12 research outputs found
Using PIV to measure granular temperature in saturated unsteady polydisperse granular flows
The motion of debris flows, gravity-driven fast
moving mixtures of rock, soil and water can be interpreted
using the theories developed to describe the shearing motion
of highly concentrated granular fluid flows. Frictional, collisional
and viscous stress transfer between particles and
fluid characterizes the mechanics of debris flows. To quantify
the influence of collisional stress transfer, kinetic models
have been proposed. Collisions among particles result in random
fluctuations in their velocity that can be represented by
their granular temperature, T. In this paper particle image
velocimetry, PIV, is used to measure the instantaneous velocity
field found internally to a physical model of an unsteady
debris flow created by using “transparent soil”—i.e. a mixture
of graded glass particles and a refractively matched fluid.
The ensemble possesses bulk properties similar to that of
real soil-pore fluid mixtures, but has the advantage of giving
optical access to the interior of the flow by use of plane laser
induced fluorescence, PLIF. The relationship between PIV
patch size and particle size distribution for the front and tail
of the flows is examined in order to assess their influences
on the measured granular temperature of the system. We find
that while PIV can be used to ascertain values of granular
temperature in dense granular flows, due to increasing spatial
correlation with widening gradation, a technique proposed to
infer the true granular temperature may be limited to flows
of relatively uniform particle size or large bulk
Additive rheology of complex granular flows
Granular flows are omnipresent in nature and industrial processes, but their rheological properties such as apparent friction and packing fraction are still elusive when inertial, cohesive and viscous interactions occur between particles in addition to frictional and elastic forces. Here we report on extensive particle dynamics simulations of such complex flows for a model granular system composed of perfectly rigid particles. We show that, when the apparent friction and packing fraction are normalized by their cohesion-dependent quasistatic values, they are governed by a single dimensionless number that, by virtue of stress additivity, accounts for all interactions. We also find that this dimensionless parameter, as a generalized inertial number, describes the texture variables such as the bond network connectivity and anisotropy. Encompassing various stress sources, this unified framework considerably simplifies and extends the modeling scope for granular dynamics, with potential applications to powder technology and natural flows. Granular materials are abundant in nature, but we haven't fully understood their rheological properties as complex interactions between particles are involved. Here, Vo et al. show that granular flows can be described by a generalized dimensionless number based on stress additivity