scholarly journals Large particle segregation, transport and accumulation in granular free-surface flows

2010 ◽  
Vol 652 ◽  
pp. 105-137 ◽  
Author(s):  
J. M. N. T. GRAY ◽  
B. P. KOKELAAR
Keyword(s):  
Particle Size ◽  
Transport Equation ◽  
Particle Transport ◽  
Large Particle ◽  
Size Segregation ◽  
Feedback Effects ◽  
Full Theory ◽  
Large Particles ◽  
Mass Flows ◽  
Geophysical Mass Flows

Particle size segregation can have a significant feedback on the motion of many hazardous geophysical mass flows such as debris flows, dense pyroclastic flows and snow avalanches. This paper develops a new depth-averaged theory for segregation that can easily be incorporated into the existing depth-averaged structure of typical models of geophysical mass flows. The theory is derived by depth-averaging the segregation-remixing equation for a bi-disperse mixture of large and small particles and assuming that (i) the avalanche is always inversely graded and (ii) there is a linear downslope velocity profile through the avalanche depth. Remarkably, the resulting ‘large particle transport equation’ is very closely related to the segregation equation from which it is derived. Large particles are preferentially transported towards the avalanche front and then accumulate there. This is important, because when this is combined with mobility feedback effects, the larger less mobile particles at the front can be continuously shouldered aside to spontaneously form lateral levees that channelize the flow and enhance run-out. The theory provides a general framework that will enable segregation-mobility feedback effects to be studied in detail for the first time. While the large particle transport equation has a very simple representation of the particle size distribution, it does a surprisingly good job of capturing solutions to the full theory once the grains have segregated into inversely graded layers. In particular, we show that provided the inversely graded interface does not break it has precisely the same solution as the full theory. When the interface does break, a concentration shock forms instead of a breaking size segregation wave, but the net transport of large particles towards the flow front is exactly the same. The theory can also model more complex effects in small-scale stratification experiments, where particles may either be brought to rest by basal deposition or by the upslope propagation of a granular bore. In the former case the resulting deposit is normally graded, while in the latter case it is inversely graded. These completely opposite gradings in the deposit arise from a parent flow that is inversely graded, which raises many questions about how to interpret geological deposits.

scholarly journals Asymmetric flux models for particle-size segregation in granular avalanches

2014 ◽  
Vol 757 ◽  
pp. 297-329 ◽  
Author(s):  
P. Gajjar ◽  
J. M. N. T. Gray
Keyword(s):  
Particle Size ◽  
Small Particle ◽  
Volume Fraction ◽  
Free Surface Flows ◽  
Two Dimensions ◽  
Combined Processes ◽  
Maximum Point ◽  
Size Segregation ◽  
Flux Function ◽  
Large Particles

AbstractParticle-size segregation commonly occurs in both wet and dry granular free-surface flows through the combined processes of kinetic sieving and squeeze expulsion. As the granular material is sheared downslope, the particle matrix dilates slightly and small grains tend to percolate down through the gaps, because they are more likely than the large grains to fit into the available space. Larger particles are then levered upwards in order to maintain an almost uniform solids volume fraction through the depth. Recent experimental observations suggest that a single small particle can percolate downwards through a matrix of large particles faster than a large particle can be levered upwards through a matrix of fines. In this paper, this effect is modelled by using a flux function that is asymmetric about its maximum point, differing from the symmetric quadratic form used in recent models of particle-size segregation. For illustration, a cubic flux function is examined in this paper, which can be either a convex or a non-convex function of the small-particle concentration. The method of characteristics is used to derive exact steady-state solutions for non-diffuse segregation in two dimensions, with an inflow concentration that is (i) homogeneous and (ii) normally graded, with small particles above the large. As well as generating shocks and expansion fans, the new asymmetric flux function generates semi-shocks, which have characteristics intersecting with the shock just from one side. In the absence of diffusive remixing, these can significantly enhance the distance over which complete segregation occurs.

