Sub menu

Data Quality and Repeatability

The increasing demand for high quality data and comparisons of detailed differences between configurations means that data quality and repeatability must be maintained for all forms of testing. To achieve this objective, it is essential to have a very thorough understanding of the quality of flow in a tunnel and to be able to monitor and attain very high standards of repeatability of data measurement.

As part of our ISO 9001-2000 QA Certification, ARA ensures that the highest of standards of data quality for our wind tunnels are maintained.  This is achieved through the use of a calibration procedure which provides a continuous monitoring of data quality by use of:

  • A reference databank of flow angularity measurements, formed  from all models tested, which enables values computed for new, similar models to be verified.
  • A reference model to provide a means of demonstrating repeatability, not just of data measurement, but also of the complete wind tunnel testing process.
  • A centreline calibration probe to measure longitudinal Mach number distributions in the wind tunnel.
  • A rotating flow survey rig fitted with alternative probes to measure flow angularity and unsteadiness.

Data Quality

From the measurements made in the ARA transonic wind tunnel using the centre-line calibration probe and the rotating flow survey rig, the set of statements given below can be made regarding flow uniformity throughout the working section.

ARA TWT Flow Uniformity
 Mach Number
Variation
 
 <0.0005  Subsonic, fan only
 <0.0010  Subsonic, auxiliary suction ( typically
M > 0.85 )
 <0.0050  Low supersonic
 <0.010  Moderate supersonic
 Flow Angle
Variation
 
 <0.01 °  Repeatability in a test series
   0.02 °  Repeatability between test series
   0.10 °  absolute
 Turbulence 
 ~0.10%  longitudinally
 ~0.20%  laterally
 Noise 
 <0.50%  rms Cp

Repeatability

The ARA reference model is a civil aircraft research model designed and manufactured to provide a means of monitoring the complete model preparation / testing / data acquisition / data reduction process.  It is tested at least once every 2 years. Repeatability is monitored and assessed for three alternative categories of testing; within the same wind on run; within the same test series; between test series.  The former two of these can be regarded as short-term repeatability, the latter as long-term repeatability.  Over a series of eight test entries, each of which started with complete reassembly of the model, it has been demonstrated that the following levels of repeatability can be achieved:

Data repeatability for ARA reference model
 Within a test seriesSeparate tests series
Mach Number ∆M = ±0.001 ∆M = ±0.001
Drag ∆CD = ±0.00005 ∆CD = ±0.00010
Lift ∆CL = ±0.0005 ∆CL = ±0.001
Pitching Moment ∆Cm = ±0.0002 ∆Cm = ±0.0004