# fix massflow/mesh/sieve command

## Syntax

    fix id group massflow/mesh/sieve mesh mesh-ID point_at_outlet pointX pointY pointZ keyword value ...

- ID, group-ID are documented in fix command
- massflow/mesh/sieve = style name of this fix command
- mesh = obligatory keyword
- mesh-ID = ID of a fix mesh/surface command
- point_at_outlet = obligatory keyword
- pointX pointY pointZ = vector components defining the “outside” point of the mesh
- zero or more keyword/value pairs may be appended to args
- keywords = same as for fix massflow/mesh, in addition: sieveSize and sieveSpacing and sieveStiffness, and sieveDampings, or sieveMultiSphereCanPass

    sieveSize value = x in meters
      this is the diameter of the sieve opening. Used to compute the probability of sieve passage.
    sieveSpacing value = x in meters
      this is the spacing between sieve openings. Used to compute the probability of sieve passage.
    sieveStiffness value = x in Newton per meter
      value to compute the linear spring part of the normal repulsive force. WARNING: use a reasonable value to avoid popcorn!
    sieveDamping value = x in Newton per meter/s
      value to compute the dashpot part of the normal repulsive force. ARNING: use a reasonable value to avoid popcorn!
    sieveMultiSphereCanPass
      add this keyword to also calculate the probability of sieve passage for multisphere particles. If not set, it is impossible for multisphere particles to pass the sieve.

Example:

    fix     massFlow1 all massflow/mesh/sieve mesh massFlow1M point_at_outlet 0 0 -1000 count once delete_atoms no file massFlow1.dat sieveSize 2e-3 sieveSpacing 2.5e-3 sieveStiffness 1e5 sieveDamping 1e3

## Description

This fix is an extension of the fix “massflow/mesh”, so the reader may wish to consult also the documentation of this fix for further details.

Fix massflow/mesh/sieve models a sieve, and counts the particles that cross the sieve (same as in massflow/mesh). The sieve is modeled with circular holes in a the mesh (the holes have a certain spacing), and a passage probability is computed based on the particle diameter. Thus, a probability is computed based on the relative area that would result into particle passage divided by the total cross-sectional area of the sieve, assuming particles move perfectly normal to the mesh. This probability is evaluated at the first time step of every collision event, and compared with a uniformly-distributed random number. Thus, sliding contacts, or multiple collision may not by modelled accurately.

## Restrictions

Currently, this feature does not allow deleting multi-sphere particles when crossing the sieve. Also, it is strongly recommended to NOT activate sieveMultiSphereCanPass, since this feature is not tested in high detail. For example, it may occur that multisphere particles may get stuck when passing the sieve.

## Related commands

compute nparticles/tracer/region fix massflow/mesh

## Default

sieveDamping = 0; sieveMultiSphereCanPass = false; all other parameters need to be set by the user.