Comparison of hot-wire measurement techniques in a Mach 3 pilot quiet tunnel

Disturbance measurements were made in the free stream of a small Mach 3 quiet tunnel using constant-current and constant-temperature anemometers (CCA and CTA). Data from the two types of instruments are compared in terms of frequency response and normalized rms levels of mass flow fluctuations. The mode-diagram analysis of the CCA data produces reliable results because the frequency response is consistent for a wide range of overheat ratios. However, the mode-diagram results for the CTA data cannot be used due to the rapidly decreasing frequency response with decreasing overheat ratio. Only the mass flow fluctuations at high overheat ratio can be obtained with the CTA system, and they can be as much as 50 percent higher than those from the CCA system. Possible reasons for these measurement differences between the two systems are considered.