An Interferometric Study of Supersonic Channel Flow

Abstract

The Zehnder‐Mach interferometer has been applied to the study of supersonic flow of air expanding from a pressure tank into the atmosphere through channels of rectangular cross section. This was made possible by using plane‐parallel glass plates for a pair of opposite walls of the channel. Two such channels were used, each about one centimeter in width and two centimeters in depth, one being a divergent channel (half‐angle=4.0°) and the other a Laval nozzle constructed by the ``characteristics'' method for M=1.7. It was found possible to measure the density distribution with a rather small estimated probable error (two percent or less). Knowledge of the stagnation temperature and pressure made it possible also to compute pressure, temperature, and velocity. Results obtained for the potential flow agree, for the most part, within one percent with the well‐known theory of isentropic channel flow. The interferometric technique has been used also as a guide in improving the Laval nozzle, giving quantitative data on the inhomogeneities resulting from small errors in construction. Further quantitative data was obtained on the degree of non‐uniformity of the flow in the throat regions of both channels. The boundary layers, which reach a thickness of about ten percent of the channel width near the orifice of the Laval Nozzle, introduce further accuracy problems into the study of channel flow by interferometry. These are taken up in some detail. Some of the considerations here will also apply to the interferometric analysis of flow in larger supersonic wind tunnels whose working sections are essentially channels of rectangular cross section.