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Coupling Code CoMA

CoMA (Coupling for Multiphysics Analysis) is a general code coupling tool, used in the simulation of surface coupled problems. It is developed since some years at the institute. The tasks of CoMA within a coupled analysis are:

  • Control of the coupled computation and the single-field solvers
  • Convergence control
  • Coupling algorithms for implicit computations
  • Data transfer between distributed interface meshes
  • Communication between parallel single-field solvers and CoMA

This wiki should just give a short overview on how to use CoMA, being specific on FSI-simulation with Carat and OpenFOAM. Detailed information on programming concepts and theoretical aspects can be found in the dissertation of Thomas Gallinger.

Installation and Compilation

CoMA has to installed under Linux operating system, because FSI-jobs only run under Linux. There exists an svn-repository, which contains all CoMA sources. The svn-repository is hosted on the same server as the carat repository. The current IP is and the folder is named "CoMA". Checkout the svn-repository. Open the file "" in the src-directory. You have to adapt the variable "CoMA_DIR", which points to your CoMA installation path, and the variable "MPIHOME", pointing to your MPI installation path. Execute the shellscript "make_obj_directories" to set the directorires for the objety, and then type "make" to build CoMA software.

Problem Setup

The structure of the input file reflects the underlying programming concepts of CoMA. An example input file can be found in the svn-repository under the directory "example". The input file is block-structured. At least five different blocks have to specified:

  • General
  • Two Processgroups
  • ExchangedQuantity
  • Mapping
  • Output

For an implicit FSI-simulation, three additional blocks have to be specified:

  • ConvergenceCalculation
  • Predictor
  • CouplingAlgo

For a restart run, one block is necessary:

  • Restart

Input File: Compulsory Block Description

Block General: General control of application flow
Type fsi or ffi or wind defines MPI Ident tag of CoMA within MPI Environment, used by other codes to identify CoMA process
Method implicit or explicit implicit or explicit coupling, implicit with, explicit without inner iteration loops

Block Processgroup: Defines a group of processes, which belong to one field
ID 0 or 1 used to order processgroups and as reference for exchanged quantities
Name string for output only
InterfaceType mpi or file type of communication between CoMA and processgroup, Warning: type file only partly implemented
MPIIdentTag 1 or 2 used to identify processes of processgroup in MPI-universe

Block ExchangedQuantity: Defines a quantitiy to be exchanged between processgroups
ID 0 or 1 or 2 ... used to order quantities (0 exchanged before 1 before 2...)
Name string for output only
Dimension int dofs per node, e.g. displacements 3
InterpolationType 1 or 2 1 = flux mapping (summation e.g. for forces), 2 = field mapping (interpolation e.g. for displacements)
SenderID int ID of processgroups, which sends this quantitiy
RecieverID int ID of processgroups, which recieves this quantitiy
MpiTagBasis int used for MPI-data exchange of this quantitiy
DoFType node dummy paramter, quantitiy specified per node
Predictor - optional parameter yes or no use a predictor for this quantitiy in the first inner iteration and do not recieve from sender, block "Predictor" is necessary
CouplingAlgo - optional parameter yes or no use a coupling algorithm for this quantitiy, block "CouplingAlgo" is necessary
Convergence - optional parameter yes or no use a convergence criterion for this quantitiy, block "ConvergenceCalculation" is necessary

Block Mapping: Defines mapping method of exchanged quantities between surface meshes.
Type TNM, MM or NLM TNM: Triangular Nonmatching Mesh Mapper, MM: Matching Meshes, NLM: NURBS-Linear Mapper

Block Output: General output of CoMA for postprocessing and simulation control
Path string absolute or relative path to directory for output files, created if necessary
Name string prefix for output file names
Format gid, vtk or rhino write ouput in Gid-,Vtk(Paraview), or Rhino-format, Warning: Vtk writes out the mesh in every step, so huge data amount possible
Frequency int ouput ever int step
Energy yes or no additional output of energy
Convergence yes or no write additional file Path/ with iteration information
QuantityID int which quantity to examine in .cc-file
Dof yes or no write dof-history in .cc-file, yes -> define node by following four entries:
ProcessgroupIndex int processgroup of dof
MeshTagIndex int meshTag of dof
NodeIndex int node index of dof
DofIndex int dof index of dof

