Material

The Material section defines the physical properties of the simulated fluid inside the Fluid model. For a gas, this is air by default, and you can edit its viscosity model and Prandtl numbers. For a liquid, it is water, defined by its density and dynamic viscosity.

Note

The material type (Air or Water) follows the Gas/Liquid selection made in the Operating Condition: choosing Gas exposes the Air parameters, choosing Liquid exposes the Water parameters. The freestream state (velocity, density, temperature, etc.) is still defined in the Operating Condition; the Material section only describes the substance itself.

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Available Options

Air (gas)

Option	Description	Applicable
Dynamic viscosity	Method used to evaluate the dynamic viscosity: Sutherland (temperature-dependent) or Constant	always
Reference viscosity	Reference dynamic viscosity \(\mu_{ref}\) in Sutherland’s law	Dynamic viscosity is Sutherland
Reference temperature	Reference temperature \(T_{ref}\) associated with the reference viscosity	Dynamic viscosity is Sutherland
Effective temperature	Sutherland constant \(S\)	Dynamic viscosity is Sutherland
Value	A single constant dynamic viscosity	Dynamic viscosity is Constant
Prandtl number	Laminar Prandtl number	always
Turbulent Prandtl number	Turbulent Prandtl number	always

Water (liquid)

Option	Description	Applicable
Density	Density of the liquid	always
Dynamic viscosity	Dynamic viscosity of the liquid	always

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Detailed Descriptions

Dynamic viscosity (Air)

Selects how the dynamic viscosity of the gas is evaluated.

• Default: Sutherland

• Options:

  • Sutherland: temperature-dependent viscosity following Sutherland’s law: \(\mu(T) = \mu_{ref} \left(\dfrac{T}{T_{ref}}\right)^{1.5} \dfrac{T_{ref} + S}{T + S}\)

  • Constant: a single, temperature-independent viscosity value.

Reference viscosity

The reference dynamic viscosity \(\mu_{ref}\) at the reference temperature.

• Required when Dynamic viscosity is Sutherland

• Default: 1.716e-5 Pa·s

Reference temperature

The reference temperature \(T_{ref}\) associated with the reference viscosity.

• Required when Dynamic viscosity is Sutherland

• Default: 273.15 K

Effective temperature

The Sutherland constant \(S\) used in Sutherland’s formula.

• Required when Dynamic viscosity is Sutherland

• Default: 110.4 K

Value

A single constant dynamic viscosity, used instead of the Sutherland model.

• Required when Dynamic viscosity is Constant

• Example: 1.825e-5 Pa·s

Prandtl number

The laminar Prandtl number of the gas.

• Required

• Default: 0.72

Turbulent Prandtl number

The turbulent Prandtl number of the gas.

• Required

• Default: 0.9

Density (Water)

The density of the liquid.

• Required

• Default: 1000 kg/m³

Dynamic viscosity (Water)

The dynamic viscosity of the liquid.

• Required

• Default: 0.001002 Pa·s

Note

A liquid is described only by its density and dynamic viscosity; it carries no thermal properties such as thermal conductivity or specific heat. Conjugate heat transfer (CHT) is therefore only available with a gas.

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Available Through the Python API

These settings have no control in the Material panel. Configure them through the Python API.

Gas

A gas other than the air preset, defined by its species with temperature-dependent (NASA 9-coefficient) thermodynamics. The species either form a mixture of fixed composition (FrozenSpeciesSolver) or are transported through the domain as mass-fraction fields (SpeciesTransportSolver), as chosen on the Fluid model’s species_transport_solver.

• Set it as the material of the operating condition’s thermal state, material.

• See Gas Mixtures for how to choose between the two and how to set up species, boundary composition, and the species transport solver.

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🐍 Python Example Usage

See also

Python API:

• Air

• Water

• Sutherland

• Gas

• ThermalState

• AerospaceCondition

• LiquidOperatingCondition

The material is carried by the operating condition, not by the Fluid model. For a gas it sits on the operating condition’s thermal state; for a liquid it sits directly on LiquidOperatingCondition.

import flow360 as fl
from flow360 import u

with fl.SI_unit_system:
    # Default air: Sutherland viscosity, calorically perfect gas (gamma = 1.4)
    condition = fl.AerospaceCondition(
        velocity_magnitude=100,
        thermal_state=fl.ThermalState(material=fl.Air()),
    )

    # Air with a custom Sutherland model and Prandtl numbers
    condition_custom = fl.AerospaceCondition(
        velocity_magnitude=100,
        thermal_state=fl.ThermalState(
            material=fl.Air(
                dynamic_viscosity=fl.Sutherland(
                    reference_viscosity=1.716e-5 * u.Pa * u.s,
                    reference_temperature=273.15 * u.K,
                    effective_temperature=110.4 * u.K,
                ),
                prandtl_number=0.72,
                turbulent_prandtl_number=0.9,
            ),
        ),
    )

    # Air as a constant-viscosity gas
    condition_constant = fl.AerospaceCondition(
        velocity_magnitude=100,
        thermal_state=fl.ThermalState(
            material=fl.Air(dynamic_viscosity=1.825e-5 * u.Pa * u.s),
        ),
    )

    # Liquid (water) defined by density and dynamic viscosity. Liquids carry the
    # material on the operating condition itself, not on a thermal state.
    condition_liquid = fl.LiquidOperatingCondition(
        velocity_magnitude=5,
        material=fl.Water(
            name="Water",
            density=1000 * u.kg / u.m**3,
            dynamic_viscosity=0.001002 * u.Pa * u.s,
        ),
    )