class 9 - aerodynamic testing - virginia tech
TRANSCRIPT
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Week of 3/26
• In lab – 4th instrumentation lab on Spectral Analysis
and LabView– Preparation: Read Spectral Analysis section
of manual– Homework on website, due at start of lab.
• Next Class– This Wednesday (3/28) on Fracture
Toughness Testing
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The Rest of Course…
Week of 4/30
Week of 4/23
Week of 4/16
Week of 4/9
Week of 4/2
Week of 3/26
- No lab -Final exam (Mon.)
- No lab -Last class (Mon.)
Experiment 7Experiment 5
- No class -
InstrumentationLab 5
- No class -
Experiment 5Experiment 7
- No class -
InstrumentationLab 4
Ex. 7 class (Mon.)Ex. 5 class (Wed.)
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Experiments 5 and 7
• Experiment 5 takes place in 127 Norris• Experiment 7 takes place in the Stability
Tunnel• Visiting times are much more restricted
– Will be part of Friday and/or Monday prior to test
– Times to be posted on web• Have pre-defined goals.
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Aerodynamic Testing1. Why do we do aerodynamic testing?2. What is measured?3. Where do people test?4. What conditions do you test at?5. How is a test actually performed?6. Experiment 7
• Ocean vehicles and devices go through closely analogous testing using towing tanks, water and wind tunnels.
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1. Aerodynamic Testing – Why?• Fundamental component of developing new
aerodynamic vehicle and hardware designs– Prediction
• To develop design• To initially evaluate design
– Wind Tunnel Testing• To evaluate design• To confirm (or not) predictions• Testing of sub-component, component,
whole vehicle models to determine performance• Obtaining performance data for control systems
– Flight Testing
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How big a deal is wind tunnel testing?
0 500 1000 1500 2000
general arrangementwing planform,camber
LE & TE flapsstrake developmentcontrol deflections
storesstore loads
pressure loadsinlet
flutterstore separation
spin/stallspillage and nozzle
miscellaneous
Testing hours
F-16
$M’s
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2. What is measured?
• Mostly forces and moments– Since these are what matters to the vehicle
performance– Since these are the data needed to define the
airplane control system• Flow diagnostics
– When forces and moments aren’t behaving has expected, or to confirm expected behavior
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Measuring forces and momentsStrain gage balance. Model mounted on the end of a beam. Deflection /twist/bending of beam sensed by strain gages, and then used to infer forces and moments
Simplest possible strain gage balance –would sense a combination of drag and pitch
moment
Gages operated with Wheatstone
bridge circuit1 2
3 4
Power
OutputVoltage
Load
Strain Gages
Moment
Actual 6 component balance –
senses all 6 forces and moments separately
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Flow diagnostics
• Flow visualization (smoke, oil-flow, tufts)• LDV, Hot-wire velocity/turbulence
measurements• Mean surface pressure measurements
using taps• Pitot static traverses to look at wake, flow
patterns
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3. Where do you test?In a Wind Tunnel…• Government
– NASA (Langley, Ames, Glenn)– AEDC (TN)
• Commercial– Boeing (12')– DTW
• Universities – VT (6'), ODU, MD (11')…
• Europe – QinetiQ (5m), DNW (8m)
0 to 80m/s, 6' x 6'M=0 to 0.25
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4. What conditions do you test at?
• The equations governing aerodynamics dictate that the form of the flow depends on a series of non-dimensional parameters of which the most important are
– Reynolds number = UL/υ
– Mach number = U/c
Aerodynamic Similarity
Flow speed
Size of device
Kinematic viscosity
Speed of sound
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Reynolds Number• Controls how important viscous effects are (big
Re = small viscous effects)
Boundary layer begins laminar
Transitions to turbulence
Transition occurs earlier the bigger Re is
Turbulent boundary layers create more friction but much less likely to stall
• So, Reynolds number influences the flow through the boundary layer
• Boundary layers control the drag and critical things like the stall behavior.
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Mach number
• Controls how important compressibility effects are
At transonic and supersonic speeds compressibility effects result in the generation of shock and other waves. These will likely dominate the aerodynamics.
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Aerodynamic Simulation
• Ideally, a wind tunnel test at the same Re and Mexperienced by a full scale vehicle we would have the same flow (ignoring the effects of the walls and any imperfections).
• In most circumstances this can’t be done– If you are testing in air at standard conditions, you
need to test at the full flight speed to reproduce the Mach number (i.e. this fixes U)
– Unless your model is full scale (?), this means you will be testing at much lower Reynolds number.
Re=UL/υ M=U/c
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Transition fixing
• To reproduce the effects of a high Re at a low Re, we use transition fixing:
High Re (full scale) Low Re (wind tunnel)
Low Re with transition fixing
Transition
‘Boundary layer trip’ causes early transition simulating high Re flow. E.g.
• Serrated tape• Wire across airfoil span• Sandpaper strip
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Serrated Tape Trip
3' chord airfoil in the VT Stability tunnel
Leading edge
Trailing edge
Serrated tape at 10% chord with serrations facing forward to trip boundary layer
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5. How is a test performed?• Tests are very expensive so
planning/organization is critical to minimize tunnel time– Begin with clear goals for the test (often set by mgmt)– Investigate the facility and the quality of its flow, the
model(s) to be used and their characteristics, the instrumentation. Models often to be built.
