Use the Wall node to determine what happens to the particles when contact with a wall is made. The Accumulator (Boundary) subnode is available from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu. If Newtonian, Newtonian, first order, Lagrangian, or Hamiltonian is selected from the Formulation list in the physics interface Particle Release and Propagation section, the Secondary Emission subnode is also available.

The Wall node can be applied to interior or exterior boundaries, but the instance of this node that is created by default only applies to exterior boundaries.

Select a Wall condition: Freeze (the default), Bounce, Stick, Disappear, Pass through, Diffuse scattering, Isotropic scattering, Mixed diffuse and specular reflection, or General reflection.

If Massless is selected from the Formulation list in the physics interface Particle Release and Propagation section, only the Freeze (default), Stick, and Disappear conditions are available.

This section is available when General reflection (see Table 3-2) is selected as the Wall condition.

Enter values for the Reflected particle velocity vp (SI unit: m/s) either in Cartesian coordinates (x, y, z) (the default) or select the Specify tangential and normal velocity components check box to enter coordinates in the tangent-normal coordinate system (t1, t2, n). In this case the normal direction is selected so that an incident particle is reflected back into the domain it previously occupied if the specified normal velocity component is positive. The tangential directions are oriented so that they form a right-handed coordinate system, together with the normal direction. As a result, the tangential and normal directions may point in the opposite direction of the corresponding vectors defined for the geometry (for example, root.nx) and for the physics interface (for example, pt.nx).

Select a Primary particle condition: None (the default), Probability, or Expression. When the default, None, is kept, it means that the Wall condition is always respected by the incident particles.

If Probability is selected, the Wall condition is applied with a certain probability. Enter a value for the Probability γ (dimensionless) of the particle behaving according to the Wall condition. Otherwise the particle behaves according to the Otherwise setting.

For example, if the Wall condition is set to:

If Expression is selected, the Evaluation expression e (dimensionless) is evaluated whenever the particle strikes the wall. The default value is 1. If the Evaluation expression is nonzero, the particle behaves according to the Wall condition, otherwise the particle behaves according to the Otherwise setting.

The options available for the Otherwise setting are the same as for the Wall Condition, except that Mixed diffuse and specular reflection and General reflection are not available. The Otherwise setting can be used to make particles interact with a wall differently when a certain condition is satisfied. For example, to specify that particles with a diameter greater than 1 μm stick to an interior boundary, and the rest pass through:

This section is available if an Auxiliary Dependent Variable has been added to the model.

Select the Assign new value to auxiliary variable check box or boxes based on the number of auxiliary variables in the model. Then enter the new value or expression in the field. For example, if there is an auxiliary variable, psi, then enter a value for psinew in the field. So, to increment the value of psi by one when a particle touches or crosses a boundary, enter psi+1 in the text field for psinew.

If the Primary particle condition is set to Probability, or if the Diffuse scattering or Isotropic scattering wall condition is used, then the Wall feature generates random numbers. If, in addition, the Arguments for random number generation setting is User defined, the Advanced Settings section is available. Enter the Additional input argument to random number generator. The default value is 1.