# Farfield 

*A farfield volume zone defines the outer boundaries of your computational domain. It establishes the limits where freestream conditions are applied and ensures proper flow development around your geometry.*

## Available Options
| *Option* | *Description* | *Applicable* |
|----------|---------------|--------------|
| [**Type**](#type) | Method of constructing the farfield | always |
| [**Symmetry plane**](#symmetry-plane) | Whether to perform a half-domain symmetrical simulation | always |
| [**Volume to include**](#volume-to-include) | Which domain side to include | **Symmetry plane** is `On` |
| [**Method**](#method) | Method for determining farfield shape | **Type** is `Automated farfield` |
| [**Width**](#width) | Width of the wind tunnel (Y-direction) | **Type** is `Wind tunnel` |
| [**Height**](#height) | Height of the wind tunnel (Z-direction) | **Type** is `Wind tunnel` |
| [**Inlet X position**](#inlet-x-position) | X-position of the tunnel inlet | **Type** is `Wind tunnel` |
| [**Outlet X position**](#outlet-x-position) | X-position of the tunnel outlet | **Type** is `Wind tunnel` |
| [**Floor Z position**](#floor-z-position) | Z-position of the tunnel floor | **Type** is `Wind tunnel` |
| [**Floor geometry**](#floor-geometry) | Type of floor configuration | **Type** is `Wind tunnel` |
| [**Friction patch X**](#friction-patch-x) | X-range of the friction patch on the floor | **Floor geometry** is `Fixed floor` |
| [**Friction patch width**](#friction-patch-width) | Width of the friction patch | **Floor geometry** is `Fixed floor` |
| [**Center belt X**](#center-belt-x) | X-range of the central moving belt | **Floor geometry** is `Center-belt wind tunnel` or `Wheel-belt wind tunnel` |
| [**Center belt width**](#center-belt-width) | Width of the central moving belt | **Floor geometry** is `Center-belt wind tunnel` or `Wheel-belt wind tunnel` |
| [**Front wheel belts X**](#front-wheel-belts-x) | X-range of the front wheel belts | **Floor geometry** is `Wheel-belt wind tunnel` |
| [**Front wheel belts Y**](#front-wheel-belts-y) | Y-range (inner to outer) of the front wheel belts | **Floor geometry** is `Wheel-belt wind tunnel` |
| [**Rear wheel belts X**](#rear-wheel-belts-x) | X-range of the rear wheel belts | **Floor geometry** is `Wheel-belt wind tunnel` |
| [**Rear wheel belts Y**](#rear-wheel-belts-y) | Y-range (inner to outer) of the rear wheel belts | **Floor geometry** is `Wheel-belt wind tunnel` |


## Detailed descriptions

### Type

*Setting allowing the user to choose between generating the farfield in the workflow and providing the farfield as a part of uploaded geometry.*

**Possible selections:**
- `Automated farfield` - Farfield generated by Flow360 around the geometry
- `User defined farfield` - Farfield geometry provided by the user, has to be assigned to an appropriate boundary condition in [Boundary conditions](../03.flow-solver/01.boundary-conditions/02.freestream.md)
- `Wind tunnel` - Analytic wind tunnel geometry with configurable dimensions and floor types

>**Notes:**
>  - `Wind tunnel` option is only available with Geometry AI.
>  - By default, the volume mesher will grow boundary layers on `User defined farfield` and `Wind tunnel` boundaries. Use [Passive Spacing](./06.refinements/04.passive-spacing.md) to project or disable boundary layer growth. 

### Symmetry plane

*Toggle to enable half-domain simulation with a symmetry boundary condition at Y=0.*

- **Default:** `Off`
>**Notes:**
>  - When enabled, the domain is split at Y=0 and only one half is meshed and simulated
>  - Significantly reduces computational cost for symmetric geometries
>  - Only available when using Geometry AI with the beta volume mesher

### Volume to include

*Specifies which side of the Y=0 plane to retain when symmetry plane is enabled.*

**Possible selections:**
- `Full body` - Keep the entire domain without applying symmetry (overrides the symmetry plane toggle behavior)
- `+Y` - Keep the positive Y side of the domain
- `-Y` - Keep the negative Y side of the domain

>**Note:** The geometry must span across Y=0 for this setting to take effect.

### Method

*Method of defining the size of an auto-generated farfield.*

**Possible selections:**

- `Auto`: Automatically determines appropriate farfield dimensions based on geometry (default radius is 50× the maximum bounding box dimension)
  - Generates a full sphere if geometry extends across Y=0
  - Creates +Y semi-sphere if geometry is entirely above Y=0
  - Creates -Y semi-sphere if geometry is entirely below Y=0
- `Quasi-3D`: Creates a thin disk for quasi-3D simulations
  - Both sides of the farfield disk are treated as symmetric planes

---

### Wind tunnel parameters

The following parameters define the wind tunnel geometry when **Type** is set to `Wind tunnel`.

