Crossed beam correlation method measures turbulent fluctuations in transonic and supersonic flows. Two collimated beams of radiation are crossed at the point of interest in the flow, and the power loss of each beam is measured with two independent photodetectors, which yield information about the turbulent properties

Fisher, M. J.

English

NASA Technical Reports Server

February 1967
	
Brief 67-10030 
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. 
Local Measurements in Turbulent Flows Through Cross Correlation 
of Optical Signals 
Various methods for measuring turbulent fluctua-
tions in transonic and supersonic flows (at tempera-
tures ranging from 1000 to 30000 K) have been studied 
in an effort to find suitable instruments which com-
bine the necessary linear and time invariant frequency 
response with a sufficiently high temporal and spatial 
resolution. This study shows that a method of meas-
urement based on the cross correlation of optical sig-
nals will provide the required characteristics. In this 
method, two collimated beams of radiation are 
crossed at the point of interest in the flow, and the 
power loss of each beam through the flow is measured 
with two independent photodetectors. Although each 
detector alone yields only an integrated effect along 
the entire beam path, the technique of forming a cross 
correlation between the two detector signals removes 
the unwanted portions of the signals and yields infor-
mation about the turbulent properties. An additional 
possibility offered by the technique is that of meas-
uring the correlation between either different thermo-
dynamic properties or different species concentrations. 
This measurement, in principle, can be achieved by 
arranging one beam to be sensitive to one property, 
while the second detects fluctuations of the other 
property. 
The crossed beam correlation method estimates 
local power spectra, turbulence scales, convection
velocities, and eddy lifetimes with the same data re-
duction procedures that have been developed for con-
ventional two-point measurements using standard 
probes. In addition, an area integral of the space-
time correlation can be obtained so that "one shot" 
estimates of forcing functions and true three-dimen-
sional wave number components become possible, 
avoiding the prohibitively expensive translation of 
point probes across the source area of interest. Be-
cause the optical wavelength can be chosen from any 
portion of the spectrum, this method appears to offer 
a versatility and/or selectivity not available with any 
standard solid probe. 
Note: 
Inquiries concerning this invention may be directed 
to:
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B67-10030 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546.
Source: M. J. Fisher 
(M-FS-1268) 
Category 01 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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Local measurements in turbulent flows through cross correlation of optical signals

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