# 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](../../../01.flow-conditions/README.md)**: 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. ``` --- ## 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 | --- ## 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. ``` --- ## 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 ({py:class}`~flow360.FrozenSpeciesSolver`) or are transported through the domain as mass-fraction fields ({py:class}`~flow360.SpeciesTransportSolver`), as chosen on the `Fluid` model's {py:attr}`~flow360.Fluid.species_transport_solver`.* - Set it as the material of the operating condition's thermal state, {py:attr}`~flow360.ThermalState.material`. - See {doc}`Gas Mixtures ` for how to choose between the two and how to set up species, boundary composition, and the species transport solver. ---

🐍 Python Example Usage

 ```{seealso} Python API: - {py:class}`~flow360.Air` - {py:class}`~flow360.Water` - {py:class}`~flow360.Sutherland` - {py:class}`~flow360.Gas` - {py:class}`~flow360.ThermalState` - {py:class}`~flow360.AerospaceCondition` - {py:class}`~flow360.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 {py:class}`~flow360.LiquidOperatingCondition`. ```python 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, ), ) ```