Planar particle imaging and doppler velocimetry system and method

• US 6,542,226 B1
• Filed: 06/04/2001
• Issued: 04/01/2003
• Est. Priority Date: 06/04/2001
• Status: Expired due to Fees

• Current Assignee: U.S. Government Administrator of NASA (Government of the United States of America)
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Wernet, Mark P.
• Primary Examiner(s): Buczinski, Stephen C.
• Application Number:
  US09/874,956
• Time in Patent Office: 666 Days
• Field of Search: 356/28, 356/28.5
• US Class Current: 356/28.5
• CPC Class Codes: G01P 5/001 Full-field flow measurement, e.g. determining flow velocity and direction in a whole region at the same time, flow visualisation
• CPC Class Codes: G01P 5/26 by measuring the direct influence of the streaming fluid on the properties of a detecting optical wave
• CPC Class Codes: G01S 17/58 Velocity or trajectory determination systems; Sense-of-movement determination systems
• CPC Class Codes: G01S 17/95 for meteorological use
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating
• CPC Class Codes: Y02A 90/10 Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation

Abstract

A planar velocity measurement system (100) is operative to measure all three velocity components of a flowing fluid (106) across an illuminated plane (108) using only a single line of sight. The fluid flow is seeded with small particles which accurately follow the flow field fluctuations. The seeded flow field is illuminated with pulsed laser light source (102) and the positions of the particles in the flow are recorded on CCD cameras (122,124). The in-plane velocities are measured by determining the in-plane particle displacements. The out-of-plane velocity component is determined by measuring the Doppler shift of the light scattered by the particles. Both gas and liquid velocities can be measured, as well as two-phase flows.

Claims

1. A method for obtaining 3-component velocity measurements of fluid flows comprising

a) providing a fluid flow seeded with particles;

b) directing at least two pulses of laser light through the fluid flow, wherein each pulse projects an illuminated light plane through the fluid flow;

c) splitting scattered light from the light plane into two beams, wherein the two beams include a reference beam, and a signal beam;

d) directing the signal beam through a molecular filter;

e) capturing image frames from each of the pulses of laser light, wherein the image frames include at least one reference image frame of the reference beam and at least one signal image frame of the signal beam after being directed through the molecular filter;

f) calculating a plurality of in-plane velocity components of the fluid flow responsive to at least two of the image frames each corresponding to a different pulse of the laser light; and

g) calculating a plurality of out-of-plane velocity components of the fluid across the light plane responsive to at least a portion of the in-plane velocity components and responsive to at least one reference image frame and at least one signal image frame corresponding to a common pulse of the laser light.

2. The method according to claim 1, wherein in step (c), the scattered light from the light plane passes through a single optical access point adjacent the light plane.

3. The method according to claim 1, wherein in step (c), the scattered light split by the beam splitter includes light scattered from the light plane in a direction generally perpendicular to the light plane.

4. The method according to claim 1, wherein step (g) includes measuring a plurality of Doppler frequency shifts of the scattered light from the fluid flow across the light plane.

5. The method according to claim 4, wherein step (g) includes determining differences in light intensity between portions of the at least one signal frame and corresponding portions of the at least one reference frame.

6. The method according to claim 5, further comprising:

h) tuning a laser injection seeder voltage associated with the common pulse of the laser light so that the Doppler frequency shifts are, on average, at about the midpoint of the transition edge in between the maximum absorption and maximum transmission of the molecular filter.

7. The method according to claim 6, further comprising:

i) adjusting the temperature and pressure of the molecular filter to achieve an absorption profile slope that encompasses a range of the Doppler frequency shifts.

8. The method according to claim 4, wherein step (g) includes calculating a plurality of oblique velocity components of the fluid flow responsive to the Doppler frequency shifts and the wavelength of the at least one pulse of laser light, wherein the oblique velocity components lie at an oblique angle with respect to the light plane, and wherein step (g) further includes calculating the out-of-plane velocity components of the fluid flow responsive to the oblique velocity components, the oblique angle, and the in-plane velocity components.

