Flow Quality
ETW has a flow quality at least as good as the quality obtained in the best conventional transonic wind tunnels and, in some ways better, being consistent with high Reynolds number testing.
The flow quality requirements are applicable in the model test volume, i.e. over a test section domain centred on the sting mounted model centre of rotation, with a length of 2.4m and a cross section extending to within 2 boundary layer thicknesses from the walls.
The Mach number spatial uniformity shows a maximum deviation of ±0.001 in subsonic conditions and ±0.008 in supersonic conditions within the test volume. These deviations are applicable for an empty test section and are to be seen as 3σ values i.e. virtually peak to peak values.
Thermal distortions after a temperature change are within tolerance 2.5 minutes after a 40K change at 3K/sec, or 5 minutes after inserting a model cart in the tunnel, or 30 minutes after a full range tunnel cooldown at 80K/hour.
The control system is designed to maintain the time variations of the Mach number during a polar below ±0.001 over the full M range. It has a set point change capability (resolution) of 0.001 in Mach number. It is also designed to prevent the relative time variations of the total pressure in the tunnel from exceeding 0.2%, i.e. ΔPtotal/Ptotal ≤ 0.002, during a polar.
The temperature set point during a polar is kept to within ±0.5K. This corresponds to the specified temperature uniformity within the test volume.
The above capabilities in pressure, temperature and Mach number stability result in a Reynolds number stability of better than ΔRe ≤ 0.005 x 106.
As far as the flow direction in the test section is concerned, ETW achieves a deviation of less than 0.1 degree, repeatable and measurable to within ±0.01 degree.
High Reynolds number simulation requires the turbulence intensity in the test section not exceeding 0.05%. This ambitious goal is achieved by using 4 screens and a honeycomb of large depth/cell diameter ratio in the stilling chamber, and a nozzle contraction with a 12 to 1 ratio.