Advanced CFD Analysis of Aerodynamics Using CFX
description
Transcript of Advanced CFD Analysis of Aerodynamics Using CFX
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!