Advanced CFD Analysis of Aerodynamics Using CFX

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Advanced CFD Analysis ofAerodynamics Using CFX

• Jorge Carregal Ferreira
• Achim Holzwarth, Florian Menter

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Outline

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

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CFX Member of AEA Technology
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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

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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

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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

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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.

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

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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.

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Standard k-? Model (Wilcox)
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Standard k-? Model (Wilcox)

• Advantages
• Lower length scales near wall.
• Robust sublayer formulation (low-Re).
• Problem
• Free stream sensitivity.
• Has not replaced k-? models.

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k-? Model Free Stream Problem
Change of w in freestream
Eddy viscosity profile
Velocity profile
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k-? vs. k-? Formulation
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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.
• BSLLimiter gives SST model.

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Diffuser Flow, 1
Experiment Gersten et al.
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Diffuser Flow, 2
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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.

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Scaling of Variables near Wall
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Flat Plate Velocity Profile
Intersection
Finer Grids
Standard Wall Function
New Wall Function
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Flate Plate Wall Friction
Finer Grids
Finer Grids
Standard Wall Function
New Wall Function
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Low-Re k-? Model

• Viscous sublayer resolution.
• Simple formulation.
• Numerically robust.
• Grid resolution near wall ylt1-2.
• Improved adverse pressure gradient behaviour.
• Non-trivial boundary conditions.
• Free stream dependency problem.
• Blending possible.

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k-? Automatic Switch
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k-? Automatic Switch
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Pipe Expansion with Heat Transfer
Structured Grid (150.000 nodes) Reynolds Number
ReD 23210 Fully Developed Turbulent Flow at
Inlet Experiments by Baughn et al. (1984)
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Pipe Expansion with Heat Transfer
k-? Model, Standard Wall Functions
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Pipe Expansion with Heat Transfer
SST Model, Low-Re Wall Treatment
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Pipe Expansion with Heat Transfer
SST Model, Automatic Wall Treatment
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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

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Thank you!