The potential of laser Doppler velocimetry as a technique for use in mapping flows in the several fluid systems under development for doing research on low-gravity processes, is investigated. Laser Doppler velocimetry techniques, equipment, and applications are summarized

Owen, R. B.

English

NASA Technical Reports Server

NASA CONFERENCE PUBLICATION 
NASA CP-2084 
LASER DOPPLER VELOC IMETRY WORKSHOP 
The Summary of a Workshop Held February 12, 1979 
at Marshall Space Flight Center 
Robert B. Owen 
Space Sciences Laboratory 
March 1979 
Prepared by 
NASA - George C. Marshall Space Flight Center 
Marshall Space Flight Center, Alabama 35812 
https://ntrs.nasa.gov/search.jsp?R=19790014284 2020-03-21T23:46:19+00:00Z
LASER DOPPLER VELOCIMETRY WORKSHOP 
A workshop on Laser Doppler Velocimetry was held at Marshall Space 
Flight Center on February 12, 1979. (An agenda is included in the Appendix.) 
The purposes of the workshop were to investigate the potential of laser doppler 
velocimetry (LDV) as a technique to use in mapping flows in the several fluid 
systems under development by NASA for doing research on low-g processes, to 
familiarize the appropriate MSFC personnel with LDV, and to answer certain 
questions which have repeatedly arisen in our previous investigations of LDV 
techniques. These questions will be specifically addressed in this report, after 
some general statements concerning LDV. 
Laser doppler systems measure the local , instantaneous velocity of 
tracer particles. This means that the relationship between the flow of interest 
and the particle velocity must be known. Appropriate particles often already 
exist in many liquids and gases, and more can generally be added if  needed. 
The most common optical arrangement used in laser doppler studies is 
the dual beam mode (Figure 1). The dual beam or  fringe system uses two 
intersecting light beams of equal intensity to produce a pattern within their 
volume of intersection. A s  each particle crosses the fringes, the intensity of 
light scattered onto a photodetector rises and falls at a rate directly proportional 
to the velocity. 
MASK 
Figure 1. Basic optical arrangement for "dual beam anemometer.'' 
There are several other optical arrangements which can be used. In the 
reference beam mode (Figure 2) ,  the laser beam is split into an intense incident 
beam and a weak reference beam. The reference beam is directed onto a photo- 
cathode where it beats with light scattered from the strong beam by particles 
moving with the flow; the frequency of the scattered light has been altered by the 
doppler effect, and the interference with the reference beam provides a frequency 
difference which is directly proportional to the particle velocity. 
PHOTODETECTOR . . 
FOCUSING LENS 
- 
Figure 2, Basic optical arrangement for "reference-beam anemometer, t f  
In the two-scattered beam or  differential doppler mode. (Figure 3) ,  a 
single focused laser beam is directed into the flow, and light scattered by a 
particle in two directions is collected symmetrically about the system axis. 
When the scattered beams are combined, the relative phase of their wave fronts 
depends on the distances of the particle from each light collecting aperture; 
hence as the particle moves across the beam, the scattered light beams interfere 
constructively and destructively, leading to a light intensity at the photocathode 
which fluctuates at the doppler frequency. This system offers no clear advantage 
over the fringe mode other than its use in m ing simultaneously two velocity 
components by collecting pairs of scattered 
planes. 
in mutually perpendicular 
LENS 
Figure 3. 
MASU 
a 
* 6 
Basic optical arrangement for "two-scattered beam anemometer." 
2 
In a fourth optical arrangement, two beams are focused to two separate 
spots in space. This forms the basis of the "two-spot'7 system (Figure 4) which 
is currently being developed at Spectron Development Labs by J. Trolinger and 
associates. This system can be interpreted as the Fourier transform of LDV. 
A tracer particle traveling from one spot to another scatters at each point; the 
spots are made as small as possible (for example, 10-micron spots separated 
by perhaps 100 microns), and the scattered signals are correlated to eliminate 
erroneous data, The resultant signal/noise ratio is 100 to 1000 times better 
than that in standard systems. This arrangement is, therefore, suited for very 
small particles o r  for a case in which test cell geometry precludes the use of a 
dual-beam arrangement. The spots can be rotated about the optical axis by use 
of a dove prism; one can then specify the velocity vector (in the plane perpen- 
dicular to the optical axis) by rotating the spots until the correlation function is 
maximized. The improved system signal/noise ratio is realized primarily 
because the scattered light is more intense due to the focused beams and because 
the spots can be imaged on two separate photomultiplier tubes. There is a slight 
velocity error  (approximately 0.2 percent) which arises from tracer particles 
passing through different parts of the spots. It should be noted that the correla- 
tion technique requires time, typically 3 to 20 seconds, o r  even several minutes 
under extreme conditions such as very low tracer particle densities. Also, 
standard LDV geometries are better for turbulent flow measurement. However, 
the two-spot geometry is superior to standard geometries near a boundary. 
