fix sedimentation command

Warning

GPU support for this command has not been tested and may not work as expected.

Syntax

fix ID group-ID sedimentation velocity_threshold vthresh time_threshold tthresh mesh mesh-ID keyword value ...

  • ID, group-ID are documented in fix command

  • sedimentation = style name of this fix command

  • velocity_threshold = obligatory keyword

  • vthresh = velocity threshold which particles must fall below to be eligible for deletion (velocity units)

  • time_threshold = obligatory keyword

  • tthresh = minimum time below velocity threshold for particles to be eligible for deletion (time units)

  • mesh = obligatory keyword

  • mesh-ID = ID for the fix mesh that the fix move/mesh is applied to

  • keyword = contact_deletion_method, probability_saltation, probability_rebound or k_epsilon_model

contact_deletion_method value = remove or deactivate
  remove = completely remove particles from the simulation when criteria are fulfilled
  deactivate = exclude particle from integration and neighbor list build when criteria are fulfilled
probability_saltation value = prob_saltation
  prob_saltation = probability of saltation occurring
probability_rebound value = prob_rebound
  prob_saltation = probability of rebound occurring
k_epsilon_model value = {constant, one_way, from_cfd}
  constant k kvalue epsilon epsvalue
    kvalue = turbulent kinetic energy
    epsvalue = turbulent dissipation rate
  one_way k_file kfile eps_file epsfile binsize bs
    kfile = file to read the turbulent kinetic energy from
    epsfile = file to read the turbulent dissipation from
    bs = binsize
  from_cfd kscale kscvalue
    kscvalue = scaling factor for turbulent kinetic energy
seed value = seed_value
    seed_value = seed for random number generator

Examples

fix sed all sedimentation velocity_threshold 1e-3 time_threshold 1e-2 probability_saltation 0.2 probability_rebound 0.5 k_epsilon_model from_cfd kscale 10 seed 1 mesh walls contact_deletion_method deactivate
fix sed all sedimentation velocity_threshold 1e-2 time_threshold 1e-3 k_epsilon_model one_way k_file data/k.dat eps_file data/eps.dat binsize 0.1 seed 7 mesh sedWalls
fix sed all sedimentation velocity_threshold 0.7 time_threshold 1e-3 k_epsilon_model constant k 1e-2 epsilon 100 seed 7 mesh sedWalls

Description

This fix evaluates particles which are: - in contact with the wall and - the relative velocity with the wall is below velocity_threshold for at least time_threshold

If the above criteria are fulfilled, particles are either flagged for deletion from the simulation (if contact_deletion_method = remove) or are deactivated (if contact_deletion_method = deactivate)

Particles will rebound with the probability probability_rebound, if the particles are below the velocity_threshold they are will saltate with the probability probability_saltation. If one of these effects is in place, the particles will experience a perturbation velocity away from the wall in the magnitude and duration of

Uturb = sqrt(2./3.*k*kscale)
tturb = 0.15*k*kscale/(epsilon);

To use the k_epsilon_model one_way variant, turbulent kinetic energy and dissipation must be provided as files in the same fashion as for dragforce/field or enable_one_way_coupling, i.e. on regular grid with a header line in the file:

x y z               k_field 1
-0.50 -0.50 0.50    100.
-0.50  0.50 0.50    100.
 0.50 -0.50 0.50      0.
 0.50  0.50 0.50      0.

The field names in the data files must be k_field for k and eps_field for epsilon.

Note

In mode k_epsilon_model one_way, the mesh must be identical for both fields, k and epsilon.

The removal itself is performed with the next neighbor list build. In case of deactivation, the user has to define two groups with names ‘is_active’ and ‘is_inactive’ which are then used by this command. Group ‘is_active’ has to be used for the integration commands. Error checks are in place to ensure there are no user error with this.

The following per-mesh element properties are registered by the fix:

mass_deleted_per_area (mass per area)
n_deleted_per_area (1 per area)
mass_deleted_per_area_step (mass per area)
n_deleted_per_area_step (1 per area)

The two former properties refer to all particles deleted from the simulation, and the two latter to particles flagged for deletion in the current time-step. Please note that the counters are increased when particles are flagged for deletion, not when actually deleted, so minimal time-lags may be possible (if contact_deletion_method = remove) .

All of these 4 properties can be can be dumped into VTK format using output_settings.

Restart, fix_modify, output, run start/stop

This fix writes the total deleted particle mass and the total number of deleted particles to binary restart files. No fix_modify option applies to this fix. This fix computes no output, which can be accessed by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command.

This vector holds a two component vector, making the total mass and the total number of deactivated/removed particles available.

Restrictions

  • must be used with CFDEMcoupling

  • if from_cfd turbulence is active, the transferTurbulence forceModel must be active

Default

contact_deletion_method = remove, probability_saltation = 0, probability_rebound = 0, k_epsilon_model value = constant, k = 0, epsilon = 0, seed = 1