fix pour command

Syntax:

fix ID group-ID pour N type seed keyword values ... 

• ID, group-ID are documented in fix command
• pour = style name of this fix command
• N = # of atoms to insert
• type = atom type to assign to inserted atoms
• seed = random # seed (positive integer)
• one or more keyword/value pairs may be appended to args
• keyword = region or diam or dens or vol or rate or vel

    region value = region-ID
      region-ID = ID of region to use as insertion volume
    diam values = dstyle args
      dstyle = one or range or poly
        one args = D
          D = single diameter for inserted particles (distance units)
        range args = Dlo Dhi
          Dlo,Dhi = range of diameters for inserted particles (distance units)
        poly args = Npoly D1 P1 D2 P2 ...
          Npoly = # of (D,P) pairs
          D1,D2,... = diameter for subset of inserted particles (distance units)
          P1,P2,... = percentage of inserted particles with this diameter (0-1)
    vol values = fraction Nattempt
      fraction = desired volume fraction for filling insertion volume
      Nattempt = max # of insertion attempts per atom
    rate value = V
      V = z velocity (3d) or y velocity (2d) at which
          insertion volume moves (velocity units)
    vel values (3d) = vxlo vxhi vylo vyhi vz
    vel values (2d) = vxlo vxhi vy
      vxlo,vxhi = range of x velocities for inserted particles (velocity units)
      vylo,vyhi = range of y velocities for inserted particles (velocity units)
      vz = z velocity (3d) assigned to inserted particles (velocity units)
      vy = y velocity (2d) assigned to inserted particles (velocity units) 
  

Examples:

fix 3 all pour 1000 2 29494 region myblock
fix 2 all pour 10000 1 19985583 region disk vol 0.33 100 rate 1.0 diam range 0.9 1.1 
fix 2 all pour 10000 1 19985583 region disk diam poly 2 0.7 0.4 1.5 0.6 

Description:

Insert particles into a granular run every few timesteps within a specified region until N particles have been inserted. This is useful for simulating the pouring of particles into a container under the influence of gravity.

Inserted particles are assigned the specified atom type and are assigned to two groups: the default group "all" and the group specified in the fix pour command (which can also be "all").

This command must use the region keyword to define an insertion volume. The specified region must have been previously defined with a region command. It must be of type block or a z-axis cylinder and must be defined with side = in. The cylinder style of region can only be used with 3d simulations.

Each timestep particles are inserted, they are placed randomly inside the insertion volume so as to mimic a stream of poured particles. The larger the volume, the more particles that can be inserted at any one timestep. Particles are inserted again after enough time has elapsed that the previously inserted particles fall out of the insertion volume under the influence of gravity. Insertions continue every so many timesteps until the desired # of particles has been inserted.

All other keywords are optional with defaults as shown below.

The diam option specifes the diameters of inserted particles. There are 3 styles: one, range, or poly. For one, all particles will have diameter D. For range, the diameter of each particle will be chosen randomly and uniformly between the specified Dlo and Dhi bounds. For poly, a series of Npoly diameters is specified. For each diameter a percentage value from 0.0 to 1.0 is also specified. The Npoly percentages must sum to 1.0. For the example shown above with "diam 2 0.7 0.4 1.5 0.6", all inserted particles will have a diameter of 0.7 or 1.5. 40% of the particles will be small; 60% will be large.

The dens and vel options enable inserted particles to have a range of densities or xy velocities. The specific values for a particular inserted particle will be chosen randomly and uniformly between the specified bounds. The vz or vy value for option vel assigns a z-velocity (3d) or y-velocity (2d) to each inserted particle.

The vol option specifies what volume fraction of the insertion volume will be filled with particles. For particles with a size specified by the diam range keyword, they are assumed to all be of maximum diamter Dhi for purposes of computing their contribution to the volume fraction.

The higher the volume fraction value, the more particles are inserted each timestep. Since inserted particles cannot overlap, the maximum volume fraction should be no higher than about 0.6. Each timestep particles are inserted, LAMMPS will make up to a total of M tries to insert the new particles without overlaps, where M = # of inserted particles * Nattempt. If LAMMPS is unsuccessful at completing all insertions, it prints a warning.

The rate option moves the insertion volume in the z direction (3d) or y direction (2d). This enables pouring particles from a successively higher height over time.

Restart, fix_modify, output, run start/stop, minimize info:

No information about this fix is written to binary restart files. This means you must be careful when restarting a pouring simulation, when the restart file was written in the middle of the pouring operation. Specifically, you should use a new fix pour command in the input script for the restarted simulation that continues the operation. You will need to adjust the arguments of the original fix pour command to do this.

Also note that because the state of the random number generator is not saved in restart files, you cannot do "exact" restarts with this fix, where the simulation continues on the same as if no restart had taken place. However, in a statistical sense, a restarted simulation should produce the same behavior if you adjust the fix pour parameters appropriately.

None of the fix_modify options are relevant to this fix. No global or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

Restrictions:

This fix is part of the GRANULAR package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

For 3d simulations, a gravity fix in the -z direction must be defined for use in conjunction with this fix. For 2d simulations, gravity must be defined in the -y direction.

The specified insertion region cannot be a "dynamic" region, as defined by the region command.

Related commands:

fix_deposit, fix_gravity, region

Default:

The option defaults are diam = one 1.0, dens = 1.0 1.0, vol = 0.25 50, rate = 0.0, vel = 0.0 0.0 0.0 0.0 0.0.