Nonisothermal Flow
Use the Nonisothermal Flow multiphysics coupling () to simulate fluid flows where the fluid properties depend on temperature. Models can also include heat transfer in solids. The physics interface supports low Mach numbers (typically less than 0.3).
The Nonisothermal Flow, Laminar Flow and Conjugate Heat Transfer, Laminar Flow interfaces solve for conservation of energy, mass, and momentum in fluids and for conservation of energy in solids.
This multiphysics coupling defines p and u variables in order to set the Absolute pressure in the Model Input section and the Velocity field in the Heat Convection section of the Fluid feature in the Heat Transfer interface. In addition, it provides all the fluids quantities that may be needed by the Heat Transfer interface (for example, viscosity).
In the Fluid Flow interface, it sets the Temperature in the Model Input section and defines the Density in the Fluid Properties section of the Fluid Properties feature.
It also synchronizes the definition of the reference temperature to be used for incompressible flows. In addition, it also accounts for the multiphysics stabilization terms.
The Nonisothermal Flow coupling node triggers pseudo time stepping when Use pseudo time stepping for stationary equation form in the Fluid Flow interface is set to Automatic from physics.
The multiphysics stabilizations (streamline diffusion and crosswind diffusion) are controlled by the Fluid Flow interface. For example, the multiphysics streamline diffusion can be disabled in a Laminar Flow physics node, in the Stabilization section. The stabilization selected in the Heat Transfer interface has no effect if the multiphysics coupling stabilization is active but remains active if not. However, the isotropic diffusion is not a multiphysics stabilization and is controlled by each physics interface.
Finally, when one of the physics interfaces or the multiphysics coupling is not solved in a study step, then the stabilization of each solved physics is used instead of the coupled stabilization, and the solver suggestions are uncoupled.
Settings
The Label is the default multiphysics coupling feature name.
The Name is used primarily as a scope prefix for variables defined by the coupling node. Refer to such variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different coupling nodes or physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first multiphysics coupling feature in the model) is nitf1.
Domain Selection
When nodes are added from the context menu, you can select Manual (the default) from the Selection list to choose specific domains to define the nonisothermal flow, or select All domains as needed.
Coupled Interfaces
This section defines the physics involved in the multiphysics coupling. The Fluid flow and Heat transfer lists include all applicable physics interfaces.
The default values depend on how this coupling node is created.
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If it is added from the Physics ribbon (Windows users), Physics contextual toolbar (Mac and Linux users), or context menu (all users), then the first physics interface of each type in the component is selected as the default.
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If it is added automatically when a multiphysics interface is chosen in the Model Wizard or Add Physics window, then the two participating physics interfaces are selected.
You can also select None from either list to uncouple the node from a physics interface. If the physics interface is removed from the Model Builder — for example, Heat Transfer in Fluids is deleted — then the Heat transfer list defaults to None as there is nothing to couple to.
Material Properties
Select an option from the Specify density list: From heat transfer interface (the default), From fluid flow interface, Custom, linearized density, or Custom.
For Custom, linearized density, enter the Reference density ρref (SI unit: kg/m3) and the Coefficient of thermal expansion αp(SI unit:1/K), or select From material, or select a variable in the list if available. When Custom, linearized density is selected, regardless how the properties are defined they should be constant. If material properties are not constant you should consider using any of the other options to define the density.
For Custom, enter a Density ρ (SI unit: kg/m3), or select a density in the list if available.
The density definition in the Nonisothermal Flow node ensures that the same definition of the density is used on the fluid flow and heat transfer interfaces. When the fluid flow compressibility setting is set to Incompressible then the thermal conductivity and the heat capacity are evaluated at the Reference temperature defined in the fluid flow interface.
Location in User Interface
Context Menus
 Multiphysics>Nonisothermal Flow
when any version of the Single-Phase Flow interface is added together with Heat Transfer in Fluids (or another version of the Heat Transfer Interface, with a Fluid feature active).