title: Friction-induced amplification of acoustic waves in a low Mach number granular flow.
abstract: We show experimentally that, contrary to ordinary fluids, low Mach number granular pipe flows are linearly unstable toward the emission of acoustic waves. Exponential amplification of the waves propagating in the direction opposite to the flow is directly demonstrated. We relate the observed instability to the friction of the grains on the pipe, which pumps energy from the mean flow to coherent elastic waves. We show that the most amplified wavelength is proportional to the ratio of the tube radius to the friction coefficient.
authors.affiliation: Physique et Mécanique des Milieux Hétérogènes, UMR 7636, CNRS-ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France.
authors.forename: Lénaic
authors.initials: L
authors.lastname: Bonneau
authors.forename: Tristan
authors.initials: T
authors.lastname: Catelin-Jullien
authors.forename: B
authors.initials: B
authors.lastname: Andreotti