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!