1. Advanced CFD Analysis of Aerodynamics Using CFX
Jorge Carregal Ferreira
Achim Holzwarth, Florian Menter

2. Outline CFX: Advanced CFD software Turbulence Modells in CFX
The company
The products
Turbulence Modells in CFX
Near wall treatment in CFX
Examples:
Duct with adverse pressure gradient
Airfoils
Heat transfer

3. CFX: Member of AEA Technology
Engineering Software
Computational Fluid DynamiX
Plant Simulation Software

4. CFX: Global Position
CFD (Computational Fluid Dynamics) group of AEA Technology
Largest European CFD company
210 employees
8 main offices
Strong industrial presence
Growth rate approx. 25% per year
More than 1500 installed licenses

5. CFD-Analysis Generate geometry: fluid domain
Generate mesh: discrete representation of fluid domain
Solve Navier-Stokes Equiations
Analyse Results
Coupling: Optimisation, fluid-structure coupling, accoustic analysis, design improvements

6. Leading Technology in CFX-5
Easy to use
Pre-Processor CFX-Build based on MSC.Patran:
Unstructured hybrid grids
Coupled algebraic multigrid-solver (AMG): Accurate, robust and fast
Solution time scales linear with grid size
Excellent parallel performance
Grid adaptation
UNIX, NT, Linux
HEX
TET
WEDGE
PYRAMID

7. Leading Technology in CFX-5
Laminar and turbulent flows.
Stationary and transient solutions.
Large variaty of turbulence models.
Transport equations for additional scalars.
Multi-component and multi-phase fluids.
Coupling with solid heat conduction.
Solution depended mesh adaptation.
Linear scaling of solver with grid size.
Scalable parallel performance.

8. Preprocessing with CFX-Build
Geometry modeller based
On MSC.Patran
Native CAD interfaces:
Pro/Engineer, CATIA,
Unigraphics, IDEAS, etc.

9. Turbulence Models in CFX-5
Release of the latest turbulence models
k- Model Variants
k- Model and BSL Model (Wilcox, Menter)
SST Model (Menter, Blending between k- and k-)
Reynolds Stress Models
Extended near-wall treatments
Scalable wall functions for k-
Automatic near-wall treatment for k- and SST
LES model (Smagorinski)
Documented validation cases on these models are available
Future: Improved LES and transition modelling

10. Problems of Standard k- Model
Two Problems:
Missing transport effects.
Too large length scales.
Result:
Reduced or omitted separation.
Very often: Too optimistic machine performance.

11. Standard k- Model (Wilcox)

12. Standard k- Model (Wilcox)
Advantages:
Lower length scales near wall.
Robust sublayer formulation (low-Re).
Problem:
Free stream sensitivity.
Has not replaced k- models.

13. k- Model Free Stream Problem
Change of w in freestream
Eddy viscosity profile
Velocity profile

14. k- vs. k- Formulation

15. Optimal Two Equation Model
Combination of k-w and k-e model:
k-w model near the surface
k-e model for free shear flows (e equation is transformed to w)
Blending is performed automatically based on solution and distance from the surface.
This model is called “Baseline Model – BSL”
Combined with “Shear-Stress-Transport” limiter offers optimal boundary layer simulation capabilities.
BSL+Limiter gives SST model.

16. Experiment Gersten et al.
Diffuser Flow, 1
k-e model
SST model
Experiment Gersten et al.

17. Diffuser Flow, 2

18. Wall Boundary Treatment
Standard wall function boundary conditions are the single most limiting factor in industrial CFD computations regarding accuracy!
“y+ has to be between 25 and 500” type statements are problematic!
Boundary layer resolution requirements have to be satisfied.
Log. Profile assumptions have to be satisfied.
To satisfy both at the same time is the challenge.

19. Scaling of Variables near Wall
Log. region
Outer region
Sub-layer

20. Flat Plate: Velocity Profile
Intersection
Finer Grids
Standard Wall Function
New Wall Function

21. Flate Plate: Wall Friction
Finer Grids
Finer Grids
Standard Wall Function
New Wall Function

22. Low-Re k- Model Viscous sublayer resolution. Simple formulation.
Numerically robust.
Grid resolution near wall y+<1-2.
Improved adverse pressure gradient behaviour.
Non-trivial boundary conditions.
Free stream dependency problem.
Blending possible.

23. k- Automatic Switch

24. k- Automatic Switch

25. Pipe Expansion with Heat Transfer
Structured Grid ( nodes)
Reynolds Number ReD= 23210
Fully Developed Turbulent Flow at Inlet
Experiments by Baughn et al. (1984)
Outlet
axis H
40 x H
Inlet
q=0 .
q=const d D

26. Pipe Expansion with Heat Transfer
k- Model, Standard Wall Functions

27. Pipe Expansion with Heat Transfer
SST Model,
Low-Re Wall Treatment

28. Pipe Expansion with Heat Transfer
SST Model,
Automatic Wall Treatment

29. Summary CFX: Advanced CFD software Fast and robust solver technology
Powerful Pre- and Postprocessing tools
Leading Turbulence Modells
Robust near wall treatment
Allows for
Accurate solutions
Reliable Predictions

30. Thank you!