Acoustic emissions of nearly steady and uniform granular flows: a proxy for flow dynamics and velocity fluctuations.

The seismic waves emitted during granular flows are generated by different sources: high frequencies by inter-particle shocks and low frequencies by global motion and large scale deformation. To unravel these different mechanisms, an experimental study has been performed on the seismic waves emitted by dry quasi steady granular flows. The emitted seismic waves were recorded using shock accelerometers and the flow dynamics were captured with a fast camera. The mechanical characteristics of the particle shocks were analyzed, along with the duration between shocks and the correlations in the particle motion. The high-frequency seismic waves (1-50 kHz) were found to originate from particle shocks and waves trapped in the flowing layer. The low-frequency waves (20-60 Hz) were generated by the oscillations of the particles along their trajectories, i.e. from cycles of dilation/compression during the shear. The profiles of granular temperature (i.e. the square of particle velocity fluctuations) and average velocity were measured and related to the average properties of the flow as well as to the slope angle and flow thickness. These profiles were then used in a simple steady granular flow model to predict the radiated seismic energy and the energetic efficiency, i.e. the fraction of the flow potential energy converted to seismic energy. Scaling laws relating the seismic power, the shear strain rate and the inertial number were derived. In particular, the emitted seismic power is proportional to the granular temperature, which is also related to the mean flow velocity.