The stratified nature of dense granular flows supported by fluctuation velocities and volume fraction measurements from laboratory flume experiments

The dynamics of granular media, involved in several hazardous geophysical phenomena such as debris flows and avalanches, is extremely complex and still represents a hot topic for the scientific community and specialists. When choosing a mathematical tool to describe such flows, depth-averaged models remain the first choice especially in large field-scale applications, while three-dimensional and discrete element models are more complete but very computationally expensive. However, the dynamics variations along the flow depth cannot be described by classical depth-averaged models. With the aim of getting a better insight into the dense regime of granular flows, which is the most common in nature, we report a laboratory investigation where a number of dense dry granular flows with different basal boundary conditions and flow rates are studied in a 2m-long Plexiglas flume. The employed granular medium consists of small spheroidal beads (d≈3mm), made of acetal resin (POM). The flume is instrumented with a high-speed digital camera and a no-flicker planar lamp, so that reliable measurements of the velocity and of the volume fraction at the side wall are obtained by using a multi-pass particle image velocimetry (PIV) approach [Sarno et al., Adv. Powder Tech., 2018] and a stochastic-optical method (SOM) [Sarno et al., Granul. Matter, 2016]. By iteratively decreasing the interrogation window in the PIV analysis down to approximately half the grain size, it is possible to estimate the magnitude of the fluctuation velocities along normal-to-bed and stream-wise directions. Small normal fluctuation velocities and relatively large volume fractions (≈0.6) are observed in the major part of the flow, where the chief resistance mechanism is frictional. At the uppermost region, close to the free surface, slightly larger values of the fluctuation velocities and lower values of the volume fraction are observed, due to the increasingly collisional behavior. These findings indicate that, owing to the particles non-penetration condition and weak collisionality, the mass exchanges from one layer to the neighboring ones are rather limited in the dense regime. Therefore, dense granular flows exhibit a clear stratified nature and, thus, they may be regarded as composed of different superimposed layers, partially coupled each other. It is worth noting that this behavior is considerably different from turbulent incompressible fluids and also from chiefly collisional granular flows, where mass and momentum exchanges are considerable along the entire flow depth. These experimental findings suggest that a multi-layer depth-averaged mathematical approach would be a suitable tool for improving the modeling of these flows without increasing significantly the computational costs.