Input File: Optional Block Description

Block Predictor: Defines a predictor for an exchanged Quantitiy used in the first inner itertation of a time step.
Type polynomial or newmark or adaptive Predictor type, for details see [1]
Order - for Type polynomial and newmark int polynomial: 0, 1, 2, 3; newmark: 1, 2;
Beta - for Type newmark float time integration parameter of Newmark scheme, standard: 0.25
Gamma - for Type newmark float time integration parameter of Newmark scheme, standard: 0.50

Block CouplingAlgo: Defines a coupling algorithm for an exchanged Quantitiy used in every inner itertation except the first of a time step.
Type constant or aitken or quasinewton Coupling algorithm type, for details see [1]
Factor - for Type constant, aitken and quasinewton float underrelaxation factor used always in constant relaxation, in the first iteration with Aitken and Quasi-Newton method
ConstRelax - for Type quasinewton yes or no use constant relaxation in first coupling iteration
NumHistoryDelta - for Type quasinewton int include delta information from preceeding timesteps into minimization problem
MinResChange - for Type quasinewton float dummy factor - not used

Block ConvergenceCalculation: Defines convergence properties for coupled computation.
By CoMA or Processgroup Which code performs convergence calculation
ProcessgroupID - for By processgroup int which group
Type - for By CoMA absolute or relative which convergence criterion should CoMA use, a absolute or relatve one
Limit - for By CoMA float convergence limit, if below=converged
CouplingSteps - for By CoMA int number of coupling steps (time steps) to be performed in total
MaxInnerLoopSteps - for By CoMA int number of maximum inner iteration loops

Block Restart: Used for computation with restart possibility
RestartRun yes or no Is this run a restart run
RestartFromStep - for RestartRun yes int from which step should the restart be performed
RestartOutput yes or no shoul restart-information be written ot output
RestartFrequency - for RestartOutput yes int write restart info every int step
RestartPath - for RestartOutput yes string absolute or relative path to directory for output, created if necessary
RestartFileName - for RestartOutput yes string prefix of restart info file names
RestartInfoInstances - for RestartOutput yes int instances of restart information in output files to be kept
PersistPredictorOrder - for RestartOutput yes yes or no write out all info necessary to preserve predictor order in restart

Restart Functionality

The Fsi environemt offers the possibility to do a restart from a certain timestep. This may be helpful in 2 cases:

  • If computing time restrictions, e.g. on clusters, exist, the simulation has to finish before cpu time ends and do a restart from the last timestep.
  • If convergence problems during a long run show up, the simulation may be stopped and a restart form an earlier timestep with smaller time step size or anothe coupling algorithm may be performed.

For a restart, all three programs have to be touched. The procedure consists of two steps:

  • During the run, all three prgrams have to write additional output necessary for a restart in certain intervalls.
  • For a restart, the programs have to read this information to initialize everything properly.

Hint 1: During the run, define all three programs to have the same frequency of restart info output.

Hint 2: For a restart run, just adapt the starttime in Carat++ and OpenFOAM, adapt the restartFromStep command in CoMA and set "restartRun yes" in all three programs.

The three different programs are explained in detail:

Restart in Carat++

Restart is possible for dynamic analysis only (linear and nonlinear) and consists of different additional instructions in th dynamic analysis block. They are explained in the corresponding wiki entry.

Restart in OpenFOAM

OpenFOAM needs an additonal field "totalDisps" in the starttime directory. It is the same as the already existing pointMotionU file and used to store the deformation history of the coupled patch. Restart info is written to this file automatically together with all other fields. For a restart-run, set the parameter "restartRun yes;" in the file couplingProperties and adapt the startTime.

Restart in CoMA

Same as in Carat++. Set "RestartOutput yes" and define frequency, path, filename and instance level. During restart run, set "RestartRun yes" and specify the step.


  1. 1.0 1.1 Gallinger, T.G.: Effiziente Algorithmen zur partitionierten Lösung stark gekoppelter Probleme der Fluid-Struktur-Wechselwirkung, Dissertation, Lehrstuhl für Statik, Technische Universität München, 2010

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