Abstract
This paper refers to complex granular-fluid mixtures involved into geophysical flows, such as debris and hyper-concentrated flows. For such phenomena, the interstitial fluids play a role when they are in the viscous regime. Referring to experiments on granular-fluid mixture carried out with pressure imposed annular shear cell, we study the rheological behaviour of dense mixture involving both spheres and irregular-shaped particles. For the case of viscous suspensions with irregular grains, a significant scatter of data from the trend observed for mixtures with spherical particles was evident. In effect, the shape of the particles likely plays a fundamental role in the flow dynamics, and the constitutive laws proposed by the frictional theory for the spheres are no longer valid. Starting from the frictional approach successfully applied to suspension of spheres, we demonstrate that also in case of irregular particles the mixture rheology may be fully characterized by the two relationships involving friction coefficient µ and volume concentration Ф as a function of the dimensionless viscous number Iv. To this goal, we provided a new consistent general model, referring to the volume fraction law and friction law, which accounts for the particle shape. In this way, the fitting parameters reduce just to the static friction angle µ1, and the two parameters, k and fs related to the grain shape. The resulting general model may apply to steady fully developed flows of saturated granular fluid mixture in the viscous regime, no matter of granular characteristics.
Highlights
The granular fluid mixture involved in geophysical flow phenomena can be considered a complex particle suspension, since the entire system is composed of particles with different shapes and sizes that are dispersed and/or suspended in a Newtonian or non-Newtonian fluid [1,2]
The rheology of viscous suspensions with spheres under imposed-pressure flow conditions could be described using constitutive laws expressed as two functions of Iv) results: μ ( (Iv) (see Equation (1)) τ = μ(Iv ) and
In case of granular avalanches in fluid, Courrech du Pont et al [19] derived the boundaries for the three different regimes, and they are reported in Figure 4, along with the experimental data considered composed by glycerin
Summary
The granular fluid mixture involved in geophysical flow phenomena can be considered a complex particle suspension, since the entire system is composed of particles with different shapes and sizes that are dispersed and/or suspended in a Newtonian or non-Newtonian fluid [1,2]. Describing the complete behavior of these materials is relevant considering the variety of natural phenomena in which they are involved such as, the rapid mass movement conveyed by a viscous interstitial fluid, the sediment transport in bodies of water, landslides, pyroclastic flows, debris, mud and hyper-concentrated flows [3,4,5,6,7]. A lack of studies focused on the properties of the ‘post-failure’ material, i.e., the soil–water mixture that flows after the triggering phase, still exists. After starting, these materials are in a viscous-like regime, where they flow to a fluid [4,5,6,7,8,9,10]
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