9. The method according to claim 8, wherein in step (g) the oblique angle is about forty-five degrees.

10. The method according to claim 1, wherein in step (g) the common pulse of the laser light corresponds to laser light from an injection seeded laser.

11. The method according to claim 1, wherein step (g) includes performing Doppler global velocimetry image processing responsive to the at least one reference image frame and the at least one signal image frame.

12. The method according to claim 11, wherein step (g) includes performing Doppler global velocimetry image processing responsive to at least two reference image frames and at least two signal image frames corresponding to the at least two of the pulses of laser light.

13. The method according to claim 11, wherein step (f) includes performing particle imaging velocimetry image processing responsive to at least two signal image frames corresponding to the different pulses of the laser light.

14. The method according to claim 11, wherein step (f) includes performing particle imaging velocimetry image processing responsive to at least two reference image frames corresponding to the different pulses of the laser light.

15. The method according to claim 1, wherein step (f) includes performing particle imaging velocimetry image processing responsive to the signal image frames and the reference image frames corresponding to the different pulses of the laser light.

16. The method according to claim 1, wherein step (f) includes calculating the plurality of in-plane velocity components of the fluid flow responsive to at least two reference image frames corresponding to the different pulses of the laser light.

17. The method according to claim 16, wherein step (f) further includes calculating the plurality of in-plane velocity components of the fluid flow responsive to at least two signal image frames corresponding to the different pulses of the laser light.

18. The method according to claim 1, wherein in step (e) the image frames are captured by at least one frame straddling camera.

19. The method according to claim 18, wherein in step (e) the at least one reference image frame and the at least one signal image frame are captured by adjacent portions of the frame straddling camera.

20. The method according to claim 18, wherein in step (e) the at least one reference image frame is captured by a first frame straddling camera and the at least one signal image frame is captured by a second frame straddling camera.

21. The method according to claim 1, wherein after step (a) further comprising:

h) directing a portion of each pulse of laser light to a frequency monitoring device;

i) measuring frequency variations of the portion of each pulse of laser light with the frequency monitoring device; and

wherein in step (g) the plurality of out-of-plane velocity components are further calculated responsive to the frequency variations.

22. The method according to claim 21 further comprising:

j) splitting the portion of each pulse of laser light into two further beams;

wherein the two further beams include a further reference beam, and a further signal beam;

k) directing the further signal beam through a further molecular filter;

l) capturing further image frames from each of the pulses of laser light, wherein the further image frames include a further reference image frame of the further reference beam and a further reference signal image frame of the further signal beam after being directed through the further molecular filter; and

wherein in step (i) the frequency variations are calculated responsive to captured further image frames.

23. A velocimetry apparatus comprising:

a laser light source, wherein the laser light source is operative to output at least two pulses of laser light through a fluid flow seeded with particles, wherein the laser light forms an illuminated light plane through the fluid flow;

a beam splitter, wherein the beam splitter is operative to split scattered light from the light plane into a reference beam and a signal beam for each pulse of the laser light;

a molecular filter, wherein the molecular filter includes a molecular absorption feature that is operative to filter the signal beam responsive to the Doppler frequency shift associated with particles illuminated in the light plane, wherein an operating frequency of the laser is tuned to coincide with the molecular absorption feature;

at least one camera, wherein the at least one camera is operative to capture a plurality of image frames corresponding to the reference beam and signal beam for the at least two, pulses of laser light, wherein the image frames include at least one reference image frame of the reference beam and at least one signal image frame of the signal beam after being filtered by the molecular filter; and

at least one computer processor in operative connection with the at least one camera, wherein the at least one computer processor is operative to calculate a plurality of in-plane velocity components of the fluid flow responsive to at least two of the image frames, each corresponding to a different pulse of the at least two pulses of laser light, and wherein the at least one computer processor is operative to calculate the out-of-plane velocity components of the fluid flow responsive to the calculated in-plane velocity components and responsive to the reference image frame and the signal image frame corresponding to a common pulse of the at least two pulses of the laser light.

