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National Advisory Committee for Aeronautics, Technical Notes - Measurements of the Arithmetic Mean Velocity of a Pulsating Flow of High Velocity by the Hot Wire Method

Frequently it is necessary to determine the mean veloc~
ity in a stream of fluctuating velocity. If the direction of
flow remains constant, the mean velocity may be determined
from pitot tube measurements, provided the magnitude and char—
acter of the deviations from mean velocity are known. If,
however, both the velocity and the direction of flow vary over
a considerable range, for example. at the outlet of blade rows
or compressor impellers, conventional methods of aerodynamic
measurement fail to give accurate results.

It was for the purpose of developing a satisfactory method
of measurement in instances similar to that cited that a pro—
gram of investigation of the hot—wire technique was undertaken
at Case School of Applied Science. This work was carried out
under the auspices of the National Advisory Committee for _'
Aeronautics,‘which also provided funds. Of necessity the meas—
urement of instantaneous velocity profiles has been included
in theflprogram. Because of repeated delay in the deliv ery of
testing equipment it was not possible to extend the investin
gation to velocities of the order of sound velocities, as in—
tended. Instead the analysis of the instrument characteristics
has been developed in detail. which should simplify further
work in the high~velooity region.

The hotrwire instrument Consists~of the following compo—
nent elements shown schematically in figure 1:

(a) A Wheatstone bridge, -one branch of which is formed
by the hot wire "" _

(b) A regulating circuit for the bridge current. includ—
ing a battery or power supply and an electronic
valve

(0) An electronic direct—current amplifier Operated by
slight variations of the unbalance of the bridge
and controlling the current regulator

In addition to the above-mentioned three components
which constitute the basic circuit, an additional element
also shown in figure 1 may be provided; namely, '

(d), A nonlinear amplifier stage; this' stage is required’
only if instrument characteristics. other than '
those resulting from the heat transfer charac—f_
teristics of the hot wire are desired and-makes
it poSsible, in combination.w1th the basic cir—
cuit, to obtain readings proportional to the
wind velocity-past the hot wire.