Do you know that CFD refers to Colors For Directors? It sure is a bad joke but it emphasis the importance of beautiful post-processing when you are running a fluid mecanics simulation. The aim of this section is to give you the key to create beautiful pictures and videos from your amazing TRUST simulations.

Before trying to post-processing something, make sure that the information has not already been printed out in one of the files created by a TRUST run.

Automatically outputted files

After running, you will find different files in your directory. Here is a short explaination of what you will find in each type of file depending on its extension.

Even if you don't post-process anything, you will have output files which are listed here:

File	Contents
[my_data_file].dt_ev	Time steps, facsec, equation residuals
[my_data_file].stop	Stop file ('0', '1' or 'Finished correctly')
[my_data_file].log	Journal logging
[my_data_file].TU	CPU/GPU performances
[my_data_file]_csv.TU	CPU/GPU performance formatted as a CSV file
[my_data_file_problem_name].sauv or .xyz	Saving 2D/3D results for resume
or [specified_name].sauv or .xyz	(binary files)

and the listing of boundary fluxes where:

[my_data_file]_Contrainte_visqueuse.out correspond to the friction drag exerted by the fluid.
[my_data_file]_Convection_qdm.out contains the momentum flow rate.
[my_data_file]_Debit.out is the volumetric flow rate.
[my_data_file]_Force_pression.out correspond to the pressure drag exerted by the fluid.

If you add post-processings in your data files, you will find:

File	Contents
[my_data_file].sons	1D probes list
[my_data_file_probe_name].son	1D results with probes
[my_data_file_probe_name].plan	3D results with probes
[my_data_file].lml (default format)
[my_data_file].lata (with all .lata.* files)*
[my_data_file].med	2D/3D results
or [specified_name].lml or .lata or .med

The screen outputs are automatically redirected in my_data_file.out and my_data_file.err files if you run a parallel calculation or if you use the -evol option of the trust script.

You can also redirect them in two files with the following command:

# Source TRUST env if not already done
source $my_path_to_TRUST_installation/env_TRUST.sh
 
# then
trust my_data_file 1>file_out.out 2>file_err.err

In the .out file, you will find the listing of physical infos with mass balance and in the .err file, the listing of warnings, errors and domain infos.

Post-processing block

Several keywords can be used to create a post-processing block, into a problem. First, you can create a single post-processing task (Post_processing keyword). Generally, in this block, results will be printed with a specified format at a specified time period.

Post_processing
{
   Postraitement_definition
   ...
}

But you can also create a list of post-processings with Post_processings keyword (named with Post_name1, Post_name2, etc...), in order to print results into several formats or with different time periods, or into different results files:

Post_processings
{
   Post_name1 { Postraitement_definition }
   Post_name2 { Postraitement_definition }
   ...
}

Probes

Probes refer to sensors that allow a value or several points of the domain to be monitored over time. The probes are a set of points defined:

one by one: Points keyword

or
by a set of points evenly
- distributed over a straight segment: Segment keyword
  
  or
- arranged according to a layout: Plan keyword
  
  or
- arranged according to a parallelepiped Volume keyword.

Here is an example of 2D Probes block:

Probes 
{
   pressure_probe [loc] pressure Periode 0.5 Points 3 1. 0. 1. 1. 1. 2.
   velocity_probe [loc] velocity Periode 0.5 Segment 10 1. 0. 1. 4.
}

where the use of loc option allow to specify the wanted location of the probes. The available values are grav for gravity center of the element, nodes for faces and som for vertices. There is not default location. If the point does not coincide with a calculation node, the value is extrapolated linearly according to neighbouring node values.

Fields

This keyword allows to post-process fields on the whole domain, specifying the name of the backup file, its format, the post-processing time step and the name (and location) of the post-processed fields.

Here is an example of Fields block:

Fichier results
Format lata
Fields dt_post 1. 
{
   velocity [faces] [som] [elem]
   pressure [elem] [som]
   temperature [elem] [som]
}

where faces, elem and som are keywords allowed to specify the location of the field.

Note: When you don't specify the location of the field, the default value is som for values at the vertices. So fields are post-processed at the vertices of the mesh.

