# Slip Wall Boundary Condition
*The Slip Wall boundary condition allows flow to slip tangentially along a surface but not penetrate it, providing a frictionless boundary representation where shear stresses are zero.*
---
## Available Options
| *Option* | *Description* | *Applicable* |
|------------|----------------------------------|----------|
| **Assigned surfaces** | Geometric boundaries to apply the slip wall condition | always |
---
## Detailed Descriptions
### Assigned surfaces
*Specifies the geometric boundaries to which the slip wall boundary condition is applied.*
- **Accepted types:** Surface, GhostSurface, GhostCircularPlane
>**Notes:**
>- Can reference surfaces by name or pattern.
>- Compatible with the `AutomatedFarfield` feature.
## Key properties
- Zero normal velocity (no flow through the boundary)
- Zero shear stress (tangential velocity can vary freely)
- No boundary layer formation
- No viscous effects at the wall
- Unlike a moving wall, a slip wall has no defined velocity but allows tangential flow with zero friction.
---
💡 Tips
### When to Use Slip Wall
- **Inviscid Flow Approximations:**
- When viscous effects are negligible compared to pressure effects
- For preliminary design studies where boundary layer details aren't important
- When computational efficiency is prioritized over viscous accuracy
- **Specific Applications:**
- Free surface approximations
- Interface between different fluid domains
- Ground plane approximation when boundary layer effects aren't important
- Far-field boundaries in some cases
### Computational Benefits
- Requires less mesh resolution near the wall (no boundary layer to resolve)
- Can improve convergence for some flow problems
- Reduces computational cost compared to resolving viscous walls
- Allows for coarser meshes with larger y+ values at the wall
### Comparison with Other Boundary Types
- **Slip Wall vs. Regular Wall:**
- Slip wall: Zero shear stress, no boundary layer
- Regular wall: No-slip condition, develops boundary layer
- Regular wall requires fine mesh resolution near surface
- **Slip Wall vs. Symmetry:**
- Slip wall: Zero normal flow, zero shear stress
- Symmetry: Zero normal flow, mirrored flow field
- Symmetry enforces additional constraints on flow variables
- **Slip Wall vs. Moving Wall:**
- Slip wall: No defined velocity, allows tangential flow with zero friction
- Moving wall: Defined velocity, enforces no-slip condition at the specified velocity
---
❓ Frequently Asked Questions
- **When should I use a slip wall instead of a regular wall?**
> Use a slip wall when:
> - Boundary layer effects are not important for your analysis
> - You're performing preliminary design studies
> - You're approximating inviscid flow
> - You need to reduce computational cost and boundary layer resolution isn't critical
- **What's the difference between a slip wall and a symmetry boundary?**
> Both prevent flow crossing the boundary, but:
> - Slip wall only enforces zero normal velocity and zero shear stress
> - Symmetry enforces mirroring of all flow variables across the boundary
> - Slip wall can be applied to any surface, while symmetry is only appropriate for actual planes of symmetry
- **How does a slip wall affect aerodynamic forces?**
> A slip wall:
> - Will correctly capture pressure forces
> - Will NOT capture any viscous/friction forces
> - Will typically underestimate drag (sometimes significantly)
> - May overestimate lift due to absence of boundary layer effects like separation
- **Do slip walls work with turbulence models?**
> Yes, but there's an important consideration:
> - Turbulence models still operate in the flow field
> - However, no turbulence is generated at the slip wall since there's no shear
> - This creates an inconsistency if you're modeling a flow that should have wall-generated turbulence
- **Is a slip wall the same as an Euler wall?**
> Yes, a slip wall is sometimes called an Euler wall because it's consistent with Euler equations (inviscid flow equations). Both terms refer to a frictionless wall condition.
- **Can I mix slip walls and no-slip walls in the same simulation?**
> Yes, you can use slip walls for some boundaries and regular no-slip walls for others. This is common when some surfaces (like main bodies) need accurate viscous modeling while others (like far-field boundaries) don't.
---
🐍 Python Example Usage
```python
# Example of applying a slip wall boundary condition
slip_wall = fl.SlipWall(
name="frictionless_surface",
entities=volume_mesh["frictionless_surfaces"]
)
# Example of external aerodynamics with mixed boundary types
def create_mixed_boundaries():
return [
# Main body with viscous effects
fl.Wall(
name="main_body",
entities=volume_mesh["body_surfaces"],
use_wall_function=True
),
# Ground plane modeled as slip wall
fl.SlipWall(
name="ground_plane",
entities=volume_mesh["ground_plane"]
),
# Far-field boundary
fl.Freestream(
name="farfield",
entities=volume_mesh["farfield"]
)
]
# Example of simplified internal flow
def create_simplified_internal_flow():
return [
# Main flow passage with slip walls
fl.SlipWall(
name="passage_walls",
entities=volume_mesh["passage_walls"]
),
# Inlet condition
fl.Inflow(
name="inlet",
entities=volume_mesh["inlet"],
total_temperature=300 * fl.u.K,
spec=fl.TotalPressure(
value=150000 * fl.u.Pa,
velocity_direction=(1, 0, 0)
)
),
# Outlet condition
fl.Outflow(
name="outlet",
entities=volume_mesh["outlet"],
spec=fl.Pressure(value=101325 * fl.u.Pa)
)
]
```