Secondary Emission
The Secondary Emission subnode is available from the context menu (right-click the Wall, Axial Symmetry, or Velocity Reinitialization parent node) or from the Physics toolbar, Attributes menu. When Massless is selected as the Formulation, the Secondary Emission subnode is not available.
If the Secondary Emission node is added to a boundary feature such as a Wall or Axial Symmetry node, secondary particles are released when particles hit the boundary. If the Secondary Emission node is added to a Velocity Reinitialization node, secondary particles are released when the particle velocity is reinitialized.
Secondary Emission Condition
For Secondary Emission subnodes applied to boundary nodes, select an option from the Activate on wall conditions list: All, Primary only, or Otherwise only. These settings determine which wall conditions in the parent node cause secondary particles to be released.
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For All both the Wall condition and Otherwise condition cause secondary particles to be released.
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For Primary only secondary emission only occurs if the incident particle behaves according to the Wall condition setting.
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For Otherwise only secondary emission only occurs if the incident particle behaves according to the Otherwise only setting.
Select a Secondary emission condition: None, Probability, or Expression. For Probability enter the Probability γ (dimensionless) for secondary emission to occur. For Expression enter an Evaluation expression e (dimensionless) that must be nonzero for secondary emission to occur.
Secondary Particles
Number of Secondary Particles
In the Number of secondary particles field Ns, specify the number of secondary particles to release per incident particle. The default value is 1.
Initial Velocity
Select an Initial velocity. If the Secondary Emission feature is added to a Wall or Axial Symmetry node, the available options are Isotropic hemisphere (the default), Reflection of primary particle, Diffuse scattering, Thermal, and User defined. If the Secondary Emission feature is added to a domain level feature such as Velocity Reinitialization, the available options are Constant speed, spherical and User defined (default). The following briefly explains each option.
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Isotropic hemisphere: releases the secondary particles with a constant speed and hemispherical velocity direction with the north pole directed in the normal direction away from the wall. The speed of the secondary particles is equal to the speed of the incident particle, divided by the number of secondary particles.
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Constant speed, spherical: releases the secondary particles with a constant speed and spherical distribution of velocity directions. Enter a Speed with which all secondary particles are released.
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Diffuse scattering: causes secondary particles to be released with a probability distribution based on Knudsen’s cosine law, like the Diffuse scattering option for the Wall node.
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Reflection of primary particle: releases the secondary particles in the direction the primary particle would go if it were specularly reflected, like the Bounce option for the Wall node.
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Thermal: The distribution of particle directions follows the cosine law, as in the Diffuse scattering option. Rather than being released at uniform speed, however, the secondary particle speeds are sampled from a distribution based on the surface Temperature, which can either be User defined or coupled to another field that has units of temperature, similar to the Thermal Re-Emission boundary condition, or to the Thermal velocity distribution for the Inlet node.
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User Defined: allows for an arbitrary velocity vector to be set for the secondary particles. Enter values or expressions for the Initial particle velocity v0 (SI unit: m/s) either in Cartesian coordinates (x, y, and z for 3D) (the default) or select the Specify tangential and normal velocity components check box (available only when the parent node is a Wall or Axial Symmetry node) to enter coordinates in the tangent-normal coordinate system (t1, t2, n), a specification of the tangential components of the velocity and a normal component (in 2D) and two tangential components and a normal component (in 3D). In this case, the normal is directed away from the wall on which the particle is incident. 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).
Secondary Particle Speed
For Secondary Emission nodes added to boundary features, the Secondary particle speed list is shown if either Isotropic hemisphere, Diffuse scattering, or Reflection of primary particle is selected from the Initial velocity list. Select one of the following options from this list:
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For Same as incident particle, the initial speed of every released secondary particle is equal to that of the incident particle.
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For Scaled by number of secondary particles (the default), the initial speed of every released particle is equal to that of the incident particle, divided by the number or expression in the Number of secondary particles field.
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For User defined, enter a value or expression for the secondary particle speed Vs (SI unit: m). The default is 0.
For example, suppose that you set Ns = 4, select Diffuse scattering from the Initial velocity list, and then select Scaled by number of secondary particles from the Secondary particle speed list. If an incident particle with speed 2 m/s hits the wall, then 4 secondary particles will be released, with randomly generated velocity directions following the cosine law and initial speeds of 0.5 m/s.
Offset Initial Position
This section is only shown if the Secondary Emission node is a subnode of a domain-level feature such as the Velocity Reinitialization node, not a boundary feature such as Wall node.
Select the Offset initial position check box to offset the initial positions of the secondary particles before releasing them. The secondary particles can then appear in the region surrounding the parent particle, instead of its exact position.
Select an Offset method: Using initial velocity (the default) or Isotropic sphere.
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For Using initial velocity enter a Time interval Δt (SI unit: s). The default is 0. Each secondary particle is then displaced by the product of its initial velocity and the specified time interval before being released.
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For Isotropic sphere enter a Particle displacement magnitude Δr (SI unit: m). The default is 0. All secondary particles are then moved a distance equal to the displacement magnitude before they are released. If the offset would cause secondary particles to be placed outside of the modeling domain, they are instead released at the location of the primary particle.
Advanced Settings
For any of the following combinations of settings, the Secondary Emission feature generates random numbers:
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Probability is selected from the Secondary emission condition list.
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Isotropic sphere, Diffuse scattering, or Constant speed, spherical, is selected from the Initial velocity list.
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The secondary particle positions are offset using the Isotropic sphere offset method (shown for domain-level secondary emission only).
If, in addition, User Defined is selected from the Arguments for random number generation list in the physics interface Advanced Settings section, the Advanced Settings section is available. Enter the Additional input argument to random number generator. The default value is 1.
Released Particle Properties
Select an option from the Released particle properties list. At least one instance of the Particle Properties node is always available, because it is a default feature. If other instances of the Particle Properties feature have been added to the model, then they can be selected from the list. Use this input to specify which type of secondary particle is released when modeling multiple particle species.
Initial Value of Auxiliary Dependent Variables
This section is available if an Auxiliary Dependent Variable has been added to the model. For each of the active Auxiliary Dependent Variable features in the model, enter a value or expression for its initial value. For example, if the auxiliary dependent variable has the default name rp, enter a value or expression for its initial value rp0.
About the Wall Boundary Conditions