24. The apparatus according to claim 23, wherein the laser light source is operative to emit at least one pulse of laser light that has a bandwidth of about 100 MHz.

25. The apparatus according to claim 23, wherein the laser light source includes at least one injection seeded Nd:

YAG laser.

26. The apparatus according to claim 23, wherein the laser light source includes two injection seeded lasers.

27. The apparatus according to claim 23, farther comprising an optical access port between the fluid flow and the beam splitter, wherein the beam splitter and the optical access port are orientated along a line that is generally perpendicular to the light plane.

28. The apparatus according to claim 23, wherein a molecular filter includes an iodine cell.

29. The apparatus according to claim 23, wherein the at least one camera includes a frame straddling camera.

30. The apparatus according to claim 29, and further comprising a second camera, wherein the first camera is operative to capture reference image frames, wherein the second camera is operative to capture the signal image frames.

31. The apparatus according to claim 23, wherein the computer processor is operative to perform particle imaging velocimetry image processing on at least two reference image frames, each corresponding to a different pulse of the at least two pulses of laser light.

32. The apparatus according to claim 23, wherein the computer processor is operative to perform Doppler global velocimetry image processing on the reference image frame and the signal image frame corresponding to the common pulse of the at least two pulses of the laser light.

33. The apparatus according to claim 32, wherein the computer processor is operative to calculate a plurality of Doppler frequency shifts of the scattered light responsive to the reference image frame and the signal image frame corresponding to the common pulse of the at least two pulses of the laser light.

34. The apparatus according to claim 33, wherein the computer processor is operative to calculate a plurality of oblique velocity components of the fluid flow responsive to the calculated Doppler frequency shifts and a value that corresponds to the wavelength of the laser light.

35. The apparatus according to claim 34, wherein the computer processor is operative to calculate the out-of-plane velocity components responsive to the calculated oblique velocity components, an oblique angle associated with the oblique velocity components, and the calculated in-plane velocity components.

36. The apparatus according to claim 23, further comprising a frequency monitoring device, wherein the frequency monitor device is operative to measure frequency variations of the pulses of laser light, wherein the computer processor is operative to calculate the out-of-plane velocity components of the fluid flow responsive to the frequency monitoring device.

37. A method for obtaining 3-component velocity measurements of fluid flows comprising:

a) providing a fluid flow seeded with particles;

b) directing two pulses of laser light through the fluid flow, wherein each pulse projects an illuminated light plane through the fluid flow;

c) receiving scattered light from the light plane which is adjacent a common line of sight to the light plane for each pulse of the laser light;

d) splitting the scattered light for each pulse of laser light into two beams with a beam splitter, wherein the two beams include a reference beam and a signal beam;

e) directing the signal beam through a molecular filter;

f) capturing at least two reference image frames of the reference beam, wherein each of the two reference image frames corresponds to a different pulse of the laser light;

g) capturing at least one signal image frame of the signal beam for at least one of the two pulses of the laser light after the signal beam has passed through the molecular filter;

h) determining in-plane particle displacements between the two pulses of laser light, responsive to the two reference image frames;

i) calculating a plurality of in-plane velocity components of the fluid flow responsive to the determined in-plane particle displacements and a time interval between the two pulses of laser light;

j) determining the Doppler shift of the light scattered by the particles responsive to both the signal image frame captured in step (g) and at least one reference image frame captured in step (f) which corresponds to the same pulse of the laser light as the signal image frame captured in step (g); and

k) calculating a plurality of out-of-plane velocity components of the fluid across the light plane responsive to the determined Doppler shift of the light scattered by the particles and at least a portion of the in-plane velocity components calculated in step (h), whereby from the common line of sight three dimensional velocity measurements for the fluid flow are obtained.