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&Vehicle Aerodynamics & Aeroacoustics DevelopmentUsing Open-Source CFDDr. Moni Islam, 08. 11. 2013

Aerodynamics Development Using Open-Source CFDContentsContents

• Motivation for and Potential of Improved Vehicle Aerodynamics

• Tools of the Aerodynamicist of the Future

• Application of Open-Source CFD Technology in the Aerodynamics Development Process

• Challenges of Open-Source Software in an Industrial Environment

d l• Summary and Conclusions

2 Dr. Moni Islam, Wind-Tunnel Centre, 08. 11. 2013

Motivation for and Potential of Improved Vehicle Aerodynamics

Aerodynamics Development Using Open-Source CFDCO2 Targets for Audi FleetCO2 Targets for Audi Fleet

20202012 2014 2016 2018 2022 2024 2025

?130 g/km

2008

95 g/km

Reduction for all EU OEMs – 4,1 % p.a. – 4 % to -7 % p.a. (expected)

?

155 g/km

101 g/km

2008 R d ti f ll USA OEM 4 0 %

162 g/km

117 g/km

5 0 l/100 km

2008 Reduction for all USA OEMs – 4,0 % p.a.

• Pressure on CO2-Value of entire Audi fleet due to worldwide legislation

6,9 l/100 km5,0 l/100 km

2008 Reduction for all chinese OEMs – 3,9 % p.a.


2 g

Aerodynamics Development Using Open-Source CFDAerodynamics in Vehicle Driving CyclesAerodynamics in Vehicle Driving Cycles

• Contributors to CO2 emissions during 120140

driving cycle

• Inertial forces (vehicle mass)

• Rolling resistance of tyres20406080

100120

eed

(km

/h)

NEDC: U ~ 33.6 km/h

g y

• Aerodynamic drag

• Auxiliary systems

020

0 200 400 600 800 1000 1200 1400 1600 1800

Spe

Time (sec.)

• Weighting of each component depends on driving cycle due to different speeds and cycle dynamics

80

100

120

140

km/h

)

WLTP: U ~ 46.5 km/h

cycle dynamics

• Influence of aerodynamics to increase with WLTP (due to take effect in 2020)

0

20

40

60

0 200 400 600 800 1000 1200 1400 1600 1800

Spee

d (k

Time (sec )


WLTP (due to take effect in 2020) Time (sec.)

Aerodynamics Development Using Open-Source CFDContribution of Aerodynamics in Driving CycleContribution of Aerodynamics in Driving Cycle

• Aerodynamics plays a significant role in energy consumption i d i i lin driving cycle

• Relative contribution to C02 emissions for example2 p

• Audi A8 (cD = 0.26, A = 2.41 m2)

• Audi customer statistical driving cycle (Vmean = 61 km/h)

l ( )

Rolling resistance

(16.4%)

• Conventional powertrain (4.2 FSI)

• Largest losses incurred by overcoming aerodynamic drag Inertia

Aerodynamic drag

(43.4%)g y g y g

• Acceleration of vehicle mass nearly equal in contribution

Inertia

(40.2%)


Aerodynamics Development Using Open-Source CFDContribution of Aerodynamics in Driving CycleContribution of Aerodynamics in Driving Cycle

• Significant increase in importance of aerodynamics for f t l t ifi d t i tfuture, electrified powertrain concepts

• Modifications to previous example Rolling resistancep p

• Hybridised e-tron powertrain

• Recuperation of kinetic energy during braking with 66% efficiency

Aerodynamic drag

(62 0%)

g

(25.7%)

efficiency

• Relative increase of 43% in contribution of aerodynamics to

(62.0%)Inertia

(12.4%)

CO2 emissions due to recuperation

• See also article by J Wiedemann ATZ 2009


See also article by J. Wiedemann, ATZ 2009

Aerodynamics Development Using Open-Source CFDHistorical Development of Aerodynamic DragHistorical Development of Aerodynamic Drag

How to make Audi A2

cD = 0 25 How to make further progress

for real production vehicles, given

cD 0.25

Audi A8 3.0 TDI vehicles, given diminishing

returns?

cD = 0.26


Aerodynamics Development Using Open-Source CFDPotential for Aerodynamics Optimisation (1)Potential for Aerodynamics Optimisation (1)

