Use the Wall node to determine what happens to the rays when contact with a boundary is made. The Wall node is the default boundary condition on all exterior boundaries.

The Accumulator (Boundary) subnode is available from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu.

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

This section is available when General reflection (see Table 8-2) is selected as the Wall condition.

Enter values for the Reflected ray direction vector Lp (dimensionless) either in Cartesian coordinates (x, y, z) (the default) or select the Specify tangential and normal direction vector 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 ray is reflected back into the domain it previously occupied if the specified normal direction vector component is positive. The tangential directions are oriented so that they form a right-handed coordinate system, together with the normal direction.

Select a Primary ray 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 rays.

If Probability is selected, the Wall condition is applied with a certain probability. Enter a value for the Probability, γ (dimensionless). If the Wall condition is not used, the ray instead 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 a ray strikes the wall. The default expression is 1. If the Evaluation expression is nonzero, the ray behaves according to the Wall condition, otherwise the ray behaves according to the Otherwise setting.

The options available for the Otherwise setting are the same as for the Wall Condition, except that General reflection and Mixed diffuse and specular reflection are not available. The Otherwise setting can be used to make rays interact with a wall differently with a certain probability or when a certain condition is satisfied. For example, to model reflection at a partially specular surface in which 50% of the ray intensity is absorbed and 30% is reflected specularly:

Select the Apply manual phase shift check box to specify the phase shift applied to reflected rays. By default this check box is cleared, causing the phase shift be derived from the reflection coefficient.

If the Apply manual phase shift check box is selected, enter a Phase shift ΔΨ (SI unit: rad). The default is 0.

If the Wall condition is set to Mixed diffuse and specular reflection, this section is instead called Phase shifts and the phase shifts for the diffusely and specularly reflected rays are specified separately. All of the text fields are given subscripts s or d for specularly and diffusely reflected rays, respectively.

Select one of the following from the Compute reflected intensity using list: Absorption coefficients, Absorber, specified impedance, Reflection coefficients, Fluid-fluid interface, Fluid-solid interface, or Layered fluid half space.

For Absorption coefficients enter the Absorption coefficient α (dimensionless). The default is 0. The intensity or power of the reflected ray will be proportional to 1 − α.

For Absorber, specified impedance enter the Characteristic impedance of absorber Z1 (SI unit: Pa·s/m). The default value is 1 GPa·s/m, corresponding to a hard wall.

For Reflection coefficients enter the Reflection coefficient R (dimensionless). The default value is 1. The intensity or power of the reflected ray will be proportional to R2.

For Fluid-fluid interface enter the following:

For Fluid-solid interface enter the following:

For Layered fluid half-space enter the following:

If the Wall condition is set to Mixed diffuse and specular reflection, this section is instead called Absorption Coefficients and the absorption coefficients for the diffusely and specularly reflected rays are specified separately. All of the text fields are given subscripts s and d for specularly and diffusely reflected rays, respectively.

Select an option from the Surface roughness model list: None (the default) or Rayleigh roughness. If Rayleigh roughness is selected, enter the RMS roughness σ (SI unit: m). The default is 0. Use this setting to reduce the intensity of rays that are reflected by a rough wall.

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 1 when a ray touches or crosses a boundary, enter psi+1 in the text field for psinew.

If the Primary ray condition is set to Probability, or if the Diffuse scattering, Isotropic scattering, or Mixed diffuse and specular reflection wall condition is used, then the Wall feature generates random numbers.

If, in addition, the Arguments for random number generation setting is set to User defined in the physics interface Advanced Settings section, the Advanced Settings section is available.

Enter the Additional input argument to random number generator i (dimensionless). The default value is 1.