The Nonisothermal Flow, Laminar Flow and Viscoelastic Flow Interfaces
The Nonisothermal Flow and Conjugate Heat Transfer Interfaces that combine the heat equation with laminar flow in a fluid domain are described in the COMSOL Multiphysics Reference Manual. The Polymer Flow module has additional functionality to couple the viscoelastic flow and heat transfer.
When the Viscoelastic Flow multiphysics interface () is added from the Fluid Flow > Nonisothermal Flow branch of the Model Wizard or Add Physics windows, the Heat Transfer in Fluids and the Viscoelastic Flow interfaces are added to the Model Builder. In addition, the Multiphysics node is added, which automatically includes the multiphysics coupling feature Nonisothermal Flow.
Energy balance
If the coupled interface is Laminar Flow, the Include viscous dissipation checkbox 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.
If the coupled interface is Viscoelastic Flow, the Include irreversible losses checkbox is available. The checkbox is selected by default to account for the heat source corresponding to the heating due to irreversible losses.
The Polymer Flow module has additional functionality to simplify the solution of the natural convection problems. The Boussinesq approximation can be used to include buoyancy effects, without having to solve the compressible formulation of the Navier–Stokes equation. The approximation is accurate when density variations are small, that reduces the nonlinearity of the problem. The Boussinesq approximation assumes that variations in density have no effect on the flow field, except that they give rise to buoyancy forces.
Material Properties
Boussinesq Approximation
When the Compressibility setting in the fluid flow interface is set to Incompressible, select the Boussinesq approximation checkbox in order to use material data evaluated at the reference temperature and reference pressure. If gravity is included in the physics, the buoyancy contribution in the momentum equation is linearized with respect to temperature.
Density
Select an option from the Specify density list: From heat transfer interface (the default); From fluid flow interface; Custom, linearized density; or Custom:
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For From heat transfer interface: define the Density ρ in the Thermodynamics, Fluid section of the Fluid node, in the Heat Transfer coupled interface. Depending on the Fluid type 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 Fluid Properties section of the Fluid Properties node, in the Fluid Flow coupled interface.
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For From fluid flow interface: define the Density ρ in the Fluid Properties section of the Fluid Properties node, in the Fluid Flow coupled interface. The same value is automatically set in the Thermodynamics, Fluid section of the Fluid node, in the Heat Transfer coupled interface.
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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. 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 Custom, linearized density is used for incompressible flow, the density ρ is evaluated to ρref. In this case αp is not used unless Boussinesq approximation is selected. The same value is automatically set in the Fluid Properties section of the Fluid Properties node, in the Fluid Flow coupled interface, and in the Thermodynamics, Fluid section of the Fluid node, in the Heat Transfer coupled interface.
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For Custom, enter a Density ρ (SI unit: kg/m3), or select a density in the list if available. The same value is automatically set in the Fluid Properties section of the Fluid Properties node, in the Fluid Flow coupled interface, and in the Thermodynamics, Fluid section of the Fluid node, in the Heat Transfer coupled interface.
The density definition in the Nonisothermal Flow node ensures that the same definition of the density is used in the fluid flow and heat transfer interfaces. When Include gravity is selected and the Compressibility is set to Incompressible flow 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 the Boussinesq approximation. Unless the density is defined as Custom, linearized density the coefficient of thermal expansion is evaluated from the fluid density.
The Viscoelastic Flow interface only supports incompressible flow.