To visualize your post-processed fields, you can use open source softwares like:

VisIt (included in TRUST package) or SALOME.

Statistics

Using this keyword, you will compute statistics on your unknows. You must specify the begining and ending time for the statistics, the post-processing time step, the statistic method, the name (and location) of your post-processed field.

Here is an example of Statistiques block:

Statistiques dt_post 0.1 
{
   t_deb 1. t_fin 5.
   moyenne velocity [faces] [elem] [som]
   ecart_type pressure [elem] [som]
   correlation pressure velocity [elem] [som]
}

This block will write at every dt_post the average of the velocity \(\overline{V(t)}\):

\[\overline{V(t)}=\left\{ \begin{array}{ll} 0 & ,\mbox{ for }t\leq t_{deb}\\ \frac{1}{t-t_{deb}}{\displaystyle \int_{t_{deb}}^{t}V(t)dt} & ,\mbox{ for }t_{deb}<t\leq t_{fin}\\ \frac{1}{t_{fin}-t_{deb}}{\displaystyle \int_{t_{deb}}^{t_{fin}}V(t)dt} & ,\mbox{ for }t>t_{fin} \end{array}\right. \]

the standard deviation of the pressure \(\left\langle P(t)\right\rangle\):

\[\left\langle P(t)\right\rangle=\left\{ \begin{array}{ll} 0 & ,\mbox{ for }t\leq t_{deb}\\ \frac{1}{t-t_{deb}}{\displaystyle \sqrt{\int_{t_{deb}}^{t}\left[P(t)-\overline{P(t)}\right]^{2}dt}} & ,\mbox{ for }t_{deb}<t\leq t_{fin}\\ \frac{1}{t_{fin}-t_{deb}}{\displaystyle \sqrt{\int_{t_{deb}}^{t_{fin}}\left[P(t)-\overline{P(t)}\right]^{2}dt}} & ,\mbox{ for }t>t_{fin} \end{array}\right. \]

and correlation between the pressure and the velocity \(\left\langle P(t).V(t)\right\rangle\) like:

\[\left\langle P(t).V(t)\right\rangle=\left\{ \begin{array}{ll} 0 & ,\mbox{ for }t\leq t_{deb}\\ \frac{1}{t-t_{deb}}{\displaystyle \int_{t_{deb}}^{t}\left[P(t)-\overline{P(t)}\right]\cdot\left[V(t)-\overline{V(t)}\right]dt} & ,\mbox{ for }t_{deb}<t\leq t_{fin}\\ \frac{1}{t_{fin}-t_{deb}}{\displaystyle \int_{t_{deb}}^{t_{fin}}\left[P(t)-\overline{P(t)}\right]\cdot\left[V(t)-\overline{V(t)}\right]dt} & ,\mbox{ for }t>t_{fin} \end{array}\right. \]

Remark: Statistical fields can be plotted with probes with the keyword "operator_field_name" like for example: Moyenne_Vitesse or Ecart_Type_Pression or Correlation_Vitesse_Vitesse. For that, it is mandatory to have the statistical calculation of this fields defined with the keyword Statistiques.

Field names

Existing & predefined fields

You can post-process predefined fields and already existing fields. Here is a list of post-processable fields, but it is not the only ones.

