Molecular Tagging in Air and Nitrogen

Transcription

1

2 Graduate Students: Christopher Peters (NASA-NSTRF), Nathan Calvert (NSF Final Public Oral scheduled July 11, currently at Lockheed), Yibin Zhang (NDSEG -AFOSR), Research Scientists: Dr. Arthur Dogariu, Dr. Mikhail Shneider Post Doctoral Associate: Dr. Ben Goldberg Visiting Scientists: Prof. Albina Tropina, University of Automobiles and Highways, Kharkov, Ukraine Prof. Sean O Byrne, University of New South Wales, Australia Dr. Kiyoshi Kinefuchi, Japan Aerospace Exploration Agency (JAXA) Collaborations and related contracts Dr. Paul Danehy, NASA Langley Dr. James Gord, AFRL Dr. Jacob George MetroLaser Dr. Sukesh Roy, Spectral Energies Dr. Eric Marineau, AEDC 2

3 Molecular Tagging in Air and Nitrogen Avoid seeding with either particles or foreign gases Follow the motion of the air or nitrogen in real time Determine transport properties and flow structure from displacement and distortion of lines, crosses or more complex patterns. Approach Femtosecond Laser Electronic Excitation Tagging (FLEET) 3

4 1. No seeding required Operates in air and nitrogen and other gas mixtures containing nitrogen 2. Instantaneous velocity profiles 3. No intrusive probe required 4. High resolution (better than 40 microns) 5. Simplicity (one laser and one camera) 6. Grids and crosses give vorticity and shear stresses 7. Operation at pressures as low as 1 Torr to > 1 atm 8. Operation at temperatures from condensation to combustion (<100K to > 2000K) 9. Capable of imaging close to surfaces.

5 Establish whether FLEET can be implemented in the near wall region for quantitative measurements of Wall heating Shear stresses Near wall transport parameters. Compare FLEET to other methods of flow measurement Particle imaging velocimetry (PIV) Hot wire Explore methods to improve FLEET capabilities Minimize perturbation to the flow Determine potential for measuring other flow parameters Temperature Mixture fraction Develop a more thorough understanding of the dissociation and recombination physics associated with FLEET Explore applications in wind tunnels and in flight.

6 Molecular Nitrogen Excitation Dissociation Ground State N 2 B State N 2 A State N 2 Ground State N Laser Photon First Positive Photon Recombination Emission

7 Dissociative recombination N e - N + N Forms free nitrogen atoms Multiphoton excitation Other dissociation paths may also be important fluorescence used for flow tagging is generated in a four-step process of 1. ser excitat ion from the N 2 ground state, 2. dissociation, 3. recombination tate, and finally 4. first positive emission. Figure adapted from C. C. Lin, or. physi cs. wi sc. edu/ dat a/ N2_ener gy. pdf >. 7

8 First Positive emission persists as long as nitrogen atoms are recombining. Lasts 10 to >50 microseconds (shorter in air than in pure nitrogen) Emission is red to near infrared It provides the light for flow tracking 8

9 Nitrogen jet 10 microsecond intervals Each progression includes about 10 line displacement shots due to the long lifetime in pure N 2 Measured centerline velocity ~150m/s

10 Velocity (m/s) X/D Height X (cm) Can be used to monitor flow structures FLEET results Laser moved with respect to air jet and then imaged Shadowgraph FLEET Slip line 10

11 Can be used to monitor flow structures Laser kept at jet exit and imaged with 1 MHz camera Allows for accurate determination of both flow speed and structure 11

12 Experiments completed with varying pressures for air Signal intensity goes up at lower pressures Likely due to fewer collisions between species 100,000 foot altitude 12

13 13

14 14

15 Measurements in Boundary Layers Boresight tagging in separated flows Laser wavelength optimization Physics of the Tagging Process Imaging of time evolving density and temperature perturbations Comparison with hot wires in oscillating flows FLEET in Gas Mixtures Data on mixtures of nitrogen with oxygen and argon Argon increases the FLEET signal Modeling of argon effects 15

16 Implementation of Boresight FLEET for boundary layer separation studies CMOS camera Image intensifier Mirror Neutral density filter 400 nm fs laser beam (1.2 mj/pulse) Flow Filter (cut < 550 nm, Φ1 ) Plano-convex lens (f35 mm, Φ1 ) Quartz window FLEET signal (mostly 580 nm) 16

