Section on fix_cpl_init

For CPL library, the fix we use is fix_cpl_init, which handles setup of the CPL library, using code in the "setup" hook to set up a coupled topology, extracting all required parameters from LAMMPS and sending these via MPI (to coupled code). The required averages are performed in the post_force hook, then information is exchanged between the codes and the constraint forces applied.

The coupling fix, fix_cpl_init, is turned by by adding a command of this form to LAMMPS input:

fix ID group-ID cpl/init region all forcetype X sendtype Y

The first two arguments are standard LAMMPS: ID = user-assigned name for the fix group-ID = ID of the group of atoms to apply the fix to The next part, cpl/init, is the "style", i.e. the fixes' name. "region all" specifies that the fix is applied to the entire box in space. Note that in the granular case, this is clearly consistent (as CFD overlaps the whole domain). For domain decompositional coupling, region should still be all, as we specify the extents of coupling through the file. The remaining words are the args which are currently as follows:

Section on forcetype

The syntax of forcetypes is as follows:

forcetype X args

This allows you to specify which constraint force to use and set a range of args. The options for X include:

1) forcetype test -- A simple test for debugging (sends cell indices)
2) forcetype Flekkoy -- stress based coupling using the form of Flekkoy et al (2000)
3) forcetype Velocity -- applies (U_particle - U_CFD) which is the important part of the Nie, Chen, E and Robbins (2004) constraint.
4) forcetype Drag -- base drag class for drag type forces
5) forcetype Stokes -- inherits from base drag and includes stokes and other granular type forces
6) forcetype Ergun -- an example granular drag class which applies the Ergun drag correlation (untested!!)
7) forcetype Di_Felice -- an example granular drag class which applies the Di Felice drag correlation (untested!!)
8) forcetype Tang -- an example granular drag class which applies the Tang drag correlation (untested!!)
9) forcetype BVK -- an example granular drag class which applies the BVK drag correlation (untested!!)
10+) This has been design so the user can add anything they want here easily. 

For the args, the forcetype 1) test, there are no arguments, it simply applies the received values as a force, similarly for 2) Flekkoy and 3) velocity. Forces 4) to 8) are all granular forces and have the same set of options. These can be set to on or off by the next keyword with true or false (case insensitive, 0 or 1 also work), e.g.

forcetype Drag overlap false 

The args are as follows, available options are shown in square brackets and the default value is denoted by bold:

a) gradP [true/false] -- Apply the gradient of pressure as a force.
b) divStress [true/false] -- Apply the divergence of stress as a force.
c) overlap [true/false] -- Use fraction of sphere in volume to get porosity (instead of add whole sphere based on centre location)
d) interpolate [true/false] -- Use interpolation of CFD fields to centre of particle when calculating forces 
e) preforce_everytime [true/false] -- Work out pre_force fields such as porosity every timestep or only every time information is received from coupled codes
f) Cd [0.0000001] -- Drag coefficient, used for Drag only
g) mu [0.001] -- viscosity used in granular force calculations (Tang, Ergun, Di Felice and BVK)
h) rho [1000] -- density used in granular force calculations (Tang, Ergun, Di Felice and BVK)

Section on sendtypes

sendtype Y -- Specifiy which data is sent to the CFD solver. This Y can be any combination of space-separated inputs (note they are additive)

1) Pick 'n' mix send types
 a) VEL
 b) NBIN 
2) Pre-defined collections
 a) velocity --4 values including 3 velocity sums VEL and number of particles NBIN
 b) gran -- which sends voidratio VOIDRATIO and force FORCE 
 c) granfull - is designed for SediFOAM and sends velocity VEL, force FORCE, sum of drag coefficients FORCECOEFF and VOIDRATIO.

So, for example, you could use gran and append an extra VEL to send VOIDRATIO, FORCE and VEL.

Optional: bndryavg Z -- the location of the bottom cell in domain-decompositional coupling, Z can be either below, above or midplane.