Python Script Tutorial: Writing Output File

In this tutorial the procedure for writing the mesh and data from a ModelPart to output files will be described briefly. More information can be found here

Starting

First of all we need to create a python file with following code to import the Kratos, create a ModelPart and read it from input as described in the previous tutorial :

from KratosMultiphysics import *
import KratosMultiphysics.FluidDynamicsApplication

this_model = Model()
fluid_model_part = this_model.CreateModelPart("FluidPart")

fluid_model_part.AddNodalSolutionStepVariable(VELOCITY)
fluid_model_part.AddNodalSolutionStepVariable(PRESSURE)

fluid_model_part_io = ModelPartIO("path/to/file/example")
fluid_model_part_io.ReadModelPart(fluid_model_part)

Creating an instance of the GiD output utility

The GiDOutputProcess helper class can be used to write GiD output from Python. It works mostly as a standard Kratos Process but it has an extra methods to write the to the output file.

Before we begin, we will need to import the module that defines the GiDOutputProcess class:

from gid_output_process import GiDOutputProcess

The GiD output class can be instantiated by passing the ModelPart you intend to print, the name of the file, and the configuration Parameters to it. The example *.json file for this tutorial already contains the required block, so we just need to retrieve it:

gid_output = GiDOutputProcess(
    fluid_model_part,
    'FluidModelPart',
    ProjectParameters["output_configuration"]
)

We can check the settings passed as Parameters to see a few common configuration options:

print(ProjectParameters["output_configuration"].PrettyPrintJsonString())

"result_file_configuration" : {
    "gidpost_flags"       : {
        "GiDPostMode"           : "GiD_PostBinary",
        "WriteDeformedMeshFlag" : "WriteDeformed",
        "WriteConditionsFlag"   : "WriteConditions",
        "MultiFileFlag"         : "SingleFile"
    },
    "file_label"          : "time",
    "output_control_type" : "step",
    "output_frequency"    : 1,
    "body_output"         : true,
    "node_output"         : false,
    "skin_output"         : false,
    "plane_output"        : [],
    "nodal_results"       : ["VELOCITY","PRESSURE"],
    "gauss_point_results" : []
},
"point_data_configuration"  : []

Output configuration

• The GiDPostMode option controls whether the output will be text-based ("GiD_PostAscii") or binary ("GiD_PostBinary"). The ASCII format is only recommended for debugging, where the user needs to manually access and read the output data. In general, the binary format should be preferred as it yields much smaller output files and can be read and written much faster.

• WriteDeformedMeshFlag specifies whether the meshes are written in deformed or undeformed configuration. Setting it to "WriteDeformedMesh" will cause the meshes to be written in deformed state, while "WriteUndeformedMesh" causes the meshes to be written in their original configuration.

• WriteConditionsFlag is used to switch between the option to write only the elements of each mesh (option "WriteElementsOnly") or to write additionally the Condition objects in the current model part (option "WriteConditions").

• MultiFileFlag determines whether all are written in a single file or to individual files for each time step. If the flag is set to "SingleFile", all meshes are written into one file. Note that this is only possible in Binary mode. If instead the flag is set to "MultipleFiles", a new file (or, in ASCII mode, set of mesh and result files) is generated for each step. If multiple files are used, the names for the files are chosen automatically according to the file_label option (both "time" and "step" based labels are supported).

Choosing output variables

The configuration also gives us the option to select which results will be printed on the nodes (and integration points) of the mesh. Additional results can be selected by adding the corresponding variable name (as a string argument) to the nodal_results or gauss_point_results list arguments.

Note that it is currently not possible to print the same variable both on nodes and on integration points.

Using the GiD output utility

The usage of the GiD output class follows the structure of Kratos processes, with one exception. The following methods should be called before starting the solution loop:

gid_output.ExecuteInitialize()
gid_output.ExecuteBeforeSolutionLoop()

During the solution loop, one should call, at each time step:

gid_output.ExecuteInitializeSolutionStep()

## The actual solution goes here

if gid_output.IsOutputStep():

    # Call ExecuteBeforeOutputStep on any auxiliar processes you have here

    gid_output.PrintOutput()

    # Call AfterBeforeOutputStep on any auxiliar processes you have here

gid_output.ExecuteFinalizeSolutionStep()

Finally, once the solution is done, call

gid_output.ExcecuteFinalize()

to close any remaining open files.

Next Nodes and Nodal Data
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