Wall Boundary Condition

Wall Boundary Condition#

The Wall boundary condition represents a solid surface where the fluid cannot penetrate, typically enforcing the no-slip condition for viscous flows.

Available Options#

Option	Description	Applicable
Use wall function	Near-wall modeling approach	always
Motion	Type of wall movement	always
Center	Coordinates of the center of rotation	Motion is `Rotating wall`
Axis	Components of the axis of rotation	Motion is `Rotating wall`
Angular velocity	Angular velocity of the wall’s rotation	Motion is `Rotating wall`
Static pressure	Pressure of the bleed	Motion is `Slater porous bleed`
Porosity	Porosity of the bleed surface	Motion is `Slater porous bleed`
Activation step	Pseudo step at which the bleed is activated	Motion is `Slater porous bleed`
Velocity components	Definition of the wall velocity	Motion is `User-defined`
Heat flux	Heat flux through the wall	when `Heat flux` is selected
Temperature	Temperature of the wall	when `Temperature` is selected
Roughness height	Surface roughness	always
Assigned surfaces	Geometric boundaries to apply the wall condition	always

Detailed Descriptions#

Use wall function#

Enables wall function approximation to model the near-wall region without resolving it.

Options: Enabled/Disabled
Default: Disabled
Note: When enabled, allows the use of coarser meshes with larger y+ values (30-300).

Motion#

Defines how the wall moves relative to the fluid.

Options:
- None (Fixed wall)
- Rotating wall
- Slater porous bleed
- User-defined
Default: None (Fixed wall)

Notes:

Fixed walls have zero velocity, while other types require additional specifications.

When the wall is within the rotating zone:

Choosing fixed wall means that the wall will have zero velocity in relation to the rotating zone.

When using the Rotating wall or User-defined option, the specified velocity will be in reference to the absolute reference frame.

If the wall is supposed to be stationary in the absolute reference frame, the velocity should be set to 0 using Rotating wall or User-defined option.

Center#

Defines the coordinates of the center of rotation for a rotating wall.

Required when Rotating wall selected.
Example: (0, 0, 0) m

Axis#

Defines the rotation axis components for a rotating wall.

Required when Rotating wall selected.
Example: (0, 0, 1)

Angular velocity#

Defines the rate of rotation for a rotating wall.

Required when Rotating wall selected.
Example: 300 RPM

Static pressure#

Static Pressure for Slater porous bleed model.

Required when Slater porous bleed selected.
Example: 101325 Pa

Porosity#

Porosity for Slater porous bleed model.

Required when Slater porous bleed selected.
Example: 0.3

Notes:

Can be between 0 (exclusive) and 1 (inclusive).

Porosity of 0 is equivalent to a solid wall, and porosity of 1 is equivalent to a fluid interface.

Activation step#

Pseudo step in the simulation at which Slater porous bleed model is activated.

Default: 0
Example: 10

Velocity components#

Specifies the velocity vector for moving walls.

Required when User-defined selected.
Example: (10, 0, 0) m/s for a wall moving in the x-direction

Heat flux#

Defines the thermal behavior of the wall using its heat flux.

Default: 0
Example: -30 W/m²

Notes: Heat flux of 0 means an adiabatic wall.

Temperature#

Defines the static temperature of the wall.

Required when Temperature selected.
Example: 300 K

Roughness height#

Specifies the equivalent sand grain roughness height of the wall surface.

Default: 0 (smooth wall)
Example: 0.0001 m

Note: Only available for boundaries associated with a Fluid zone.

Assigned surfaces#

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

Accepted types: Surface

Note: Assign the boundaries by selecting from the list using the + button or select graphically in the viewer region.