17 Wall Boundary layer Sonic line 247m/s Compression waves u=454m/s v=2m/s Subsonic region v=76m/s u=168m/s 184m/s Expansion fan Main flow 599 m/s Incoming shock Reflected shock Flow becomes subsonic in the separation region and the flow vector is turned away from the wall 17

18 Three experiments set into one layout FLEET beam directly into gas flow Planar Rayleigh perpendicular to gas flow Nd:YAG also used for Rayleigh polarimetry measurements Rayleigh polarimetry used to measure dissociation fraction Parallel scattering due to atoms Perpendicular scattering due to molecules Parallel Perpendicular 18

19 µj, 175 mm focusing Comparison between FLEET (top) and planar Rayleigh scattering (bottom) Close correspondence between emission and scattering profiles observed Before t=500ns, emission appears to be longer than density Indicates that second positive emission and heating are not closely related Exact opposite appears to be true after 1 µs Pressure Waves 19

20 Can track pressure wave movement to determine bulk gas temperature Monitor wave position in time to determine speed of sound a 2 = γrt M T = a 2 x i M i x j c v,j R x k c p,k 20

21 FLEET Profile Along the Beam (320μJ) Compared to temperature and emission profiles Fitted FLEET Intensity Rayleigh intensity Raw FLEET Intensity Emission Intensity Data taken 1µs after fs pulse Fitted intensity agrees when raised to 20 th power Indicates 10 photon interaction Agreement persists for >50µs mm

22 Can also determine temperature rise along laser axis T = T 0 I

23 Feedback Loop Streamwise Spanwise SWJ Actuator Main Channel Throat Active flow control with no moving parts Velocity and sweep frequency depend upon nozzle pressure ratio (NPR) NPR = 1.4 Limited direct quantitative 3.2 mm, throat measurements of supersonic sweeping jet actuators 2.5 mm FLEET Velocity + Concentration (Initial and Delayed) 23

24 HWA (const-temperature, single wire) 50-µm diameter by 2.5-mm long thin-film probe, 1.8 overheat ratio, kHz acq rate Three-axis traverse, mm spanwise increments Unable to insert HWA probe into device Unsteady wind loading restricted probes to 2.5 mm away Compressible HWA measurements Sensitive to mass flux, ρu Qualitative comparison of FLEET spatial profiles only HWA Probe HWA Probe Tip Spanwise Traverse SWJ Actuator 24

25 Mean Profiles ( mm and NPRs of ) Velocity increases with NPR (expected) Black error bars denote 95% confidence Sweep angle decreases with NPR (expected) Transonic flow control, fast speeds AND large region of influence Jet loses velocity and broadens with distance downstream (expected) Spreading rate, dy 1/2 /dx, of 1.2 to 0.5 (decreases with NPR) vs. 0.1 for steady turbulent jet FLEET (2.4k shots) HWA (400k shots) 25

26 Steered laser beam into device through throat Shorter focal length (125 mm) to generate spot, lower pulse energy (0.5 mj) to prevent burning plastic Pure nitrogen for longer FLEET lifetime and good SNR at low energy NPR = 1.4, 1.8, 2.0, 2.5 and frame bursts with fixed 5-µs delay between frames Significant strain and break-up of tagged region after about five frames Two-dimensional cross-correlation to determine displacement Red box approximate reporting domain > 2.6k 15-frame bursts for each NPR, spatial averaging (uniform grid) of position-velocity ensemble NPR = 1.4 NPR =

27 Initial O 2 addition drops FLEET intensity to ~85% of max value Peak emission intensity occurs at ~50% O 2 Likely due to a combination of effects O 2 quenching of excited N 2 species Secondary oxygen species such as (O, O 2+, or O + ) generated by the laser could open pathways for N 2 dissociation Signal with no N 2 present due to expected atomic emission Air 27

28 Additional Ar enhances FLEET signal, even in low N 2 density regimes Likely due to collisional interaction between Ar and N 2 Fs pulse creates excited Ar species through some multiphoton process Collisional energy transfer between Ar* and N2 promotes N 2 to either a predissociative state or the typical electronic excitation pathways Still currently under investigation 28

29 Broadband N2 emission spectrum with Ar enhancement Enhancement seen across all vibrational features Highlighting the transitions from the 10 th and 11 th vibrational levels of the B state show the strongest enhancement Peaks with the highest overall magnitude are traced back to v=3 and 4 29

