# Gravity

*The gravity model applies a gravitational body force to the fluid momentum and energy equations, enabling simulation of buoyancy-driven flows. Gravity is applied globally to all fluid zones in the simulation and is disabled by default.*

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## Available Parameters

| *Parameter* | *Description* |
|-------------|---------------|
| **Gravity** (toggle) | Enable or disable the gravitational body force |
| **Direction** | Unit vector defining the direction of gravitational acceleration [X, Y, Z] |
| **Magnitude** | Magnitude of the gravitational acceleration |

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## Detailed Descriptions

### Gravity (toggle)

*Enables or disables the gravitational body force. When enabled, gravity is applied globally to all fluid zones by adding a body force to the momentum equations and a work term to the energy equation: $\mathbf{f}_{\text{mom}} = \rho \, \mathbf{g}, \qquad f_{\text{energy}} = \rho \, (\mathbf{g} \cdot \mathbf{u})$*

- **Default:** Disabled (off)

### Direction

*The unit vector specifying the direction of the gravitational acceleration. Defines the orientation of the gravity vector in the simulation coordinate system.*

- **Default:** `X: 0, Y: 0, Z: -1` (pointing in the negative Z-direction)
- **Example:** `(0, 0, -1)` for vertical-down in a Z-up coordinate system, `(0, -1, 0)` for a Y-up coordinate system
>**Notes:**
>- The direction vector will be normalised automatically; you do not need to supply a unit vector.
>- Only available when the Gravity toggle is enabled.

### Magnitude

*The magnitude of the gravitational acceleration.*

- **Default:** `9.81 m/s²` (Earth surface gravity)
- **Example:** `9.81 m/s²`, `1.62 m/s²` (lunar gravity)
>**Notes:**
>- Must be a positive value.
>- Only available when the Gravity toggle is enabled.

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

- Enable gravity only when buoyancy effects are physically significant (e.g., natural convection, thermally stratified flows, or flows where density differences drive the motion).
- For aerodynamic simulations where density variations are small and driven by compressibility rather than temperature gradients, gravity can typically be omitted.
- Ensure the Direction vector is aligned with the physical orientation of your geometry in the simulation coordinate system.
- When using a Z-up coordinate system, the default direction `(0, 0, -1)` corresponds to Earth's downward gravity and requires no adjustment.
- For conjugate heat transfer (CHT) simulations with buoyancy-driven convection, enabling gravity is essential for physically accurate results.

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