Advancements of the fluctuating density gradient cross beam laser Schlieren technique, the fluctuating line-reversal temperature measurement and the development of the two-dimensional drag-sensing probe to a three-dimensional drag-sensing probe are discussed. The three-dimensionality of the instantaneous momentum vector can shed some light on the nature of turbulence especially with swirling flow. All three measured fluctuating quantities (density, temperature, and momentum) can provide valuable information for theoreticians

Battle, T.

Cheng, D. Y.

Wang, P.

English

NASA Technical Reports Server

RECENT ADVANCEMENT OF TURBULENT FLOW MEASUREMENT TECHNIQUES*
Tom Battle, Peter Wang**, and Dah Yu Cheng
Mechanical Engineering.Department, University of Santa Clara
Santa Clara, CA. 95053
In a turbulent flow, pressure, density, temperature, velocity, momentum,
and chemical-species-concentration are the fluctuating quantities to be
measured. After a search of the current measuring techniques, it is found.
that piezoelectric pressure probes are well developed; and the hot-wire, the
pitot probe, and .the laser doppler velocimeter have been developed for velo-
city measurements to a certain degree of sophistication. The Raman Scattering
technique, which had a history in analytic chemistry, is capable of determining
the chemical species concentration even in a chemically reacting region. Since
these techniques have had some years of development history and many more
papers probably will be devoted to some specific advances in their respective
methods, they will not be discussed any further.
Instead, attention is paid to recent advancements of the fluctuating
density gradient cross beam laser Schlieren technique, the fluctuating line-
reversal temperature measurement and the development of the two-dimensional
drag-sensing probe to a three-dimensional drag-sensing probe. So the three-
dimensionality of the instantaneous momentum vector can shed some light on
the nature of turbulence especially with swirling flow. All three measured
fluctuating quantities (density, temperature, and momentum) can provide valu-
able information for theoreticians.
This work is supported under NASA-University Grant No. 05-017-033.
Presently affiliated with Raman Aeronautics Research & Engineerin
of Palo Alto, California.
((NASA-CR-141007 ) RECENT ADVANCEMENT OF N75-13191
TURBULENT FLOW MEASUREMENT TECHNIQUES .r)
(Santa Clara Univ.) 7 p HC $3.25 -
CSCL 20D Unclas go
G3/34 03890
https://ntrs.nasa.gov/search.jsp?R=19750005119 2020-03-23T00:57:48+00:00Z
(1) Cross beam laser Schlieren system
The cross beam laser radiation absorption technique was first developed
around 1966, (Ref. 1-3), where the chemical species fluctuations were assumed
to be in a one-to-one correspondence with density fluctuations in a flow.
Not only was this assumption difficult to verify but also the fluctuating
signal had only a small value on top of a large DC signal and the fluctuating
signal caused by the index of refraction could not be separated from the ab-
sorption signals. The development of the laser Schlieren system (Ref. 4-7),
which measures the density gradient fluctuation, became both a logical step
and a simple workable system.
Recently, we examined the laser Schlieren from an experimental point of
view, concerning the choice of the laser system, the additional information
of signal correlation with a rotating knife edge and the effect of the angle
of the knife edge relative to the plane of polarization of a linearly polarized
laser. Experiments were conducted to determine the correlation length of a
small turbulent air jet and the mean rotational velocity in the shear region
of the turbulent jet with one circular and one plane polarized laser. On the
one hand, the laser beam is bent due to index of refraction changes; on .the
other hand, the index of refraction is a function of dielectric constant of
the fluid. The magnetic permeability is constant for most non-ferro materials.
Hence, the light is bent differently in the same fluid depending on the rela-
tive vector angle between the E-field in the laser light beam and the gradient
of the index of refraction of a fluid. From the measurements, we concluded
that the circular polarized laser (inexpensive one) is more suitable than the
higher quality linearly polarized laser. We also found that the correlation
signal can be 20 percent to 30 percent lower if the plane of polarization is
perpendicular to the knife edge. It is also found that from the cross beam
-2-
laser Schlieren that it is possible to obtain information on mean rotational
velocities. This is due to the fact that in correlating the signals of two
laser beams with a fixed separating distance, one is trying to identify the
same fluctuating component downstream of the flow. Since the Schlieren
system only measures the density gradient (a vector quantity) perpendicular
to the knife edge, and since the same fluctuating gradient will rotate to a
certain angle flowing downstream, then rotation of the knife edge to find the
maximum cross-correlation signal will yield the mean rotational speed as well
as the correlation distance in a turbulent flow.
