1. 29 aug 2006Molecular Tagging Velocimetry Molecules, the ultimate flow tracers (or are they?) Nico Dam Applied Molecular Physics Radboud University of Nijmegen Nijmegen, NL The real work: Jeroen Bominaar Thijs Elenbaas Mira Pashtrapanska Coralie Schoemaecker Margriet Verkuijlen Willem van de Water

2. 29 aug 2006Molecular Tagging Velocimetry Writing in air Introduction Molecules vs. particulates Practical stuff Examples & image processing Conclusions

3. 29 aug 2006Molecular Tagging Velocimetry  563: Anonymous, unknown technique 1984: Hiller et al., biacetyl 1997: Noullez et al., RELIEF –first application to turbulence  2000: 3 new NO-based techniques present: –ca. 10 MTV-schemes (gas phase) –few in-depth studies Writing in air: a concise history

4. 29 aug 2006Molecular Tagging Velocimetry Three steps: 1.Create well-defined tracer distribution 2.Wait 3.Visualize advected tracer distribution molecular A velocimetry technique based on displacement of molecular clouds

5. 29 aug 2006Molecular Tagging Velocimetry Phosphorescence MTV biacetyl Room temperature vapour pressure: 5 kPa (50 mbar) Legend: internal (vibronic) energy state optical transition internal transition (relaxation) Energy

6. 29 aug 2006Molecular Tagging Velocimetry Phosphorescence MTV biacetyl Absorption spectrumLuminescence spectrum 355532416

7. 29 aug 2006Molecular Tagging Velocimetry Phosphorescence MTV biacetyl Example: microjet visualisation (1 mm orifice) (flow control, orbit maintenance) N 2 in Experimental setup

8. 29 aug 2006Molecular Tagging Velocimetry Phosphorescence MTV biacetyl laser beam nozzle 1 mm

9. Phosphorescence MTV biacetyl P s  100 kPa; P 0  few kPa (estimate) Center-line velocity: 330.6  0.7 m/s (false colour scale) ss ss ss

10. 29 aug 2006Molecular Tagging Velocimetry Phosphorescence MTV biacetyl

11. 29 aug 2006Molecular Tagging Velocimetry Phosphorescence MTV biacetyl Observations: Tracers are created locally, seeding is global Very good flow visualization Single write, multiple read Precise velocity determination Tracer distribution broadens in time Relatively straightforward laser & detection system But: Single shot images are rather poor in contrast Requires O 2 -free flow Biacetyl will ruin your social life

12. 29 aug 2006Molecular Tagging Velocimetry Molecules versus Particulates Particularities of MTV: Create Well-defined Molecular tracers local seeding taylored to situation the real flow (?) non-intrusive (?)

13. 29 aug 2006Molecular Tagging Velocimetry Molecules versus Particulates The role of diffusion - 1 (x 0 ; t 0 ) (x; t)

14. 29 aug 2006Molecular Tagging Velocimetry Molecules versus Particulates The role of diffusion - 2 Written structure blurred by molecular diffusion

15. 29 aug 2006Molecular Tagging Velocimetry Molecules versus Particulates The role of diffusion - 3 (x 0 ; t 0 ) (x; t) Schmidt number: Sc = / D  1 ( air ) Komogorov scale remains unresolved

16. 29 aug 2006Molecular Tagging Velocimetry Molecular Tagging Velocimetry practical stuff Two steps: 1.Create well-defined tracer distribution 2.Visualize advected tracer distribution Crucial issues  How to create molecular tracers?  How to visualize them?

17. 29 aug 2006Molecular Tagging Velocimetry Tracers for MTV Requirements: distinguishable visible (“visualizable”) persistent producible in sufficiently large amounts convenient (non-toxic!)

18. 29 aug 2006Molecular Tagging Velocimetry Marking Molecules Modify the default composition...... by creating new molecules... by changing the internal energy of existing molecules Both can be done locally and instantaneously by (laser-)optical means chemistry physics photochemistry photophysics e.g. N 2 + O 2 → 2NO e.g. biacetyl → biacetyl*

19. 29 aug 2006Molecular Tagging Velocimetry MTV implementations (gas phase)

20. 29 aug 2006Molecular Tagging Velocimetry APaRT Air Photolysis and Recombination Tracking N 2 + O 2 + lots of h (193 nm) → NO Chemical pathway unknown (but it works) Highly localized NO creation Visualisation of NO by LIF Applicable to “air” and combustion

