Author(s): Aronne Armanini; Luigi Fraccarollo; Michele Larcher
Keywords: Kinetic theory; Collisional regime; Frictional regime; Intermittency; Experiments
Abstract: In the paper a detailed analysis of the rheological stratification of a liquid-granular flow is presented, focusing on the mechanisms governing the transition from the frictional to the collisional regime. In particular, in the experiments it was observed that in opportune conditions high-concentrated flows show locally a typical intermittency of the flow regimes, switching alternatively from frictional to collisional. In the experiments a mixture of water and perfectly spherical, identical particles was introduced steadily in the upstream part of a flume and filmed through the sidewall of the channel utilizing a high-speed camera. The reconstruction of detailed particle kinematics and of the effect of the intermittence of the flow regime was obtained utilizing imaging techniques based on the Voronoi polygons. On the base of the experimental evidence, the intermittency factor was defined in analogy with what happens in the border of a turbulent boundary layer and determined trough the analysis of the actual velocity signals. The intermittency function was then introduced in the classical formulation of kinetic theories, allowing for the numerical integration of the complete set of equations and giving the profiles of the mean velocity, the granular temperature, the granular concentration, the normal and tangential stresses and single terms of the balance of the particles kinetic energy (the rates of energy production, dissipation and diffusion). The comparison between the experimental data and the numerical integration confirms satisfactorily the applicability of the dense gas analogy to the motion of hard spheres in water, during the collisional spots.