• Induced drag

• Contribution to drag of downwash generated by lift “avoidable” drag

• Minimised by overall form optimisation of vehicle• Minimised by overall form optimisation of vehicle

• Cooling drag

• Ram drag

Dictated by cooling requirements of engine

Active control: Inlet louvers (BMW Mercedes)Active control: Inlet louvers (BMW, Mercedes)

• Spillage / interference

Aerodynamic interaction between cooling air and vehicle


Details of wheels and underbody particularly important

Aerodynamics Development Using Open-Source CFDPotential for Aerodynamics Optimisation (2)Potential for Aerodynamics Optimisation (2)

• Underbody

• Potential conflict with thermal management integrated development process required

• Must be optimised for basic vehicle shape • Must be optimised for basic vehicle shape (different measures for Avant than for saloon)

• Package

• Examples: Audi A2, VW XL1 concept car

• Paradigm-shift required for customers, Paradigm shift required for customers, stylists … and engineers

• Maintaining brand identity a challenge


Tools of the Aerodynamicist of the Future

Aerodynamics Development Using Open-Source CFDDevelopment Tools – ExperimentalDevelopment Tools Experimental

• High accuracy required!

• Audi Aeroacoustic Wind Tunnel (1998)

• Open test section• Open test section

• 11 m2 nozzle

• Full ground simulation

5-belt system and BL suction

• 6-component balance for forces and moments up to U∞ = 300 km/h

• Demand now significantly exceeds capacity


Aerodynamics Development Using Open-Source CFDDevelopment Tools – Ground Simulation in Wind Tunnel

• Ground simulation very important for proper accuracy

Development Tools Ground Simulation in Wind Tunnel

• Fundamental influence on vehicle’s aerodynamics

• Relevant not only for underbody-component development but also vehicle body optimisationoptimisation

Without GS With GS

• For details, see SAE 2005-01-0872 and 2006-01-0339 and references contained therein


Aerodynamics Development Using Open-Source CFDDevelopment Tools – Numerical SimulationsDevelopment Tools Numerical Simulations

• Advanced aerodynamics development no longer possible without CFD due to complexity of problems to be solved and accuracy required

• CFD for vehicle aerodynamics standard component of development process with multiple goals

• Evaluation of styling models in early development phase

• Substitution of wind-tunnel experiments to f i ffi i i icompensate for insufficient testing capacity

• Supplementary information to wind-tunnel data for analysis of phenomena of interest


Aerodynamics Development Using Open-Source CFDDevelopment Tools – Requirements of CFD in AerodynamicsDevelopment Tools Requirements of CFD in Aerodynamics

• Very short turn-around times / high process integration to keep pace with development cycle

• <3 days from new geometry to aerodynamics result

• High robustness of solver

• U bl l b t • Useable also by non-expert users

• High accuracy of resultsg y

• Trends found in experiments must be captured

• Accuracy must be reliable, especially where no experiments are available

• Acceptable costs

• Must be competitive with wind tunnel experiments


p p

Aerodynamics Development Using Open-Source CFDDevelopment Tools – Motivation for Considering Open-Source CFDDevelopment Tools Motivation for Considering Open Source CFD

• Commercial environment for CFD codes

• Very small number of commercial codes truly viable for productive use

• Proprietary technology offering limited insight or black-box approach

• Li f i ith i i • License fees increase with increasing use

• Code development driven primarily by vendor’s interest

• Very high overhead associated with switching to alternative producty g g p

• Limitations to meeting requirements for aerodynamics development process

• Audi’s conclusion: Alternative approach needed!


Aerodynamics Development Using Open-Source CFDDevelopment Tools – Open-Source CFD (1)Development Tools Open Source CFD (1)

• Solution to many observed problems provided by open-source model for CFD code

• High process integration

• R b t f d li ti d hi d b li ti ifi • Robustness, ease of use and application speed achieved by application-specific customisation

• High accuracy in principle

• Full transparency of technology (vs. black-box approach) permits complete analysis and solution of problemssolution of problems

• New / alternative technology can be implemented rapidly on demand


Aerodynamics Development Using Open-Source CFDDevelopment Tools – Open-Source CFD (2)Development Tools Open Source CFD (2)

