CFD Module

New Functionality in Version 4.3

Turbulent Mixing

The user interfaces for Transport of Diluted Species and Transport of Concentrated Species include a new Turbulent Mixing subnode to the Convection and Diffusion material model. The Turbulent Mixing node models the additional mixing caused by turbulence by adding turbulent diffusivity to the molecular diffusivity.

Fluid Flow

• Stabilization has been updated to increase accuracy for meshes with high aspect ratio. This update affects the following interfaces:

• Single Phase Flow

• High Mach Number Flow

• Two-Phase Flow

• Mixture Model

• Bubbly Flow

• Euler-Euler Model

• Vacuum Pump is now a separate feature instead of an option in the Outlet feature. The Vacuum Pump feature has enhanced graphical support that shows the intended flow direction during physics set up.

• New scaling has been introduced for time-dependent turbulence problems. The new scaling makes the simulation more robust but still accurate. This makes, for example, the Airlift Loop Reactor model easier to solve.

Rotating Machinery

Automatic support for a time-dependent angular velocity has been added in the Rotating Domain feature. In the new implementation the displacement angle of the rotating domain is defined as an ODE variable and integrated using the rotational frequency input. The implementation supports all types of time-dependent frequencies (analytic, interpolation functions, and so on).

Backward compatibility:

• The new implementation does not solve for the rotating wall velocities. Due to this, along with the addition of the displacement angle in the solution, the number of degrees of freedom (DOFs) in a model created in 4.3 will differ compared to that of a model created in a previous version.

• When you open an model using rotating machinery created in a previous version, the previous implementation will be used, retaining the previous number of degrees of freedom.

• Any attempt to access to the wall velocity variables in a Java script using 4.3 will fail.

Non-Isothermal Flow and Heat Transfer

• A new boundary condition, Interior Wall, is available on interior boundaries in the Non-Isothermal Flow interfaces. It makes it possible to define a wall condition between two fluid domains. This is especially useful to model thin walls as boundaries. You no longer need to define a solid domain with a wall boundary condition on both sides, which can result in a dense mesh. This boundary condition implements wall functions when using the k-ε or k-ω turbulence model.

• The inflow heat flux boundary condition has been improved so that heat transport into the domain is controlled by the flow convection. This change prevents unphysical results like computing high temperatures in no-flow parts of inflow heat flux boundaries.

• Enthalpy and internal energy are now calculated using state integrals. This gives increased accuracy for heat and energy balances.

Backward Compatibility vs. 4.2a

Discontinuous Galerkin Conditions

The variables used to set up discontinuous Galerkin conditions have been revised. Neither of those variables were available as postprocessing variables; they were only visible in the Equation View nodes’ Variables lists. The most notable change is that the variable K_Stress_Tensor has changed sign in order to conform with the literature. Please contact support for further information.

This compatibility information affects the following fluid-flow interfaces:

• Single-Phase Flow

• Brinkman Equations

• Free and Porous Media Flow

• Two-Phase Flow, Level-Set

• Two-Phase Flow, Phase Field

• Bubbly Flow

• Mixture Model

Fan, Grille, and Vacuum Pump Features

The Fan, Grille, and Vacuum Pump features are now separate features instead of options in the Inlet and Outlet features.

Old models with Inlet or Outlet features get the new features instead when opened. The Model Java-file backward compatibility is extensive but not complete. If you experience problems with old Model Java- files, please contact COMSOL’s technical support.

This compatibility information affects the following fluid-flow interfaces:

• Creeping Flow

• Laminar Flow

• Fluid-Structure Interaction

• Non-Isothermal Flow

• Slip Flow

Heat Transfer

• Due to the new Inflow Heat Flux boundary condition behavior, the results of models using it are affected. This should lead to better physical results. It is, however, possible to add a compensation flux to the model to retrieve the old behavior. Please contact COMSOL’s technical support for this purpose.

• Heat Continuity and Periodicity features from Non-Isothermal flow interface have been updated to support wall function when using a k-ε or k-ω turbulence model.

Dependent Variable Change in the Rotating Machinery Interfaces

When building models in the current release using the Rotating Machinery interfaces, the dependent variable vwall, describing the rotating wall velocity, will no longer be solved for. Opening a model built in a previous version, the previous implementation will be retained (including vwall). A Java file generated from the previous model may fail to run using the current version. This happens when the no longer present vwall variable is accessed, or if a solver setting for this variable is applied. To avoid this, remove any access to vwall in the generated Java file.