The motion of debris flows, gravity-driven fastudmoving mixtures of rock, soil and water can be interpretedudusing the theories developed to describe the shearing motionudof highly concentrated granular fluid flows. Frictional, collisionaludand viscous stress transfer between particles andudfluid characterizes the mechanics of debris flows. To quantifyudthe influence of collisional stress transfer, kinetic modelsudhave been proposed. Collisions among particles result in randomudfluctuations in their velocity that can be represented byudtheir granular temperature, T. In this paper particle imageudvelocimetry, PIV, is used to measure the instantaneous velocityudfield found internally to a physical model of an unsteadyuddebris flow created by using “transparent soil”—i.e. a mixtureudof graded glass particles and a refractively matched fluid.udThe ensemble possesses bulk properties similar to that ofudreal soil-pore fluid mixtures, but has the advantage of givingudoptical access to the interior of the flow by use of plane laserudinduced fluorescence, PLIF. The relationship between PIVudpatch size and particle size distribution for the front and tailudof the flows is examined in order to assess their influencesudon the measured granular temperature of the system. We findudthat while PIV can be used to ascertain values of granularudtemperature in dense granular flows, due to increasing spatialudcorrelation with widening gradation, a technique proposed toudinfer the true granular temperature may be limited to flowsudof relatively uniform particle size or large bulk.
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