Post on 03-Feb-2016
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Advanced CFD Analysis ofAerodynamics Using CFX
Jorge Carregal Ferreira
Achim Holzwarth, Florian Menter
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
CFX: Member of AEA Technology
Engineering Software
Engineering Software
Computational Fluid DynamiX
Computational Fluid DynamiX
Plant Simulation Software
Plant Simulation Software
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
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
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
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.
Preprocessing with CFX-Build
Geometry modeller basedOn MSC.Patran
Native CAD interfaces: Pro/Engineer, CATIA, Unigraphics, IDEAS, etc.
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
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.
Standard k- Model (Wilcox)
j
t
jk
j
ji
x
k
xkP
x
kU
t
k)(
)()(
j
t
jk
j
ji
xxP
kx
U
t
)()()( 2
k
t
Standard k- Model (Wilcox)
• Advantages:– Lower length scales near wall.– Robust sublayer formulation (low-Re).
• Problem:– Free stream sensitivity.– Has not replaced k- models.
k- Model Free Stream Problem
Change of in freestream
Velocity profile Eddy viscosity profile
k- vs. k- Formulation Model k- k-
Sublayer Robust Simple Accurate
Stiff Less accurate Complex
Log. Layer Accurate Large length scales
Wake Region
Missing transport effects
Missing transport effects
Boundary layer edge
Free-stream sensitive
Well defined
Optimal Two Equation Model
• Combination of k- and k- model:– k- model near the surface– k- model for free shear flows ( equation
is transformed to )• 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.
Diffuser Flow, 1k- model
SST model
Experiment Gersten et al.
Diffuser Flow, 2
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.
Scaling of Variables near Wall
Log. region
Outer region
yU
Sub-layer
CyU )log(1
Flat Plate: Velocity Profile
Intersection
Standard Wall Function New Wall Function
Finer Grids
Flate Plate: Wall Friction
Standard Wall Function New Wall Function
Finer GridsFiner Grids
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.
k- Automatic Switch
k- Automatic Switch
Pipe Expansion with Heat TransferStructured Grid (150.000 nodes)
Reynolds Number ReD= 23210
Fully Developed Turbulent Flow at Inlet
Experiments by Baughn et al. (1984)
Outlet
axis
H
H 40 x H
Inlet
q=0.
q=const.
d
D
Pipe Expansion with Heat Transfer
k- Model, Standard Wall Functions
Pipe Expansion with Heat Transfer
SST Model,
Low-Re Wall Treatment
Pipe Expansion with Heat Transfer
SST Model,
Automatic Wall Treatment
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
Thank you!