The influence of particle density and fluid viscosity on the rheological behaviour of dense granular-fluid involved in flow phenomena

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Anna Maria Pellegrino, Leonardo Schippa

Tuesday 30 june 2015

8:45 - 9:00h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 4A. Sediment - Erosion

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Suspensions of solid particles are commonly encountered in natural flows. Typical examples include soil avalanches, rock-falls, debris flows and torrential process, which cause disasters and catastrophes. A laboratory activity on high concentrated granular-fluid mixtures was carried out in order to study the flow behavior of natural materials involved in fast-moving phenomena. We transferred the approach using for dry granular media (i.e., a frictional rheology) to wet suspensions, using different combinations of granulars and viscous fluid sheared with two pressure-imposed rheometers. The shear stress as a function of the shear rate at imposed values of the normal stress was measured for granular-fluid mixtures with particles denser than interstitial fluid in order to investigate how the particles weight (i.e., the sedimentation) affects the experimental results. The suspensions considered exhibit a viscoplastic behavior similarly to granular media and they are fully characterized by the evolution of the friction coefficient _ and the volume fraction _ with a dimensionless viscous number Iv. In particular, we have different _-Iv curves each one associated to a different value of the pressure imposed. The effects of the gravity seems more relevant in case of low value of vertical pressure imposed. Indeed, when the imposed normal stress is reduced, the relative density play a relevant role. Moreover, also the interstitial fluid viscosity affects the results, influencing the value of the static friction coefficient _1 (i.e., _=_1 at Iv_0) and the value of the maximum concentration of the mixture during the shearing _m (i.e., _= _m at Iv_0).