Three-D instrument using a laser light source measures both turbulence and mean velocity of subsonic and supersonic gas flows. This instrument is based on the measurement of the Doppler frequency shift of light waves scattered by moving particles in the gas stream

Booth, S.

Meister, K.

Rolfe, E.

Silk, J. K.

Young, R. M.

English

NASA Technical Reports Server

I	 I 
September 1968
	 Brief 68-10349 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151., 
Laser Doppler Gas-Velocity Instrument 
Wind Tunnel Facility 
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L8 
L6 '/	 I Optical 
Attenuator	 BS 
Li	 NO 
BS BS
L4
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	 O4	 L5 Bea
P war 
Prism P4q 
Electronics 2	 BA-2 
CF CF	 Power CF 
Supply 
Broadband Broadband Broadband 
Arnpl. Arnpl.	 ' Arnpl.
I Velocity J	 Frey. I	 Frey,	 _j Velocity	 I Velocity j Freq. 
Readout 1 Discrm	 Oiscrm.	 1 Readout	 Readoutil Discrrr, 
A 3-D instrument using a laser light source has been 
designed to measure both turbulence and mean veloc-
ity of subsonic and supersonic gas flows. This instru-
ment is based on the measurement of the Doppler 
frequency shift of light waves scattered by moving 
particles in the gas stream. Using an experimental 
laboratory model of the system, gas velocities of up 
to Mach 2 were measured in the NASA seven-inch 
wind tunnel facilities. With smoke injected into the 
gas flow to provide scattering particles, signal-to-noise 
ratios of 20 to 30 dB were obtained using a 1-watt 
argon laser.
The schematic diagram outlines the essential fea-
tures of the instrument. The light from the laser passes 
through lens LI and a window in the wind tunnel or 
test facility to focus at a selected point in the flow 
stream. The uriscattered beam is collected after 
passing through a boresight alignment aperture (BA-1) 
and rendered parallel by lens L2. The parallel beam 
passes through an optical attenuator and neutral 
density filter (ND) for adjustment of beam intensity, 
in accordance with specified operating criteria. The 
optical local oscillator beam is obtained for the three 
photomultiplier (PM) mixers by dividing the beam 
(continued overleal) 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights. 
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with a tetrahedral beam splitting prism, finally 
reflecting in each scattered beam. Lenses L3, L4, and 
L5 focus the local oscillator signals onto the photo-
multiplier cathodes. The top corner of the tetrahedral 
prism is ground off to allow a small portion of the 
beam to pass straight through the prism, out of the 
base, and through a second boresight aperture (BA-2). 
By using the two boresight apertures, a reference 
baseline is established for alignment of the whole 
instrument. 
Each scattered beam is picked up at a fixed angle to 
the laser beam, the angles being symmetrically situated 
around the axis of the reference beam. Lenses L6, 
L7, and L8, behind adjustable apertures, collect the 
scattered light at the chosen angle. After passing 
through suitable optical collimating and focusing 
components, the scattered beams pass through the 
beam-splitter mirrors (BS), and onto the photomulti-
plier cathode where the scattered signal is mixed with 
the local oscillator (unscattered reference) beam. 
The scattered beam lens system is somewhat more 
complicated than shown schematically, to provide 
independent control of the scattering volume and 
instrument broadening at one end and of the mixing 
process at the opposite end of the optical systems. 
Refractor plates are situated at several places in 
the optical paths to permit independent parallel 
alignment with respect to transverse locations of the 
beam at the photomultiplier cathodes. Angular beam 
adjustment at the photocathodes is provided by the 
beam-splitters. 
At each photomultiplier active surface, the scattered 
light heterodynes with the unscattered reference beam 
(local oscillator beam). The heterodyne outputs of 
the photomultipliers pass thrcugh impedance match-
ing preamplifiers (CF). The signals are then amplified 
and further processed in the electronics section of the 
system. Spectrum analyzers are used as monitors, 
while the turbulence and velocity information is 
obtained by processing through special discriminator
circuits. To optimize the signal-to-noise ratio, fre-
quency tracking discriminators are used which contin-
uously track the frequency of the fluctuating turbu-
lence Doppler signals. The three orthogonal compon-
ents of the gas velocity vector are calculated from 
measurements of the Doppler shift at the three scat-
tering angles. 
Further work remains to be done on several prob-
lems: (I) the construction of a special signal frequency 
tracker, which should result in an improved signal-
to-noise ratio and wider operating range; (2) the 
establishment of a more acceptable injectant material 
for the light scattering process; (3) the acquisition 
of turbulence data in a number of different systems, 
using the 3-13 vector velocity instrument; (4) the 
investigation of methods of reducing or eliminating 
instrumental frequency broadening; and (5) the 
further development of the device for cross-correlation 
measurements. Experiments have shown that with an 
increased instrument sensitivity, scattering from 
natural air contaminants, such as dust and water 
drops may prove feasible, eliminating the use of arti-
ficial injectants. 
Notes: 
I. A similar instrument for a single scattering angle 
is described in Tech Brief 66-10693. 
2. Complete details may be obtained from: 
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B68-10349 
Patent status: 
No patent action is contemplated by NASA. 
Source: E. Rolfe, J. K. Silk, 
S. Booth, K. Meister, and R. M. Young 
of Raytheon Company 
under contract to 
Marshall Space Flight Center 
(M FS-20039) 
Brief 68-10349	 Category 02.


Laser-Doppler gas-velocity instrument

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