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National Advisory Committee for Aeronautics, Technical Notes - Auxiliary Equipment and Techniques for Adapting the Constant Temperature Hot Wire Anemometer to Specific Problems in Air Flow Measurements

The constant-temperature hot-wire anemometer amplifier and acces-
sories have been developed to provide an instrument with wide frequency
response, good stability, and ease of operation: Auxiliary equipment
has been developed to provide heating currents for large wires, to make
average-square computations, and to make double—correlation coefficient
measurements.

Techniques are described for using this equipment to study periodic
phenomena such as surge, rotating stall, and wake surveys in centrifugal-
and axial-flow compressors. The application of the equipment to the
study of nonperiodic phenomena such as intensity, scale, and spectra of
isotropic turbulence is also discussed.

Heat—loss data for standardized tungsten wire probes show that no
wire calibration is necessary if accuracies of i5 percent are sufficient.
The problems associated with the evaluation of compressor and tur—
bine performance, as well as the flow fluctuations in combustion phenom-
ena, are of such a nature that a knowledge of the instantaneous flow
patterns is of considerable importance. These measurements in many cases
are beyond the range of conventional measuring instruments because of the
limitations of frequency response. Measurements of this type (compressor
surge and rotating stall, blade wake velocity profiles, vortex shedding
frequencies, and associated phenomena) are most readily made by means of
hot-wire anemometers.

The advantages and disadvantages of operating hotswire anemometers
at constant current and at constant resistance (temperature) have been
discussed.by several writers (references 1 to 5).

The principal advantages of the constant-temperature system are as
follows: (1) It provides a continuously varying feedback voltage which
operates the wire with continuous compensation, (2) it can be used for
large mass-flow fluctuations - over 100 percent of the mean flow, (3) in
instances.where there is a sudden decrease in flow there is no danger of
wire burnout. The prdblems of air-flow fluctuations associated with
compressors, turbines, combustion phenomena, and so forth usually involve
flow changes which are large with respect to mean flow. Experience shows
that the fluctuations found in jet-engine research are usually larger
than 1 to 2 percent and hence the main disadvantage of the constant-
temperature hot4wire anemometer, its relatively large input noise level,
is unimortant in measurements of this kind.