Test Section and Pressure Shell

Test Section

The test section is where the model is installed in the wind tunnel and where air speeds from Mach 0.2 to Mach 1.4 can be achieved. A standard model is supported by a single “sting” at the rear and the sting is mounted on a model “cart” which is rolled into the test section on railway tracks. Mobile carts allow the model to be prepared outside the tunnel while the previous test is underway – significantly improving the productivity of the facility.

Pressure Shell

The pressure shell or “Plenum” surrounds the test section. The Plenum is needed because the walls of the test section are porous. For certain tunnel operations, air is sucked out of the test section through the porous walls and into the Plenum before it is pumped out of the Plenum and injected back into the tunnel via a slot in the walls of the High Speed Diffuser.

Porous Walls and Transonic Speeds

With the onset of the jet age, the wind tunnel designers of the 1950’s faced a significant challenge. At transonic speeds, regions of the flow around an aircraft model become supersonic and are terminated by shockwaves. These shockwaves reflect off any solid surfaces – and in the contemporary solid walled tunnels of the 1930’s and 1940’s, these reflections rendered experimental results useless for most practical purposes. The solution found was to use slotted or porous walls to prevent shockwaves from reflecting back onto the aircraft model.

Test Observation

The absence of any large areas of glass for viewing means that the porous walls must contain portholes through which the model can be viewed remotely during the test with video and stills cameras.

Test Section and Model Dimensions

The test section is 2.74 metres (9 feet) wide by 2.44 metres (8 feet) high and can accommodate full models up to 1.83 metres (6 feet) long and half models up to 3.5 metres (11.5 feet) long. Typical model scales are 1/8 for a fighter jet, 1/20 for a medium sized civil aircraft and 1/60 for the very largest civil aircraft flying today.

Technical Information

The test section has perforated walls on all four sides with a variable porosity increasing from approximately 5% at the start of the test section to a maximum of 22% near the centre of the model – permitting tests to be conducted throughout the transonic regime.

The saw-tooth leading edges at the start of the walls is designed to soften the Mach wave generated at the start of the porous region which is caused by a thickening of the tunnel boundary layer.

The position of the test section side walls are adjusted for each Mach number to give the best possible flow quality in the test section; the wall positions have been defined by careful calibration and flow measurement.