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### fix mesh/surface command

### fix mesh/surface/planar command

Syntax:

    fix ID group-ID mesh/surface file filename premesh_keywords premesh_values mesh_keywords mesh_values surface_keywords surface_values
    fix ID group-ID mesh/surface/planar file filename premesh_keywords premesh_values mesh_keywords mesh_values surface_keywords surface_values

- ID, is documented in fix command.
- mesh/surface or mesh/surface/planar = style name of this fix command
- file = obligatory keyword
- filename = name of STL or VTK file containing the triangle mesh data
- zero or more premesh_keywords/premesh_value pairs may be appended
- premesh_keyword = type or precision or heal or element_exclusion_list or verbose

        type value = atom type (material type) of the wall imported from the STL file
        precision value = length mesh nodes this far away at maximum will be recognized as identical (length units)
        heal value = auto_remove_duplicates or no
        element_exclusion_list values = mode element_exlusion_file
          mode = read or write
          element_exlusion_file = name of file containing the elements to be excluded
        verbose value = yes or no

- zero or more mesh_keywords/mesh_value pairs may be appended
- mesh_keyword = scale or move or rotate or temperature or mass_temperature

        scale value = factor
          factor = factor to scale the mesh in x-, y-, and z-direction (double value)
        move values = mx my mz
          mx my mz = move the mesh by this extent in x-, y-, and z-direction (length units)
        rotate values = axis ax ay az angle ang
          axis = obligatory keyword
          ax, ay, az = axis vector for rotation (length units)
          angle = obligatory keyword
        temperature value = T0
          T0 = Temperature of the wall (temperature units)
        mass_temperature value = mt0
          mt0 = mass (mass units) assumed for temperature update in the frame of surface sphere/heatable

- zero or more surface_keywords/surface_value pairs may be appended
- surface_keyword = surface_vel or surface_ang_vel or curvature or curvature_tolerant

        surface_vel values = vx vy vz
          vx vy vz = conveyor belt surface velocity  (velocity units)
        surface_ang_vel values = origin ox oy oz axis ax ay az omega om
          origin = mandatory keyword
          ox oy oz = origin of rotation (length units)
          axis = mandatory keyword
          ax ay az = axis vector for rotation (length units)
          omega = mandatory keyword
          om = rotaional velocity around specifyied axis (rad/time units)
        curvature value = c
          c = maximum angle between mesh faces belonging to the same surface (in degree)
        curvature_tolerant value = ct
          ct = yes or no

Examples:

    fix cad all mesh/surface file mesh.stl type 1

Description:

This fix allows the import of triangual surfeace mesh wall geometry for granular simulations from ASCII STL files or legacy ASCII VTK files. Style mesh/surface is a general surface mesh, and mesh/surface/planar represents a planar mesh. mesh/surface/planar requires the mesh to consist of only 1 planar face.

Generall, you can apply scaling, offset and rotation to the imported mesh via keywords scale, move, rotate. Operations are applied in the order as they are specified. The group-ID defines which particles will "see" the mesh, in case it is used as a granular wall.

One fix represents one wall with a specific material, where the material is identified via keyword type. If multiple meshes with different materials are desired, the respective meshes must be imported with different fix mesh/surface commands.

With the temperature keyword, you can define a constant temperature for a mesh in conjunction with heat conduction via fix heat/gran. Note that the actual calculation of the heat transfer happens only if you use the mesh in conjunction with a granular wall, see fix wall/gran.

With the optional surface_vel keyword, you can specify the imported mesh as conveyor belt. The velocity direction for each mesh face is given by the projection of the conveyor belt velocity parallel to the mesh face, the velocity magnitude for each mesh face is equal to the conveyor belt velocity. This ensures the model makes sense also in case the mesh is curved. Likewise, the optional rotation model activated via keyword surface_ang_vel mimics rotational motion of the mesh (e.g. for modeling a shear cell)

Quality checks / error and warning messages:

LIGGGHTS(R)-PUBLIC checks a couple of quality criteria upon loading a mesh. LIGGGHTS(R)-PUBLIC tries to give you as much information about the issue as possible.

Warning messages:

- There should be no angle < 0.5 degrees in any element (soft limit for angles)
- All nodes should be within the simlation box

  If any of the obove rules is not fulfilled, a warning is generated. Keyword verbose controls if details about the warning are written to the screen.

  Error messages:

- The curvature must not be larger than any angle in any mesh element
- There must be no element with an angle < 0.181185 degrees in any element (hard limit for angles)
- The number of neighbor elements must be <= 5 for any element
- Mesh elements must not be duplicate
- Coplanar mesh elements that share an edge must not overlap
- Mesh elements must not leave the simulation domain (based on the center of the element)

  If any of the obove rules is not fulfilled, an error is generated and LIGGGHTS(R)-PUBLIC halts. Error messages are always verbose. If LIGGGHTS(R)-PUBLIC halts due to the last error, you might think about (a) changing the mesh import parameters (scale, move, rotate), (b) changing the mesh dynamics if a fix move/mesh is applied or using boundary m m m

  There are a couple of features that help you read / repair a mesh which can not be read correctly in the first place:

- precision
- curvature
- heal
- element_exclusion_list
- curvature_tolerant

  The precision keyword specifies how far away mesh nodes can be at maximum to be recognized as identical. This is important for building the topology of the mesh and identify elements as neighbors. Normally, this should only be changed if the mesh file you are working with is suffering from precsion / round-off issues.

  The curvature keyword lets you specify up to which angle between two triangles the triangles should be treated as belonging to the same surface (e.g. useful for bends). This angle is used to decide if (a) contact history is copied from one triangle to the other as the contact point proceeds and (b) if edge and corner interaction is calculated.

  If LIGGGHTS(R)-PUBLIC stalls because of duplicate elements, you can try setting heal to auto_remove_duplicates. LIGGGHTS(R)-PUBLIC will then try to heal the geometry by removing duplicate elements.

  With the optional element_exclusion_list keyword is used in mode 'write', LIGGGHTS(R)-PUBLIC will write a list of elements which have more than the allowed 5 face neighbors per surface element to a file, as well as those elements which have an angle below 0.181185 degrees. The 'read' mode can then use this file and will skip the elements in the list. However, you can also manually write such a file to exclude elements you do not want to have included

  IMPORTANT NOTE: The element_exclusion_list write model works in serial only. However, this is not a real restriction, since you can generate the exclusion lists in serial, but then read them to import the mesh into a parallel simulation

  IMPORTANT NOTE: If you use the 'heal' or 'element_exclusion_list' keywords, you should check the changes to the geometry, e.g. by using a dump mesh/stl command.

  The curvature_toleran keyword can simply turn off the check that the curvature must not be larger than any angle in any mesh element. This is typically not recommended, but can be used as a last resort measure.

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

  This fix writes the STL data to binary restart files to be able to correctly resume the simulation in case the mesh is moved. None of the fix_modify options are relevant to this fix. No global scalar or vector 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:

  To date, only ASCII STL and VTK files can be read (binary is not supported). In the current implementation, each processor allocates memory for the whole geometry, which may lead to memory issues for very large geometries . It is not supported to use both the moving mesh and the conveyor belt feature.

  Related commands:

  fix wall/gran

  Default: curvature = 0.256235 degrees, precision = 1e-8, verbose = no, heal = no