Particle Inital Position

Particle Positioning Weights

The particles are placed randomly in the inflow area defined in the Filter Experiment tab. The particles are placed with a probability based on the local fluid velocity, which will lead to a uniform particle concentration in the inflow.

Random Seed

The parameter Random Seed controls the underlying random number generator. The same random seed produces identical results, whereas results with different random seeds are similar but not identical.

Particle Multiplicity

When the Particle Resolution is set to Unresolved in the Constituent Materials tab, the parameter Particle Multiplicity becomes available and a value can be entered in the Max. Number of Trajectories per Type box.

Unresolved particles may be much smaller than the used voxel length and thousands of dust particles are needed to fill a single voxel. Therefore, the number of Particles per Batch (on the Solver tab) may grow very large, leading to increased simulation times. With Particle Multiplicity, the number of particles that are tracked can be restricted.

When, for example, 1,000,000 particles of diameter 0.05 µm are to be simulated in the next batch, and the user checks Particle Multiplicity and sets Max. Number of Trajectories per Type to 10,000, only the movement of 10,000 particles is tracked. However, each of these particles represents 100 particles, and the multiplicity is 100 (100 x 10,000 = 1,000,000). If one of these particles is filtered, the mass and volume fractions of 100 particles are added to the deposited dust.

To make sure that the choice of Particle Multiplicity does not introduce large numerical errors, FilterDict ensures that the particle volume multiplied by its multiplicity is less than 5% of the single voxel volume.

Particle Injection

All particles simulated in a time inteval (a batch) move independently from each other. Select, if all particles should start moving at the beginning of the time interval (Batch Start) or if starting times should be distributed uniformly over the batch interval (Continuous).

While Continuous offers a physically correct description of what happens in a real filter experiment, it may make a later visualization of the particle movement almost impossible: In a typical air filter media simulation, a batch interval may last minutes, while particles move with a velocity of several meters per second through a filter material of less than a millimeter thickness, which means that at most observation points in time, no moving particles can be seen. This can be overcome by letting all particles start at Batch Start, or distributed over a short time interval at the beginning of the batch (Batch Continuous).

The selection made here has very little influence of the computed results, because in any case all particles of the same batch do not interact with each other: they cannot collide in flight, and they cannot deposit on top of each other. They can only interact with the particles deposited in previous batches.