Particle Size Segregation in Granular Mass Flows With Different Ambient Fluids

2020 ◽  
Vol 125 (10) ◽  
Author(s):  
Gordon G. D. Zhou ◽  
Kahlil F. E. Cui ◽  
Lu Jing ◽  
Tao Zhao ◽  
Dongri Song ◽  
...  
Keyword(s):  
Particle Size ◽  
Size Segregation ◽  
Granular Mass ◽  
Mass Flows

scholarly journals Microtomy of Large Particle Zeolites for Tem

1990 ◽  
Vol 199 ◽  
Author(s):  
J. G. Ulan ◽  
C. Schooley ◽  
R. Gronsky
Keyword(s):  
Electron Microscopy ◽  
Particle Size ◽  
Large Particle ◽  
Materials Science ◽  
Oxide Powder ◽  
Size Limit ◽  
Large Particles ◽  
Powder Samples ◽  
Organic Resin

ABSTRACTUniformly thin specimens of powders, suitable for electron microscopy, can be prepared by ultramicrotomy. There is a size limit to the particles that can be studied since larger particles tend to be dislodged from the available resins when microtomed. A method is described to strengthen the binding of the organic resin to the inorganic zeolite, allowing large particles to be sectioned. The particle size limit increased from 3 μm to greater than 20 gim in diameter for FeZSM-5 aggregates. This method can be applied to a variety of oxide powder samples, extending the utility of microtomy as a materials science tool.

scholarly journals Stable solutions of a scalar conservation law for particle-size segregation in dense granular avalanches

2008 ◽  
Vol 19 (1) ◽  
pp. 61-86 ◽  
Author(s):  
M. SHEARER ◽  
J. M. N. T. GRAY ◽  
A. R. THORNTON
Keyword(s):  
Particle Size ◽  
Free Surface ◽  
Shear Flow ◽  
Free Surface Flows ◽  
Size Segregation ◽  
Small Particles ◽  
Surface Flows ◽  
Scalar Conservation Law ◽  
Large Particles ◽  
Scalar Conservation

Dense, dry granular avalanches are very efficient at sorting the larger particles towards the free surface of the flow, and finer grains towards the base, through the combined processes of kinetic sieving and squeeze expulsion. This generates an inversely graded particle-size distribution, which is fundamental to a variety of pattern formation mechanisms, as well as subtle size-mobility feedback effects, leading to the formation of coarse-grained lateral levees that create channels in geophysical flows, enhancing their run-out. In this paper we investigate some of the properties of a recent model [Gray, J. M. N. T. & Thornton, A. R. (2005) A theory for particle size segregation in shallow granular free-surface flows. Proc. R. Soc. 461, 1447–1473]; [Thornton, A. R., Gray, J. M. N. T. & Hogg, A. J. (2006) A three-phase mixture theory for particle size segregation in shallow granular free-surface flows. J. Fluid. Mech. 550, 1–25] for the segregation of particles of two sizes but the same density in a shear flow typical of shallow avalanches. The model is a scalar conservation law in space and time, for the volume fraction of smaller particles, with non-constant coefficients depending on depth within the avalanche. It is proved that for steady flow from an inlet, complete segregation occurs beyond a certain finite distance down the slope, no matter what the mixture at the inlet. In time-dependent flow, dynamic shock waves can develop; they are interfaces separating different mixes of particles. Shock waves are shown to be stable if and only if there is a greater concentration of large particles above the interface than below. Constructions with shocks and rarefaction waves are demonstrated on a pair of physically relevant initial boundary value problems, in which a region of all small particles is penetrated from the inlet by either a uniform mixture of particles or by a layer of small particles over a layer of large particles. In both cases, and under a linear shear flow, solutions are constructed for all time and shown to have similar structure for all choices of parameters.

Effects of the Particle Size Variety of Metal Oxide on the Absorption Property of Long-Wavelength Infrared Laser

2011 ◽  
Vol 418-420 ◽  
pp. 536-539 ◽  
Author(s):  
Wei Fu Wang ◽  
Yu Gui Zheng ◽  
Jian Hua Yao ◽  
Miao Zhang
Keyword(s):  
Particle Size ◽  
Aggregate Size ◽  
Absorption Property ◽  
Feedback Effects ◽  
Long Wavelength ◽  
Laser Absorption ◽  
Large Particles ◽  
Heated Area ◽  
Laser Heated ◽  
Long Wavelength Infrared

In order to study the influences of aggregate size on CO2laser absorption property, several special absorbing coatings were prepared from four kinds of different particle sizes (45 μm, 13 μm, 6.5 μm and 1.6 μm) TiO2powder. The CO2laser absorption capacities of these coatings were studied by comparative analysis of the surface morphology and laser heated areas. The results show that the absorbing capacity increases with particle size dwindling. In all samples, the 1.6 μm specimens display the best performance with the largest laser heated area on cross section. The analysis indicate that, the decreasing of particle size will lead to an increase on absorbing surfaces, which is benefit for improving the absorbing capacity. Furthermore, smaller size particles also can cause the temperature increased faster than large particles, which lead to a further increase on absorbing performance for the positive feedback effects between the temperature and the absorbing capacity. Finally, the effects of particle size variety on designing absorbing coatings are also discussed.

