From a page in the booklet "Test or be damned".

  Published by Vickers-Armstrongs (Aircraft) Ltd. C 1962

The supersonic tunnel, which was designed by the Aeronautical Research and Development Department, has a 20in. square working section. The

operating principle of the tunnel is to evacuate the stratosphere chamber (a Convenient low pressure source) to as low a pressure as possible. 

The tunnel, which consists of an intake, working section and diffuser, is connected to the stratosphere chamber through a quick opening valve. When the latter is sprung open, air is sucked in through the tunnel to fill the vacuum in the chamber. Supersonic is caused by a convergent-divergent nozzle made from a pair of detachable, profile, liners. The shape of the nozzle governs the Mach number produced in the working section; thus the Mach number can be altered by using liners of differing profile.

The duration of a run in this tunnel varies from 20 seconds at a Mach number of 1.4 to about 30 seconds at Mach 3.5 (2,700 m.p.h.) and a test run can take place every 15 minutes (the time required to evacuate the stratosphere chamber). The tunnel is electronically instrumented so that the three component forces and three moments acting on a model can be recorded automatically, for a complete range of aircraft incidence during the one test run.

 1 : Reservoir lightly inflated (l5 p.s.i.a).

 2 : Simple stop valve (to isolate in maintenance).

 3 : Main Venturi with a range of values to suit by changing the pairs of shaped pieces 

 5 : Detachable RH & LH sides with inspection-window (roll along track 6 on to traversers

 7, turn handle 8 to rack and pinion the traverser 7 with 5 into place, then turn

 9  to raise taper-pegs 10 to press 5 home on rubber seal 11.

12 : Secondary venturi with adjustment 13, 13 and 14, 14.

15 : Quick-acting (1 second) gate valve.

16 : Exhausted chamber. (Sudden opening of 15 without wire-drawing causes 

     high­pressure supersonic airflow for 30 seconds through tunnel.)

17 : Model held in airflow at any angle by quadrant 18 turned by motor 19.