Geophysical flows, like rock avalanches, snow avalanches and debris flows, involve the rapid motion of granular media. The dynamics of these flows is still an open problem due to the complexity of the flow resistance mechanisms involved. In case of dry granular flows two main resistance mechanisms have been individuated: friction and collisions. In order to better understand the constitutive behaviour, it is necessary to provide local measurements of the main flow variables: velocity and volume fraction. To do so, we set up an experimental apparatus in which a dry granular material was let flow in a flume with transparent sidewalls and recorded by two high-speed cameras to get both side-wall and free surface views. Velocity profiles were, then, derived by PIV analysis and a novel optical method was developed to estimate the near-wall volume fraction
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