General systems design and optical and signal processing requirements for laser velocimetric measurement of flows are reviewed. Bias errors which occur in measurements using burst (counter) processors are discussed and particle seeding requirements are suggested

Stevenson, W. H.

English

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LASER VZLOCIMETRY - A STATE-OF-TEE- 
ART OVERVIEW 
Warren H. Stevenson 
School of Mechanical Engineering 
Purdue University 
West Lafayette, Indiana 47907 
205 
https://ntrs.nasa.gov/search.jsp?R=19820024807 2020-03-21T06:30:25+00:00Z
III I l llllllllllll II I I 
TYPICAL DIFFtiRENTIAL DOPPLER LDV 
RECEIVING 
LENS SCATTERED 
LASER 
ENLARGED VIEW OF 
PROBE VOLUME 
E(T) 
DETECTOR 
i 
1 
E(T) 
TYPICAL BURST SIGNAL 
1 
PROCESSOR 
VARIABLE PARAMETER LDV SYSTEM 
UPPER OPTICS PACKAGE 
n 
Acousto-Optic Sliding Prism 
Modulators e Adj. Mirrors 
Beam 
Splitter 
qJq +I---- 
Optics Table 
LOWER OPTICS PACKAGE 
Minor 
I Beam Expander 
Telescope Polarization Rotator . 
--fj+ -- -- *-Qg.- -- _I 5 Watt Argon Laser 
* 
f2=68 mm f,,=44 mm 
(movable) (fixed) 
206 
FRINGE MODEL 
Fringe model indicating variations in Doppler frequency. 
Beam waists are on same side of focal plane. 
When beam intersection does not occur at the waists, 
the fringe planes in the probe volume will not be parallel 
and the observed signal frequency will depend on particle 
trajectory. This effect, though small, can lead to an 
increase in the measured turbulence intensity. Perhaps 
of more significance in most cases is that the beam 
intensity is higher at the waists than in the probe 
volume. This results in a lower signal-to-noise ratio. 
Durst, F. and Stevenson, W. H., "Influence of Gaussian 
Beam Properties on Laser Doppler Signals," Applied Optics, 
18, 516 (1979). - 
207 
NATURE OF VOLTAGE SIGNAL 
SINGLE PARTICLE SIGNAL 
MULTI PARTICLE SIGNAL 
PHOTON LIMITED SiGNAL 
I I III Ml I II I I I I Ill Ill1 I Ill I I II I I Ill1 
SIGNALS BURlED IN NOISE 
The nature of the voltage signal observed at the 
detector output will depend on several factors including 
the laser power, scattering efficiency of the particles, 
and particle seeding density. 
either single particle "burst" 
In high speed wind tunnels 
signals will be typical. 
signals or photon limited 
208 
SIGNAL PROCESSING .DEVIChS 
SPECTRUM ANALYZER 
FREQUENCY TRACKER 
BURST PROCESSOR (COUNTER) 
PHOTON CORRELATOR 
TRANSIENT RECORDER 
The parameters usually desired from the sampled 
velocity data are 
Mean Velocities ii, 7, w ? 
Turbulence Intensity 7 'u etc. 
Reynolds Stress I uv 
Power Spectrum E (WI 
209 
COMMENTS ON SEEDING 
Experience has shown that particles z 1 pm are 
needed to make accurate measurements in gas flows. 
Below 1 pm the scattered intensity drops off rapidly 
and above 1 pm the particles do not follow the flow. 
However, smaller particles may be necessary in 
situations where extremely high accelerations are 
present. 
In tunnels where "zero" seeding.is allowed, 
natural contaminants in the tunnel must be used. 
This can pose severe requirements on the LDV system, 
both in terms of the optical design and the signal 
