CFD and Wind Tunnel Testing: Complimentary Methods for Aircraft

Aeronautical Testing Service, Inc.

CFD and Wind Tunnel Testing: Complimentary Methods for Aircraft

Design

18820 59th DR NEArlington, WA 98223

USA

Copyright © 2010, Aeronautical Testing Service, Inc.

Background

• Introduction– ATS Company Background

• New and modified aircraft or UAV• Aerodynamic consulting• Wind tunnel testing • Model design and construction

– Personal Background• 14 years experience in wind tunnel

testing• Involved in 30+ wind tunnel tests• 10+ years in CFD• STAR-CCM+ user since V1.08• Primary focus is design work related to

modifications of existing and new aircraft


Background – continued

• Research and Design Programs with the University of Washington

– UW Aeronautics and Astronautics Programs• Capstone aircraft design course• Graduate and undergraduate research• UW Kirsten wind tunnel• Lamborghini composites laboratory

– Other Supported UW Programs• Formula SAE • Human Power Submarine


Common Uses and Misconceptions of CFD

• Common Uses – Design Development– Control System Analysis (S&C)– High Lift– Icing accumulation– Propulsion– Acoustics– Aeroelastics– And Many More!

• Misconceptions and Misuses– CFD is very fast– CFD is cheap– CFD is an acceptable replacement for wind

tunnel testing– If it comes out of the computer, it has to be

right!


Common Uses and Misconceptions of Wind Tunnel Testing

• Common Uses – Development testing– Control System Analysis (S&C)– High Lift– Simulator testing– Icing accumulation– Propulsion (powered testing)– Acoustics– Aeroelastics– Flutter– CFD validation test– And many more!

• Misconceptions and Misuses– Validation of “final” design (often after parts are in

production!)– Wind tunnel testing is too costly– Flight testing is an acceptable replacement


Typical Cost of CFD and Wind Tunnel Testing

• What is the “True” Cost of CFD and Wind Tunnel Testing?Estimated Cost For Preliminary Wind Tunnel and CFD Data

Based on ATS Experience and Computer Hardware Availability

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What Are Some of the Cost Drivers?

• CFD Cost Drivers– Software Costs

• Basic licensing fees• Cost per parallel process (varies depending on software)

– Hardware Costs• Cost per physical cpu core (Total system cost divided by # of cores per

box)

– Operating Costs• Time to solution and time per solution

– Dominated by required mesh size and software capabilities• Power Cost ($ per Kilowatt-hour)


What Are Some of the Cost Drivers?

• Wind Tunnel Cost Drivers– Model Cost

• Complexity of model (Number of parts, pressure taps, powered controls, etc.)

• Quality of CAD definition• Time from CAD delivery to model delivery

– Wind Tunnel Costs• Cost per hour of wind tunnel time• Cost for non-standard instrumentation

– Operating Costs• Design of the model• Efficiency of staff• Efficiency of wind tunnel equipment and data reduction tools• Power cost


Future Cost Outlook

• What does the cost of CFD and wind tunnel testing look like in the future?

Estimated Cost For Preliminary Wind Tunnel and CFD DataBased on ATS Experience and Computer Hardware Availability

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What is Changing To Reduce these Costs?

• CFD– Hardware Changes

• Increases in number of cores per socket• Increase in interconnect speeds and decrease in costs

– Software Changes• Software ease of use• Better software integration with other tools (mainly CAD)• Meshing advances

– Mesher is more tolerant of CAD errors (surface wrapper)– Surface wrapper can also be used to rapidly change

overlapping geometry to allow for configuration studies• Licensing changes

– Power session licensing and on-demand computing


What is Changing To Reduce these Costs?

• Wind Tunnel Testing– Model Design and Construction

• Model designed for quick model changes (low cost models)

• Powered controls• Reduced cost of hardware (electric motors, servos,

sensors, etc.)• Tooling improvements

– Testing Improvements• Test planning with powered models in mind• Continuous data acquisition


Why Does This Matter To A Design Engineer?

