Comparison between constant-current and constant-temperature hot-wire anemometers in high-speed flows

A comparison is made between the performance of constant-current (C.C.A.) and constant-temperature (C.T.A.) anemometers in measuring various characteristics of turbulence in supersonic flows. The two systems are comparable, with regard to the bandwidth and to the signal-to-noise ratio, provided that the overheat is not too low in the case of the C.T.A. system, and that suitable corrections be applied to the C.C.A. system. Besides, the two types of apparatus are similar in that their use is very time consuming, a separate adjustment being required for each point of measurement in the flow and each overheat, especially adjustment of the C.T.A. bandwidth, using a sine wave technique, and adjustment of the thermal lag compensating circuit of the C.C.A. system. They differ in that rigorous corrections, dealing for example with electrical noise and insufficiency of the bandpass, are more easily determined in situ and applied with the last system, while the former is better adapted to rapid measurements in blow-down facilities, or when only high overheats are required. As for the level of the electronic noise, it seems to be equivalent in the two systems. Besides, when turbulent intensities and spectral densities are measured in a continuously running tunnel, the results are more reliable if they are obtained with the C.C.A. system, except for measurements of transverse fluxes of heat or momentum performed using an inclined hot wire. Hence the two types of apparatus appear to be complementary.