Abstract

A dual-scatter laser Doppler velocimeter (LDV) system designed for measuring wind tunnel flow velocity is described. The system simultaneously measures two orthogonal velocity components of a flowing fluid at a common point in the flow. Essential single-velocity component dual-scatter concepts are presented to simplify the description of the more sophisticated two-component system. To implement the two-component system three laser beams with a 0°, 45°, and 90° polarization plane relationship are focused to a common point in the flow by the system-transmitting optics. The beams interfere to form two perpendicular sets of interference fringe planes that are orthogonally polarized. The system-receiving optics collect and separate the orthogonally polarized components of laser radiation scattered from micron-size particles moving with the flowing fluid through the fringes. The system requires no artificial seeding, since intrinsic test section aerosols are utilized for radiation scattering. The passage of each scatter particle through the interference fringes simultaneously produces two frequency-burst-type photodetected signals, the frequencies of which are directly proportional to two perpendicular components of particle velocity. The system photodetection, signal-conditioning, and data acquisition instrumentation is specifically designed to process the frequency burst information in the time domain as opposed to spectrum analysis or frequency domain processing. The system was initially evaluated in an AEDC wind tunnel operating over a Mach number range from 0.6 to 1.5. The LDV and calculated wind tunnel mean velocity data agreed to within 1.25%; flow direction deviations of a few milliradians were resolved.

© 1973 Optical Society of America

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References

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  1. J. W. Foreman, Appl. Opt. 6, 821 (1967).
    [CrossRef] [PubMed]
  2. R. J. Goldstein, D. K. Kried, Trans. ASME, Ser. E, J. Appl. Mech. 34, 813 (1967).
    [CrossRef]
  3. D. T. Davis, I.S.A. Trans. 7, 43 (1968).
  4. J. G. Angus, D. L. Morrow, J. W. Dunning, M. J. French, Ind. Eng. Chem. 61, 8 (1969).
    [CrossRef]
  5. A. E. Lennert, D. B. Brayton, F. L. Crosswy, AEDC-TR-70-101 (1970).
  6. D. B. Brayton, 1969 Proceedings Electro-Optical Systems Design Conference (Industrial and Scientific Conference Management, Inc., 222 W. Adams St., Chicago, Ill., 1970), pp. 168–177.
  7. W. T. Mayo, J. Sci. Instrum. (J. Phys. E) 3, 235 (1970).
    [CrossRef]
  8. M. J. Rudd, J. Sci. Instrum. (J. Phys. E) 2, 55 (1969).
    [CrossRef]
  9. The concept of threshold laser power level as discussed in the latter part of Sec. II.A for the dual-scatter LDV is also applicable to the reference-beam LDV. For all reference-beam configurations except for that of back-scattered radiation collection, the scattering direction must be severely restricted in range due to frequency broadening considerations.1–6 Thus, except for back scattering, the reference-beam LDV requires a higher threshold laser power level.
  10. D. B. Brayton, W. H. Goethert, Trans. Instrum. Soc. Am. 10, 40 (1971).
  11. C. M. Penny, I.E.E.E. J. Quantum Mech. QE5, 318 (1968).
  12. H. Kogelnik, Bell Syst. Tech. J. XX, 467 (March1965).
  13. M. Born, E. Wolf, Principles of Optics (Pergamon Press, New York, 1965), p. 652.
  14. M. Kerker, The Scattering of Light (Academic Press, New York, 1969).
  15. Let it be assumed that wedge refraction caused by the poor surface flatness of an ordinary window will cause the direction of each illuminating beam to be deviated by an average angular amount aw in a random direction. One can then show that the beam separation distance at the window DW must satisfy DW ≪ NF λ/αW if substantial beam overlap at the beam crossover point is to be maintained. The quantity αW has been experimentally determined to be about 1 × 104 radian by examining the crossover point after inserting many ordinary plate glass and plexiglass windows.
  16. D. B. Brayton, W. M. Farmer, “Small Particle Signal Characteristics of a Dual Scatter Laser Doppler Velocimeter,” Appl. Opt., to be published.
  17. F. H. Smith, A. E. Lennert, J. O. Hornkohl, AEDC-TR-71-165 (1971).

1971 (1)

D. B. Brayton, W. H. Goethert, Trans. Instrum. Soc. Am. 10, 40 (1971).

1970 (1)

W. T. Mayo, J. Sci. Instrum. (J. Phys. E) 3, 235 (1970).
[CrossRef]

1969 (2)

M. J. Rudd, J. Sci. Instrum. (J. Phys. E) 2, 55 (1969).
[CrossRef]

J. G. Angus, D. L. Morrow, J. W. Dunning, M. J. French, Ind. Eng. Chem. 61, 8 (1969).
[CrossRef]

1968 (2)

D. T. Davis, I.S.A. Trans. 7, 43 (1968).

C. M. Penny, I.E.E.E. J. Quantum Mech. QE5, 318 (1968).

1967 (2)

J. W. Foreman, Appl. Opt. 6, 821 (1967).
[CrossRef] [PubMed]

R. J. Goldstein, D. K. Kried, Trans. ASME, Ser. E, J. Appl. Mech. 34, 813 (1967).
[CrossRef]

1965 (1)

H. Kogelnik, Bell Syst. Tech. J. XX, 467 (March1965).

Angus, J. G.

