We propose an explanation for the onset of oscillations seen in numericalsimulations of dense, inclined flows of inelastic, frictional spheres. It isbased on a phase transition between disordered and ordered collisional statesthat may be interrupted by the formation of force chains. Low frequencyoscillations between ordered and disordered states take place over weakly bumpybases; higher-frequency oscillations over strongly bumpy bases involve theformation of particle chains that extend to the base and interrupt the phasechange. The predicted frequency and amplitude of the oscillations induced bythe unstable part of the equation of state are similar to those seen in thesimulations and they depend upon the contact stiffness in the same way. Suchoscillations could be the source of sound produced by flowing sand.
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