OpenFOAM

This is the documentation for all OpenFOAM solver wrappers. Currently, the use as the flow solver in FSI simulations is supported, no other multiphysics problems.

Parameters

This section describes the parameters in the JSON file, listed in alphabetical order.

parameter	type	description
`application`	str	Name of the (adapted) OpenFOAM-solver to be used for the flow problem. This name should start with `coconut_`. For OpenFOAM 11, only `coconut_foamRun` is possible.
`boundary_names`	list	List of names of the patches corresponding to the interface. These names should match the patch names defined in the OpenFOAM-case.
`compile_clean`	bool	(optional) Default: `false`. If set to true, the adapted application will first clean and then compile.
`debug`	bool	(optional) Default: `false`. For every iteration, additional files are saved containing information on the input and output data of the solver.
`delta_t`	double	Fixed timestep size in flow solver.
`density`	double	(optional) Density of the fluid in an incompressible case. The density is multiplied with the kinematic pressure and traction. Required if an incompressible application is used, such as coconut_pimpleFoam. More information can be found here.
`interface_input`	dict	List of dictionaries that describes the input `Interface`. This provides the interface boundary conditions for the OpenFOAM solver. Each entry in the list has two keys: `model_part` and `variables`, with values as name of the model part and list of input variables, respectively. The input variables in the list should be chosen from the `variables_dimensions` `dict` in the file `coconut/data_structure/variables.py`. The model part name must be the concatenation of an entry from `boundary_names` and the string `_input`.
`interface_output`	dict	Analogous to `interface_input`, but here the name must be the concatenation of an entry from `boundary_names` and the string `_output`. The entries in the list provides boundary conditions for the other solver(s) participating in the coupled simulation.
`parallel`	bool	Set it to `true` if OpenFOAM solver is required to run in parallel. The required decomposition method and number of cores should be provided in the `<case_directory>/system/decomposeParDict` file.
`print_coupling_convergence`	bool	(optional) Default `false`. If `true` and if the solver coupling convergence is checked, a statement is printed when the solver converges in the first solver iteration, see solver coupling convergence.
`residual_variables`	list	(optional) A list containing OpenFOAM variables whose residuals you need to output. If provided, this will output the last initial residual of the pimple iterations for each FSI-coupling iteration in `<case_directory>/residuals.csv`.
`time_precision`	int	Number of digits after the decimal sign to be used in the name of the time step directories which are made during execution.
`timestep_start`	int	Time step to (re)start a transient FSI calculation from. If 0 is given, the simulation starts from t = 0, else the code looks for the relevant case and data files.
`working_directory`	str	Directory where the OpenFOAM-case is defined (and which contains the JSON-file).

timestep_start and delta_t are necessary parameters, but are usually defined already in the parameters of the coupled solver. However, they can also be given directly as parameter of the solver wrapper (e.g. for standalone testing). If they are defined both in the coupled solver and in the solver wrapper, then the former value is used, and a warning is printed.

Setting up a new case

Following items should be present in the OpenFOAM-directory prior to launching CoCoNuT:

The entire framework of the CFD-case in OpenFOAM which is to be used in the CoCoNuT simulation (so it should contain the constant and system directory as well as the 0 directory). The working directory should be defined as if you would like to run it as a CFD case. This working directory is defined in the JSON-file.
The necessary parameters in the JSON-file containing the settings stipulated above.

Following files are modified or used as a reference to create files by CoCoNuT, and must be included in the original OpenFOAM-directory:

system/controlDict with compulsory arguments:

key	value
`writeControl`	`timeStep`
`writeInterval`	required write interval
`writeFormat`	`ascii`
`timeFormat`	`fixed`
`timePrecision`	required time precision based on `delta_t` (used in the names of time step folders)
`runTimeModifiable`	`false`
`adjustTimeStep`	`no`

constant/dynamicMeshDict which contains the settings for OpenFOAM's dynamic motion solver
system/decomposeParDict with the necessary decomposition of the fluid domain (if cores>1)
0/pointDisplacement with all the boundary conditions, including fixedValue boundary condition for the FSI boundaries. This is used as a template for the pointDisplacementTmp to supply displacement boundary condition ( from structural solver) for the FSI-interface.

Comments

It is probably best to derive a new case from the directory containing an FSI simulation with OpenFOAM in coconut/examples/ in order to copy its structure.
For OpenFOAM 8, the applications like pimpleFoam need to be adapted to accommodate the communication with the solver wrapper during the FSI-simulation. These adapted versions have the same name as the original OpenFOAM-solver but with the prefix coconut_. If you do not use an OpenFOAM-application which is already converted for operation in CoCoNuT, you will have to convert the application yourself, see also this brief set of instructions.
For OpenFOAM 11, only foamRun has been adapted but this solver can work with many solver modules (see OpenFOAM 11 documentation).