An experimental study of the motion of concentrated suspensions in two-dimensional channel flow. Part 2. Bidisperse systems

1998 ◽  
Vol 363 ◽  
pp. 57-77 ◽  
Author(s):  
M. K. LYON ◽  
L. G. LEAL
Keyword(s):  
Large Particle ◽  
Particle Concentration ◽  
Volume Fraction ◽  
Balance Model ◽  
Concentration Profiles ◽  
Size Segregation ◽  
Small Particles ◽  
Concentrated Suspensions ◽  
Particle Volume ◽  
Large Particles

In this paper we report experimental velocity and concentration profiles for suspensions possessing a bidisperse distribution of particle size undergoing pressure-driven flow through a parallel-wall channel. In addition to the overall concentration distributions determined by implementing the modified laser Doppler velocimetry method described in Part 1 (Lyon & Leal 1998), concentration profiles for the particles of each size were measured by sampling the position of marked tracer particles across 60% of the channel gap. Non-uniform overall particle concentration distributions and blunted velocity profiles were found at bulk particle volume fractions of 0.30 and 0.40, which were equal to the monodisperse data of Part 1, within experimental uncertainty. The large-particle concentration profiles were non-uniform down to a large-particle bulk volume fraction of 0.075, while non-uniform distributions of the small particles were only found when the volume fraction of small particles in the bulk was greater than or equal to 0.20. Experiments in which at least half the suspended particulate volume was occupied by large particles revealed enrichment of the large particles in the centreline region of the channel. This size segregation was found to increase as the total number of suspended particles decreased. Finally, the data from experiments in which a uniform small-particle concentration profile was measured were compared with suspension balance model (McTigue & Jenkins 1992; Nott & Brady 1994) predictions for parameter values that corresponded only to the large particles. While close agreement with the large-particle concentration profiles was found, this comparison also reflected the fact that the small particles bring the suspension viscosity to a regime that is more sensitive to the particle concentration, rather than simply providing an increment in background viscosity to the suspending liquid.

scholarly journals Grain-size segregation and levee formation in geophysical mass flows

2012 ◽  
Vol 117 (F1) ◽  
pp. n/a-n/a ◽  
Author(s):  
C. G. Johnson ◽  
B. P. Kokelaar ◽  
R. M. Iverson ◽  
M. Logan ◽  
R. G. LaHusen ◽  
...  
Keyword(s):  
Grain Size ◽  
Size Segregation ◽  
Mass Flows ◽  
Geophysical Mass Flows

Statistics of Saturn Ring occultations

2020 ◽  
Author(s):  
Larry W. Esposito ◽  
Miodrag Sremcevic ◽  
Joshua E Colwell ◽  
Stephanie Eckert
Keyword(s):  
Particle Size ◽  
Optical Depth ◽  
Large Particle ◽  
Large Particle Size ◽  
Monte Carlo Calculations ◽  
Effective Particle Size ◽  
Effective Particle ◽  
Large Particles ◽  
Upward Curvature ◽  
The Individual

<p>We give calculations for the excess variance, excess skewness and excess kurtosis with formulas that combine the effects of cylindrical shadows, along with gaps, ghosts and clumps (all calculated for the granola bar model for rectangular clumps and gaps). Wherever the rings have significant gaps or clumps, those will dominate the statistics over the individual ring particles contribution. We have refined an overlap correction for multiple shadows, which is important for larger optical depth. This correction results from summing a geometric series, and is similar to the empirical formula, eq. (22) in Colwell et al (2018). The comparison to Monte Carlo calculations is improved for large particle size by including the edge effects when large particles cross the edges of the viewing area A in Cassini UVIS occultations. As a check, we can explain the upward curvature of the dependence of normalized excess variance for Saturn’s background C ring by the observation of Jerousek etal (2018) that the increased optical depth is directly correlated with effective particle size. Assuming a linear dependence R<sub>eff</sub> = 12 * (tau – 0.08) + 1.8m, we match both the curvature of excess variance E and the skewness Gamma in the region between 78,000 and 84,600km from Saturn. This explanation requires no gaps or ghosts (Baillie etal 2013) in this region of Saturn’s C ring.</p>

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