Granular Matter

, Volume 12, Issue 5, pp 507–516 | Cite as

Numerical investigation of reverse segregation in debris flows by DEM

Article
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Abstract

Studies of the mechanisms and the effects on the flowing mobility for hazardous geophysical flows (e.g., debris flows) is crucial for hazard mitigation and prediction. Granular flows with grains of mixed sizes are numerically modeled and the contact behavior of solid particles is fundamentally studied using the discrete element method. The mechanical effects of particle contacts (shearing and collision) are contrasted with geometrical effects (kinetic sieving) to explain the mechanism of reverse segregation. Compared to granular flows with uniform solid particles, the effect of segregation on granular flowing mobility is investigated. It is found that reverse segregation can significantly influence the flowing mobility and the flowing regimes in the front head of the granular body. A mechanical explanation of the segregation mechanism can be presented by a new dimensionless number, which is correlated with the contact force.

Keywords

Discrete element method Dry granular flow Reverse segregation Flow regime 

List of symbols

\({\mathord{\buildrel\rightharpoonup\over{F}}_n,\mathord{\buildrel\rightharpoonup\over {F}}_T}\)

Contact normal/tangential force between solid particles

KN, KT

Particle stiffness in the normal/tangential direction

\({{{\mathord{\buildrel\rightharpoonup\over{\delta}}_n,\mathord{\buildrel\rightharpoonup\over {\delta}}_T}}}\)

Displacement of contact particles in the normal/tangential direction

CN, CT

Viscous damping in the normal/tangential direction

\({{{\mathord{\buildrel\rightharpoonup\over{V}{^{r}_{n}}},\mathord{\buildrel\rightharpoonup\over {V}{^{r}_{T}}}}}}\)

Sliding velocity in the normal/tangential direction

\({{\mathord{\buildrel\rightharpoonup\over{r}}}}\)

Position vector to the sphere center

βm

Critical damping ratio

M

Effective system mass

\({{\mathord{\buildrel\rightharpoonup\over{s}}_{ij}}}\)

Unit vector of the tangential direction of the contact point

\({{\mathord{\buildrel\rightharpoonup\over{F}}_{firct}}}\)

Coulomb frictional limit

μ

Frictional coefficient

\({\phi}\)

Inter-particle friction angle

Fd

Damping force

F

Out-of-balance force

V

Generalized velocity

α

A non-dimensional constant

θ

Slope angle

W

Channel width

d

Particle diameter

m

Mass of spherical balls

GS

Density of sphere ball

U

(Front) traveling velocity along the slope

L

Traveling distance

g

Gravity acceleration

U*

Dimensionless flowing (front) velocity

L0

Initial length of the deposited debris mass

L*

Lagrangian (mass) coordinate inside granular body

t

Flowing time of granular particles after releasing

t*

Dimensionless form of flowing time

\({\dot{\gamma}}\)

Shear rate

NSav

Savage number

H

Flowing height (thickness)

H0

Initial deposit thickness

H*

Dimensionless flowing height

Hmax

The largest flowing thickness in granular body

HC

Centroid height of fine, medium or coarse particles

Mi

Mass of a single particle

Hi

Flowing height of a single particle (distance from the slope bed)

N1, N2, N3

Total number of coarse, medium and fine particles

FN

Mean contact force normal to the slope due to particle interactions

ρ

Particle density

u(L)

Traveling velocity along the slope

\({F_N^\ast}\)

A dimensionless particle contact force normal to slope

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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  1. 1.Department of Civil and Environmental EngineeringHong Kong University of Science and TechnologyKowloonHong Kong
  2. 2.Institute of Mountain Hazards and EnvironmentChinese Academy of SciencesChengduChina

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