Interferometric Studies of Faster than Sound Phenomena. Part I. The Gas Flow around Various Objects in a Free, Homogeneous, Supersonic Air Stream

Abstract

A small Mach-Zehnder interferometer is used for analysis of the supersonic air flow in a free homogeneous jet of cylindrical symmetry ("open wind tunnel") at Mach number 1.70 and around various simple objects suspended in it. The interferograms obtained can be evaluated because of the axial symmetry of the phenomena, and this process is carried out by integration of the Abel equation. This "open wind tunnel" does not need any windows; however, the working section is rather restricted, especially by the intersection of the head wave of a suspended body under investigation with the free boundary of the jet. Also the turbulent free jet boundary itself is a source of disturbance. The density, pressure, temperature, velocity, and Mach number throughout the air flow have been determined with reasonable accuracy. For suspended cones of 10° and 30° semi-angle the theory of Taylor-Maccoll has been verified—except near the cone tip where a systematic density increase occurred. For a 45° semi-angle cone and a small sphere where a detached curved shock wave appears, no theoretical results exist at present. However, stream and Mach lines were constructed with the help of the measured density distribution and of the conventional equations for compressible fluids, and pressure coefficients on the surface of the objects were calculated. The results show that the rotational effects are small at the present Mach number. The experience obtained with the small interferometer indicates that it is a very valuable instrument in studying supersonic phenomena, and that it will yield accurate quantitative results not obtainable by other methods. Undoubtedly, it can be applied to a modern supersonic wind tunnel, and such investigations are being undertaken with a larger interferometer and associated equipment.