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National Advisory Committee for Aeronautics, Technical Notes - Methods of Measurement of High Air Velocities by the Hot Wire Method

Investigations of strengths of hot wires at high ve— V _~
locities were conducted with platinum, nickel, and tung~ v ~Vm2
sten at approximately 200° C hot—wire temperature. _The
results appear to diSQualify platinum for velocities ap3h
proaching the sonic range, whereas nickel withstands
sound velocity and tungsten may be used at supersonic vs?
locities. This conclusion applies to standard atmospheric
conditions of the air. ~ w

For measurements at high velocities, hot wires must
be supported by rigid prongs to avoid breakage caused by
vibration of the prongs. Tungsten wire may be soldered_mr _.u . _m
to the prongs with soft solder after a very thin film’of '
platinum has been electrolytically applied. _
Measurements of heating current of the hot wire for
constant-temperature operation show agreement with King's U‘_,,1
relation up to a velocity of 300 feet per second. Devia— C‘s
tions from this relation were observed as sonic veloci— A ——f
ties were approached, presumably because of the effect 6? "__
impact temperatures. No numerical evaluation of this A‘
aspect was undertaken,however. ' ““”

Calibration curves-of measurements with a circuit
adjusted for linearity of-reading with velocity and of
directional characteristics of hot wires adjusted for
linear reading are given. -

The results"which have been obtained to date of an
investigation undertaken at Case School of Applied
Science. under the'auspices of-the National Advisory
Committee for Aeronautics, for the purpose of investigat—
ing the possibilities and limitations7of hot—wire measure— _"
ments at high velocities are given in the present report.
A preliminary report on this subject, which was submitted
on July 25 1941, contained a general survey of the field.
The investigation has been continued and certain tech—
niques discussed hereinafter have been developed which it __
is hoped may render measurements hy the hot—wire method ___
at high velocities practicable.

Topics under consideration were listed in the prelim— .
inary report as follows: (a) measurement by the hot—wire
method of high-velocities up to and above the acoustic
velocity; (b) effect of compressibility on'hot-wire meas-
urements; (c) effect of ambient temperature as a factor
entering into the-measurements; (d) hot—wire measurements
in a flow in which fluctuations of velocity of'high free
quency and large amplitude occur; and (e) hot—wire measure—
ments- in flow in which changes of _di-rection. of high fre-
quency occur.