21. APaRT Hardware – 1

22. APaRT Hardware – 2 Excimer laser pulsed gas discharge laser (rare gas halides) for NO creation: 193 nm (ArF), ca. 50 mJ/pulse of  20 ns not or hardly (< 1 nm) tunable Nd:YAG-pumped dye laser (or OPO) pulsed liquid (or solid) state laser for NO visualisation: 226 nm, ca. 5 mJ/pulse of  5 ns widely tunable Rep. rate limited by visualisation laser & camera (10 Hz)

23. 29 aug 2006Molecular Tagging Velocimetry How non-intrusive is MTV? APART Write-laser-induced temperature rise Beam diameter ca. 60  m Pulse energy ca. 50 mJ in 20 ns Broad-band Intensity ca. 17 MJ/m 2 Instantaneous power ca. 25 MW ambient

24. 29 aug 2006Molecular Tagging Velocimetry 1 cm 40 cm 0.65 cm Nozzle diameter d = 1 cm Measurements at x/d = 40 U = 40 m/s, u’/U = 25% Jet turbulence

25. 29 aug 2006Molecular Tagging Velocimetry Jet turbulence: snap shots

26. 29 aug 2006Molecular Tagging Velocimetry Analysis: velocity extraction Cross section Sub-pixel resolution through gaussian fit

27. 29 aug 2006Molecular Tagging Velocimetry Analysis: velocity extraction Perpendicular Cross section Sub-pixel resolution through gaussian fit

28. 29 aug 2006Molecular Tagging Velocimetry Velocity u rms U

29. 29 aug 2006Molecular Tagging Velocimetry Structure functions  (p=2)=0.75 p=8

30. 29 aug 2006Molecular Tagging Velocimetry Scaled exponents  t = 10  s  t = 30  s

31. 29 aug 2006Molecular Tagging Velocimetry t Material line stretching DNS: S. Kida and S Goto, Phys. Fluids 14, 352 (2002)

32. 29 aug 2006Molecular Tagging Velocimetry Issues in data processing Finding the correct line centers can be hard in extremely curved lines with low intensity regions Vertical and perpendicular fit

33. 29 aug 2006Molecular Tagging Velocimetry Issues in data processing Vertical and perpendicular fits ? ? Highly contorted or interupted lines

34. 29 aug 2006Molecular Tagging Velocimetry Examples of badly fitted line Result of the fit program with vertical and perpendicular fit Fit done by hand

35. 29 aug 2006Molecular Tagging Velocimetry ‘Snakes’ Any line v (the ‘snake’) is assigned an energy value E snake (a cost function) This ‘energy’ depends on –the shape of the curve (smoothness constraint) –its position within the image (quality of fit) The best fit is the curve v with the lowest energy

36. 29 aug 2006Molecular Tagging Velocimetry Snake fit: example

37. 29 aug 2006Molecular Tagging Velocimetry Material line stretching l t (1) l t (n)  *  0.17

38. 29 aug 2006Molecular Tagging Velocimetry Material line stretching DNS: S. Kida and S Goto, Phys. Fluids 14, 352 (2002)

39. 29 aug 2006Molecular Tagging Velocimetry exponential spreading diffusion At short times, t~   combined Diffusion & stretching combined  = 0.25 (DNS: 0.17)

40. 29 aug 2006Molecular Tagging Velocimetry At long times exponential spreading becomes clear Again  = 0.25 Exponential spreading at larger  t

41. 29 aug 2006Molecular Tagging Velocimetry Clouds Why can you make them at all? NO creation process is (strongly) non-linear (in intensity)

42. 29 aug 2006Molecular Tagging Velocimetry Richardson: Mean-square separation should grow as t 3 for long times: Batchelor: Mean-square separation should grow as t 2 for : Spreading of molecular clouds 

43. 29 aug 2006Molecular Tagging Velocimetry READ WRITE Pulsed excimer laser lens beam splitter Pulsed dye laser spherical mirror for the “writing” of the second dot WRITE t0+tt0+t t0t0 Two clouds

44. 29 aug 2006Molecular Tagging Velocimetry  t = 10  s  t = 20  s  t = 30  s  t = 40  s Two clouds: examples

45. 29 aug 2006Molecular Tagging Velocimetry Batchelor rules

46. 29 aug 2006Molecular Tagging Velocimetry Molecules: the perfect flow tracers? yes & no, by definition diffusion –can(?) be tuned by Sc chemical & thermal intrusiveness: –can be tuned by tracer local tracer (cloud) creation image processing is an issue implementations exist for gas, liquid, flames