Abstract Geophysical granular flows, like avalanches and debris flows, are hazardous phenomena for infrastructures and human activities. The complex dynamics of granular systems, involved in these phenomena, poses important challenges to their reliable mathematical modelling. In this context, laboratory investigations on granular flows and non-invasive optical measuring methods are irreplaceable tools for getting a deeper insight into the granular dynamics, as they provide useful information for model calibration and validation. Frictional and collisional momentum flux exchanges coexist in dense granular flows. These resistance mechanisms and their complex interplay with the boundary conditions generate non-negligible fluxes of translational and fluctuation energies along the flow domain, which also influence the kinematics of the free surface. In this work we study steady granular flows in a chute geometry over a fixed bed, specifically focusing on the spatial fluctuations of the free surface. Several tests have been carried out by systematically varying the mass flow rate, the chute inclination and the basal roughness. The experimental apparatus, consisting of a narrow Plexiglas flume, is equipped with a load cell to measure the mass flow rate and a high-speed camera to get non-invasive optical measurements of the free surface fluctuations. A harmonic analysis, employing the Welch method, is performed on a time-series signal obtained from suitably binarized digital images of the free surface fluctuations, recorded at a sampling frequency of 1 kHz. Subsequently, the power spectral densities and the power content of different frequency intervals are analysed. Meaningful correlations are observed between these power components and other relevant measurements, such as the mass flow rates and the flow depths. These preliminary results may be useful for a better understanding of the momentum exchange mechanisms in granular flows and the related fluctuation energy fluxes.
Sarno et al. (Tue,) studied this question.