Optical measurements of velocity and of solid volume fraction in fast dry granular flows in a rectangular chute

Abstract

Geophysical flows, like avalanches and debris flows, are characterized by the gravity-driven motion of a granular medium immersed in an interstitial fluid. To better understand their dynamics, laboratory investigations represent invaluable tools and are essential to study several peculiar features (e.g. the effects of fixed boundaries, non-local momentum exchanges, segregation effects) that are difficult to isolate at the field scale. An experimental study on dry granular flows in a chute geometry is reported. Different basal conditions are investigated by varying the bed roughness. Several flow rates are investigated by adjusting the inflow boundary condition. By employing two high-speed cameras and particle image velocimetry (PIV) technique, accurate velocity measurements (typical error ≈0.004 m/s) could be obtained at sidewall and free surface. An innovative stochastic-optical method [Sarno et al., Granul. Matter, 2016], which exploits highly controlled illumination conditions guaranteed by a flickering- free planar lamp, allowed to obtain reliable volume fraction profiles (typical error ≈0.025). The method uses a transfer function, numerically determined on random grain distributions of known volume fraction. This function stochastically relates the near- wall volume fraction with a measurable quantity, named two-dimensional volume fraction and accessible by binarization of digital pictures, taken by a high-speed camera. The combined knowledge of velocity and volume fraction fields allowed a detailed description of the rheological behavior of channelized granular flows and of the effects of the flume boundaries. The superposition of different flow regimes is revealed by different shapes of velocity and volume fraction profiles along the flow depth. It emerges that frictional momentum exchanges increase at the expense of collisional mechanisms with increasing depth. This behavior appears related to the sidewall resistances and to the increasing normal pressures.