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Super free fall

Published online by Cambridge University Press:  15 December 2009

E. VILLERMAUX*
Affiliation:
Aix-Marseille Université, IRPHE, 13384 Marseille Cedex 13, France
Y. POMEAU
Affiliation:
Department of Mathematics, The University of Arizona, 617 N. Santa Rita Avenue, PO Box 210089 Tucson, AZ 85721-0089, USA
*
†Also at: Institut Universitaire de France. Email address for correspondence: villerma@irphe.univ-mrs.fr
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Abstract

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The free fall of a liquid mass through vertical tubes with a weakly increasing cross-section induces an acceleration of the upper liquid interface larger than gravity. The phenomenon is well described by a one-dimensional inviscid model. The super acceleration of the upper interface comes from the additional positive pressure gradient caused by the expanding geometry, which adds to the gravity body force. A perturbative expansion of this base solution further accounts for the interface shape and stability. In particular, the positive pressure gradient at the interface makes it unstable, forming a concentrated ‘nipple’ on top of the essentially flat base solution. We discuss the possible connexion of these findings with the problem of wave breaking in free surface flows.

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Copyright © Cambridge University Press 2010