Physical values	Keyword for field_name	Unit
Velocity	Vitesse or Velocity	m.s⁻¹
Velocity residual	Vitesse_residu	m.s⁻²
Kinetic energy per elements	Energie_cinetique_elem	kg.m⁻¹.s⁻²
Total kinetic energy	Energie_cinetique_totale	kg.m⁻¹.s⁻²
Vorticity	Vorticite	s⁻¹
Pressure in incompressible flow (P/ρ + gz)	Pression	Pa.m³.kg⁻¹
Pressure in incompressible flow (P+ρgz)	Pression_pa or Pressure	Pa
Pressure in compressible flow	Pression	Pa
Hydrostatic pressure (ρgz)	Pression_hydrostatique	Pa
Total pressure	Pression_tot	Pa
Pressure gradient	Gradient_pression	m.s⁻²
Velocity gradient	gradient_vitesse	s⁻¹
Temperature	Temperature	C or K
Temperature residual	Temperature_residu	C.s⁻¹ or K.s⁻¹
Temperature variance	Variance_Temperature	K²
Temperature dissipation rate	Taux_Dissipation_Temperature	K².s⁻¹
Temperature gradient	Gradient_temperature	K.m⁻¹
Heat exchange coefficient	H_echange_Tref	W.m⁻².K⁻¹
Turbulent viscosity	Viscosite_turbulente	m².s⁻¹
Turbulent dynamic viscosity	Viscosite_dynamique_turbulente	kg.m.s⁻¹
Turbulent kinetic	Energy	K m².s⁻²
Turbulent dissipation rate	Eps	m³.s⁻¹
Constituent concentration	Concentration	-
Constituent concentration residual	Concentration_residu	-
Component velocity along X	VitesseX	m.s⁻¹
Component velocity along Y	VitesseY	m.s⁻¹
Component velocity along Z	VitesseZ	m.s⁻¹
Mass balance on each cell	Divergence_U	m³.s⁻¹
Irradiancy	Irradiance	W.m⁻²
Q-criteria	Critere_Q	s⁻¹
Distance to the wall Y +	Y_plus	-
Friction velocity	U_star	m.s⁻¹
Void fraction	Alpha	-
Cell volumes	Volume_maille	m³
Source term in non Galinean referential	Acceleration_terme_source	m.s⁻²
Stability time steps	Pas_de_temps	s
Volumetric porosity	Porosite_volumique	-
Distance to the wall	Distance_Paroi	m
Volumic thermal power	Puissance_volumique	W.m⁻³
Local shear strain rate	Taux_cisaillement	s⁻¹
Cell Courant number (VDF only)	Courant_maille	-
Cell Reynolds number (VDF only)	Reynolds_maille	-
Viscous force	Viscous_force	kg.m².s⁻¹
Pressure force	Pressure_force	kg.m².s⁻¹
Total force	Total_force	kg.m².s⁻¹
Viscous force along X	Viscous_force_x	kg.m².s⁻¹
Viscous force along Y	Viscous_force_y	kg.m².s⁻¹
Viscous force along Z	Viscous_force_z	kg.m².s⁻¹
Pressure force along X	Pressure_force_x	kg.m².s⁻¹
Pressure force along Y	Pressure_force_y	kg.m².s⁻¹
Pressure force along Z	Pressure_force_z	kg.m².s⁻¹
Total force along X	Total_force_x	kg.m².s⁻¹
Total force along Y	Total_force_y	kg.m².s⁻¹
Total force along Z	Total_force_z	kg.m².s⁻¹

Note: Physical properties (conductivity, diffusivity,...) can also be post-processed.; The name of the fields and components available for post-processing is displayed in the error file after the following message: "Reading of fields to be postprocessed". Of course, this list depends of the problem being solved.

Creating new fields

The Definition_champs keyword is used to create new or more complex fields for advanced post-processing.

Definition_champs { field_name_post field_type { ... } }

field_name_post is the name of the new created field and field_type is one of the following possible type:

refChamp
Reduction_0D using for example the min, max or somme methods
Transformation

Refer to Keywords Reference Manual for more information.

Note: You can combine several field_type keywords to create your field and then use your new fields to create other ones.

Here is an example of new field named max_temperature:

Read my_problem 
{
   ...
   Postraitement 
   {
      Definition_champs 
      {
         # Creation of a 0D field: maximal temperature of the domain #
         max_temperature Reduction_0D 
         {
            methode max
            source refChamp { Pb_champ my_problem temperature }
         }
      }
 
      Probes 
      {
         # Print max(temperature) into the datafile_TMAX.son file #
         tmax max_temperature periode 0.01 point 1 0. 0.
      }
 
      Champs dt_post 1.0 { ... }
   }
}

Complete Post-Processing Example

Here is a complete post-processing example taken from the TRUST's upwind test case.