Treatment of kinematic pressure and traction

If the OpenFOAM application is intended for incompressible flows, e.g. pimpleFoam (v8) or incompressibleFluid (v11), it solves the incompressible Navier-Stokes equations. As a consequence the pressure and traction (wallShearStress) values are kinematic and have the unit m²/s². In order to couple these solvers to a structural solver, the pressure and traction are required to be expressed in Pa (kg/ms²).

For these solvers, the density parameter is required in the JSON file and will be used to calculate the actual values from the kinematic ones. In case the OpenFOAM application is compressible, this correction is not required and the actual values in Pa are obtained directly.

There is a third possibility. In some cases, the application solves the compressible equations obtaining an actual pressure, but the used momentumTransportModel is incompressible. It is the type of momentumTransportModel that determines whether the wallShearStress functionObject returns the kinematic or actual traction. In these cases, the density is not fixed and cannot simply be multiplied with the obtained kinematic values. Therefore, a new functionObject, rhoWallShearStress, is available, which is a modified version of wallShearStress and returns the actual traction, even if the momentumTransportModel is incompressible. Note that is this new functionObject is version specific, but common for all applications of one version. Upon compilation of the coconut application this functionObject is included in the binary. This was the case for, for example, coconut_interFoam and coconut_cavitatingFoam (OpenFOAM 8).

The behaviour of CoCoNuT for these different applications is implemented in the solver wrapper itself, namely in the kinematic_conversion_dict, located in the files vX.py, with X the identifier of the OpenFOAM-version (e.g. v10.py for OpenFOAM 10). This means that when a new application is added, the behaviour for this new application should be specified in that location.

Solver coupling convergence

The convergence criterion solver coupling convergence has been implemented for the OpenFOAM solver wrapper. Instead of comparing the different residuals (U, p, ...) with their respective tolerances before the first iteration, they are checked after the first iteration due to implementation restrictions. Note that OpenFOAM will always execute a final solver iteration after convergence. Depending on the settings, this final iteration may have different or no relaxation factors. Calling the solver twice with the same interface displacement, will not necessarily mean that the second call immediately converges. This is due to a (weak) nonlinearity in the calculation of the mesh deformation as a result of the explicit non-orthogonality correction in the normal gradient calculation used in the Laplacian equation. Attempts to use the option moveMeshOuterCorrectors (recalculating mesh motion in every solver iteration) showed that there exist problems with this feature: it disturbs the convergence checks and, when used in parallel, even causes the code to crash.

Version specific documentation

v10 (OpenFOAM 10)

Base version. In this OpenFOAM version some changes were made with respect to OpenFOAM 8, and there are some changes in the case setup. The following list provides some but is not all extensive:

The file constant/transportProperties has been renamed to constant/physicalProperties.
The keyword version can be omitted from the header dictionary FoamFile.
The dynamicMeshDict has been restructured, defining a fvMeshMover and removing the dynamicFvMesh class, see also the OpenFOAM documentation.

v11 (OpenFOAM 11)

In this OpenFOAM version some changes were made with respect to OpenFOAM 10. The most prominent is the replacement of separate applications like pimpleFoam by a single solver foamRun which works with solver modules. The required changes for the OpenFOAM wrapper are visible in the version specific solver wrapper Python file v11.py.

This is also reflected in the case setup:

The application in system/controlDict is now foamRun and an additional keyword solver is required, such as incompressibleFluid.

Details on the OpenFOAM solver wrapper

Communication with OpenFOAM

As with most other solver wrappers, the communication between the OpenFOAM code and the solver wrapper occurs through the use of (empty) files with extension .coco. The OpenFOAM code checks for these message files in an infinite loop. When such file is detected, for example continue.coco, the OpenFOAM code performs a certain action and creates a file continue_ready.coco. The solver wrapper is paused until it detects such a corresponding file.

Clean-up after unexpected stop

The aforementioned messaging procedure implies that OpenFOAM is constantly running during execution of the CoCoNuT-simulation. Exiting of the loop and closing of the program only occurs when the stop.coco is received. If an unexpected crash occurs, it can occasionally occur that the OpenFOAM processes are not ended properly and that the user should take care to kill that OpenFOAM-loop manually (using kill or pkill in the Linux-terminal, e.g. pkill coconut_*).

