Wall
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.
Wall Condition
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.
General Reflection Settings
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.
Primary Ray Condition
Use this section to apply two different wall conditions on the same set of boundaries based on a probability or logical expression.
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.
Probability
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.
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 rays that strike the wall, on average one freezes and the remaining 9 rays behave according to the Otherwise setting.
Stick and γ is set to 0.5 then on average half of the rays 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 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.
Otherwise
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 Mixed diffuse and specular reflection as the Wall condition.
Set the Probability of specular reflection to 0.3/(1-0.5).
Set the Primary ray condition to Probability.
Set the Probability to 1-0.5.
Select Freeze as the Otherwise option.
Phase shift
This section is available when:
The Compute phase check box is selected under the physics interface Advanced settings section.
The Wall condition is set to Specular Reflection, Diffuse scattering, Isotropic scattering, General reflection, or Mixed diffuse and specular reflection.
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.
Absorption Coefficient
This section is available when the ray intensity or power is solved for in the model, and the Wall condition is set to any option that can reflect rays, including the following:
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:
-
Speed of sound, adjacent fluid c1 (SI unit: m/s). The default is 1500 m/s.
-
Density, adjacent fluid ρ1 (SI unit: kg/m3). The default is 1000 kg/m3.
-
Attenuation coefficient, adjacent fluid α1 (SI unit: 1/m). The default is 0.
For Fluid-solid interface enter the following:
-
Compressional speed of sound, adjacent solid cp,1 (SI unit: m/s). The default is 1500 m/s.
-
Shear speed of sound, adjacent solid cs,1 (SI unit: m/s). The default is 1500 m/s.
-
Density, adjacent solid ρ1 (SI unit: kg/m3). The default is 1000 kg/m3.
-
Compressional attenuation coefficient, adjacent solid αp,1 (SI unit: 1/m). The default is 0.
-
Shear attenuation coefficient, adjacent solid αs,1 (SI unit: 1/m). The default is 0.
For Layered fluid half-space enter the following:
-
Thickness, adjacent fluid layer h (SI unit: m). The default is 1 μm.
-
Speed of sound, adjacent fluid layer c1 (SI unit: m/s). The default is 1500 m/s.
-
Density, adjacent fluid layer ρ1 (SI unit: kg/m3). The default is 1000 kg/m3.
-
Attenuation coefficient, adjacent fluid layer α1 (SI unit: 1/m). The default is 0.
-
Speed of sound, adjacent fluid domain c2 (SI unit: m/s). The default is 1500 m/s.
-
Density, adjacent fluid domain ρ2 (SI unit: kg/m3). The default is 1000 kg/m3.
-
Attenuation coefficient, adjacent fluid domain α2 (SI unit: 1/m). The default is 0.
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.
Surface Roughness
This section is available when
the Wall condition is set to Specular Reflection, General reflection, or Mixed diffuse and specular reflection. For Mixed diffuse and specular reflection the roughness only applies to the specularly reflected rays.
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.
New Value of Auxiliary Dependent Variables
This section is available if an Auxiliary Dependent Variable has been added to the model.
When a ray crosses or touches a boundary, the values of the user-defined auxiliary dependent variables can be changed. The value can be a function of any combinations of ray variables and variables defined on the boundary.
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.
Advanced Settings
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.