• Costs under GPL licensing

• Remain fixed with increasing use: No license fees coupled to solver use

• Limited and predictable: User pays for only what he needs

• General advantages

• Excellent long-term potential for technological development and process integration due g p g p p gto high customisability

• No inherent disincentives to use of technology

Closer coupling to vehicle development process through increased useCloser coupling to vehicle development process through increased use

More rapid technological development

• User has free choice of technology provider


f O S CApplication of Open-Source CFD Technology in theAerodynamics Development Process

Aerodynamics Development Using Open-Source CFDApplication of Open-Source CFD TechnologyApplication of Open Source CFD Technology

• OpenFOAM-based open-source CFD toolbox chosen by Audi

• Customised applications development, support and consulting by Icon Ltd.

• Based on public-domain OpenFOAM toolbox

• Multi-year project to fully integrate OpenFOAM-based applications into Audi aerodynamics development process

• Development and support by Icon and OpenCFD

• Validation and integration in collaboration with Volkswagen and SEAT

• Details first published in SAE 2009 01 0333• Details first published in SAE 2009-01-0333

• Full, exclusive productive use for vehicle development since January 2009


Aerodynamics Development Using Open-Source CFDAerodynamics CFD ProcessAerodynamics CFD Process

CAD Hardware via photogrammetry

CAD-surface preparation

Triangulated surface

CFD

Input to volume mesher


CFD

Aerodynamics Development Using Open-Source CFDAerodynamics CFD Applications – Mesh GeneratorAerodynamics CFD Applications Mesh Generator

• Volume mesher developed and maintained by Icon

• Based on snappyHexMesh from public OpenFOAM releasep

• Unstructured hexahedral meshes

• Local refinement

• Feature-line handling

• Cell-quality optimisation

• Fully parallel operationFully parallel operation


Aerodynamics Development Using Open-Source CFDAerodynamics CFD Applications – Flow SolverAerodynamics CFD Applications Flow Solver

• Multi-step solution procedure developed and maintained by Icon

• Incompressible LES

• DES formulation using Spalart-• DES formulation using SpalartAllmaras model

• Based on oodles solver from public OpenFOAM releaseOpenFOAM release

• Case set-up application to set initial and boundary conditions

• Local blending for differencing schemes to increase solver stability

• Function objects for on-the-fly analysis


j y y

Aerodynamics Development Using Open-Source CFDAerodynamics CFD Applications – Job HandlingAerodynamics CFD Applications Job Handling

• Post-processing using EnSight (data conversion with foamToEnsight application)

• Job handling using standard Audi CAE workflow• Job handling using standard Audi CAE workflow

• FlowGuide browser-based automated workflow

Sets job-related parameters

Executes all steps in CFD process

Manages data handling for cluster operation

• LSF queueing system• LSF queueing system

• Systems integration by S+C AG and Icon


Aerodynamics Development Using Open-Source CFDAerodynamics CFD with OpenFOAM-Based TechnologyAerodynamics CFD with OpenFOAM Based Technology

• Fully automatic volume meshing in parallel

• No manual user intervention after CAD-surface preparation

• Mesher and solver from same package provides optimum in process robustness

• Large-eddy simulation instead of RANS/k- or lattice-Boltzmann

• Higher accuracy through better fundamental modelling

f f• Detached-eddy formulation for low turn-around times and easier mesh generation

• Unsteady simulation captures important / relevant spatial and temporal scales

• Current turn-around times for full DES run starting from prepared CAD surfaces

• 2.5 days for mock-up cases

• 4 days for cases with full underbonnet flow


• 4 days for cases with full underbonnet flow

Aerodynamics Development Using Open-Source CFDCurrent Aerodynamics CFD ResultCurrent Aerodynamics CFD Result

• Example from current standard CFD setup

• Audi A4 Avant

• Includes ground simulation & underhood flow

• M d l i 100 M ll• Model size: ca. 100 M cells

• Number of cores: 256

• Simulation run time: ca. 87 h for 2 s physical timep y

cD [-] cLf [-] cLr [-]

Experiment 0.316 0.086 0.047

• Rear lift typically problematic for estate vehicle

p

Simulation 0.313 0.084 0.071


yp y p

Aerodynamics Development Using Open-Source CFDValidation Example from 2009 SAE PaperValidation Example from 2009 SAE Paper

