Wall
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.
Wall Condition
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.
 
Select to bounce particles off a wall according to Knudsen’s cosine law. That is, the probability a particle bouncing off the surface in a given direction within a solid angle dω is given by cos(θ)dω, where θ is the angle between the direction of the reflected particle and the wall normal. The total particle kinetic energy is conserved.
General Reflection Settings
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).
Primary Particle Condition
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.
Probability
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.
The value of γ should always be between 0 and 1.
For example, if the Wall condition is set to:
Freeze and γ is set to 0.1, then for every 10 particles that strike the wall, on average one freezes and the remaining 9 particles behave according to the Otherwise setting.
Stick and γ is set to 0.5, then on average half of the particles stick to the wall and the other half behave according to the Otherwise setting.
Expression
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.
Otherwise
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:
Select Stick as the Wall condition,
set the Primary particle condition to pt.dp>1E-6 (or pt.dp>1[um]), and
select Pass through as the Otherwise option.
New Value of Auxiliary Dependent Variables
This section is available if an Auxiliary Dependent Variable has been added to the model.
When a particle crosses or touches a boundary, the values of its auxiliary dependent variables can be changed. The new values can be functions of any combinations of particle variables and variables defined on the boundary. A simple application is to use this to count the number of times a particle strikes the wall.
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.
Advanced Settings
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.