The `solve_solution_step` method

This method is the core of the simulation passing on the interface displacement to OpenFOAM and receiving the resulting loads (pressure and traction). The interface displacement is converted into an OpenFOAM-readable format (with the method write_node_input), by storing it in a pointDisplacementTmp field, which is read in by OpenFOAM in every iteration (this required some adaptation of the solver, see next section). Subsequently, the mesh is updated in OpenFOAM and the flow equations are solved. The dynamic mesh motion is handled by OpenFOAM itself. Finally, the method read_node_output is called in the solver wrapper, which reads the interface loads from the directory postProcessing (more precisely from the subdirectories coconut_<boundary_name>).

Adapting a new application to be used in CoCoNuT (OpenFOAM 10)

For OpenFOAM 10, the applications like pimpleFoam need to be adapted to accommodate the communications with the solver wrapper during the FSI-simulation. These adapted versions have the same name as the original OpenFOAM-solver but with the prefix coconut_. If you do not use an OpenFOAM-application which is already converted for operation in CoCoNuT, you will have to convert the application yourself. This can be done in a rather straightforward way by taking a look at already implemented application, for example coconut_pimpleFoam. In brief, the following steps should be undertaken:

Some additional include-statements are needed: fsiDisplacement.H, waitForSync.H and <unistd.h>. Also include readCoconutControls.H to create control variables used by CoCoNuT.
Except for these initial include-statements, the entire solver code should be put in an infinite loop that starts with while (true). The code will stay in this loop and check with several conditional statements whether the solver wrapper in CoCoNuT has sent a message to the OpenFOAM-solver. These messages are sent by creating an empty file with a specific name in the OpenFOAM-directory. The following file names should be checked by the OpenFOAM-solver: next.coco, continue.coco, save.coco, stop.coco. To allow a pause of 1 ms between each loop the command usleep(1000) is used (which requires the line: #include <unistd.h> before the loop).
Once such a message is received, the OpenFOAM code first executes a command to sync all processors (only important in a parallel run), for example waitForSync("next"). Details on this function are given here.
If the file next.coco exists, a new time step is started. Due to the fact that the infinite loop is driven by while(true) and not the default OpenFOAM-expression while(runTime.run()), the statement runTime.run(); has to be added. This creates, initializes and calls the functionObjects in the controlDict file. Furthermore, as in the original application, the runTime-object should be incremented, increasing the time step.
If the file continue.coco exists, the interface has to be moved first, using the following lines:
```
forAll(boundaryNames, s)
{
    word boundaryName = boundaryNames[s];
    ApplyFSIPointDisplacement(mesh, boundaryName);
}
```
Thereafter, the flow equations need to be solved. This if-statement consequently contains most of the original solver definition, in which the flow equations are called in the same order as in the original CFD solver. Additionally, at the end of the PIMPLE corrector loop, the coupling convergence is checked, see solver coupling convergence. Finally, at the end of this block the loads are written by calling the CoCoNuT functionObjects by including executeCoconutFunctionObjects.H.
If the file save.coco exists, it is checked whether the flow fields should be stored in corresponding files according to the user-defined save interval by calling runTime.write();.
If the file stop.coco exists, a break-statement should end the infinite loop and the OpenFOAM program terminates.

The solver wrapper will automatically compile the adapted applications. This can also be done the default OpenFOAM-compilation method (loading the OpenFOAM-module and using wmake in the directory solver_wrapper/coconut_<solver_name>). To be able to compile the new application, the following files need to adapted.

Update the file Make/files to have the correct application name and change the location of the executable to $(FOAM_USER_APPBIN):
```
coconut_pimpleFoam.C

EXE = $(FOAM_USER_APPBIN)/coconut_pimpleFoam
```
Update the file Make/options such that the CoCoNuT header files are found by adding the parent directory and its parent directory to the included files:
```
EXE_INC = \
  -I.. \
  -I../.. \
  -I<INSERT OTHER LOCATIONS>
```

The waitForSync command

This command, for example waitForSync("next"), is called at the start of every message block and is required to avoid that one processor goes through the block and already removes the message file before all others have seen it. This is achieved by gathering a label on all processors, effectively syncing them. After they are synced, the message file is removed and the corresponding ready message is written, for example next_ready.coco. Thereafter, they are synced once more. This avoids that while removing the file, one processor has left and again entered the same block. Note that the solver wrapper already receives the return message, for example next_ready.coco, before the completion of the block. This results in a speed-up, but requires the solver wrapper to check if, for example, the loads have been fully written before reading them.

Disclaimer

This offering is not approved or endorsed by OpenCFD Limited, producer and distributor of the OpenFOAM software via www.openfoam.com, and owner of the OPENFOAM® and OpenCFD® trademarks, nor by OpenFOAM Foundation Limited, producer and distributor of the OpenFOAM software via www.openfoam.org.