4.3. DEM03: Two stacked, compressed particles

This case serves to verify the MFIX-DEM linear spring-dashpot collision model through analysis of a multi-particle, enduring collision. This test case is based on the work of Chen et al. [4] and the MFIX-DEM test case was originally reported in Garg et al. [8].

4.3.1. Description

Two particles of equal radius, rp, are stacked between two fixed walls such that the particles are compressed. The lower and upper walls are located at yl=0.0 and yw=3.6rp and the particle centers are initially located at y10=0.25yw and y20=0.75yw. This configuration, illustrated in Fig. 4.7, ensures that the particles remain in contact and compressed.

../_images/dem03-setup.png

Fig. 4.7 Two smooth spherical particles stacked between two fixed walls so that the system is always under compression. A sketch of the problem mechanics is provided along with force balances for the lower and upper particles.

An expression for the acceleration of the lower particle (particle 1) is

(4.12)d2y1dt2=gknwm1(y1rp)ηn1wm1dy1dtkn12m1(2rp(y2y1))ηn12m1(dy1dtdy2dt)

where y1 and y2 are the particle center positions measured from the lower wall, g is the acceleration due to gravity, knw and kn12 are the particle-wall and particle-particle spring coefficients, ηn1w and ηn12 are the particle-wall and particle-particle damping coefficients, and m1 is the mass of particle 1. Similarly, acceleration of the upper particle (particle 2) is given by

(4.13)d2y2dt2=gknwm2(rp(ywy2))ηn2wm2dy2dt+kn12m2(2rp(y2y1))+ηn12m2(dy1dtdy2dt)

where ηn2w is the particle-wall damping coefficient for the upper particle, and m2 is the mass of the upper particle.

4.3.2. Setup

Table 4.4 DEM-03 Setup, Initial and Boundary Conditions.

Computational/Physical model

1D, Transient

Granular flow (no gas)

Gravity

Thermal energy equation is not solved

Geometry

Coordinate system

Cartesian

x-length

1.0

(m)

y-length

0.0018

(m)

z-length

0.0010

(m)

Solids Properties

Normal spring coefficient, kn

103

(N·m-1)

Restitution coefficient, en

varied

( )

Friction coefficient, μ

0.0

( )

Solids 1 Type

DEM

Diameter, dp

0.001

(m)

Density, ρs

20000

(kg·m-3)

Solids 2 Type

DEM

Diameter, dp

0.001

(m)

Density, ρs

10000

(kg·m-3)

Boundary Conditions

All boundaries

Solid walls

4.3.3. Results

Analytical solutions to equations Eq.4.12 and Eq.4.13 describing the motion of the particles are readily obtainable for perfectly elastic (ηn12=ηn1w=ηn2w=1.0) particles of equal mass (m1=m2). This is not the case for inelastic particles of different mass, therefore a fourth-order Runge-Kutta method is used to calculate a secondary numerical solution which is considered to be the analytical solution during the analysis.

Simulations were conducted for six friction coefficients, [0.5, 0.6, 0.7, 0.8, 0.9, 1.0], using the Adams-Bashforth time-stepping method. Fig. 4.8 shows the motion of the lower (left) and upper (right) particles as well as the absolute value of the relative error for a restitution coefficient of 1. The percent relative difference in results remains below 0.1% for this case. This is the largest observed difference across all cases with the difference in relative error decreasing with decreasing restitution coefficient.

../_images/image541.png

Fig. 4.8 Comparison between the fourth-order Runge-Kutta solution (solid line) and MFIX-DEM simulation (open symbols) for the center position of two stacked particles compressed between fixed walls for a restitution coefficient of 1. The absolute percent relative errors are shown as dashed lines.