Diffuser Efficiency of Free-Jet Supersonic Wind Tunnels at Variable Test Chamber Pressure

Abstract

The flow process occurring in the diffuser of a supersonic wind tunnel with free jet and its efficiency have been previously investigated by the author (see Air Force Technical Report No. 6334, December, 1950) for the special case, where the pressure in the test chamber surrounding the jet is equal to the pressure in the exit cross section of the Laval nozzle. The present analysis deals with the general case, where the test chamber pressure is different from the nozzle exit pressure. By using the basic equations of continuity, energy, and momentum flow through a control surface, an exact solution for one-dimensional, nonviscous, steady flow is obtained. One of the basic results is that the flow—after the transformation process in the diffuser is completed—is subsonic if the test chamber pressure is above, equal to, or below the nozzle exit pressure. A supersonic flow is possible, too, if the test chamber pressure is smaller than a certain finite limit below the equilibbrium. The results are presented in eight graphs over a Mach Number range from 1 to 10, showing velocity and pressure after the transformation, diffuser efficiency, total pressure ratio, and diffuser throat area at variable chamber pressure. The analysis also shows, for the first time, the exact mechanism of test chamber pressure control for the subsonic solution by means of an adjustable diffuser throat and the quantitative description of a slow starting process of the tunnel.