Friction-induced amplification of acoustic waves in a low Mach number granular flow

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.