For Filter Lifetime simulations, a Collision Model must be chosen for the collision with solid materials, but also for the collision with particles that have been deposited in previous batches (the filter cake). Those particles are deposited in the pore space, therefore the Collison Model defined in the pore space is the model that applies for the filter cake.

In Resolved simulations, every voxel that exceeds the Volume Fraction Threshold is treated as solid material and arriving particles will collide with it using the defined collision model.

In Unresolved simulations, every voxel that exceeds the Max. Particle Packing Density is treated as impassable and arriving particles will collide with it using the defined collision model.

See the theoretical background for more information on the selectable collision models Caught on first touch, Hamaker and Sieving.

If Use Pass Through Model is selected on the Movement tab, a Pass Through Model must be selected for the pore space and all porous materials:

See the theoretical background for more information on the selectable absorption (or pass through) models All particles pass, Const. absorption rate, Clogging, Const. efficiency, and Velocity-dependent efficiency.

The user interface can be understood as follows: The Collision Model on the right applies only after the given Volume Fraction Threshold or Max. Particle Packing Density has been reached. If the material is solid, the Max. Particle Packing Density or Volume Fraction Threshold is fixed to 0, and the Collision Model applies directly.

On the Constituent Materials tab, you may select multiple pore materials for a structure. This allows to use different packing densities models in different zones of the structure.

Particle Density

If Individual per particle type densities are chosen, a new column called Particle Density is added to the table under the Size Distribution tab, and an individual density can be entered for each particle type or size. Otherwise, the Particle Density entered here is used for all particle sizes and types.

Particle Diffusivity

If Individual per particle type is chosen, a new column called Difusivity in Pore is added to the table under the Size Distribution tab, and an individual diffusivity can be entered for each particle type or size.

For Brownian Motion, the particle diffusivity is computed for each particle type according to equation (245) .

Particle Collision Diameter

The particle Diameter, as given in the table under the Size Distribution tab, is used when tracking the particle in the flow field. It determines the particle radius used in equations (242) , (243) and (244) to determine the mass and the friction.

The Particle Collision Diameter is used to check if the particle collides with the filter structure and should usually be set to Same as particle diameter. With this selection, the particle is consistently modeled as a sphere.

However, real dust particles may exhibit different, possibly less compact, shapes. Thus, they may have a higher probability of colliding with the structure than a spherical particles of the same mass. By setting the Particle Collision Diameter to a different value than the Diameter, you can model this effect. It is possible to set a fixed Diameter factor for all particles sizes by choosing Multiple of particle diameter or to select Individual per particle type and enter the collision diameter in the Size Distribution tab for all particle types.

Particle Sliding

Particles that collide with the surface loose some of their energy if the restitution factor is smaller than 1. In certain pore geometries it may happen that the flow moves a particle very close along a pore surface, causing many consecutive hits and thus the repeated application of the restitution factor will cause the particle to stop moving at all. This behavior is often undesired, and it may overestimate the filter efficiency when combining a Sieving collision model with a relatively low restitution value.

When Sieving is selected in the drop-down menu, all Sieving particle-wall collisions are modified, when Sieving and Hamaker is selected, all Sieving particle-wall collisions are modified and all Hamaker particle-wall collisons are modified. Caught on First Touch particle-wall collisions are not modified.

Enabling Particle Sliding for the Sieving or Hamaker collision models enlarges the tangential restitution factor when a particle hits a surface in a location that lies close to the last surface collision:

The first bounce on the wall is always treated the same, independent whether particle sliding is active or not: A particle arrives with a certain velocity, loses energy on impact, and bounces back again. The strength of the bounce-back is controlled with the given Restitution. When particle sliding is active, further impacts near the first hit are treated differently: Only the momentum perpendicular to the wall is reduced. The velocity along the wall is not reduced further, so in this direction the restitution is equal to 1.This means that particle movement along the wall is no longer slowed down. The normal restitution remains unchanged. In effect, the particle will slide along the surface.

In setups where particles are sieved by filter materials having pores of well-defined sizes (e.g. in meshes, nets or weaves), selecting particle sliding has a great influence on the computed filter efficiency and pressure drop and it is recommended to select this option.

In setups where particles are mainly caught by adhesion or where the filter material consists of irregular pores of many different sizes (e.g. nonwovens), the choice of the particle sliding model has little to no effect on the simulation results.