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, thermodynamic (thermally perfect gas) species, 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
Thermally perfect gas	List of species (with NASA 9-coefficient polynomials and mass fractions) defining the temperature-dependent thermodynamics of the gas	always
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

Thermally perfect gas

Defines the temperature-dependent thermodynamic properties of the gas through one or more species, each described by NASA 9-coefficient polynomials. By default the gas is a single species named Air whose coefficients reproduce a constant specific-heat ratio \(\gamma = 1.4\) (calorically perfect gas).

Each species is edited from its row (the cog opens the species editor) and exposes the following, all required:

• Name: the species identifier (e.g. N2, O2, Ar).

• Mass fraction: the mass fraction of this species in the mixture.

• Temperature: the lower and upper bounds of a temperature range over which a coefficient set applies. A species can hold several ranges (use Add range).

• Coefficients: the nine NASA 9-coefficient polynomial coefficients \(c_1 \ldots c_9\) for the range. They define the non-dimensional specific heat, enthalpy, and entropy:

  • \(\dfrac{c_p}{R} = c_1 T^{-2} + c_2 T^{-1} + c_3 + c_4 T + c_5 T^2 + c_6 T^3 + c_7 T^4\)

  • \(c_1\)–\(c_7\) are the \(c_p\) polynomial coefficients, \(c_8\) is the enthalpy integration constant, and \(c_9\) is the entropy integration constant.

Use Add species to add another component: the N2, O2, and Ar presets come pre-filled with standard data, while Custom starts from an empty species.

Notes:

• The mass fractions of all species must sum to 1.0. Values within a small tolerance (1e-3) are renormalised automatically; otherwise the input is rejected. This validation matches the Python client.

• Species names must be unique, and all species must use the same temperature-range boundaries.

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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❓ Frequently Asked Questions

• Why can’t I run a simulation after I added a species?

  The mass fractions of all species must sum to 1.0. After adding a species, adjust the mass fractions so they total 1.0 (values within a small tolerance are renormalised automatically; larger deviations are rejected and the simulation cannot start).

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🐍 Python Example Usage

See also

Python API:

• Air

• Water

• Sutherland

• ThermallyPerfectGas

• FrozenSpecies

• Fluid

The material is configured via the material field on the Fluid model.

import flow360 as fl
from flow360 import u

# Default air: Sutherland viscosity, single-species calorically perfect gas (gamma = 1.4)
fluid = fl.Fluid(
    material=fl.Air(),
)

# Air with a custom Sutherland model and Prandtl numbers
fluid_custom = fl.Fluid(
    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
fluid_constant = fl.Fluid(
    material=fl.Air(dynamic_viscosity=1.825e-5 * u.Pa * u.s),
)

# Liquid (water) defined by density and dynamic viscosity
fluid_liquid = fl.Fluid(
    material=fl.Water(
        name="Water",
        density=1000 * u.kg / u.m**3,
        dynamic_viscosity=0.001002 * u.Pa * u.s,
    ),
)