A variation of LDV which was  discussed at the workshop (mostly by M. 
Farmer and J. Mann) involved use of a fluorescent LDV probe. Such a system 
would use molecules which would fluoresce in the laser beam as tracer particles. 
If used in an ordinary dual-beam LDV system, the molecules would fluoresce on 
and off as  they crossed the fringe pattern, It would be possible to adjust the 
structure of the molecules (surfactants) so they would accumulate at surfaces, 
thus making it possible to distinguish between bulk and surface flows. Another 
possibility would be to use a two-dimensional Bragg cell to generate two sets of 
moving fringes. Such a system would use well-established dual-beam LDV 
technology and would eliminate many of the problems which sometimes arise 
from stray laser light, 
All  the system geometries discussed can be employed with either forward 
or backward scattered light, but most measurements are  made with forward 
scattering since this signal is much more intense (by a factor of about 1000) for 
scattering particles of the size used to represent fluid velocities. 
Some sort of signal processing system such as spectrum analyzer or 
frequency tracker must be utilized since a large number of particles must be 
3 
X 
4 
observed t6 obtain reliable results, In most flow configurations, low particle 
concentrations preclude averaging with continuous signals. A s  a result, elec- 
tronic devices are required to control the intermediate signal output when no 
doppler signal is present, Devices of this sort must be scrutinized with care. 
Another problem common to all methods of signal analysis is posed by reverse 
flow, as found in regions of high turbulence. The doppler frequency is related 
to the velocity but does not reflect the direction of flow in a given line. This is 
commonly allowed for by using optical frequency shifting devices to change the 
frequency of one beam and so displace the doppler frequency corresponding to 
zero velocity from zero to a finite positive value, thus providing the sign of the 
measured velocity. A s  a result, a carefully selected range of instruments is 
normally required to obtain useful information in different flows, 
It is also important to have information on the size and size range of the 
light scattering particles before an optical system is designed, since the relative 
magnitude of the modulated and unmodulated components of the light scattered 
from a particle depends on the size of the particle and the ratio of this size to 
the fringe spacing. Water  normally contains ample suitable particles, but air 
and most gases do not, More important in many applications is the extent to 
which particles of different diameter and density will follow the flow. For 
example, a 1-micron diameter dust particle will follow frequencies up to about 
10 kHz in air with a precision of 1 percent; if the particle size is 10 microns 
(dia) , the frequency response drops to 700 Hz. Clearly, particle size data is 
critical to successful LDV studies, 
In general, the need to match the components of an LDV system to each 
other and to the flow configuration cannot be overemphasized. Provided this is 
done, LDV systems are precise, economical, and easy to use. Many measure- 
ments cannot be made any other way, 
We are now in a position to make some specific comments on questions 
which have repeatedly arisen in our previous investigation of LDV techniques. 
1, What are the measurement capabilities of LDV in ultrapure materials 
which do not contain added tracer particles ? Must spatial and/or velocity reso- 
lution be sacrificed ? 
To obtain conditions in which molecular scattering predominates, it is 
sometimes necessary to filter for months. Under such ultrapure conditions, 
LDV probably could not be used unless the material would fluoresce in the laser 
beam. However, particles as small as 0.5 microns in diameter, which can be 
expected to occur naturally in our systems, would provide a perfectly adequate 
LDV tracer array. 
5 
By using an existing LDV system at Marshall (in the Fluid Dynamics 
Laboratory, Space Sciences Laboratory) , it was established that an LDV signal 
could be obtained from a sample of R. Kroes' crystal growth solution prepared 
as for his ground study work. The solution used filtered distilled water, but 
clean-room conditions and elaborate filtering techniques were not attempted. 