💡 Tips

Mesh Resolution Requirements#

Without Wall Functions:
- Target y+ < 1 for accurate boundary layer resolution
- Use prism layers with growth rate < 1.2
- At least 20-30 points within the boundary layer
With Wall Functions:
- Target y+ between 30-300
- At least 10 points within the boundary layer
- More efficient for high Reynolds number flows

Common Applications#

Moving Walls:
- Ground simulations with moving belt in wind tunnels
- Rotating wheels in automotive simulations
- Moving components in machinery
Thermal Conditions:
- Adiabatic for external aerodynamics where heat transfer is negligible
- Isothermal for heat transfer studies with known wall temperatures
- Heat Flux for heat transfer studies with known energy inputs

Convergence Considerations#

Start with simpler boundary conditions before introducing complexity
For moving or rotating walls, consider ramping up the motion gradually
For thermal simulations, first converge the flow field, then enable heat transfer

❓ Frequently Asked Questions

When should I use wall functions?

Wall functions are appropriate for high Reynolds number flows where resolving the entire boundary layer would be prohibitively expensive. They’re a good choice when you’re more interested in overall flow patterns and forces than detailed boundary layer behavior.
What y+ value should I target for my simulation?

For simulations without wall functions, target y+ < 1 to properly resolve the viscous sublayer. For simulations with wall functions, target y+ between 30-300, with optimal results typically around 30-100.
How do I handle complex geometries with varying y+ values?

Focus on achieving proper y+ values in regions of interest. For complex geometries, consider using wall functions in some regions and resolving the boundary layer in critical areas.
Can I model surface roughness effects?

Yes, use the Roughness Height parameter to specify an equivalent sand grain roughness height. This is particularly important for aerodynamic simulations where surface quality affects performance.
How does the Slater Porous Bleed model work?

The model controls mass flow through a boundary based on the difference between local static pressure and specified target pressure, modulated by the porosity value. It’s commonly used to simulate bleed systems in inlets or other flow control devices.
Can I model heat transfer between the wall and fluid?

Yes, by selecting either Isothermal (fixed temperature) or Heat Flux in the Thermal Condition setting. For coupled heat transfer with solids, additional setup may be required.

🐍 Python Example Usage

# Example of configuring a stationary adiabatic wall
stationary_wall = fl.Wall(
    name="wing_surface",
    entities=volume_mesh["wing"],
    heat_spec=fl.HeatFlux(0 * fl.u.W / fl.u.m**2)  # Adiabatic wall (zero heat flux)
)

# Example of a moving wall with specified velocity
moving_wall = fl.Wall(
    name="moving_ground",
    entities=volume_mesh["ground"],
    velocity=["30", "0", "0"]  # Velocity vector as strings
)

# Example of a moving wall with velocity expressions
moving_wall_expressions = fl.Wall(
    entities=volume_mesh["wall_function"],
    velocity=["min(0.2, 0.2 + 0.2*y/0.5)", "0", "0.1*y/0.5"],
    use_wall_function=True
)

# Example of a rotating wall
rotating_wall = fl.Wall(
    name="propeller",
    entities=volume_mesh["propeller"],
    velocity=fl.WallRotation(
        axis=(0, 0, 1),
        center=(0 * fl.u.m, 0 * fl.u.m, 0 * fl.u.m),
        angular_velocity=1000 * fl.u.rpm
    )
)

# Example with wall functions and roughness
rough_wall = fl.Wall(
    name="rough_surface",
    entities=volume_mesh["rough_wall"],
    use_wall_function=True,
    roughness_height=0.0002 * fl.u.m
)

# Example with isothermal wall
isothermal_wall = fl.Wall(
    entities=volume_mesh["fluid/isothermal-*"],
    heat_spec=fl.Temperature(350 * fl.u.K)
)

# Example with heat flux
heat_flux_wall = fl.Wall(
    entities=volume_mesh["solid/isoflux-*"],
    heat_spec=fl.HeatFlux(1.0 * fl.u.W / fl.u.m**2)
)

# Example with Slater porous bleed model
porous_wall = fl.Wall(
    entities=volume_mesh["fluid/SlaterBoundary-*"],
    velocity=fl.SlaterPorousBleed(
        static_pressure=1.01e6 * fl.u.Pa, 
        porosity=0.4, 
        activation_step=200
    )
)