# Operating condition

Operating condition window allows to specify the properties of the fluid that is simulated as well as its velocity in relation to the analyzed object. The values defined are also used for the calculations of force coefficients. You can choose between two types of a fluid: gas and liquid.

# Available Options

# Gas

The simulated fluid is considered an ideal gas.

Option	Description	Applicable
Type	A method of defining the fluid velocity	always
Velocity magnitude	Freestream velocity	Type is `Velocity`
Reference velocity magnitude	Reference velocity magnitude value for coefficient calculation	Type is `Velocity`
Mach	Freestream Mach number	Type is `Mach`
Reference mach	Reference Mach for coefficient calculation	Type is `Mach`
Alpha angle	Angle of attack	always
Beta angle	Sideslip angle	always
Thermal state	A method of defining the gas physical properties	always
Density	Gas density	Thermal state is `Density and temperature`
Temperature	Gas temperature	Thermal state is `Density and temperature`
Altitude	The altitude above sea level	Thermal state is `Standard atmosphere`
Temperature offset	Temperature difference from the standard atmosphere at a given altitude	Thermal state is `Standard atmosphere`

# Liquid

The simulated fluid is considered a liquid.

Option	Description	Applicable
Velocity magnitude	Freestream velocity	always
Reference velocity magnitude	Reference velocity magnitude value for coefficient calculation	always
Alpha angle	Angle of attack	always
Beta angle	Sideslip angle	always
Density	Liquid density	always
Dynamic viscosity	Liquid dynamic viscosity	always

# Detailed descriptions

# Type

Choose a method of defining the freestream velocity.

# Velocity magnitude

Define the freestream velocity using direct velocity magnitude.

Required
Example: 100 m/s

Note: For hover or static conditions, you can set this to zero but must provide a reference velocity for coefficient calculations.

# Reference velocity magnitude

Optional reference velocity used for calculating force and moment coefficients. Required when freestream velocity is zero.

Default: Same as Velocity magnitude
Example: 100 m/s

Note: Particularly important for static/hover cases or when you want coefficients referenced to a different velocity than freestream.

# Mach

Define the freestream Mach number.

Required
Example: 0.8

Note: For hover or static conditions, you can set this to zero but must provide a reference velocity for coefficient calculations.

# Reference Mach

Optional reference velocity used for calculating force and moment coefficients. Required when freestream velocity is zero.

Default: Same as Mach
Example: 0.8

Note: Particularly important for static/hover cases or when you want coefficients referenced to a different velocity than freestream.

# Alpha angle

The angle of attack defining the orientation of the freestream flow relative to the model. Positive values typically indicate nose-up orientation.

Default: 0 degrees
Example: 5 degrees

Notes:

The angle is applied around the Y-axis of the global coordinate system.

Affects velocity components according to these formulas:
$β α$

$β α$

Where:
is the velocity magnitude prescribed by either Velocity Magnitude or Mach number

is the x-component of velocity in the global coordinate system

is the z-component of velocity in the global coordinate system

# Beta angle

The sideslip angle defining the orientation of the freestream flow relative to the model. Positive values typically indicate flow from the right.

Default: 0 degrees
Example: 2 degrees

Notes:

The angle is applied around the Z-axis of the global coordinate system after alpha is applied.

Affects velocity components according to these formulas:
$β α$

$β$

$β α$

Where:
is the velocity magnitude prescribed by either Velocity Magnitude or Mach number

is the x-component of velocity in the global coordinate system

is the y-component of velocity in the global coordinate system

is the z-component of velocity in the global coordinate system

# Thermal state

Choose a method of defining the gas physical properties.

Note: Fluid material is fixed as Air in the GUI.

# Density

Defines the fluid density.

Required
Example: 1.0 kg/m³

# Temperature

Defines the fluid temperature.

Required
Example: 250K

# Altitude

Defines the altitude for which the air properties are set according to the US Standard Atmosphere.

Required
Example: 10000 m

# Temperature offset

An offset from the temperature taken from the standard atmosphere.

Default: 0 K
Example: -10 K

Note: If the altitude is set to 10000 m, the temperature is 223.3 K according to the US standard atmosphere. With the temperature offset of -10 K, the effective freestream temperature will be 213.3 K.

# Dynamic visosity

The dynamic viscosity of a liquid.

Required
Example: 0.001002 Pa*s

💡 Tips

For steady hover simulations, set freestream velocity to zero and provide a reference velocity.
Use standard atmosphere for simulations that need realistic atmospheric conditions.
When comparing with wind tunnel data, match Reynolds number by adjusting both velocity and thermal state.
For low-speed simulations (M < 0.3), using a low Mach preconditioner is recommended for better efficiency.
Reynolds number in the simulation is calculated using the mesh unit length, not necessarily a physical reference length.

