# Slip Wall Boundary Condition

# 📝 Slip Wall Boundary

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 Unit
Entities Geometric boundaries to apply the slip wall condition
No additional configuration options The slip wall condition is fully defined by its application to a boundary

# 🔍 Detailed Descriptions

# Entities

Specifies the geometric boundaries to which the slip wall boundary condition is applied.

  • Accepted types: Surface, GhostSurface, GhostCircularPlane
  • Example: entities=volume_mesh["frictionless_surfaces"] or entities=[auto_farfield.symmetry_planes]
  • Notes: Can reference surfaces by name or pattern. Compatible with the AutomatedFarfield feature.

The Slip Wall boundary condition enforces zero normal velocity at the boundary while allowing tangential flow with zero shear stress. Unlike a regular wall with the no-slip condition, a slip wall does not develop a boundary layer.

Key properties of the slip wall boundary:

  • 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

  • Default:

  • Notes: 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

 
# 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)
        )
    ]

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