# Reference Geometry

The reference geometry parameters define essential dimensional characteristics used for computing aerodynamic coefficients and non-dimensional quantities in Flow360 simulations.

# 📋 Available Parameters

Parameter Description Unit
Moment Reference Center Point coordinates defining the reference location for moment calculations length (e.g. m)
Moment Length Reference dimensions used for non-dimensionalization of moment coefficients length (e.g. m)
Area Reference area used for non-dimensionalization of force and moment coefficients area (e.g. m²)

# 🔍 Detailed Descriptions

# Moment Reference Center

The moment reference center specifies the point about which aerodynamic moments are computed. This point is typically located at a significant position on the aircraft, such as the center of gravity or a specific percentage of the mean aerodynamic chord.

  • Components: (X, Y, Z)
  • Default: (0, 0, 0)
  • Units: length (e.g. m)
  • Notes: The coordinate system follows the right-hand rule convention.

# Moment Length

Reference lengths used in the calculation of non-dimensional moment coefficients. These values are typically related to the characteristic dimensions of the geometry.

  • Components: (X, Y, Z)
  • Default: (1, 1, 1)
  • Units: length (e.g. m)
  • Notes: Each component corresponds to the respective axis of rotation.

# Area

The reference area is used to non-dimensionalize force and moment coefficients. For aircraft applications, this is typically the wing planform area, whereas for automotive cases it would be the frontal area of the car.

  • Default: 1
  • Units: area (e.g. m²)
  • Notes: Must be a positive non-zero value.

💡 Tips
  • The moment reference center location significantly impacts moment coefficient calculations.
  • For aircraft analysis:
    • Reference area is typically the wing planform area
    • Moment reference center is often at 25% of the mean aerodynamic chord
    • Longitudinal (roll) moment length is usually the wingspan
    • Lateral (pitch) moment length is typically the mean aerodynamic chord
    • Vertical (yaw) moment length is often the mean aerodynamic chord
  • For automotive analysis:
    • Reference area is typically the frontal area of the vehicle
    • Moment reference center is often at the center of the wheelbase at ground level
    • Longitudinal moment length is usually the wheelbase
    • Lateral moment length is typically the track width
    • Vertical moment length is commonly the vehicle height

❓ Frequently Asked Questions

  • How do reference geometry parameters affect coefficient calculations?

    These parameters are used to non-dimensionalize forces and moments into coefficient form. The reference area is used for all coefficient calculations, while moment lengths are used specifically for moment coefficients.

  • What happens if I use incorrect reference values?

    Using incorrect reference values will result in incorrectly scaled coefficients, making it difficult to compare results with other simulations or experimental data.

  • Can I change reference values between simulations?

    Yes, but ensure you account for the changes when comparing results between different simulations.

🐍 Python Example Usage

 
import flow360 as fl

# Define reference geometry parameters
with fl.SI_unit_system:
    params = fl.SimulationParams(
        reference_geometry=fl.ReferenceGeometry(
            moment_center=(0.25, 0.0, 0.0),
            moment_length=(1.0, 5.0, 1.0),
            reference_area=10.0
        )
    )

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