CFD Module

New Functionality in Version 4.3a

New Study Types for One-way FSI

For improved performance for certain fluid-structure interaction (FSI) models, two new study types simplify setting up and solving one-way coupled FSI models:

• Stationary, One-Way Coupled

• Time Dependent, One-Way Coupled

Both study types solve for the fluid flow first, the for the solid mechanics. Other physics can be included in either or both of these study steps.

Reacting Flow User Interface

The new Reacting Flow user interface is a multiphysics interface for mass transport in fluids. The Reacting Flow user interface combines the physics from Single Phase Flow and Transport of Concentrated Species. Key features:

• A new reaction feature.

• Pseudo time stepping for both the species and the momentum equations.

• Turbulence modeling by RANS:

• κ-ε

• κ-ω

• low-Reynolds number κ-ε

• Includes two algebraic models for the turbulent Schmidt number:

• Kays-Crawford

• High-Schmidt number model.

• Turbulent reaction modeling using Eddy Dissipation Concept (EDC).

• Wall functions for turbulent mass transport.

Spalart-Allmaras Turbulence Model with Rotation Correction

The Spalart-Allmaras model has been supplemented with a “rotation correction.” The correction is passive in thin shear layers while it gives superior predictions in for example core regions. The rotation correction can effect the solutions of old models.

Improved Meshing Defaults

The meshing defaults have been improved to include corner refinement, trimming of mesh elements at sharp corners (instead of splitting), and generally improved settings for robust and accurate meshing.

Improved Pseudo Time-stepping

The pseudo time-stepping algorithm, used per default for turbulence models and for Non-Isothermal Flow and Conjugate Heat Transfer models, has been improved. The new implementation uses a PID regulator, which compared to the 4.3 implementation makes the convergence more robust without using additional iterations. The additional robustness reduces the need for manual tuning of the solvers and makes it easier to solve stationary problems, especially for turbulent flow.

As a result of this update, some old models may experience different convergence behavior compared to version 4.3.

Flexible Default Solver Settings

The default solver settings for all fluid-flow user interfaces are now based on the number of mesh elements and use a direct solver for small models (up to 100, 000 elements in 3D or 300,000 elements in 2D). For larger models, additional multigrid levels are added for an iterative solver. The first additional level is added at 600,000 elements in 3D, and the solver uses a maximum of four levels, including the finest mesh.

Thin-film Flow Interfaces

A major upgrade has been performed on the Thin-Film Flow interfaces. New functionality has been added and errors in the weak form implementation, present in the previous versions, have been corrected. Correspondingly results may differ in comparison to previous versions. It is recommended that models from version 3.5a and earlier are rebuilt in the current version. The Thin-Film Flow user interfaces now have easier to understand names and new options for nonidentical slip coefficients at the wall and the base.

Phase Field Interface: Transiend with Initialization Study

The 3D Phase Field user interface now includes a preset Transient with Initialization study in addition to the Time Dependent study.

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.

New Models in Version 4.3a

• A new benchmark model, blasius_boundary_layer, models a Blasius boundary layer for a 2D laminar flow. The model investigates the effect of mesh refinement as well as changes in the basis function order, and the results are compared with a theoretical similarity solution.

• A new benchmark model for the Thin-Film Flow interfaces has been added (slider_bearing_1d). This model computes the flow and forces in a 1D slider bearing and compares the results with exact analytic solutions.

• A second new benchmark model for the Thin-Film Flow interfaces has been added (step_bearing_1d). This model computes the flow and forces in a 1D step bearing and compares the results with exact analytic solutions.

Backward Compatibility vs. 4.3

Fluid-structure Interaction

The Fluid-Structure Interaction (FSI) multiphysics interface has been updated. The separate vWall field is no longer required and has been removed. FSI models from 4.3 and earlier versions that include Time Dependent study steps will be affected in the following way:

• Model Java-files will fail. Any reference to the vWall field must be removed.

• Models older than 4.3 (that is, 4.2a and older) must either clear their time-dependent solutions when opened in 4.3a. An alternative procedure is to:

• Open the model in version 4.3.

• Right-click any study with a time-dependent study step and select Update Solution.

• Save the model.

• Open the model in version 4.3a.

FSI models with only stationary study steps will not be affected. Note that vWall will still be available as a variable. Hence, references to fsi.vWall made in, for example, another physics still work.