– Make a minute-by-minute run schedule that defines exactly what will be done and when to meet the goals
– Make theoretical ‘Pretest Estimates’ of the quantities to be measured.
– Execute the test with max efficiency, comparing measurements with pretest estimates as the test proceeds to catch measurement problems
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Experiment 7
• Educational goal: To provide as realistic as possible experience of an aerodynamic test
–The goals for the test–Investigate the facility and the quality of its flow, the model(s) to be used and their characteristics, the instrumentation–Make a minute-by-minute run schedule that defines exactly what will be done and when to meet the goals–Make theoretical ‘Pretest Estimates’ of the quantities to be measured. –Execute the test with max efficiency, comparing measurements with pretest estimates as the test proceeds to catch measurement problems
Logbook preparation
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Goals (predetermined by mgmt)• Determine the…
– lift curve slope – angle of attack for zero lift – maximum lift coefficient – aerodynamic center – nature of the pitching moment curve "break" near stall – zero lift drag – drag due to lift performance relative to the 0% and
100% polars…of a Clark Y rectangular wing. Also perform flow
diagnostics (flow vis)THESE ARE AMBITIOUS – YOU WILL NEED TO BE WELL ORGANIZED
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FacilityVirginia Tech Stability Wind Tunnel
Detailed information on this facility, the quality of its flow can be found at www.aoe.vt.edu/research/facilities/stab/
Facility is operated independently and performs educational, research and commercial testing.
0 to 80m/s, 6' x 6'M=0 to 0.25
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Scheduling• Two teams are scheduled to use the
wind tunnel during each 2 hour 45 minute period.
• The first team (with the lowest number) starts their test at the beginning of the lab period and has 1 hour 15 minutes to complete all their work and return the facility to its original state
• The second team has 1 hour and 15 minutes to compete their test which will start 1 hour 30 minutes into the scheduled lab period
First team test (with lower number)
Second team test
0h 0m
1h 15m
1h 30m
1h 45m
Time from start of lab period
WIND TUNNEL OPERATOR WILL BE PRESENT TO OPERATE FACILITY AND INSTRUMENTATION
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Model
Strut Mount with strain gage balance
Wing modelAngle of attack mechanism
Wing Mounting Holes
40 inches
6 inches3.7inches
Trailing edge
Nominal Planform
x/c-0.05
0.00
0.05
0.10
0.15
0.20
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
y/c
Not to scale
Model section
Measured
Theoretical (in manual)
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Prior Data Available
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
-15° -10° -5° 0° 5° 10° 15° 20° 25°
CL
α, deg.
Estimate,(Inviscid Theory)
Clark Y AirfoilAR = 5.6 Rectangular Wing
NACA dataRe = 1.0x106
VPI data,Re = .3x106
-0.50
0.00
0.50
1.00
1.50
2.00
2.50
-0.15-0.10-0.05-0.000.05
CL
Cm
Clark Y AirfoilAR = 5.6 Rectangular Wing
NACA data,Re = 1.0x106
-0.30
-0.00
0.30
0.60
0.90
1.20
1.50
0.00 0.05 0.10 0.15 0.20
CL
CD
NACA Data, Clark Y airfoil, rectangular wing, AR = 5.6, Re = 1x10 6
Theoretical 100%Suction Polar, e = .98
Theoretical 0% Suction Polar
Wind tunnel test data
Note: Theoretical polars shifted to match experimental zero lift drag
USEFUL FOR COMPARISON. NACA REPORT HYPERLINKED
FROM MANUAL
A GUIDE BUT NOT NECESSARILY
CORRECT
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Mounting / InstrumentationModel strut w/o shroud
and model removed
Computer running LabView controls angle of attack, measures 6 components of force and moment, reads wind tunnel Pitot Static, barometer and thermometer to get conditions.
Angle of attack mechanism
Strut with 6-component strain gage balance
Flow
Flow
24'
6'
12' 3'
Test section
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Flow Diagnostics (Tuft flow vis)
3 rows of tufts
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Trip and flow vis. material
Pinking shears for cutting serrated edge in tape to make boundary layer trip
Electrical tape for boundary layer trip
Scotch tape and tufted Nylon yarn to make and attach tufts
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What you have to do…• Read the manual carefully to understand the goals, and
what is expected. Look over earlier data in manual. Add any goals of your own.
• Learn about the facility (on web), and instrumentation and model, visit the tunnel.
• Define a run schedule (what angles of attack, what speeds, trip/no trip, flow speeds), be prepared to make trip strips, tufts etc.
• Make pretest estimates - techniques and tools in manual (formulas, lifting line theory program)
• Run an organized and efficient test, be ready to tell the wind tunnel engineer what you want.
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Things To Ask When You Visit The Tunnel
• What range of angles of attack can actually be run? • What maximum speed can be run?• Is there any way of using the angle of attack system to
maximize accuracy?• What is the uncertainty on angle of attack?• What is the uncertainty on the force and moments?• Can we prepare (now) a trip or set of tufts ready for our
test?• How is the tunnel flow speed measured?• Can we measure the model?• Can we see the fan?
GOOD LUCK!!