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 defines p and u variables in order to set the in the section and the in the section of the feature and subfeature, 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, turbulence parameters).

In the Fluid Flow interface, it sets the in the section and defines the in the section of the and features.

It synchronizes also the definition of the reference temperature to be used for incompressible flows, and 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.

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 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 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.


	The pressure, velocity, and temperature variables of the Nonisothermal Flow coupling node are set to the Common Model Input values of the Default Model Inputs node on its complementary selection, i.e. all domains except those from the Selection list. It allows to couple multiple fluid flow interfaces with a single heat transfer interface. See Default Model Inputs in the COMSOL Multiphysics Reference Manual for details.

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

The coupling node selection is locked to the union of

•

the intersection of the or features’ selection with the feature’s selection

•

the intersection of the feature’s selection with the feature’s selection

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

Boussinesq Approximation

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.

Density

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

•

For : define the ρ in the section of the node, in the Heat Transfer coupled interface. Depending on the option in this node, the density may bet taken from material, set directly, or computed by using the ideal gas law. The same value is automatically set in the section of the node, in the Fluid Flow coupled interface.

•

For : define the ρ in the section of the node, in the Fluid Flow coupled interface. The same value is automatically set in the section of the node, in the Heat Transfer coupled interface.

•

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. 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. Also, if is used for incompressible flow, the density ρ is evaluated to ρref. In this case αp is not used unless is selected. The same value is automatically set in the section of the node, in the Fluid Flow coupled interface, and in the section of the node, in the Heat Transfer coupled interface.

•

For , enter a ρ (SI unit: kg/m3), or select a density in the list if available. The same value is automatically set in the section of the node, in the Fluid Flow coupled interface, and in the section of the node, in the Heat Transfer coupled interface.

See Fluid Properties and Fluid in the COMSOL Multiphysics Reference Manual for details.

When the coupled Heat Transfer interface is also coupled to a Phase Transport interface via the multiphysics coupling, should be set to . See Nonisothermal Mixture Model in the CFD Module User’s Guide for details.

The density definition in the 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 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.

Reference Temperature

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

•

For , set the Tref (SI unit: K) in the section of the interface selected in the list of the section. The input in the interface is synchronized to the same value or expression, and is not editable.

•

For , set the Tref (SI unit: K) in the section of the interface selected in the list of the section. The input in the interface is synchronized to the same value or expression, and is not editable.

•

For , set a value or expression. The inputs in the sections of the and interfaces are synchronized to the same value or expression, and are not editable.

Flow Heating

The check box is selected by default to account for the heat source corresponding to viscous heating. 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 fluid flow interface should be used in at most one Nonisothermal Flow node. In cases where multiple fluid flow interfaces are used, they can be coupled with a single heat transfer interface, using an equal number of Nonisothermal Flow nodes to define proper multiphysics couplings. In the Heat transfer interface, the inputs for pressure and velocity are then taken from the first Nonisothermal Flow node.


	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 Workflow 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):