• Example: Audi A6 predecessor production vehicle (mock-up, no ground simulation)

Experiment LES (time average) LES (instantaneous)

cpt wake

Oil streak


Aerodynamics Development Using Open-Source CFDCurrent Aerodynamics CFD ProductivityCurrent Aerodynamics CFD Productivity

• Current computing resources

• NEC LX2200 cluster with 8064 cores (Intel Xeon E5-2660)

• QDR Infiniband interconnect

• J b 128 t 256 • Jobs run on 128 to 256 cores

• Queueing system configured to run up to 10 jobs simultaneously

• Approximately 2700 aerodynamics jobs run per year

• More than 12000 OpenFOAM jobs overall (aero, underhood, HVAC) run per year at AudiHVAC) run per year at Audi

Open-source CFD technology successfully applied at Audi for vehicle aerodynamics development!


for vehicle aerodynamics development!

Aerodynamics Development Using Open-Source CFDOngoing Work – Ground Simulation and Cooling Drag / Lift

• Refinement of existing methodology ongoing, as need for improved accuracy always exists

Ongoing Work Ground Simulation and Cooling Drag / Lift

• Methods development always done in conjunction with vehicle development

• Intimate understanding of all aspects of wind-tunnel testing essential for assessing accuracy of experimental data and pointing to weaknesses of current methodologyof experimental data and pointing to weaknesses of current methodology

Ground simulation Cooling drag / lift


Aerodynamics Development Using Open-Source CFDOngoing Work – AeroacousticsOngoing Work Aeroacoustics

• Aeroacoustics CFD currently not standard part of vehicle-development process

• Methods development for various applications ongoing

• Productive use only expected in the long term due to very high complexity of physics

Mirror flowCavity flow Flow-induced vibration


C f O S S fChallenges of Open-Source Software in an Industrial Environment

Aerodynamics Development Using Open-Source CFDOpen-Source Software in an Industrial Environment (1)Open Source Software in an Industrial Environment (1)

• Integrating open-source software in industrial environment poses significant challenges

• Debunking of common myths required

• O / GPL li i d t CFD t thi• Open source / GPL licensing does not mean CFD costs nothing

• Costs exist and must be borne by the user

• High level of user competence required

• Use of software as a “black box” of limited utility

h fl b l ff d b lb d h k l d• High flexibility offered by OpenFOAM toolbox requires in-depth knowledge

By end user: Process, engineering, applications CFD

By technology provider: Technical CFD, industrial process


y gy p , p

Aerodynamics Development Using Open-Source CFDOpen-Source Software in an Industrial Environment (2)Open Source Software in an Industrial Environment (2)

• Understanding of intellectual property advantageous

• General reluctance toward open-source software due to perceived risk to know-how and investment

• Need for clear demarcation between IP of public domain technology provider and end user• Need for clear demarcation between IP of public domain, technology provider and end user

• Clear conceptual understanding by both management and technical staff essential

• Legal counselling beneficial

• Long-term strategy required

• Technical overhead involved but similar to when switching between proprietary codes• Technical overhead involved, but similar to when switching between proprietary codes

• Partnership approach with technology provider essential


Aerodynamics Development Using Open-Source CFDSummary and ConclusionsSummary and Conclusions

• Increasing importance of vehicle aerodynamics requires modern development tools

• OpenFOAM toolbox most promising CFD technology p p g gyavailable

• Open-source CFD applications successfully developed and Open source CFD applications successfully developed and integrated into Audi vehicle-development process

• Long-term success possible as challenges can be • Long term success possible, as challenges can be overcome


Aerodynamics Development Using Open-Source CFDAcknowledgementsAcknowledgements

• Icon Ltd.

S. Weston, J. Papper, J. Gines, P. Rubio

• IDAJ• IDAJ

C. Hsu, T. Shimizu, K. Iida

• Audi AG

Dr. T. Blacha, Dr. F. Decker, Dr B Heinzelmann A Kolmer Dr. B. Heinzelmann, A. Kolmer, P. Rudolf, S. Schimmelpfennig, Dr. T. Schütz, Dr. P. Unterlechner, Dr G Wickern


Dr. G. Wickern

Thank you for your attention.

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