Brinkman Equations and Free and Porous Media Flow

The momentum equations in the Brinkman Equations interface and the Free and Porous Media Flow interface have been corrected. The term -Q_br·u/ε_p²in the right hand side previously lacked the factor 1/ε_p² , where ε_p is the porosity.

Reacting Flow Interface Name Change

The Reacting Flow interfaces have been renamed Reacting Flow in Porous Media. Opening a model using either the Reacting Flow, Diluted Species or the Reacting Flow, Concentrated Species interface, the interface is renamed Reacting Flow in Porous Media.

Weak Constraints Update for Fluid Flow Interfaces

The weak constraints formulation for the following boundary conditions in the following interfaces has been updated:

Laminar Flow and Creeping Flow

• Symmetry

• The Slip boundary condition in the Wall feature

Turbulent Flow, κ-ε and Turbulent Flow, κ-ω

• Symmetry

• The Wall functions boundary condition in the Wall feature

• The Moving wall (wall functions) boundary condition in the Wall feature

• The Slip boundary condition in the Wall feature

Turbulent Flow, low-Re κ-ε and Turbulent Flow, Spalart-Allmaras

• Symmetry

• The Slip option in the Wall feature

Rotating Machinery

• Symmetry

• The Slip boundary condition in the Wall feature

• The Wall functions boundary condition in the Wall feature (turbulent flow only)

• The Moving wall (wall functions) boundary condition in the Wall feature (turbulent flow only)

• Rotating Wall (turbulent flow only)

Bubbly Flow

• Symmetry

• The Slip boundary condition for the liquid phase in the Wall feature

• The Wall functions boundary condition in the Wall feature (turbulent flow only)

Euler-Euler Model, Laminar Flow

• Symmetry

• The Slip boundary condition for either phase in the Wall feature

• The Slip boundary condition for either phase in the Inlet feature

Brinkman Equations and Free and Porous Media Flow

• Symmetry

• The Slip boundary condition in the Wall feature

Two-Phase Flow, Level Set

• Symmetry

• The Slip boundary condition in the Wall feature

• The Wetted wall boundary condition in the Wall feature

• The Moving wetted wall boundary condition in the Wall feature

• The Wall function boundary condition in the Wall feature (turbulent flow only)

• The Moving wall (wall functions) boundary condition in the Wall feature (turbulent flow only)

Two-Phase Flow, Phase Field

• Symmetry

• The Slip boundary condition in the Wall feature

• The Wall function boundary condition in the Wall feature (turbulent flow only)

• The Moving wall (wall functions) boundary condition in the Wall feature (turbulent flow only)

Non-Isothermal Flow and Conjugate Heat Transfer

• Symmetry

• The Slip boundary condition in the Wall feature

• The Wall functions boundary condition in the Wall feature (turbulent flow k-ε and turbulent flow k-ω only)

• The Moving wall (wall functions) boundary condition in the Wall feature (turbulent flow k-ε and turbulent flow k-ω only)

High Mach Number Flow

• Symmetry

• The Slip boundary condition in the Wall feature

• The Wall functions boundary condition in the Wall feature (turbulent flow k-ε only)

• The Moving wall (wall functions) boundary condition in the Wall feature (turbulent flow k-ε only)

These boundary condition are now formulated using the same set of Lagrange multipliers as all the other boundary conditions for the dependent velocity variables. The previously used Lagrange multiplier un_lm has been removed.

Models saved in version 4.3 can be opened and postprocessing will include un_lm until the model is re-solved. In some cases, occurrences of un_lm in the solver sequence must be replaced manually. This is the case if un_lm was the only Lagrange multiplier component in a segregated group or the only Lagrange multiplier component of a Vanka smoother. Alternatively, generate a new automatic solver sequence. Models saved in versions previous to version 4.3 must either be re-solved in version 4.3a to be possible to postprocess, or opened and re-saved in version 4.3 before opened in version 4.3a.

Weak constraints for the Interior Wall feature are no longer available.

Revision of the Turbulence Models

The formulations of some variables in the turbulence models have been revised in order to improve accuracy. Models using a turbulence model can display a different convergence behavior in version 4.3a compared to version 4.3 and, the result can differ slightly between the versions.