The Hot-Wire Technique in Supersonic Research

Abstract

For the last few years considerable attention has been turned to the problem of turbulence in supersonic flows. There are several new aspects to the compressible case that are of great academic and practical interest. In particular, one should mention the problems of the onset of supersonic turbulence (transition) and of the aerodynamic noise. Although considerable theoretical research has been done on these subjects, progress is seriously hampered by the lack of experimental work.

The experimental difficulties encountered in studying this problem are obvious. One deals with a flow field in which all the fluid properties, thermodynamic and flow variables, are random functions of the time. Furthermore, the frequencies of the fluctuations are orders of magnitudes higher than those encountered in low speed flows. Finally, length scales are usually very much smaller which limits the physical size of the sensing element. These facts make the choice of a proper experimental device to study supersonic turbulence extremely difficult. Until the present time, such devices have been tried as the corona discharge, glow discharge, pressure transducers, the hot-wire and some optical techniques. All of these have some serious disadvantages. The hot wire has proved to be the only instrument with which some quantitative measurements have been made so far. The technique, as used at present, was developed by Kovasznay and Morkovin (Refs. 1 and 2). This note will essentially outline their method.