advances in star-ccm+ dem models for simulating...
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Advances in STAR-CCM+ DEM models for simulating
deformation, breakage, and flow of solids
Satish Bonthu
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Overview of DEM in STAR-CCM+
Recent DEM improvements
– Parallel Bonds in STAR-CCM+
– Constant Rate Damage Model
– Maximum Packing Random Injector
– Particle Depletion Model
– Abrasive Wear Model
Simulation studies
– Compression of brittle material
– Rock drilling
– Erosion in a pipe bend
– Erosion due to water jet
Summary
Outline
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DEM is used to model particles of different sizes and shapes
Discrete Element Modeling (DEM) - Overview
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• DEM resolves the collisions between particles
• Particles can be bonded together to form deformable / breakable
material
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Momentum conservation
𝑚𝑖𝑑𝑣𝑖𝑑𝑡= 𝐹𝑖𝑗 + 𝐹𝑔 +
𝑗
𝐹𝑓𝑙𝑢𝑖𝑑
𝑚𝑖 and 𝑣𝑖 are mass and velocity of particle 𝑖, 𝐹𝑔 = 𝑚𝑖𝑔 is gravity force, 𝐹𝑖𝑗 is contact
force between particle 𝑖 and element 𝑗
- DEM is a meshless method
- DEM is computationally intensive method
Conservation of angular momentum 𝑑
𝑑𝑡𝐼𝑖𝜔𝑖 = 𝑇𝑖𝑗
𝑗
– 𝑖, 𝐼𝑖 and 𝜔𝑖 are the momentum on inertia and rotational velocity of particle 𝑖. 𝑇𝑖𝑗 = 𝑟𝑖𝑗(𝐹𝑖𝑗 + 𝐹𝑟) is the torque produced at the point of contact and it is the
function of the rolling friction force 𝐹𝑟
DEM Governing Equations
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Contact forces
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Normal component
of contact force
Tangential component
of contact force
Particle A
Particle B
Friction
Young’s
modulus
(stiffness) Normal
restitution
Tangential
restitution
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Contact models in STAR-CCM+
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Optional models
Basic models
Rolling Resistance Three models for resisting rolling
Linear Cohesion Constant attractive force
Artificial Viscosity Velocity dependent damping model
Heat Conduction Heat flow through contact
Parallel Bonds Bonds resisting to tension, bending, twist
Hertz-Mindlin Classical nonlinear contact force model for rigid bodies
Walton-Braun Linear model for deformable particles
Linear Spring Linear model for rigid bodies
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Two STAR-CCM+ models use same bond physics
– Parallel bond contact model
• Bonds are formed after injection at each new contact
• User specifies time interval of bonding
– Bonded particle model
• Used to create clumped particles:
• Bonds are formed at the moment of injection
Recent improvements
– Bond strength distribution, visualization, and…
Parallel bonds in STAR-CCM+
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Massless bar, subject to breaking under load
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Bond Failure Models in STAR-CCM+
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F
F
t t+dt
F F F
Constant Rate Damage Model
New in version 10.04
Simple Failure Model
t t + n dt
t+dt
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Constant Rate Damage Model
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kr is the fracture softening modulus, model parameter
Bond tensile strain
Str
es
s o
n b
on
d
O
εf kr εf
σm
σl
A0
B0
A
B
M
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Standard test to determine the strength of brittle material
Solid material is modeled as particles bonded together
Example of Brazilian compression test
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New Maximum Packing option in Random Injector
– Particles “grow from seeds” until whole region is fully packed
– Resulting configuration has no overlaps between particles
Sample preparation
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200 seeds 10000 seeds
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Geometry involves stationary bottom wall and moving down top platen
– Particles are injected using table injector with “Allow overlap” option
• Particle diameters are “inflated” by factor 1.01 to ensure small overlaps
– Bonds are set to form for first 0.1 s before starting compression
Running compression test
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Animation of compression test
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Crack formation
Both “Simple Failure” and
“Constant Rate Damage” models
were tested:
– Only “Constant Rate Damage”
model reproduced the vertical
crack observed in experiment
Bonds model parameters (Normal
and Tangential strength of bonds,
its distribution) were calibrated to
reproduce the target material
– Soft sandstone in our tests
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Rock is permeable with void
fraction =0.4
Overset mesh is used to rotate
and advance the drill-bit down
Solution for drilling fluid flow was
obtained using 2-way coupling