It is almost impossible to maintain a fluid of this type in a condition that will 
not produce LDV signals. Spatial and velocity resolutions are tied to the fluid 
characteristics and the required data rate, not tracer particle size. However, 
a limiting factor might be refractive index gradients caused by thermal or  
density gradients which could cause the illumination beam to wander. Some 
modeling of the flow is necessary to determine the seriousness of this problem. 
2. What sizes and densities of tracer particles are required to measure 
submillimeter/second flows ? 
Some elementary computations performed during the workshop led to the 
conclusion that particles as small as 0.2 micron in diameter would be sufficiently 
large not to be adversely affected by Brownian motion. No upper limit is 
required on such tracer particles in this type of fluid for low velocities. The 
size of the tracer particle required depends critically upon the specific laser 
velocimeter system used. Generally speaking, for particles 0 . 5  micron in 
diameter and smaller, a forward scatter laser velocimeter would be required 
unless a photon correlation may permit use of a backscatter system. The 
required number density of particles is closely connected to the required data 
rate. 
3, What configurations are most suitable for mapping a flow field? 
What are the tradeoffs for measuring three directional components of the flow, 
versus two components , versus one component ? 
The most suitable configuration for mapping a flow field such as this is 
a backscatter system because traversing and scanning can be accomplished with 
zoom lenses and scanning mirrors. This problem must be traded off against the 
required tracer particles and laser power. There was general agreement during 
the workshop that a compromise was required to make this system practical. 
The compromise would include such things as reducing the measurement to a 
one- or two-component velocity measurement with mapping limited to a few line 
scans through the flow field. The possibility of producing an array detection 
system was discussed, and it would be possible to use an array of probe volumes 
with the same set of electronics serving several directional components and 
probe volumes. This seems to be a highly practical and desirable configuration. 
Also, the Fourier transform analyzer described by M. Fingerson offers a method 
of simplifying the electronics. 
6 
It is very difficult to measure a three-dimensional flow field from a 
single position. Such a measurement would probably require a separate system 
oriented at 90 degrees to the basic measuring system. The benefits of such an 
arrangement are rather dubious. A two-dimensional system is relatively 
straightforward, but even two-dimensional scanning would require considerable 
hardware and sophistication. It is possible to design a system which could be 
one or two dimensional as required. 
4. What resolution can be expected from an LDV system? 
Again, specific answers are highly dependent on the available particle 
size and number density distribution in the fluid. However, a spatial resolution 
of 100 micrometers should be easily obtained, with 10 micrometers a possibility, 
assuming no significant refractive index gradients. Specific information about 
the experimental fluid must be available to discuss this question. 
5. How accessible must the test zone be for these various configurations 
to be effective? 
The accessibility of a test zone and the locations and sizes of the various 
optics depend critically upon the particle size and number density distribution as 
well as upon the required data rate in the measurement. The required data rate 
also depends critically upon the type of signal processing used. 
6. Is it feasible to measure flows in opaque materials such as molten 
metals using surface reflections, optical fiber probes, etc. ? 
Surface measurements are possible using a dual and/or reference beam 
LDV configuration. 
In summary, LDV appears suitable for making flow measurements in our 
fluid systems provided that the systems are adequately defined, the flows are 
not dominated by transients, and the system contains no significant variations in 
refractive index. More detailed discussion of these three problem areas follows. 
A s  was repeatedly emphasized during the workshop, it will be necessary 
to specify our measurement requirements to specify system design parameters. 
Because the LDV configuration must be matched to the experiment to obtain 
optimum results, it is difficult to design a general purpose LDV system which 
will operate at maximum capacity for all fluids experiments. However, it is 
probably possible to design an LDV system which will yield adequate results for 
most experimental configurations. To insure this, it will be necessary to do 
7 
ground-based studies to avoid relying on the LDV system to measure every 
unknown during flight. One such approach would be to do a computer study of 
the fluid dynamics of each experiment and check the computer results on the 
ground. When the experiment is flown, one need only make measurements in  
areas which the computer results indicate are critical. This will reduce the 
measurement requirements as mentioned previously (question 3) . One then has 
a well-defined experiment with a greatly simplified LDV system. Such an 
approach should be within the capabilities of current LDV technology. 
Because LDV measures velocities only at a point, transient flows are 
difficult to measure using this technique. If the flows contain significant 
transients, it is desirable to map the entire flow field, which is difficult to do 
rapidly using LDV. 
