Boundary Conditions
The following options are available for the boundary conditions for particles when they make contact with a wall: Bounce, Freeze, Stick, Disappear, Pass through, Diffuse scattering, Isotropic scattering, Mixed diffuse and specular reflection, and General reflection. Some of the available options are illustrated in the figure below.
Demonstration of the Bounce, Freeze, Diffuse scattering, and Disappear Wall conditions.
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The Freeze, Stick, and Disappear options all stop a particle when it hits a boundary. The Stick option allows the particle to move if the boundary moves, as in the Moving Mesh interface (). The Freeze option freezes the particle in place when it interacts with a boundary, even if the boundary moves. The Disappear option completely removes particles from the simulation when they hit a surface.
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The Bounce option specularly reflects from the wall such that the particle kinetic energy is conserved.
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The Diffuse scattering option bounces particles off a wall according to the cosine law, sometimes called Knudsen’s cosine law or Lambertian reflection depending on the application area. The probability of a particle bouncing off the surface in a given direction within a differential 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 kinetic energy of the particle is conserved.
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The Isotropic scattering option bounces particles off a wall in a random direction. In contrast with the Diffuse scattering option, the particle has an equal probability of being reflected in any direction. The kinetic energy of the particle is conserved.
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The Mixed diffuse and specular reflection condition is a combination of the Bounce and Diffuse scattering conditions, each with a given probability.
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The Pass through option allows particles to cross boundaries unimpeded. By combining this with other Wall conditions, it is possible to filter particles based on their properties or with a given probability.
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The General reflection option allows an arbitrary post-collision velocity to be specified. This can either be done in Cartesian coordinates or in the tangent-normal coordinate system. The velocity components can be functions of the incident particle velocity, energy, or any other quantity.
Conditional Particle-Wall Interactions
The above boundary conditions for particle-wall interaction can all be conditional, either assigned a probability or contingent on a logical expression being satisfied.
A conditional Pass through Wall condition can be used to filter particles based on properties such as particle mass or diameter. Here the interior boundary acts as a screen that blocks large particles (marked mostly in shades of red and orange) but allows smaller particles to pass through unimpeded.
Releasing Secondary Particles
Secondary particles can be introduced into the modeling domain when a primary particle strikes the wall. Any number of secondary particles can be emitted during a single wall collision. The released particles can be assigned initial velocity based on an expression or sampled from a Lambertian or isotropic distribution.
The degrees of freedom for secondary particles are allocated when the study begins, not when the secondary particles are emitted. Therefore, enabling the release of secondary particles increases the number of degrees of freedom in the solution at all time steps.