# Freestream Boundary Condition

# 📝 Freestream Boundary

The Freestream boundary condition represents the far-field conditions in external aerodynamics simulations, defining the flow conditions at the external boundaries of the computational domain.

# 📋 Available Options

Option	Description	Unit
`Entities`	Geometric boundaries to apply the freestream condition
`Override Operating Condition Settings`	Option to use local settings rather than those from Operating Condition	N/A
`Velocity Components`	Components of velocity vector (constants or expressions)	velocity (e.g., m/s)
`Turbulence Quantities`	Specification of turbulence parameters	various

# 🔍 Detailed Descriptions

# `Entities`

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

Accepted types: Surface
Example: entities=volume_mesh["farfield"] or entities=[auto_farfield.farfield]
Notes: Can reference surfaces by name or pattern. Compatible with the AutomatedFarfield feature that creates appropriate far-field boundaries.

# `Override Operating Condition Settings`

Allows specification of local freestream conditions for this boundary instead of using the freestream conditions defined in the Operating Condition section.

Options: Enabled/Disabled
Default: Disabled (uses Operating Condition settings)
Notes: When disabled, the boundary uses the freestream conditions defined in the Operating Condition section.

# `Velocity Components`

Explicit values or expressions for the freestream velocity vector, available when overriding Operating Condition settings.

Default: None (uses values derived from Operating Condition)
Example (constants): X=100 m/s, Y=0 m/s, Z=0 m/s
Example (expressions, Python API only): X="min(100, 100t/10)" m/s, Y="10sin(t)" m/s, Z=0 m/s
Notes:
- When specified, these values override the velocity derived from velocity or Mach number, alpha, and beta angles in the Operating Condition section
- Each component can be a constant value or a mathematical expression using variables like x, y, z, t
- Mathematical expressions are only available through the Python API, not in the GUI

# `Turbulence Quantities`

Optional specification of turbulence parameters at the boundary.

Options: Various parameter combinations
Default: Uses defaults based on turbulence model defined in Operating Condition
Example: Turbulence Intensity = 0.01, Turbulent Length Scale = 0.1 m
Notes:
- Only specify at most two turbulence parameters
- See Turbulence Quantities documentation for valid combinations and detailed explanations

💡 Tips

# Domain Size Considerations

Place freestream boundaries 50-100 characteristic lengths away from the object of interest
For subsonic flows, ensure boundaries are far enough to avoid artificial acceleration effects
For supersonic flows, place the inlet boundary at least 3-5 characteristic lengths upstream

# When to Override Operating Condition Settings

When modeling regions with varying freestream conditions
When creating a localized flow field different from the conditions specified in Operating Condition
For boundary layer ingestion studies where a non-uniform flow enters the domain
When modeling time-varying or spatially-varying freestream conditions using expressions (Python API only)

# Automated Farfield Creation

Flow360 provides an AutomatedFarfield feature that generates appropriate far-field boundaries
These automated boundaries can be used with the Freestream condition for simplified setup
Particularly useful for external aerodynamics simulations

# Turbulence Specification Guidelines

For external aerodynamics:
- Use lower turbulence intensity (0.1-1%)
- Set turbulent length scale proportional to characteristic length (1-10%)
For specific atmospheric conditions:
- Calm air: Turbulence intensity ~0.1%
- Moderate atmospheric turbulence: 1-5%
- High atmospheric turbulence: 5-10%

❓ Frequently Asked Questions

How far should I place the freestream boundary from my geometry?

For subsonic flows, place boundaries at least 50-100 characteristic lengths away. For supersonic flows, the outlet can be closer (10-20 lengths), but the inlet should still be at least 3-5 lengths upstream to allow proper bow shock formation.
When should I specify explicit turbulence quantities?

Specify turbulence quantities when you need to match specific experimental conditions or when modeling environments with known turbulence levels, such as wind tunnel tests or atmospheric flight conditions.
Can I use varying freestream conditions at different boundaries?

Yes, by overriding the Operating Condition settings and specifying local velocity components for each freestream boundary. This is useful for modeling non-uniform approach flows.
Can I use expressions for velocity components?

Yes, but only through the Python API (not available in the GUI). In the Python API, each velocity component can be specified as a mathematical expression using variables like x, y, z, and t. This allows for spatially or temporally varying freestream conditions in programmatically created simulations.
What happens at a freestream boundary for subsonic vs. supersonic flow?

For subsonic flow, both inflow and outflow can occur across a freestream boundary based on local flow conditions. For supersonic flow, information only travels downstream, so inflow boundaries should be treated carefully.
How do the turbulence settings affect my results?

Turbulence settings at freestream boundaries influence the development of boundary layers and can affect transition location. Higher turbulence intensities typically promote earlier transition from laminar to turbulent flow.
Should I adjust freestream settings for low-speed flows?

For very low Mach numbers (M < 0.1), consider using pressure inlet/outlet boundaries instead of freestream, as these may provide better convergence for nearly incompressible flows.

🐍 Python Example Usage

# Example using operating condition settings (default velocity)
freestream_bc = fl.Freestream(
    name="farfield",
    entities=volume_mesh["farfield"]
)

# Example with explicit velocity components as a list
freestream_with_velocity = fl.Freestream(
    name="farfield_custom",
    entities=volume_mesh["farfield"],
    velocity=[100 , 10, 0 ] * fl.u.m / fl.u.s
)

# Example with velocity expressions
freestream_with_expressions = fl.Freestream(
    name="farfield_varying",
    entities=[volume_mesh["blk-1/freestream-part1"], 
              volume_mesh["blk-1/freestream-part2"]],
    velocity=["min(100, 100*t/10)", "10*sin(t)", "0"]  # Using expressions for time-varying flow
)

# Example with turbulence specifications for k-ω SST model
freestream_with_turbulence = fl.Freestream(
    name="farfield_turbulent",
    entities=volume_mesh["farfield"],
    turbulence_quantities=fl.TurbulenceQuantities(
        turbulent_intensity=0.01,  # 1% turbulence intensity
        turbulent_length_scale=0.1 * fl.u.m
    )
)

# Example with automated farfield
auto_farfield = fl.AutomatedFarfield()
freestream_auto = fl.Freestream(
    name="auto_farfield",
    entities=[auto_farfield.farfield],
    turbulence_quantities=fl.TurbulenceQuantities(
        modified_viscosity_ratio=10
    )
)

# Example for Spalart-Allmaras model
freestream_sa = fl.Freestream(
    name="farfield_sa",
    entities=volume_mesh["farfield"],
    turbulence_quantities=fl.TurbulenceQuantities(
        viscosity_ratio=5.0  # Ratio of turbulent to laminar viscosity
    )
)

# Turbulence Quantities

For detailed information on available turbulence parameters, valid combinations, and usage examples, see the Turbulence Quantities documentation.

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# Freestream Boundary Condition