# 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

 
# 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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