# Freestream Boundary Condition
*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* | *Applicable* |
|------------|----------------------------------|----------|
| **Use operating condition's velocity** | The velocity specified at the boundary condition is the same as in [Operating condition](../../01.flow-conditions/README.md) | always |
| **Custom** | Option to use local settings rather than those from Operating Condition | always |
| **Velocity Components** | Components of velocity vector (constants or expressions) | **Custom** is selected |
| **Turbulence Quantities** | Specification of turbulence parameters | always |
| **Assigned surfaces** | Geometric boundaries to apply the freestream condition | always |
---
## Detailed Descriptions
### Use operating condition's velocity
*Uses freestream conditions defined in the [Operating condition](../../01.flow-conditions/README.md) section.*
- **Default:** Selected
### Custom
*Allows specification of local freestream conditions for this boundary instead of using the freestream conditions defined in the Operating Condition section.*
- **Default:** Unselected
### Velocity Components
*Explicit values or expressions for the freestream velocity vector, available when overriding Operating Condition settings.*
- **Required when `Custom` is selected**
- **Example (constants):** `(100, 0, 0) m/s`
- **Example (expressions, Python API only):** `X="min(100, 100*t/10)" m/s, Y="10*sin(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](./08.turbulence-quantities.md) for valid combinations and detailed explanations
### Assigned surfaces
*Specifies the geometric boundaries to which the freestream boundary condition is applied.*
- **Accepted types:** Surface
>**Notes:**
>- Assign the boundaries by selecting from the list using the + button or select graphically in the viewer region.
>- Compatible with the `AutomatedFarfield` feature that creates appropriate far-field boundaries.
---
💡 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
```python
# 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
)
)
```