Post_processing
{
    Probes
    {
        sonde_pression pression periode 0.005 points 2 0.13 0.105 0.13 0.115
        sonde_vitesse vitesse periode 0.005 points 2 0.14 0.105 0.14 0.115
        sonde_vitesse_bis vitesse periode 0.005 position_like sonde_vitesse
        sonde_vitesse_ter vitesse periode 1e-5 position_like sonde_vitesse
    }

    Definition_champs
    {
        # Calcul du produit scalaire grad(Pression).Vitesse #
        pscal_gradP_vit Transformation {
            methode produit_scalaire
            sources {
                Interpolation { localisation elem source refChamp { Pb_champ pb gradient_pression } } ,
                Interpolation { localisation elem source refChamp { Pb_champ pb vitesse } }
            }
        }
        # Calcul du produit scalaire Vitesse.Vitesse #
        pscal_vit_vit_elem Transformation {
            methode produit_scalaire
            sources {
                refChamp { Pb_champ pb vitesse } ,
                refChamp { Pb_champ pb vitesse }
            }
            localisation elem
        }

        pscal_vit_vit_elem_interp Transformation {
            methode produit_scalaire
            sources {
                Interpolation { localisation elem source refChamp { Pb_champ pb vitesse } } ,
                Interpolation { localisation elem source refChamp { Pb_champ pb vitesse } }
            }
        }

        pscal_vit_vit_som Interpolation { localisation som sources_reference { pscal_vit_vit_elem_interp } }

        pscal_vit_vit_faces Interpolation { localisation faces sources_reference { pscal_vit_vit_elem_interp } }

        norme_transfo_vit Transformation {
            methode norme
            source Interpolation {
                localisation elem
                source refChamp { Pb_champ pb vitesse }
            }
        }

        vecteur_unitairex transformation { methode vecteur expression 2 1. 0. localisation faces }

        vecteur_transfo transformation { methode vecteur  expression 2 x t localisation faces }

        vecteur_source_elem Transformation {
            methode vecteur  expression 2 pression_natif_dom y
            source refChamp { Pb_champ pb pression }
            localisation elem
        }
        vecteur_source_faces Transformation {
            methode vecteur expression  2 pression_natif_dom y
            source refChamp { Pb_champ pb pression }
            localisation Faces
        }

        compo_vitessex Transformation {
            methode composante numero 0
            source refChamp { Pb_champ pb vitesse }
            localisation elem
        }

        compo_vitessex_elem Interpolation { localisation elem sources_reference { compo_vitessex } }

        # Calcul de la composante selon X du gradient de pression #
        compo_graPx Transformation {
            methode composante numero 0
            source refChamp { Pb_champ pb gradient_pression }
            localisation elem
        }
        # Interpolation de cette composante aux elements #
        compo_graPx_elem Interpolation { localisation elem sources_reference { compo_graPx } }
    }
    Format Lata
    fields dt_post 1.3 /* physical time */
    {
        pression elem
        vitesse elem
        masse_volumique elem
        gradient_pression elem
        pscal_gradP_vit elem
        pscal_vit_vit_elem
        pscal_vit_vit_elem_interp
        pscal_vit_vit_som
        norme_transfo_vit elem
        vecteur_unitairex elem
        vecteur_transfo elem
        vecteur_source_elem
        compo_vitessex_elem
        compo_graPx_elem
    }
}

liste_de_postraitements {
    test1 Post_processing {
        Definition_champs {
            test_nom reduction_0D {
                methode weighted_average
                sources { refchamp { pb_champ pb pression } }
            }
        }
        format lata
        fields dt_post 1.3
        {
            pression elem
            test_nom
        }
    }

    test2 Post_processing {
        format lata
        fields dt_post 1.3e9
        {
            pression elem
        }
    }
}

Visualisation Tools

To open your 3D results in lata format, you can use VisIt which is an open source software included in TRUST package. For that you may "source" TRUST environment and launch VisIt:

# Source TRUST env if not already done
source $my_path_to_TRUST_installation/env_TRUST.sh
# then
visit -o my_data_file.lata &

To open your 3D results in med format, you can also use VisIt, SALOME or Paraview.

Here are some actions that you can perform when your simulation is finished:

To visualize the positions of your probes in function of the 2D/3D mesh, you can open your .son files at the same time of the .lata file in VisIt.
If you need more probes, you can create them with VisIt (if you have post-processed the good fields) or with MEDCoupling.
You can use the option -evol of the trust script, like:

trust -evol my_data_file

and access to the probes or open VisIt for 2D/3D visualizations via this tool.