# Mesh for coupled problems

This exercise demonstrates creating meshes for coupled multi-domain problems where interface connectivity is critical.

## Problem Description

Consider the cooling of a solid block by a fluid flowing through circular cross-section channels. The channel is centered within a square cross-section block. The outer boundaries of the solid are adiabatic.

![Coupled problem schematic](/_static/FIGURES/Coupled_problem.png)

Two domains need to be created:
- **Domain 1:** Solid block
- **Domain 2:** Fluid channel

**Key requirement:** Mesh elements must be connected at the interface between domains for TRUST to read the mesh correctly.

## Setup

```bash
mkdir -p TRUST_tutorials/salome/exo4
cd TRUST_tutorials/salome/exo4
$PathToSALOME/salome &
```

- Create a new study: File → New
- Select the Geometry module
- Save frequently in HDF format

## Creating the Geometry

- **Create the solid block:** New Entity → Primitives → Box
  - Dx = `200`, Dy = `200`, Dz = `400`
  - Click "Apply and Close"

- **Create a vertex for the cylinder base:** New Entity → Basic → Point
  - X = `100`, Y = `100`, Z = `0`
  - Click "Apply and Close"

- **Create the fluid channel:** New Entity → Primitives → Cylinder
  - Base Point: Vertex_1
  - Vector: OZ
  - Radius R = `40`
  - Height H = `400`
  - Click "Apply and Close"

- **Cut the cylinder from the block:** Operations → Boolean → Cut
  - Main Object: Box_1
  - Tool Objects: Cylinder_1
  - Click "Apply and Close"

- **Create a partition:** Operations → Partition
  - Objects: Select both Cylinder_1 and Cut_1
  - Click "Apply and Close"

**Note:** This partition ensures mesh connectivity at the interface.

## Defining Volume Groups

Create groups for each domain:

- **Solid domain:** New Entity → Group → Create Group
  - Shape Type: Volume
  - Name: `Solid`
  - Main Shape: Partition_1
  - Select the hollow box
  - Click "Add" → "Apply"

- **Fluid domain:** Continue in the same dialog
  - Name: `Fluid`
  - Main Shape: Partition_1
  - Select the cylindrical channel
  - Click "Add" → "Apply and Close"

## Defining Boundary Groups

Create surface groups for all boundaries:

- **Fluid inlet:** New Entity → Group → Create Group
  - Shape Type: Surface
  - Name: `Fluid_inlet`
  - Main Shape: Partition_1
  - Select the bottom of the cylinder → "Add" → "Apply"

- **Fluid outlet:**
  - Name: `Fluid_outlet`
  - Select the top circular boundary → "Add" → "Apply"

- **Solid top:**
  - Name: `Solid_top`
  - Select the top of the box → "Add" → "Apply"

- **Solid bottom:**
  - Name: `Solid_bottom`
  - Select the bottom of the box → "Add" → "Apply"

- **Solid lateral walls:**
  - Name: `Solid_lateral_walls`
  - Select the four lateral boundaries of the box → "Add" → "Apply"

- **Solid-Fluid interface:**
  - Name: `Solid_Fluid_Interface`
  - Select the top of the box → "Hide selected"
  - Select a lateral boundary → "Hide selected"
  - The lateral surface of the cylinder should now be visible
  - Select it → "Add" → "Apply and Close"

## Creating the Mesh

- **Switch to the Mesh module**

- **Create the mesh:** Mesh → Create Mesh
  - Name: `Mesh_1`
  - Geometry: Partition_1
  - 3D algorithm: NETGEN 1D-2D-3D

- **Configure parameters:**
  - Click the wheel icon next to "Hypothesis" → "NETGEN 3D Parameters"
  - In "Arguments": Change fineness from "Moderate" to "Fine"
  - Click "OK" → "Apply and Close"

- **Compute the mesh:**
  - Right-click on "Mesh_1" → Compute

- **Verify groups:**
  - Check that six boundaries appear under "Group of Faces" of **Mesh_1**
  - Check that two volume groups appear under "Group of Volumes" of **Mesh_1**

## Exporting the Mesh

- **Export to MED format:**
  - Select "Mesh_1"
  - Right Click → Export → MED file
  - Choose MED 3.2 if possible
  - Save as `Mesh_1.med`

- **Dump the study:** File → Dump Study
  - Save as a Python script (needed for the next exercise)

## Running the Coupled Problem

Copy and run the TRUST data file:

```bash
cp $TRUST_ROOT/doc/TRUST/exercices/salome/Coupled_pb.data .
trust Coupled_pb.data
```

Visualize the results with VisIt:

```bash
visit -o Coupled_pb.lata
```

Plot the temperature field on both domains and set the color bar min/max to 300 and 400 respectively. You will observe the solid cooling over time. Given sufficient simulation time, the solid temperature will eventually match the fluid temperature at steady state.