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

Type

Wall function model type (Boundary layer / Inner layer)

Use wall function is enabled

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

Estimate rotation from selected faces via OBB

Helper that auto-fills Center, Axis and Angular velocity from the selected faces

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).

Type#

Selects the wall function model used when wall functions are enabled.

  • Options:

    • Boundary layer (default) — Uses integral flat-plate boundary-layer theory to predict wall shear stress. Performs well across all y+ ranges.

    • Inner layer — Uses the inner-layer behaviour of the turbulent boundary layer. Offers better accuracy for y+ values in the log layer and below (y+ ≈ 30–100).

  • Default: Boundary layer

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

Estimate rotation from selected faces via OBB#

Helper that automatically fills in the rotation Center, Axis and Angular velocity from the geometry of the selected faces, so you do not have to measure a wheel (or other rotating component) by hand.

  • Available when Rotating wall is selected.

  • Select the rotating faces (for example a wheel) in Assigned surfaces, then click Estimate.

  • Flow360 fits an oriented bounding box (OBB) to those faces and uses it to fill in:

    • the Center of the selected faces,

    • the rotation Axis, and

    • the Angular velocity, derived from the freestream velocity (from the operating conditions) and the estimated radius.

Notes:

  • Requires at least one face to be selected. The angular velocity estimate also requires the operating-condition velocity to be defined.

  • The estimated values are written into the Center, Axis and Angular velocity fields and can be edited afterwards.

  • The same oriented bounding box can be computed from the Python API through DraftContext.compute_obb.

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 (adds energy to the fluid, heating it)

Notes:

  • Heat flux of 0 means an adiabatic wall.

  • Sign convention: a positive heat flux removes energy from the fluid (heat flows from the fluid into the wall, cooling the fluid); a negative heat flux adds energy to the fluid (heat flows from the wall into the fluid, heating it).

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

❓ 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?

    \(y^+\) is a non-dimensional distance that measures how well your mesh resolves the flow boundary layer. Because it relies on wall shear stress, its value changes dynamically with flow velocity and fluid properties, not just mesh size. For simulations without wall functions (resolved approach), target \(y^+ \approx 1\) to resolve near-wall physics. For simulations with wall functions, target \(30 < y^+ < 300\) to approximate them mathematically, with optimal results typically around 30-100. The Spalart-Allmaras model tolerates \(y^+\) values up to approximately 2, while the k-ω SST model recommends \(y^+ < 1\). See y+ estimation for formulas to estimate the required first layer thickness.

  • 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.

🐍 Python Example Usage

See also

Python API:

# 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 (default Boundary layer wall function)
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=fl.WallFunction()
)

# Example of a moving wall with Inner layer wall function
moving_wall_inner_layer = 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=fl.WallFunction(type_name="InnerLayer")
)

# 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=fl.WallFunction(),
    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
    )
)
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