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The 1-4-1 system of jack movements for the flexible liners of supersonic wind tunnels

A method of improving the flow angle and Mach number distribution of a flexible-plate supersonic wind tunnel by small movements of the top and bottom liners has been known for a number of years. Once the influence on the flow, at a given Mach number, of small displacements of jacks supporting the liners are known, jack movements can be used to reduce non-uniformities in the working section flow. Such a procedure is being studied for adjusting the flow in the 8 ft x 8 ft Supersonic Wind Tunnel at RAE Bedford. However, one of the main difficulties encountered has been that of accurately determining the influence of each jack. The extended rippling in the plate which results from a displacement of a single jack generates a Mach number disturbance which is difficult to trace. This is particularly true at lower Mach numbers (i.e. near M=l.4) where the reflection of the disturbance from the opposite wall is observed to combine with the ‘tail’ of the direct effect, thereby causing interference effects in the data. Even with a 9-tube pitot rake spanning the tunnel it is difficult to ascertain which part of the disturbance is coming from the top of the tunnel and which part from the bottom. Ways have therefore been considered of moving the plate in the region of a particular jack in such a manner that the resulting waveform is shorter and more readily traced. A simple but effective system of jack movement has been derived and is described here as the 1-4-1 system.