model
Jet flow form nozzles results in
large drag forces on bonded
grains (jetted erosion)
Example of rock drilling
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Erosion and Wear in Solids Processing Equipment
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Slurry flow in pipes
erosion
Sediment flow, Gravel Packing
Drilling rock
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New in version 10.04
Relevant for flow regimes
– With high solids loading
– Prolonged “sliding” contact between particles and geometry
Abrasive Wear Model in STAR-CCM+
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Archard Model:
𝑴𝒆 = 𝑪 𝑭𝒄𝒏𝒗𝑐𝒕𝒅𝒕𝒄𝒐𝒏𝒕𝒂𝒄𝒕𝒔 is mass
eroded from single surface cell
– 𝑪 is abrasive wear coefficient in units of kg/J (model parameter)
– 𝐹𝑐𝑛 is normal component of particle-cell contact force
– 𝒗𝑐𝒕 is tangential component of particle-cell relative velocity at the point of contact
– 𝒅𝒕 is DEM timestep
Field Function option:
𝑴𝒆 = 𝑪 𝐹(𝑐)𝒅𝒕
𝒄𝒐𝒏𝒕𝒂𝒄𝒕𝒔
– 𝐹(𝑐) is user field function
Abrasive Wear Formulation
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Surface cell
motion
Contact path
𝐹𝑐𝑛
𝒅𝒕 At the end of each timestep, for each surface cell,
– Erosion rate calculated as
𝐸𝑟 =1
𝑇
1
𝐴𝑐𝑒𝑙𝑙𝑀𝑒
• 𝑇 is the timestep
• 𝐴𝑐𝑒𝑙𝑙 is the area of surface cell
– 𝑀𝑒 is reset to zero
– 𝐸𝑟 is available for postprocessing
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Model properly accounts for
– All contacts during timestep
– Variation of contact force strength
and relative velocity for single
particle during prolonged “sliding
contact”
Abrasive wear coefficient is often
related to “hardness” of boundary
Best for flows when harder
particles are sliding along softer
boundary
Abrasive wear formulation summary
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Abrasive wear model is compatible
with coarse grain model
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Elbow pipe example, top view
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2 m
1 m
0.4 m
Inlet
Mean Parcel Diameter = 10 mm
Mesh size 20 mm
Outlet
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Random injector
– Uses “Maximum Independent Set” Algorithm to provide High Solid Loading flow
– Fast, mesh independent
– Initial particle velocity:
• Horizontal component = inlet fluid velocity
• Small “down component”
Particle Injection
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Injection Volume
gravity
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Initial Results for Elbow flow
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DEM Postprocessing
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View from bottom
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Segment of pipe (or solids buildup
inside pipe) can be modeled using
DEM particles bonded together
– D_inner = 0.3 m
– D_outer = 0.4 m
Water flow is set with inlet at one
end of DEM pipe and outlet at
another end
Inlet velocity is tilted 10 deg with
respect to inlet normal
– One side of pipe should
experience more erosion
Erosion due to water jet example
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0.4 m
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Initial Results with Constant Rate Damage Model
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Inlet velocity in aggressively
ramped from 0 till 20 m/s over
simulated time
Fully 2-way coupling between DEM
and CFD
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DEM Post-processing
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Maximum packing option in random injector
– Allows obtaining tight packing of particles without timestepping
– For size distribution, smaller particles are injected after larger one are in the
system
Particle depletion model
– New in version 10.04
– Allows removing subset of particles (clean central core area)
Table injector
– Allows loading the saved particle assembly with “inflated diameters”
Features used to create bonded assembly
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Summary
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Improved parallel bonds New injection and depletion options
Improved accuracy in modeling brittle materials processing
New abrasive wear model
Accurate prediction of equipment damage
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Back-up slides
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B. C. Trent, L. G. Margolin, P. A. Cundall, E. S. Gaffney, The Micro-mechanics
of Cemented Granular Material, Second International Conference on
Constitutive Laws for Engineering Materials, 1987
G.T. Gamacho and M. Ortiz, Computational Modelling of Impact Damage in
Brittle Materials, Int. J. Solids, 33, pp. 2899-2938, 1996
Luis Kosteski , Ricardo Barrios D’Ambra, Ignacio Iturrioz, Crack propagation
in elastic solids using the truss-like discrete element method, Int J Fract
(2012) 174:139–161
Constant rate damage model references
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