# Passive Spacing

*Passive spacing provides indirect control over mesh behavior without explicit refinement specification, enabling smooth transitions between different mesh regions and maintaining mesh continuity across interfaces.*

---

## Available Options

| *Option* | *Description*                  |
|------------|------------------------------|
| **Type** | Spacing control methodology |
| **Assigned surfaces** | Target surfaces for spacing control |

---

## Detailed Descriptions

### Type

*Specifies the methodology for spacing control.*

- **Required**
- **Available options:** 
  - `projected`: Projects spacing from adjacent volumes.
  - `unchanged`: Preserves existing surface mesh.
>**Note:** Selection determines how mesh spacing is controlled.

```{important}
The `projected` spacing type works only for planar surfaces.
```

### Assigned surfaces

*Identifies the surfaces where spacing control will be applied.*

- **Required**
>**Notes:** 
>  - Must reference valid surface entities in the geometry. 
>  - Assign the boundaries by selecting from the list using the + button or select graphically in the viewer region.

<details>
<summary><h3 style="display:inline-block"> 🐍 Python Example Usage</h3></summary>

```{seealso}
Python API:
- {py:class}`~flow360.PassiveSpacing`
```

```python
import flow360 as fl

# Projected spacing for interface region
interface_spacing = fl.PassiveSpacing(
    name="interface_region",
    type="projected",
    faces=[interface_surface]
)

# Unchanged spacing for optimized surface
preserved_spacing = fl.PassiveSpacing(
    name="optimized_surface",
    type="unchanged",
    faces=[optimized_surface]
)
```
</details>