Document
Background Oriented Schlieren (BOS) of a
Supersonic Aircraft in Flight
James T. Heineck, NASA Ames Daniel W. Banks, NASA Armstrong th 17 International Symposium on Flow Visualization June 17 - 22, 2016 Gatlinburg, TN
Introduction
• Schlieren imaging for aerodynamics research was limited to ground test facilities • Weinstein introduced the first reliable method for flight test in 1994 • Retroreflective Background Oriented Schlieren was demonstrated to work Weinstein’s sun - edge streak camera image of a T - 38 for full - scale aircraft in flight in 2012 (DLR Goettingen) • First AirBOS flight in April, 2011 was successful, but restricted.
RBOS of a BO - 105 in slow forward flight Raffel et. al. , DLR Goettingen
NASA CST Project
Artist rendering of Locheed Demonstrator Artist rendering of Boeing Demonstrator • Research on the reduction of sonic boom for land overflight: current barrier to return to civil supersonic aviation • An X - plane proposed: Low - Boom demonstrator called QueSST for boom research • The Program invested in three schlieren methods: AirBOS, Ground - to - Air Schlieren Photography System (GASPS), and a hybrid method dubbed Background Oriented Schlieren with Celestial Objects (BOSCO) to support QueSST
Simulated Sonic Booms
Sonic Boom rendering of Concorde Sonic Boom rendering of Locheed Demonstrator Sonic Boom rendering of Boeing Demonstrator NASA CST Project needs schlieren imaging to see these booms
The BOS Method
Speckle Background 2008 Wind tunnel test: reference, data image and result of an abort motor tower at M=1.3
The AirBOS Method
AOX - Experimental Aero - Physics Branch Record the under - pass of target plane Use the flora as speckle background
AirBOS Implementation
• Fly in the Black Mountain Supersonic Corridor near Edwards AFB • Characterize the Mojave Desert flora in the Supersonic Corridor: Creosote bushes with scattered Joshua trees • Bushes average 10 feet (3.1 m) diameter; too few trees to be of concern • Dark green against light gray soil; red filter enhances contrast
Observer plane
NASA Beechcraft B - 200 Super King Air
• Fly at 30,000 ft MSL ( Highest practical altitude) • Low stall speed – 99 knots (75 with full flaps) • Already equipped with high - quality nadir port window • GPS navigation
Target plane
Air Force T - 38, operated by the Test Pilot School at Edwards AFB Supersonic flight achieved by full acceleration during a shallow dive, leveling for the flyby
Imaging system design
• Calculate the proper lens focal length to optimize speckle size • Phantom V641, with 2650 x 1600 pixel and 10 – micron pitch • Speckle distribution should be 2 - 5 pixels • Spreadsheet calculates pixel resolution and field of view on ground and at target location Lens Camera Half Angles Altitudes FOV at Target Aircraft FOV at Ground Observer Target Lens fl ccd nx ccd ny pixel size X Y a/c a/c Ratio Δ xfov Δ yfov Resolution Δ xfov Δ yfov Resolution (mm) (pixels) (pixels) (μ m) ( Deg ) (Deg) (Ft) (Ft) (Ft) (Ft) (pixels/ft) (Ft) (Ft) (pixel / ft) 105 640 512 25 4.36 3.49 27000 13500 0.5 2057.14 1645.71 0.31 4114.29 3291.43 0.16 180 2560 1600 10 4.07 2.54 30000 26000 0.75 568.89 355.56 4.50 4266.67 2666.67 0.60 180 2560 1600 10 4.07 2.54 30000 28000 0.9 284.44 177.78 9.00 4266.67 2666.67 0.60 Sample of table for two cameras and target aircraft separation distances
AirBOS Implementation
• Survey the Black Mountain SS Flight Corridor at 30,000 ft altitude • Photographically survey large area, find consistent flora • Test for cross correlation performance • Design flight pattern to hit the “sweet spot” where the acceleration can be achieved, but turn around is within the corridor Black Mountain SSC Area Survey area Edwards AFB 11 Google Earth view of area of Supersonic Corridor and Edwards AFB
Determining the Sweet - spot
Evaluate two successive frames from the reconnaissance flight using cross correlation
Determining the Sweet - spot
Assure SNR of 5 or higher in the cross - correlation product using the anticipated window size
Flight Plan
Cameras and Layout
Phantom V641 monochrome, 2560 x 1600 pixels, 10 micron pitch, 180 mm lens - 8 GB of internal memory, ~ 2 seconds of record time @ 1000 fps - #25 Red filter, enhance contrast of bushes against the bright soil - “Pickle” switch trigger by operator Two cameras: redundancy and potential for stereo and multi - stream referencing Legacy camera for 2011 work: Goodrich SUI SU640 - SDWHVis - 1.7RT InGaS - 640 x 512 pixel sensor, 25 micrometer pixel pitch, and fitted with a 105 mm lens - Used mainly as real - time spotting camera Two cameras, mounted vertically Schematic of cabin layout
Data Acquisition
Pilots flew identical tracks using independent GPS units Radio communication between target plane pilot, observer plane, and control room - Countdown provided by Control Room based on radar tracking - Camera operator set recording in circular buffer mode, watched live feed - M anually triggered “record” point.
