Artificial Viscosity Model Reference

When two particles collide, or when a particle collides with a wall, the DEM module uses the impact velocity to predict the outcome. Impact velocities can increase to the same order of magnitude as the speed of sound in the particle material, which can cause the numerical models to over-predict the overlap between the colliding objects. The large overlap results in inaccurate results or unphysical ones, such as particles passing through walls. You can avoid or reduce these problems by applying the artificial viscosity model.

The energy that artificial viscosity dissipates represents unresolved processes of damage, deformation, and entropy. Do not use the artificial viscosity model if you want to study these or similar collision effects in detail. Instead, use models that address those effects; for example, to study particle breakage, use the Bonded Particles model.


Theory	See Artificial Viscosity.
Provided By	Continua > Physics 1 > Models > Multiphase Interaction > Phase Interactions > Phase Interaction 1 > Models > Optional Models
Example Node Path	Continua > Physics 1 > Models > Multiphase Interaction > Phase Interactions > Phase Interaction 1 > Models > Artificial Viscosity
Requires	Under Lagrangian Multiphase, select Particle Type: DEM Particles Under Multiphase Interaction select: Phase Interaction Topology: DEM Phase Interaction if it has not been automatically selected. Select Optional Models: any
Properties	Key properties are: Linear Coefficient, Quadratic Coefficient, Upper ramp limit, Lower ramp limit. See Artificial Viscosity Properties.

Artificial Viscosity Properties

Linear Coefficient: The linear coefficient $Q_{1}$ in Eqn. (3248), which must be nonnegative. The default value is 0.3.
Quadratic Coefficient: The quadratic coefficient $Q_{2}$ in Eqn. (3248), which must be nonnegative. The default value is 1.
Upper ramp limit: The upper ramp limit, $p_{upper}$ in Eqn. (3249). The value must lie between zero and one, and $p_{upper}$ must be larger than $p_{lower}$ . The default value is 0.2.
Lower ramp limit: The lower ramp limit, $p_{lower}$ in Eqn. (3249). The value must lie between zero and one, and $p_{lower}$ must be smaller than $p_{upper}$ . The default value is 0.1.

All these values are dimensionless.

The default value of $Q_{1}$ , 0.3, is high, to enhance stopping at bounding surfaces. A value of 0.06 is more typical of shock problems.

The default value of $Q_{2}$ , 1, is the value for an elastic fluid. Values typically range from 1.5 to 0.6.

The default values for $p_{upper}$ and $p_{lower}$ , 0.2 and 0.1, are high. Contact force models are valid only for values lower than 0.05, but the high values create a default situation where artificial viscosity comes into play even in borderline cases. Bring values down for stiff particles and up for soft ones.