30 Note the dramatic increase in excited state population when argon is present 30

31 The bad news: FLEET intensity at atmospheric pressure is near the minimum Argon/Nitrogen Oxygen/Nitrogen The good news: Signals are lower, but still measurable Emission still lasts for >20µs, allowing for accurate flow speed measurements Argon enhancement is clear, but not yet well understood Possibility exists to tailor the fs beam frequency to exploit this enhancement 31

32 Application of boresight configuration for boundary layer measurements of 2D velocity vectors Study of shock induced separation Measurement of FLEET dissociation physics and flow perturbations. Imaging density and temperature perturbations with time Indication that 10 photon absorption is the dominant process, suggesting the formation of N 2 + followed by dissociative recombination Comparison of FLEET with hot wire in oscillating flows Measured impact of gas mixtures on FLEET Increase in FLEET signal by the introduction of argon Decrease in FLEET signal with oxygen, but recovery with higher oxygen concentrations

33 Further comparison with hot wire Determination of the strengths and weaknesses of each for turbulent flow and boundary layer measurements Examination of the use of FLEET for mixture fractions (with NASA) Extension to pumping at nm (4 th harmonic of Ti:sapphire) to overlap the two photon transition to the a Rydberg state of nitrogen(selective twophoton absorptive resonance femtosecond-laser electronic-excitation tagging velocimetry ---STARFLEET --- AFRL & NASA) Reduced perturbations to the flow May enable accurate temperature measurements Determination of the accuracy limitations for turbulence and near wall applications Effects of tagging perturbations Trade off between position and velocity (uncertainty principle ΔxΔv = constant the question is what is that constant for FLEET)

34 Simultaneous measurement of velocity, temperature, density and mixture fractions Use of FLEET signal level to measure oxygen concentration (with NASA Langley) Use of Rayleigh scattering for density Use of Second positive emission spectroscopy for temperature Use of femtosecond laser induced breakdown spectroscopy (LIBS) to identify atomic species in gas mixtures Simultaneous multipoint measurements Use of lenslet arrays for the simultaneous tagging of multiple lines that can be tracked Use of plenoptic camera lenses to recover the focus as the lines move and track multiple lines.

35 Peer Reviewed papers C. Limbach and R. Miles, Rayleigh Scattering Measurements of Heating and Gas Perturbations Accompanying Femtosecond Laser Tagging, AIAA Journal (accepted for publication) Conference manuscripts Nathan Calvert, Yibin Zhang, Richard B. Miles Characterizing FLEET for Aerodynamic Measurements in Various Gas Mixtures and non- Air Environments, (AIAA ) 32nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference, 2016, / Tat Loon Chng, Richard B. Miles A Comparison of Radar REMPI and Laser Induced Fluorescence for Concentration Measurements, (AIAA )54th AIAA Aerospace Sciences Meeting, 2016, / Yibin Zhang, Nathan Calvert, Arthur Dogariu, Richard B. Miles Towards shear flow measurements using FLEET, (AIAA ) 54th AIAA Aerospace Sciences Meeting, 2016, / Albina Tropina, Mikhail N. Shneider, Richard B. Miles, Femtosecond laser discharge and small scale turbulence, (AIAA ) 54th AIAA Aerospace Sciences Meeting, 2016, / Yibin Zhang, Nathan Calvert, Mikhail N. Shneider, Richard B. Miles, Enhancement of FLEET in Argon Gas Mixtures, (AIAA )32nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference, 2016, / Jacob George, Christopher Limbach, Tom P. Jenkins, Richard B. Miles, Measurement of Dissociation Fraction and Temperature using Laser Rayleigh Scattering Methods, (AIAA ) 32nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference, 2016, / Christopher J. Peters, Richard B. Miles, Ross A. Burns, Paul M. Danehy, Brett F. Bathel, Gregory S. Jones, Femtosecond Laser Tagging Characterization of a Sweeping Jet Actuator Operating in the Compressible Regime, (AIAA ) 32nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference, 2016, / Kiyoshi Kinefuchi, Andrey Y. Starikovskiy and Richard B. Miles, Control of Shock Wave Boundary Layer Interaction Using Nanosecond Dielectric Barrier Discharge Plasma Actuators, AIAA Propulsion and Energy Forum,, July Conference presentations Y. Zhang, Femtoscond Laser Electronic Excitation Tagging (FLEET) in Non-air Flows APS Division of Fluid Dynamics Conference, Nov 20-22, 2015 R. Miles, Walter Lempert s Decade at Princeton : The technologies he advanced and the impact he has had. 54th AIAA Aerospace Sciences Meeting, 2016

36 36

37 Metastable argon stores energy and transfers it to nitrogen 37