# Quasi Compressible flows

As always when using TRUST, start by loading your **TRUST** environment, [see](../quick_start.md).

The case we will work with in this tutorial is called `TP_Temp_QC_VEF` in the **TRUST** non-regression database. It is a 2D simulation of helium gas flow in a rectangular channel from left to right between two heated walls.

Start by copying it into your folder:
```
source $my_path_to_TRUST_installation/env_TRUST.sh
mkdir -p TRUST_tutorials
cd TRUST_tutorials
trust -copy TP_Temp_QC_VEF
```

Then, open the [](../user_guide/reference/index.rst) in another tab, as it will be useful for looking up keywords throughout this exercise.

Edit the data file with your favorite editor, or using gedit:
```
gedit TP_Temp_QC_VEF.data &
```

You will make several changes to the `.data` file:

- First, modify the geometry and the mesh as shown in {numref}`fig:lowmach`.

```{figure} /_static/FIGURES/low_mach.png
:class: custom-image-class
:name: fig:lowmach
:alt: low_mach

Geometry modification for the Low-Mach case
```

- Then, add several probes (velocity, density, temperature) near the upper right corner of the geometry at location (x,y)=(4,1).

- Add a **segment** of probes (with 9 points) between locations (x,y)=(4,0.05) and (x,y)=(4,0.95) for the temperature field.

- Write the results in **lata** format and change the **dt\_post** period to 1s.

- The goal of the calculation is to reach steady state, so remove the **tmax** keyword and change the **seuil\_statio** $\varepsilon$ value to 10 ($|dT/dt|<\varepsilon$ and $dt \sim 0.001s$, so $|dT|<0.01$).

- Add the keyword **impr** to the pressure solver to print the convergence residuals of the solver.

- Save and close your `.data` file.

- Run the simulation with the `evol` tool (or using command lines):

       trust -evol TP_Temp_QC_VEF &

- Check the mass flow rate (absolute and relative values) in the `TP_Temp_QC_VEF.out` file.

- Once the calculation is finished, visualize the results using **VisIt**:

       visit -o TP_Temp_QC_VEF.lata &

   - Display the mesh and visualize the temperature field by selecting the last time step with the slider, then plot it: `Add` $\rightarrow$ `Pseudo Color` $\rightarrow$ `TEMPERATURE_SOM_dom` $\rightarrow$ `Draw`.

   - Hide the mesh and visualize the velocity field: `Add` $\rightarrow$ `Vector` $\rightarrow$ `VITESSE_SOM_dom` $\rightarrow$ `Draw`.

   - Save a screenshot of your visualization: `File` $\rightarrow$ `Set Save options` $\rightarrow$ `File type` $\rightarrow$ `Select a type` $\rightarrow$ `Save`. An image file named `visit***` will be created in your working directory.

   - Add a second screen with `Window` $\rightarrow$ `Layout` $\rightarrow$ `1x2` and plot a horizontal temperature profile. To do so, select the temperature field, right-click and select `Mode` $\rightarrow$ `Lineout`, then define your profile with the left button. The profile should appear in the second window.

   - Close **VisIt**.

Going back to your `.data` file, replace the time scheme with an implicit time scheme such as {ref}`schema_euler_implicite`.

Use the **implicite** solver and specify the **facsec** and **facsec\_max** parameters according to the advice given in {ref}`facsec_expert`.

Run the calculation again with this time scheme using the `evol` tool or with:
```
trust TP_Temp_QC_VEF.data 1>TP_Temp_QC_VEF.out 2>TP_Temp_QC_VEF.err
```

You can edit the files containing information about the time step and residual evolution for each equation:
```
gedit TP_Temp_QC_VEF.dt_ev &
```

If everything looks correct, try to improve the convergence speed of the implicit solver by adjusting the **seuil\_convergence\_implicite** keyword.
If the number of GMRES iterations is between 3 and 5, convergence is fast enough. You can find this information in the `TP_Temp_QC_VEF.out` file.

To resume the calculation, change the **tinit** value in the data file to the one saved in the `.err` file. You also need to insert the keyword **reprise binaire TP\_Temp\_QC\_VEF\_pb.sauv** in the problem definition block of your `.data` file.

Then, restart the calculation with:
```
trust TP_Temp_QC_VEF.data 1>TP_Temp_QC_VEF.out 2>TP_Temp_QC_VEF.err
```
or
```
trust -evol TP_Temp_QC_VEF.data &
```
The `evol` option automatically creates the `TP_Temp_QC_VEF.out` and `TP_Temp_QC_VEF.err` files.