processing. 
Various methods of particle generation are 
available including liquid atomization, solid particle 
dispersal, and "chemical" seeding with reacting gases. 
Yanta, W. J. "The Use of the Laser Doppler Velocimeter 
in Aerodynamic Facilities," Paper 80-0435, 10th AIAA 
Aerodynamics Testing Conference (1980). 
IMPORTANT BIAS ERRORS WHICH OCCUR IN LDV MEASUREMENTS 
USING BURST (COUNTER) PROCESSORS 
BIAS SOURCE' 
Particle Lag Particle Inertia 
Directional Ambiguity Bias Lack of Directional 
Sensitivity in LDV 
Velocity Bias Particle Controlled 
Sampling 
Incomplete Signal Bias Signal Validation Based 
on Fixed Number of Cycles 
Both directional ambiguity and incomplete signal bias 
may be eliminated by frequency shifting. 
210 
VELOCITY BIAS 
ARISES WHEN A PARTICLE (ENSEMBLE) AVERAGE RATHER THAN 
A TIMe AVERAGE OF THE DATA IS USED, 
/Measured Mean” 
=True Mean 
INCOMPLETE SIGNAL BIAS 
ARISES IN HIGHLY TURBULENT FLOWS WHEN PARTICLES AT A 
LARGE flANGLE OF ATTACKN FAIL TO CROSS THE NUMBER OF FRINGES 
NEEDED TO TRIGGER AN OUTPUT FROM THE PROCESSOR, THIS 
LEADS TO AN ERRONEOUSLY HIGH MEASURED MEAN VELOCITY WHEN 
THE FRINGES ARE ALIGNED PERPENDICULAR TO THE LOCAL MEAN 
VELOCITY VECTOR, 
211 
FLOW GEOMETRY OF VELOCITY BIAS STUDY 
POINT OF HIGH TUFWJLENCE 
POINT OF LOW TURBULENCE INTENSITY MEflSUREMENTS 
INTENSITY MEfWUREMENTS 
Q 
J 
ALL DIMENSIONS 
IN MILLIMETERS 
Flow geometry used in Purdue study of velocity 
bias is shown above. 
laminar. 
At point A the flow was essentially 
At points B and C the turbulence intensity was 
20% and 35% respectively. 
212 
EFFECT OF DATA RATE ON MEAN VELOCITY 27 2 22 1 0 0 0 0 00 0 0 
3 0 I’ I 
I IO3 IO4 IO5 
DATA RATE (Part icks/sec) 
Effect of Data Rate on Mean Velocity at Point A 
0 0 0 
o” 0 0 
0 I- D Correct ion 0 
I& I I O0 0 J 
IO2 IO” IO4 IO5 
DATA RATE (Pmticleskec) 
Effect of Data Rate on Mean Velocity at Point B 
In the above figures "data rate" refers to the number of validated 
Doppler signals per second when the processor was uncontrolled. This 
depends on seeding density. The velocity data shown were taken with 
the processor controlled by a microcomputer so that a fixed waiting 
time occurred after a velocity sample was stored before the processor 
was available to accept a new signal. At the high data rates (high 
seeding density) this resulted in essentially equal time interval 
sampling of the velocity at a sample rate of 250 Hz. 
The analytically corrected data point shown in the second figure 
is based on the 1-D correction of McLaughlin and Tiederman. 
213 
EFFECT OF DATA RATE ON MEAN VELOCTTY (Concluded) 
0 
X 0 X Q 
@QQ 25 
Effect of Data Rate on Mean Velocity at Point C 
X- 25 Hz Sampling Rate 
0' 250 Hz Sampling Rate 
Roesler, T. C., Stevenson, W. H., Thompson, H. D., 
"Investigation of Bias Errors in Laser Doppler 
Velocimeter Measurements," AFWAL-TR-80-2105, December 
1980. 
- 
AREAS REQUIRING FURTHER STUDY 
Velocity Bias - “The Final Word” 
Improved Designs for 3-D Systems 
Measurements Near Surfaces 
214 


Laser velocimetry: A state-of-the-art overview

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