J. G. Angus, D. L. Morrow, J. W. Dunning, M. J. French, Ind. Eng. Chem. 61, 8 (1969).
[CrossRef]

Born, M.

M. Born, E. Wolf, Principles of Optics (Pergamon Press, New York, 1965), p. 652.

Brayton, D. B.

D. B. Brayton, W. H. Goethert, Trans. Instrum. Soc. Am. 10, 40 (1971).

D. B. Brayton, 1969 Proceedings Electro-Optical Systems Design Conference (Industrial and Scientific Conference Management, Inc., 222 W. Adams St., Chicago, Ill., 1970), pp. 168–177.

A. E. Lennert, D. B. Brayton, F. L. Crosswy, AEDC-TR-70-101 (1970).

D. B. Brayton, W. M. Farmer, “Small Particle Signal Characteristics of a Dual Scatter Laser Doppler Velocimeter,” Appl. Opt., to be published.

Crosswy, F. L.

A. E. Lennert, D. B. Brayton, F. L. Crosswy, AEDC-TR-70-101 (1970).

Davis, D. T.

D. T. Davis, I.S.A. Trans. 7, 43 (1968).

Dunning, J. W.

J. G. Angus, D. L. Morrow, J. W. Dunning, M. J. French, Ind. Eng. Chem. 61, 8 (1969).
[CrossRef]

Farmer, W. M.

D. B. Brayton, W. M. Farmer, “Small Particle Signal Characteristics of a Dual Scatter Laser Doppler Velocimeter,” Appl. Opt., to be published.

Foreman, J. W.

French, M. J.

J. G. Angus, D. L. Morrow, J. W. Dunning, M. J. French, Ind. Eng. Chem. 61, 8 (1969).
[CrossRef]

Goethert, W. H.

D. B. Brayton, W. H. Goethert, Trans. Instrum. Soc. Am. 10, 40 (1971).

Goldstein, R. J.

R. J. Goldstein, D. K. Kried, Trans. ASME, Ser. E, J. Appl. Mech. 34, 813 (1967).
[CrossRef]

Hornkohl, J. O.

F. H. Smith, A. E. Lennert, J. O. Hornkohl, AEDC-TR-71-165 (1971).

Kerker, M.

M. Kerker, The Scattering of Light (Academic Press, New York, 1969).

Kogelnik, H.

H. Kogelnik, Bell Syst. Tech. J. XX, 467 (March1965).

Kried, D. K.

R. J. Goldstein, D. K. Kried, Trans. ASME, Ser. E, J. Appl. Mech. 34, 813 (1967).
[CrossRef]

Lennert, A. E.

A. E. Lennert, D. B. Brayton, F. L. Crosswy, AEDC-TR-70-101 (1970).

F. H. Smith, A. E. Lennert, J. O. Hornkohl, AEDC-TR-71-165 (1971).

Mayo, W. T.

W. T. Mayo, J. Sci. Instrum. (J. Phys. E) 3, 235 (1970).
[CrossRef]

Morrow, D. L.

J. G. Angus, D. L. Morrow, J. W. Dunning, M. J. French, Ind. Eng. Chem. 61, 8 (1969).
[CrossRef]

Penny, C. M.

C. M. Penny, I.E.E.E. J. Quantum Mech. QE5, 318 (1968).

Rudd, M. J.

M. J. Rudd, J. Sci. Instrum. (J. Phys. E) 2, 55 (1969).
[CrossRef]

Smith, F. H.

F. H. Smith, A. E. Lennert, J. O. Hornkohl, AEDC-TR-71-165 (1971).

Wolf, E.

M. Born, E. Wolf, Principles of Optics (Pergamon Press, New York, 1965), p. 652.

Appl. Opt. (1)

Bell Syst. Tech. J. (1)

H. Kogelnik, Bell Syst. Tech. J. XX, 467 (March1965).

I.E.E.E. J. Quantum Mech. (1)

C. M. Penny, I.E.E.E. J. Quantum Mech. QE5, 318 (1968).

I.S.A. Trans. (1)

D. T. Davis, I.S.A. Trans. 7, 43 (1968).

Ind. Eng. Chem. (1)

J. G. Angus, D. L. Morrow, J. W. Dunning, M. J. French, Ind. Eng. Chem. 61, 8 (1969).
[CrossRef]

J. Appl. Mech. (1)

R. J. Goldstein, D. K. Kried, Trans. ASME, Ser. E, J. Appl. Mech. 34, 813 (1967).
[CrossRef]

J. Sci. Instrum. (J. Phys. E) (2)

W. T. Mayo, J. Sci. Instrum. (J. Phys. E) 3, 235 (1970).
[CrossRef]

M. J. Rudd, J. Sci. Instrum. (J. Phys. E) 2, 55 (1969).
[CrossRef]

Trans. Instrum. Soc. Am. (1)

D. B. Brayton, W. H. Goethert, Trans. Instrum. Soc. Am. 10, 40 (1971).

Other (8)

M. Born, E. Wolf, Principles of Optics (Pergamon Press, New York, 1965), p. 652.

M. Kerker, The Scattering of Light (Academic Press, New York, 1969).

Let it be assumed that wedge refraction caused by the poor surface flatness of an ordinary window will cause the direction of each illuminating beam to be deviated by an average angular amount aw in a random direction. One can then show that the beam separation distance at the window DW must satisfy DW ≪ NF λ/αW if substantial beam overlap at the beam crossover point is to be maintained. The quantity αW has been experimentally determined to be about 1 × 104 radian by examining the crossover point after inserting many ordinary plate glass and plexiglass windows.

