000219784 001__ 219784
000219784 005__ 20190317000502.0
000219784 0247_ $$2doi$$a10.1017/jfm.2016.170
000219784 022__ $$a0022-1120
000219784 02470 $$2ISI$$a000373937400023
000219784 037__ $$aARTICLE
000219784 245__ $$aAsymmetric breaking size-segregation waves in dense granular free-surface flows
000219784 260__ $$bCambridge University Press$$c2016$$aNew York
000219784 269__ $$a2016
000219784 300__ $$a46
000219784 336__ $$aJournal Articles
000219784 520__ $$aDebris and pyroclastic flows often have bouldery flow fronts, which act as a natural dam resisting further advance. Counter intuitively, these resistive fronts can lead to enhanced run-out, because they can be shouldered aside to form static levees that self-channelise the flow. At the heart of this behaviour is the inherent process of size segregation, with different sized particles readily separating into distinct vertical layers through a combination of kinetic sieving and squeeze expulsion. The result is an upward coarsening of the size distribution with the largest grains collecting at the top of the flow, where the flow velocity is greatest, allowing them to be preferentially transported to the front. Here, the large grains may be overrun, resegregated towards the surface and recirculated before being shouldered aside into lateral levees. A key element of this recirculation mechanism is the formation of a breaking size-segregation wave, which allows large particles that have been overrun to rise up into the faster moving parts of the flow as small particles are sheared over the top. Observations from experiments and discrete particle simulations in a moving-bed flume indicate that, whilst most large particles recirculate quickly at the front, a few recirculate very slowly through regions of many small particles at the rear. This behaviour is modelled in this paper using asymmetric segregation flux functions. Exact non-diffuse solutions are derived for the steady wave structure using the method of characteristics with a cubic segregation flux. Three different structures emerge, dependent on the degree of asymmetry and the non-convexity of the segregation flux function. In particular, a novel 'lens-tail' solution is found for segregation fluxes that have a large amount of non-convexity, with an additional expansion fan and compression wave forming a 'tail' upstream of the 'lens' region. Analysis of exact solutions for the particle motion shows that the large particle motion through the 'lens-tail' is fundamentally different to the classical 'lens' solutions. A few large particles starting near the bottom of the breaking wave pass through the 'tail', where they travel in a region of many small particles with a very small vertical velocity, and take significantly longer to recirculate.
000219784 6531_ $$agranular media
000219784 6531_ $$amixing
000219784 6531_ $$apattern formation
000219784 700__ $$uUniv Manchester, Sch Math, Manchester M13 9PL, Lancs, England$$aGajjar, P.
000219784 700__ $$0246054$$g218932$$aVan Der Vaart, K.
000219784 700__ $$aThornton, A. R.
000219784 700__ $$uUniv Manchester, Sch Math, Manchester M13 9PL, Lancs, England$$aJohnson, C. G.
000219784 700__ $$0240366$$g148669$$uEcole Polytech Fed Lausanne, Environm Hydraul Lab, CH-1015 Lausanne, Switzerland$$aAncey, C.
000219784 700__ $$aGray, J. M. N. T.$$uUniv Manchester, Sch Math, Manchester M13 9PL, Lancs, England
000219784 773__ $$j794$$tJournal Of Fluid Mechanics$$q460-505
000219784 8564_ $$uhttps://infoscience.epfl.ch/record/219784/files/2016JFM%20Asymetric...%20flow_1.pdf$$zn/a$$s16644311
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000219784 937__ $$aEPFL-ARTICLE-219784
000219784 973__ $$rREVIEWED$$sPUBLISHED$$aEPFL
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