To measure spectrum, magnitude, and time-average value of turbulent shear stress in flow of gas use small, hollow sphere suspended in flow to measure drag fluctuations in two 90 deg-directions as function of time

Cheng, D. Y.

English

NASA Technical Reports Server

June 1974	 B74-10072 
NASA TECH BRIEF 
Ames Research Center 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Probe for Measuring Turbulent Real-Time Shear-Stress Waves 
.
The problem: 
To measure the spectrum, magnitude, and time-
average value of the turbulent shear stress in a flow 
of gas. 
The solution: 
Use a small, hollow sphere suspended in the flow 
to measure drag fluctuations in two 90°-directions as 
a function of time. 
How it's done: 
The probe design is based on the principle that the 
drag coefficient of a sphere and the cross-flow over a 
cylinder are constant in flows where Reynolds num-
bers range between 2 x 103 and 2 x 10; therefore 
the drag force in the sphere is directly proportional to 
the dynamic head (½ pv2) of the flow. As shown in 
the diagrams, the probe is constructed by mounting a 
small sphere, of a diameter suitable for the proper 
Reynolds number range for the flow condition, on
the tip of a stereo phonograph-cartridge stylus so 
that the sphere can move with complete freedom in 
the plane of the measurements. The movements give 
signals resolved in two channels which represent 90° 
separation of the movements of any orthogonal co-
ordinate system. A stereo cartridge has a frequency 
response from 8 to 33,000 Hz, and channel separa-
tion of 30 db is common. 
The signals are proportional to the rate change of 
the drag, and thus are also separated from the con-
stant time-averaging components. They can be am-
plified, integrated with time, and multiplied to give 
instantaneous correlated drag forces, pu'v' drag-fluc-
tuations, etc., independent of temperature, radiation, 
and chemical-reaction effects. 
Total weight of the sensing elements is less than 
1/10 gram; drag-force measurements up to 10 grams 
about the small (3.8-mm diameter) sphere is possible 
in a jet exhaust stream. The theoretical frequency 
response can be estimated as the time delay for the 
sphere to transmit inertial forces through a small 
aluminum support tube to the magnetic sensing el-
ement inside the cartridge. Since the length of the 
support tube is 12.7 mm and the speed of sound of 
aluminum is 4543 m/sec, the time delay is 2.5 x 10 
second. Turbulent fluctuation usually has a frequency 
below 20,000 Hz, so the risetime of the sensing 
elements is more than ample to detect the signals. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 74-10072
(continued overleaf) 
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.
https://ntrs.nasa.gov/search.jsp?R=19740000072 2020-03-17T14:02:56+00:00Z
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
NASA Patent Counsel 
Mail Code 200-1 1A 
Ames Research Center 
Moffett Field, California 94035 
Source: Dah Yu Cheng 
Ames Research Center
(ARC-10755)
. 
B74-10072	 Category 03


Probe for measuring turbulent real-time shear-stress waves

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