• Understanding the cost of the tools we use– Cost per data point for CFD

• Higher cost per data point• Lower initial cost

– Cost per data point for wind tunnel data• Lower cost per data point• Higher initial cost


Case Study - 2010 UW Capstone Program

• 2010 Capstone Design Class– Airplane design from RFP to flying prototype

– Follow-on program to 2009 class– Handling qualities and noise research for a supersonic class airliner

– Development cycle of 6 months– Supported by state of the art computational and experimental resources


Case Study - 2010 UW Capstone ProgramResources and Limitations

• Two quarter class• Undergraduate students• Software

– Unigraphics NX6– Xfoil, VLAero+, and STAR-CCM+

• Students start with no CAD or CFD experience

• Workstations limited to 4GB or RAM (4 workstations total)

• STAR-CCM+ cases limited to ~4 Million Cells Maximum

• Access to 96 Core Linux cluster at ATS


• CFD Data Available On the First Day of Wind Tunnel Test

Case Study - 2010 UW Capstone ProgramComparison of Data


• CFD Data Available On the First Day of Wind Tunnel Test (cont.)

Case Study - 2010 UW Capstone ProgramComparison of Data


Case Study - 2010 UW Capstone ProgramMethodology

• Students Used Simplest Tools First– Xfoil, VLAERO+, etc.

• CFD is Used Early in Design Cycle– CFD is used as soon as basic geometry is available– Simple 3D wing cases were used to develop planform using STAR-CCM+– Half and Full model STAR-CCM+ cases are run after the preliminary planform and

basic configuration are selected– After basic configuration is selected STAR-CCM+ is used for detailed nacelle design

• Wind Tunnel Testing is Done As Early As Practical– Wind tunnel testing is expected but only after initial concepts are investigated and

reduced to most promising design– Testing is more focused but model and schedule allow for development work– Commercial cost of wind tunnel model would be approximately $45K– Approximately four weeks from CAD definition to final model

• CFD and Wind Tunnel Testing Are Integrated Parts of the Design Cycle– Strengths of each tool are exploited at the proper time and area in the design cycle


Case Study #2 - Estimation of Downwash for High Powered Propeller Driven Airplane

• Equilibrium and static longitudinal stability can be seriously affected

by the downwash at the horizontal tail

• Downwash at the tail is driven by the wing and propeller affects

• Handbook methods can estimate wing contribution reasonably well for preliminary design but do not estimate propeller contribution well

• Propeller contribution to downwash can be large in high powered aircraft

• Measuring downwash in the wind tunnel requires a powered model and multi-hole probe


• First Problem: Can CFD Accurately Predict Propeller Performance?



• Using Rigid Body Motion STAR-CCM+ Results Agree Well With Experiment


NACA 4-(5)(05)-041 Four Blade PropellerTunnel Mach = .2, 30 Degree Nominal Blade Angle at .75 Radius

STAR-CCM+ Blade Angle = 32 Degrees(Loaded blade angle change of +2 degrees estimated from Summary of Propeller Design Procedures and Data, Volume II)

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Unsteady Ridgid Body Motion Model (RBM)1.5x10^6 Cell Trimmed Hexahedral ModelCoupled SolverK-Epsilon Turbulence Model



• Second Problem: Can CFD Accurately Predict Propeller and Wing

Affects At the Horizontal Tail (Downwash)?



• Predicted Downwash Angle Vs Wind Tunnel Results For Similar Configurations

STAR-CCM+ Predicted Downwash Angle Vs SpanAlpha = 0, RPM=1700, Mach=.15

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• Influence of Propeller is Well Predicted and Highly Useful for Preliminary Design• Good Example of How CFD Can Be Used Early in the Design Cycle to Predict

Parameters That Can Have a Significant Impact On the Design

NOTE: Configurations Are Not The Same


How Can CFD and Wind Tunnel Testing Benefit Each Other

• CFD Can Be Used To Help Us Better Understand How To Improve Wind Tunnel Testing

The effect of a business jet model on the UWAL diffuser was investigated

Methods of improving the diffuser flow are currently being studied


How Can CFD and Wind Tunnel Testing Benefit Each Other

• Wind Tunnel Testing Can Continue to Help Us Understand How to Improve Flow Solvers – Some Challenges For CFD

• Accurate and robust high-lift prediction• S&C characteristics in highly separated conditions• Turbulence modeling in separated conditions• Understanding fluid-structure interactions for complex unsteady flow cases

– Ways Wind Tunnel Testing Can Help• Using low cost models where appropriate to allow for more validation testing• When necessary use highly instrumented models to gather data that is useful for

CFD development• Use of advanced dynamically scaled models for investigation of fluid-structure

interactions


Closing Thoughts

• CFD and wind tunnel testing each have inherent strengths and weaknesses

• By understanding these strengths and weaknesses we can:

– Exploit the strengths to allow for more efficient design cycles

– Reduce the overall cost of using these tools– Use these tools for their mutual benefit

Documents

CFD and Wind Tunnel Testing: Complimentary Methods for Aircraft