- Captured images before and after trigger point - Downloaded buffer to laptop – up to 15 minutes, but usually trimmed to 7 min - Reported to Control Room “Love” or “No Love” - Love got a High - Five by operators, Control Room explodes as if we landed on Mars
Data Processing
1. Reference - to - data registration: First - order projective transform - Aligns the displaced backgrounds caused moving observer - Corrects perspective distortion caused by pitch and roll during acquisition ′ ′ 𝑎 𝑥 + 𝑎 𝑦 + 𝑎 1 2 3 𝑥 = ′ 𝑐 𝑥 + 𝑐 𝑦′ + 1 1 2 ′ ′ 𝑏 𝑥 + 𝑏 𝑦 + 𝑏 1 2 3 𝑦 = ′ 𝑐 𝑥 + 𝑐 1 2𝑦′ 1 + - Four points at corner of images are chosen, large - window CC performed - Cross correlation between the two images yields ∆ 𝑥 and ∆ 𝑦 at each ′ ′ location, 𝑥 and 𝑦 are solved to then calculate the eight coefficients ′ 𝑥 = 𝑥 + ∆ 𝑥 ′ 𝑦 = 𝑦 + ∆ 𝑦 2. Image c ross correlation at defined grid nodes yields Dx and Dy due to density gradient shift
AirBOS Results
5000 - foot Separation Distance
Reference plus three raw image data sequences Cross correlation product sof above data sequences - 32 x 32 refined to 16x16 IA, - 9 x9 pixel High Pass filter, - 3 pixel Grid n ode density - 3 - point Gaussian peak finder
AirBOS Results – Data Averaging
Track the movement of the aircraft using cc of glint, realign correlation grids
AirBOS Results – Data Averaging
Compute the simple average of aligned grids
2000 - foot Separation, Raw images
21 Sample movie of raw imagery, two frames skipped for brevity
Single Frame Results
T - 38 at 45 deg. r oll, M=1.05, s ingle frame, 16x16 IA, 3 pixel grid
AirBOS Results
Results from 200 sequences aligned and averaged, Dy (horizontal knife edge)
AirBOS Results
Results from 200 sequences aligned and averaged, magnitude of displacement
AirBOS Results
Results from 200 sequences aligned and averaged, Dx (vertical knife edge)
AirBOS Results
Results from 200 sequences aligned and averaged, Dx (vertical knife edge)
Summary
Background Oriented Schlieren has been successfully adapted to full - scale supersonic flight The planning and system design permit predictable results Technique permits testing of maneuvers, monitoring tip vortex trajectories, and subsonic wakes LATER in this Session: Optical Flow solutions triple the resolution: Smith, et. al .
Acknowledgements
Peter Coen , CST P roject Manager, for years of support for this research Nils Larson, NASA Armstrong Chief Test Pilot, and the rest of the flight crew Maj. Jonathan Orso and Maj. Jeremy Vanderhal , Instructor Pilots from Air Force Test Pilot School Brett Pauer, for flight test management Ed Schairer, NASA Ames, for continuously adapting his program to the specifics of this technique Ed Haering, NASA Armstrong, for the Intangibles