Incompressible-compressible transition in falling granular jets

Abstract

-Through a systematic experimental investigation of the behavior of falling granular jets under the action of gravity for different particle sizes, funnel diameters and ambient air pressures, necessary conditions to obtain incompressible granular jets are identified. A transition from compressible (characterized by a significant density decrease along the propagation) to incompressible granular jets (characterized by a constant density) is observed. This transition depends solely on the aspect ratio between the diameter of the particles and the diameter of the funnel. Surprisingly, the incompressible liquid-like behavior observed here seems to find its origin in the balance between the heat flux and the dissipation in the funnel independently of the ambient fluid pressure. A simple granular hydrodynamic model provides a good description of the transition.

Eulerian modelling of the powder discharge of a silo: Attempting to shed some light on the origin of jet expansion

“…This instability appears even without air and could be provoked by a cohesion effect linked to the humidity and particle charge. The authors proposed the introduction of an effective granular surface tension to describe such an instability [14,16,17]. However, the instability could also find its origin at the granular flow transition at the opened bottom of the silo, or in the granular flow inside the silo (non-sphericity of particles, wall roughness, particle-wall interactions or arches formation).…”

Subharmonic instability of a self-organized granular jet

“…Initially homogeneous granular jets falling in gravity undergo a similar instability, e.g. 3 4 5 6 , despite the fact that there is no surface tension. The second similarity concerns the interaction of a granular jet with a solid obstacle, e.g.…”

Sedimentation of granular columns in the viscous and weakly inertial regimes

“…The sedimentation of spherical clouds of particles, in an external fluid of variable viscosity, has been recently investigated experimentally and numerically [6]. With the exception of the experimental work of Nicolas [7], investigations related to jets or column of particles focused mainly on highly viscous fluids (i.e., zero Reynolds numbers limit) [8,9], air, and moderate vacuum (large Reynolds numbers limit) [10][11][12] or other kinds of interactions: capillary bridges, van der Waals forces [13-16], etc.In this paper we present an investigation that fully characterizes, using point-particle simulations, the dynamics of freely falling granular columns in different flow regimes, clarifying the dependence on the Reynolds number, the aspect ratio, and the particle density. The main characteristics of the present system are (i) solid particles suspended in a viscous fluid and * h.chraibi@loma.u-bordeaux1.fr † y.amarouchene@loma.u-bordeaux1.fr interacting by virtue of the fluid; (ii) particles heavier than the fluid, thus sedimenting on account of gravity; and (iii) no continuous supply of particles in the granular cylinder.…”

“…The sedimentation of spherical clouds of particles, in an external fluid of variable viscosity, has been recently investigated experimentally and numerically [6]. With the exception of the experimental work of Nicolas [7], investigations related to jets or column of particles focused mainly on highly viscous fluids (i.e., zero Reynolds numbers limit) [8,9], air, and moderate vacuum (large Reynolds numbers limit) [10][11][12] or other kinds of interactions: capillary bridges, van der Waals forces [13-16], etc.…”

“…At the exception of the experimental work of Nicolas [7], investigations related to jets or column of particles focused mainly on highly viscous fluids (i.e. zero Reynolds numbers limit) [8,9], air and moderate vacuum (Large Reynolds numbers limit) [10][11][12] or other kinds of interactions : capillary bridges, Wan Der Waals forces [13][14][15][16]... In this letter, we present an investigation that fully characterizes, using point-particle simulations, the dynamics of freely falling granular columns in different flow regimes, clarifying the dependence to the Reynolds number, the aspect ratio and the particle density.…”