Design parameters and performance of two-dimensional, asymmetric, ‘sliding block’, variable Mach number, supersonic nozzles

Abstract

The ‘sliding block’ asymmetric nozzle, which is mechanically simple and robust, is ideal for a small, variable Mach number supersonic wind tunnel installation, due the fact that it is affordable and relatively easy to manufacture. Until now there has been no rational procedure for selecting the correct nozzle profiles to satisfy certain predetermined nozzle performance criteria, ie, a range of operating Mach numbers and an acceptable test section flow non-uniformity over the given range. The present paper considers the minimum requirements which are sufficient and necessary to design an asymmetric nozzle and shows that these can be met by nozzles defined in terms of a shape parameter, an angular parameter and a length parameter. The lower end of the range of operating Mach numbers is determined by the choice of the overall flow turning angle. For a family of nozzles defined by a particular shape parameter, an heuristic analysis indicates how the test section flow non-uniformity must vary as a function of a variable which is a particular grouping of the angular and length parameters, and the range of operating Mach numbers. The dependence of the nozzle performance on the parameters is investigated numerically, assuming inviscid flow and utilising the method of characteristics. The result shows that the best nozzle shape is where the flow is expanded very rapidly immediately downstream of the throat. For a particular family of nozzles, the numerical analysis shows the complex relationship between nozzle performance and the angular and length parameters, which is best described in a graphical representation. As a result, the task of selecting correct profiles for sliding block nozzles is reduced to simply selecting the appropriate values of nozzle parameters from the curves supplied.

Aerodynamic Analyses of Variable Geometry Asymmetric Supersonic Nozzles

“…The design parameters for an asymmetric nozzle are the total flow turn angle from inlet to exit, lower wall length, inlet and exit areas, and inlet and exit Mach numbers. Also, it was unknown what design parameters resulted in an asymmetric nozzle with acceptable flow quality at a required Mach number [9], [12].…”

“…The effect of changing nozzle parameters on the test section flow quality was investigated, and this made it possible for a designer to select nozzle parameters in advance that will produce acceptable flow quality for a given Mach range. These parameters being the inlet and exit Mach numbers, the total flow turning angle, the lower wall length and the shape parameter [9], [12], [6].…”

“…In supersonic nozzle design, the shape of both walls is determined by the choice of the design Mach number. In the case of an asymmetric supersonic nozzle, the shape of the lower profile is initially designed [12].…”

“…That is, two separate nozzles with two different Mach ranges. Both nozzles are variable geometry due to their ability to vary the exit Mach number by displacing the upper wall relative to the lower wall [9], [12], [6] .…”

“…In contrast, a symmetric nozzle required to produce a flow of Mach 4 would require the length to be approximately 5 times the width. Therefore, a designer has to consider available space as well as mechanical robustness and simplicity when deciding which type of nozzle to use [9], [12], [6].…”