Advanced Mach and Reynolds Number Considerations

For transonic flows (0.8 < M < 1.2), use finer meshes near shock regions to capture discontinuities.
When matching experimental data, ensure you're using the same reference values for non-dimensionalization.
Remember that the Reynolds number affects boundary layer thickness—higher Reynolds numbers result in thinner boundary layers requiring finer near-wall mesh resolution.
For simulations containing multiple components (e.g., many propellers), the same reference values will be used across all components.
When using Standard Atmosphere, be aware that density decreases exponentially with altitude, which affects Reynolds number significantly.

❓ Frequently Asked Questions

What is the difference between velocity magnitude and reference velocity?

Velocity magnitude defines the actual freestream flow speed, while reference velocity is used for calculating non-dimensional coefficients. For most cases, they are the same, but for hover or special cases, you might want different values.
How is Reynolds number calculated in Flow360?

Reynolds number is calculated as Re = ρ·V·L/μ, where ρ is density, V is reference velocity, L is the reference length unit in the mesh, and μ is dynamic viscosity.
Can I simulate altitude effects?

Yes, by using the Standard Atmosphere option in Thermal State. This automatically sets temperature and density based on the specified altitude.
What happens if I leave Reference Velocity blank?

If freestream velocity is non-zero, the reference velocity defaults to the freestream value. If freestream velocity is zero, a reference velocity must be provided.
How do alpha and beta affect the simulation?
Rather than rotating the geometry, Flow360 adjusts the freestream flow direction based on alpha and beta using the formulas:
- U_∞ = U_mag · cos(β) · cos(α)
- V_∞ = -U_mag · sin(β)
- W_∞ = U_mag · cos(β) · sin(α)
Where U_mag is the velocity magnitude prescribed by either Velocity Magnitude or Mach number and U_∞, V_∞, and W_∞ represent the x, y, and z components of velocity in the global coordinate system, respectively. This simplifies comparing results at different angles without modifying the mesh.
Can I specify a different material other than Air?

In the GUI, only Air is available. For custom materials, you need to use the Python API.

🐍 Python Example Usage

Below is a Python code example showing how to configure operating conditions using the Flow360 Python API:

import flow360 as fl
from flow360 import u

# Example 1: Setting up a condition with velocity magnitude
condition = fl.AerospaceCondition(
    velocity_magnitude=100 * u.m / u.s,
    alpha=5 * u.degree,
    beta=0 * u.degree,
    thermal_state=fl.ThermalState(
        temperature=288.15 * u.K,
        density=1.225 * u.kg / u.m**3
    )
)

# Example 2: Setting up a condition using from_mach with a reference Mach
condition = fl.AerospaceCondition.from_mach(
    mach=0.8,
    alpha=2 * u.degree,
    beta=1 * u.degree,
    thermal_state=fl.ThermalState(
        temperature=250 * u.K,
        density=0.9 * u.kg / u.m**3
    ),
    reference_mach=0.75  # For coefficient calculations
)

# Example 3: Setting up a hover condition (zero freestream) with reference values
hover_condition = fl.AerospaceCondition(
    velocity_magnitude=0 * u.m / u.s,
    alpha=0 * u.degree,
    beta=0 * u.degree,
    thermal_state=fl.ThermalState(
        temperature=288.15 * u.K,
        density=1.225 * u.kg / u.m**3
    ),
    reference_velocity_magnitude=100 * u.m / u.s  # Required for hover cases
)

# Example 4: Using standard atmosphere model for thermal state
condition = fl.AerospaceCondition(
    velocity_magnitude=200 * u.m / u.s,
    alpha=3 * u.degree,
    beta=0 * u.degree,
    thermal_state=fl.ThermalState.from_standard_atmosphere(
        altitude=10000 * u.m,
        temperature_offset=-5 * u.K
    )
)

# Example 5: Creating a condition from Mach and Reynolds number

condition = fl.AerospaceCondition.from_mach_reynolds(
    mach=0.85,
    reynolds=1e6,
    project_length_unit=1 * u.m,
    temperature=288.15 * u.K,
    alpha=2.0 * u.degree,
    beta=0.0 * u.degree,
    reference_mach=0.85
)

# Example 6: Calculating Reynolds number for an existing condition
project_length_unit = 1 * u.m  # Physical length represented by unit length in mesh
reynolds = condition.flow360_reynolds_number(length_unit=project_length_unit)
print(f"Reynolds number: {reynolds}")

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