#### Width

*Width of the wind tunnel in the Y-direction.*

- **Default:** `10 m`
- **Units:** Length

#### Height

*Height of the wind tunnel in the Z-direction.*

- **Default:** `6 m`
- **Units:** Length

#### Inlet X position

*X-coordinate of the tunnel inlet plane.*

- **Default:** `-20 m`
- **Units:** Length
>**Note:** Must be less than **Outlet X position**.

#### Outlet X position

*X-coordinate of the tunnel outlet plane.*

- **Default:** `40 m`
- **Units:** Length
>**Note:** Must be greater than **Inlet X position**.

#### Floor Z position

*Z-coordinate of the tunnel floor.*

- **Default:** `0 m`
- **Units:** Length

---

### Floor geometry

*Specifies the type of floor configuration for the wind tunnel.*

**Possible selections:**
- `Fixed floor` - Static floor with a friction patch region where wall boundary conditions apply
- `Fully-moving road` - Entire floor moves at freestream velocity to simulate road motion
- `Center-belt wind tunnel` - Floor with a central moving belt surrounded by stationary regions
- `Wheel-belt wind tunnel` - Floor with a central belt plus four additional wheel belt regions for automotive simulations

---

### Fixed floor parameters

The following parameters are available when **Floor geometry** is set to `Fixed floor`.

#### Friction patch X

*X-range (minimum, maximum) defining the extent of the friction patch on the floor.*

- **Default:** `(-3, 6) m`
- **Units:** Length
>**Notes:**
>  - The friction patch applies wall boundary conditions within this X-range
>  - Must be within the inlet and outlet X positions

#### Friction patch width

*Width of the friction patch centered at Y=0.*

- **Default:** `2 m`
- **Units:** Length
>**Note:** Must be less than the wind tunnel width.

---

### Center-belt wind tunnel parameters

The following parameters are available when **Floor geometry** is set to `Center-belt wind tunnel` or `Wheel-belt wind tunnel`.

#### Center belt X

*X-range (minimum, maximum) defining the extent of the central moving belt.*

- **Default:** `(-2, 2) m`
- **Units:** Length
>**Note:** Must be within the inlet and outlet X positions.

#### Center belt width

*Width of the central moving belt centered at Y=0.*

- **Default:** `1.2 m`
- **Units:** Length
>**Note:** Must be less than the wind tunnel width.

---

### Wheel-belt wind tunnel parameters

The following parameters are available when **Floor geometry** is set to `Wheel-belt wind tunnel`. These are in addition to the center belt parameters.

#### Front wheel belts X

*X-range (minimum, maximum) defining the extent of the front wheel belts.*

- **Required**
- **Units:** Length
>**Note:** Maximum X must be less than the minimum X of the rear wheel belts.

#### Front wheel belts Y

*Y-range (inner edge, outer edge) defining the lateral position of the front wheel belts.*

- **Required**
- **Units:** Length
>**Notes:**
>  - The inner edge must be greater than half the center belt width
>  - The outer edge must be less than half the wind tunnel width
>  - Belts are symmetric about Y=0

#### Rear wheel belts X

*X-range (minimum, maximum) defining the extent of the rear wheel belts.*

- **Required**
- **Units:** Length
>**Note:** Minimum X must be greater than the maximum X of the front wheel belts.

#### Rear wheel belts Y

*Y-range (inner edge, outer edge) defining the lateral position of the rear wheel belts.*

- **Required**
- **Units:** Length
>**Notes:**
>  - The inner edge must be greater than half the center belt width
>  - The outer edge must be less than half the wind tunnel width
>  - Belts are symmetric about Y=0

---

<details>
<summary><h3 style="display:inline-block"> 💡 Tips</h3></summary>

- `Automated farfield` is generally preferred for aerospace applications
- `Wind tunnel` is ideal for automotive simulations requiring realistic ground effects
- Use `Wheel-belt wind tunnel` floor geometry to match physical wind tunnel configurations with separate wheel pads
- When using symmetry plane with wind tunnels, ensure your geometry is symmetric about Y=0

</details>

---

<details>
<summary><h3 style="display:inline-block"> ❓ Frequently Asked Questions</h3></summary>

- **How do I choose between Auto and Quasi-3D farfield methods?**
  > Use Auto for full 3D simulations and Quasi-3D for 2D or axisymmetric cases where the flow is primarily in one plane.

- **When should I use the Wind tunnel farfield type?**
  > Use Wind tunnel for automotive CFD simulations where you need to model realistic wind tunnel conditions including moving floor/belts and proper inlet/outlet boundary conditions.

- **What is the difference between Fixed floor and Fully-moving road?**
  > Fixed floor has a stationary floor with a friction patch where boundary layer develops. Fully-moving road simulates the entire floor moving at freestream velocity, eliminating floor boundary layer effects—ideal for simulating on-road conditions.

- **How do wheel belts work?**
  > Wheel belts are small moving belt regions positioned under each wheel location. They move at freestream velocity to simulate tire contact with the road while the surrounding floor remains stationary.

</details>