Using PIV to measure granular temperature in saturated unsteady polydisperse granular flows

Nicoletta Sanvitale,Elisabeth T Bowman

doi:10.1007/s10035-016-0620-6

Abstract

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.

Highlights

Debris flows are highly dangerous gravity-driven mass flows of soil, rock and water [1]
In doing so we examine some issues associated with the PIV technique as applied to granular flow related to the behaviour of the interior of polydisperse, unsteady and segregating flows over rigid beds
Particle size has been taken as D50 to normalise the velocity and granular temperature profiles, in keeping with common practice

Summary

Introduction

Debris flows are highly dangerous gravity-driven mass flows of soil, rock and water [1] The motion of such flows may be considered in the light of granular flow theory which has been developed to describe the behaviour of ensembles of particles undergoing shearing motion under gravity—encompassing pseudo-static, frictional, collisional and kinetic regimes [2]. The highly dilute, saturated tail, which is dominated by fines, may predispose the channel to further erosion by the gradual transfer of moisture from the flow into the bed, preconditioning it to subsequent flow surges These elements point to the consideration of the diverse roles that particles of different size may play within a given flow—and that these can be as important as their ensemble behaviour. These unique mechanical characteristics have led to a particular interest in understanding the stress transfer behaviour between different elements of the flow

Methods

Results

Conclusion