LDV always involves two unknowns, the tracer velocity and the integrated 
refractive index over the light paths in the fluid. At  the low velocities e-xpected 
in our fluid systems a knowledge of refractive index variations becomes more 
critical. Most applications of LDV involve systems with a constant refractive 
index. If the fluid systems have varying and unknown refractive indices, LDV 
cannot be accurately applied. 
Thus while low velocities and tracer particle requirements should present 
no problems in making LDV measurements, there are problems in current 
measurement requirement definition, flow field visualization, and possible fluid 
refractive index variations. For these reasons it has been decided not to base- 
line LDV for the first fluid experiment system. The technique is, however, 
quite promising for later experiments. Some type of time-exposure photographs 
of tracer particles muld probably be more feasible for initial experiments. 
8 
APPENDIX 
LDV WORKSHOP AGENDA 
February 12, 1979 
Main Conference Room/ Building 4481 
Chairman: J. Williams 
8:30 
9 :oo 
9:30 
1o:oo 
10:30 
11:oo 
12:oo 
12:30 
1 :oo 
1:30 
2 :oo 
2:30 
MPS Fluid Systems Overview 
The Limits of Low Flow Speed 
Measurement with LDV 
Applications of LDV at AEDC 
Typical LDV Configurations 
Surface and Bulk Flows by LDV 
Lunch 
Low Velocity LDV Measurements and 
Particle S iz e Requirements 
Particle Measurements and Particle 
Requirements in Sub-mm Flow and 
Surface Velocity Measurements 
Instrumentation for Application 
in Extremely Low-Seeded Cases 
An Example System for Measuring Low 
Flow Velocities 
Comments Concerning LDV Applicability 
to Solution Crystal Growth 
Discussion and Comments 
R. Ruff 
MSFC 
W. Fowlis 
MSFC 
T. Bentley 
AEDC 
D. Reed 
Texas A&M 
J. Mann 
Case Western 
R. Adrian 
U. of Ill. (Urbana) 
M. Farmer 
UTSI 
J. Trolinger 
Spectron Development Labs 
M. Fingerson 
Thermo-Systems 
R. Kroes 
MSFC 
9 
APPROVAL 
LASER DOPPLER VELOCIMETRY WORKSHOP 
By Robert B. Owen 
The information in this report has been reviewed for technical content. 
Review of any information concerning Department of Defense o r  nuclear energy 
activities or programs has been made by the MSFC Security Classification 
Officer. This report, in its entirety, has been determined to be unclassified. 
Chief, Solid State Branch 
Chief, Space Processing Division 
Director, Space Sciences L'aboratory 
+U.S. GOVERNMENT PRINTING OFFICE 1979--640-081/503 REGION NO. 4 
10 
TECHNICAL REPORT STANDARD TITLE PAGE 
1 .  REPORT NO. 2. GOVERNMENT ACCESSlON NO. 3. RECIPIENT'S CATALOG NO. 
NASA CP- 2084 
I I 
a T ITLE AND SUBTITLE 15. REPORT DATE 
Laser Doppler Velocimetry Workshop 
March 1979 
6. PERFORMING ORGANIZATION CODE 
7. AUTHOR(S) 8. PERFORMING ORGANIZATION REPORT 
Robert B. Owen 
9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. WORK UNIT NO. 
George 6. Marshall Space Flight Center 11. CONTRACT OR GRANT NO. 
Marshall  Space Flight Center, Alabama 35812 
13, TYPE OF REPORT & PERIOD COVEREI 
2 SPONSORING AGENCY NAME AND ADDRESS 
National Aeronautics and Space Administration 
Washington, D.C. 20546 
Conference Publication 
14. SPONSORING AGENCY CODE 
I 
15, SUPPLEMENTARY NOTES 
Prepared by Space Sciences Laboratory, Science and Engineering 
A workshop on laser doppler velocimetry was held at Marshall Space Flight Center 
on February 12, 1979. The main purpose of the workshop was to investigate the potential 
of laser doppler velocimetry as a technique to use in mapping flows in the several fluid 
systems under development by NASA for doing research on low-gravity processes. This 
summary presents the results of the worlxshop. 
7. KEY WORDS 10. DISTRIBUTION STATEMENT 
ssified - Unlimited 
I S M :  - Form 3 2 92 (Rev December 19  7 2 )  For sale by National Technical Ififormation Sewice, Springfield, Virginia 22 15 1 


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