D. B. Brayton, W. M. Farmer, “Small Particle Signal Characteristics of a Dual Scatter Laser Doppler Velocimeter,” Appl. Opt., to be published.

F. H. Smith, A. E. Lennert, J. O. Hornkohl, AEDC-TR-71-165 (1971).

The concept of threshold laser power level as discussed in the latter part of Sec. II.A for the dual-scatter LDV is also applicable to the reference-beam LDV. For all reference-beam configurations except for that of back-scattered radiation collection, the scattering direction must be severely restricted in range due to frequency broadening considerations.1–6 Thus, except for back scattering, the reference-beam LDV requires a higher threshold laser power level.

A. E. Lennert, D. B. Brayton, F. L. Crosswy, AEDC-TR-70-101 (1970).

D. B. Brayton, 1969 Proceedings Electro-Optical Systems Design Conference (Industrial and Scientific Conference Management, Inc., 222 W. Adams St., Chicago, Ill., 1970), pp. 168–177.

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Figures (9)

Fig. 1
Fig. 1

(a) Single-velocity-component dual-scatter laser velocimeter detection scheme. The parallel-surface coated optical flats 2a and 2b cause the laser beam 1 to be split into two equal-intensity, parallel path beams 3 and 4. Lens 5 causes these beams to both cross and focus at a common point F. Light 6, scattered from particles moving through the adjacently bright and dark interference fringe planes established near F, is first selectively collected by the lens–pinhole aperture combination 7 and 8, and then detected by the photodetector system 9. Light intensity vs time is displayed on the CRT oscilloscope 10 and the signal period is simultaneously determined by the Doppler burst signal processor 11. (b) Enlarged cross-sectional views of the beam-crossover region F of (a). The indicated fringe width is proportional to the local peak fringe intensity.

Fig. 2
Fig. 2

Signal amplitude vs particle position near X = 0 for a number of particle trajectories. The indicated fringe width is proportional to the local peak fringe intensity.

Fig. 3
Fig. 3

Two orthogonal views of the two-component velocity detection system as applied to the transonic wind tunnel PWT-IT located at the AEDC. The scale factors indicated are accurate for all component dimensions and relative component positions except for components 19–29 inclusively.

Fig. 4
Fig. 4

Enlarged cross-sectional views of the three illuminating beams of Fig. 3 before (AA) and after (BB) polarization plane rotation. Electric field vector amplitude and direction is indicated.

Figure 5
Figure 5

Enlarged cross-sectional view (CC) of the focal region 13 of Fig. 3 showing the fringe planes established by beams 5, 6, and 7.

Fig. 6
Fig. 6

Examples of two simultaneously recorded signals due to the passage of a single, submicron-size scatter particle through two crossed-polarized, orthogonally located sets of interference fringes; polarization separation was used to discriminate between the two signals and the laser velocimeter system of Fig. 3 was employed. Signal channel alternating rate, 1 MHz; sweep rate, 10 μsec/div.

Fig. 7
Fig. 7

Doppler data processor.

Fig. 8
Fig. 8

Doppler data processor signal waveforms.

Fig. 9
Fig. 9

Six CRT photographed signal bursts at Mach 1.0 with the corresponding period recorded by the Doppler data processor (DDP), Vertical sensitivity, 2.3 × 10−9 cathode amp/cm (S-20 response); horizontal, 0.2 μsec/cm.

Equations (14)

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D = 4 λ / π Δ θ ,
D F = λ / 2 sin ( θ / 2 ) ,
f = V 0 / D F = 2 V 0 sin ( θ / 2 ) / λ ,
D w 0.5 N F cm ,
z 2 + y 2 cos 2 ( θ / 2 ) + x 2 sin 2 ( θ / 2 ) = 2 a ( λ / π Δ θ ) 2 ,
Δ z = 4 λ / π Δ θ ,
Δ y = 4 λ / π Δ θ cos ( θ / 2 ) ,
Δ x = 4 λ / π Δ θ sin ( θ / 2 ) ;
N F = 8 tan ( θ / 2 ) / π Δ θ .
Δ y Δ z 0.001 cm , Δ x 0.004 cm
Δ x Δ x = Effective probe volume x dim . Unapertured or maximum probe volume x dim . = e θ Δ θ s ,
N F = N F cos β ,
N F = N F cos β N e .
d V 0 / V 0 = ( d f / f ) - [ d θ / 2 tan ( θ / 2 ) ] ,

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