(2) Line-reversal temperature fluctuation measurement.
When instantaneous temperature fluctuation data is needed, only the
radiation line-reversal or a fast response thermocouple are feasible. The
Raman Scattering Method requires a very high repetition rate and high power
laser; thus it is not likely to be available for small laboratories. A fast
response thermocouple contains velocity signals as well as temperature signals,
and therefore is not the best instrument if one has a choice. The radiation
line reversal method is an old technique based on the principle that if a
background radiation source is focused at a point in a fluid, the chemical
species will absorb and re-radiate light at certain discrete radiation lines.
When the neighborhood wave length has the same intensity as the discrete line
intensity, then-the chemical species has absorbed and re-radiated equal
amounts of photons at that time and the background radiation source has the
same temperature as the fluid. The most commonly used discrete line is the
sodium D-line, due to the fact that seeding sodium compounds in a hot fluid
can be relatively simple. Recently, the technique has been modified in several
ways. Light pipes are used at the exit slit of a monochronometer to direct the
-3-
sodium-D line and the neighboring line into two PMT's. The PMT outputs
contain DC and AC components. The DC signals are used to balance the back-
ground radiation source strength, and the fluctuating signal (AC component)
of the sodium line is used as a measurement of the fluctuating temperature.
The system can be pre-calibrated so
AV/V = 4 AT
DC To
In a simple experimental setup, large temperature fluctuations were created
by pulsating the pre-mixing chamber of a Mecker burner with a small air jet.
Responses and the fluctuating data obtained will be presented .Other lines
can also be used for different temperature regimes.
(3) Three-dimensional drag-sensing probe.
Less than two years ago, we demonstrated that a small light sphere
mounted on a stereo magnetic phono cartridge could simultaneously monitor
u' and v' in..a cold small jet (Ref. 8-9). Direct multiplication of the u'
and v' signals yielded the instantaneous momentum flux. The DC average of
u', v' yielded the Reynolds stress pu'v'. Many advancements of the two-
dimensional probe has been made, such as the measurement of Reynolds stress
profiles of a turbulent jet. Comparisons of velocity fluctuation profiles
with hot-wire measured profiles were also made. The prototype two-dimensional
probe had many disadvantages.due to the fact that a commercially purchased
cartridge was the only source of a magnetic sensor available. Disturbances
due to the body of a cartridge and the 900 supporting sting were undesirably
present. This problem was compounded by the difficulty in finding the true
magnetic axis.
-4-
It would be desirable to have a streamline support probe with the
supporting sting hiding behind the sphere. A new concept of how to use
differential and total magnetic flux to separate three signal channels has
been discovered. This makes a three-dimensional drag-sensing probe (Fig. 1)
design possible. Preliminary results of the three-dimensional signal will
be presented. This enables us to track the rotational velocity fluctuation
as well as momentum fluctuation, and in principle the signal can be displayed
in any coordinate system of the experimenter's choice. Work is still in
progress to adopt the.new 3-D probe to boundary layer measurements in a
high subsonic wind tunnel.
-5-
REFERENCES
1. Fisher, M. J. & Krause, F. R., "The Crossed-beam Correlation Technique",
J. Fluid Mechanics (1967), Vol. 28, part 4, pp. 705-717.
2. Krause, F. R. & Wilson, L. N., "Optical Crossed-beam Investigation of
Local Sound Generation in Jets", NASA SP-189, 1968.
3. Fisher, M. J. & Damkevala, R. J., "Fundamental Considerations of the
Crossed Beam Correlation Technique", NASA CR-61252, Jan. 1969.
4. Funk, B. H., and Cikanek, H. A., Jr., "Optical Probing of Supersonic
Aerodynamic Turbulence with Statistical Correlation. Phase I: Feasi-
bility. NASA TM X-53850, June 9, 1969.
5. Funk, B. H., "A Direct Measurement of the Most Probable Preferred
Angular Velocity of Turbulent Structures by Optical Correlation of
Laser Schlieren Signals", NASA TN D-6029, Oct., 1970.
6. Parthasarathy, S. P., Massier, P. F. & Cuffel, R. F., "Comparison of
Results Obtained with Various Sensors Used to Measure Fluctuating
Quantities in:Jets", AIAA Paper No. 73-1043, Oct. 1973.
7. Massier, P. F., Parthasarathy, S. P., & Cuffel, R. F., "Experimental
Evaluation of Fluctuating Density and Radiated Noise from a High-Temper-
ature Jet", AGARD Paper No. 131', 1973.
8. Cheng, D. Y., & Wang, P., "Viscous Force Sensing Fluctuating Probe
Technique", AIAA Progress in Astronautics & Aeronautics Series, Vol. 2,
MIT Press, 1974. (Presented as Paper 73-1044 at AIAA Aero-Acoustics
Conference, Seattle, Wash., Oct. 15-17, 1973.)
9. Cheng, D. Y., & Wang, P., "A Simple Method of Measuring Two Orthogonal
Components of Fluid Velocity", accepted for publication by Journal of
Fluid Mechanics.
-6-



Recent advancement of turbulent flow measurement techniques

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