, CetécoulementCetécoulement peutêtrepeutêtre lent et continu, mais est très fréquemment intermittent Ceci dépend principalement de l'´ etat de surface des billes et du taux d'hygrométrie. L'analyse spatio-temporelle du régime compact intermittent a permis d'identifier la propagation vers le haut de fronts de compaction et de décompaction lors du blocage et du redémarrage de l'´ ecoulement. Une corrélation forte entre pression et compacitécompacitéà l'intérieur du tube a ´ eté mise enévidenceenévidence. Ainsi, la mise en mouvement des grains (front de décompaction) s'accompagne d'une décompaction locale qui, bien que faible (moins de 2%), entra??neentra??ne néanmoins des variations de pression significatives (de l'ordre de 3000 Pa) Ces fluctuations de pression s'amplifient fortement au fur etàetà mesure de la propagation

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