region command¶

fix ave/euler/region/universe command¶

Syntax¶

fix ID group-ID ave/euler/region nevery N region reg-ID keywords values
fix ID group-ID ave/euler/region/universe nevery N region reg-ID keywords values ukeywords uvalues

ID, group-ID are documented in fix command
ave/euler/region = style name of this fix command
nevery = obligatory keyword
N = calculate average values every this many timesteps (also sending interval in universe version)
region = obligatory keyword
reg-ID = ID of region with style mesh/hex defining the grid
zero or more keyword/value pairs may be appended

keyword = basevolume_region

basevolume_region value = region-ID
  region-ID = correct grid cell volume based on this region

one or more ukeyword/uvalue pairs must be appended for the universe version of this command

ukeywords = send_to_partition (obligatory) or sync

send_to_partition value = partition
  partition = partition to send data to in multi-partition simulations
sync value = mode
  mode = yes to use MPI_Ssend, no to use MPI_Bsend for communication between partitions

Examples¶

fix 1 all ave/euler/region nevery 10 region ave_reg
fix 2 all ave/euler/region/universe nevery 10 region ave_reg send_to_partition 2 sync yes

Description¶

Calculate cell-based averages of velocity, radius, volume fraction, and pressure (-1/3 * trace of the stress tensor) every few timesteps, as specified by the nevery keyword. The cells are taken from the specified region.

Note that velocity is favre (mass) averaged, whereas radius is arithmetically averaged. To calculate the stress, this command internally uses a compute stress/atom . It includes the convective term correctly for granular particles with non-zero average velocity (which is not included in compute stress/atom). However, it does not include bond, angle, diahedral or kspace contributions so that the stress tensor finally reads

where vave is the (cell-based) average velocity. The first term is a kinetic energy contribution for atom I. The second term is a pairwise energy contribution where n loops over the Np neighbors of atom I, r1 and r2 are the positions of the 2 atoms in the pairwise interaction, and F1 and F2 are the forces on the 2 atoms resulting from the pairwise interaction.

The basevolume_region option allows to specify a region that represents the volume which can theoretically be filled with particles. This will then be used to correct the basis of the averaging volume for each cell in the grid. For example, if you use a cylindrical wall, it makes sense to use an identical cylindrical region for the basevolume_region option, and the command will correctly calculate the volume fraction in the near-wall cells. the calculation of overlap between grid cells and the region is done using a Monte-Carlo approach.

If LIGGGHTS is invoked in multi-partition mode (cf. Command-line options), where each partition represents a separate coarse-graining level of the multi-level coarse-grain model, the universe version of this command needs to be used to provide input data for a fix of style forcecontrol/region/universe on the coupled partition. The coupled partition must be specified via the send_to_partition option. Furthermore, in this mode the nevery option also specifies the sending interval.

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

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix.

This fix computes the above-mentioned quantities for output via a dump euler/vtk command. The values can only be accessed on timesteps that are multiples of nevery since that is when calculations are performed.

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¶

Volume fractions and stresses are calculated based on the specified grid, so volume fractions and stresses near walls that are not aligned with the grid will be incorrect.