Nonisothermal Flow

Use the multiphysics coupling (

) to simulate fluid flows where the fluid properties depend on temperature. Models can also include heat transfer in solids or in porous media as well as surface-to-surface radiation and radiation in participating media, with the Heat Transfer Module. The physics interface supports low Mach numbers (typically less than 0.3).

The Nonisothermal Flow, Laminar Flow interface solves for conservation of energy, mass and momentum in fluids and porous media and for conservation of energy in solids. It synchronizes the features from the and interfaces when a turbulent flow regime is defined. It also complements the and feature from the flow interface to account for thermal effects.


	When the Nonisothermal Flow is used, there is no need to add a Flow Coupling or Temperature Coupling. Indeed, Nonisothermal Flow combines the effects of both of them. In addition, it also accounts for the multiphysics stabilization terms, for the heat transfer changes in the turbulent regime (for example, thermal wall functions), for work due to pressure forces and viscous dissipation, and for natural convection, including a Boussinesq approximation.


	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 physics 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 is the default multiphysics coupling feature name.

The 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 field. The first character must be a letter.

The default (for the first multiphysics coupling feature in the model) is nitf1.

Domain Selection

When nodes are added from the context menu, you can select (the default) from the list to choose specific domains to define the nonisothermal flow, or select as needed.

Coupled Interfaces

This section defines the physics involved in the multiphysics coupling. The and lists include all applicable physics interfaces.

The default values depend on how this coupling node is created.

•

If it is added from the ribbon (Windows users), 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.

•

If it is added automatically when a multiphysics interface is chosen in the or window, then the two participating physics interfaces are selected.

You can also select from either list to uncouple the node from a physics interface. If the physics interface is removed from the — for example, is deleted — then the list defaults to as there is nothing to couple to.

Heat Transfer Turbulence Model

This section is available when the fluid flow interface uses a turbulence model. Select an option from the list: (the default), , or .

For , enter a Reinf (dimensionless).

For , enter a prT (dimensionless).

When the flow interface uses a RANS turbulence model, the conductive heat flux is defined as

with the turbulent thermal conductivity defined as

where μT is defined by the flow interface, and PrT depends on the . See Turbulent Conductivity for details.

The used by the fluid flow interface can be displayed by selecting the button at the right of the list.

Material Properties

When the setting in the fluid flow interface is set to , select the check box in order to use material data evaluated at the reference temperature and reference pressure. If gravity is included in the physics, it is linearized with respect to temperature.

Select an option from the list: (the default), , , or .

For , enter the ρref (SI unit: kg/m3) and the αp(SI unit:1/K), or select or select a variable in the list if available. When 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 , enter a ρ (SI unit: kg/m3), or select a density in the list if available.

The density definition in the node ensure 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 then the thermal conductivity and the heat capacity are evaluated at the defined in the fluid flow interface. When is selected and the is set to in the fluid interface properties, the gravity forces are defined using the coefficient of thermal expansion. Along with the fact that the material properties are evaluated for a constant temperature and pressure, this gravity force definition corresponds to Boussinesq approximation. Unless the density is defined as the coefficient of thermal expansion is evaluated from the fluid density.

Flow Heating

When the setting in the fluid flow interface is set to or , select the check box to account for the heat source due to pressure changes:

By default this option is not selected; however, it should be selected for compressible fluids as soon as significant pressure gradients occur.

Select the check box to account for the heat source corresponding to viscous heating. This option is not selected by default. Because it may induce an extra computational cost it should be only selected in application where such effect is expected. If no information on this is available, selecting the option ensures that the energy balance for the heat and the flow equation is respected.


	When an interface is selected from the Heat transfer list, some of its model inputs are forced with values from the Nonisothermal Flow node. In addition, it defines how the turbulence has to be accounted for, depending on the Fluid flow interface’s turbulence settings. Therefore, each heat transfer or fluid flow interface should be used in at most one Nonisothermal Flow node. In cases where multiple fluid flow interfaces are used, an equal number of heat transfer interfaces and Nonisothermal Flow nodes are needed to define proper multiphysics couplings.


	If a physics interface is deleted and then added to the model again, then in order to reestablish the coupling, you need to choose the physics interface again from the Fluid flow or Heat transfer lists. This behavior is applicable to all multiphysics coupling nodes that would normally default to the once present interface. See Multiphysics Modeling Approaches in the COMSOL Multiphysics Reference Manual.


	Heat Sink: Application Library path Heat_Transfer_Module/Tutorials,_Forced_and_Natural_Convection/heat_sink

Location in User Interface

Context menus

when any of the following interface is added together with (or another version of the Heat Transfer Interface):
(any version)