Design, construction and calibration of a transonic wind tunnel
Date
2013-01-31
Authors
Nash, Jonathan
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Abstract
A transonic wind tunnel was designed, constructed and calibrated in order to provide a valuable
tool for the study of transonic flow phenomena. The wind tunnel makes use of flow properties
surrounding the propagation of a shock wave along a tube in order to create the transonic flow.
As a result, the wind tunnel is a modified shock tube, with its layout being optimised for
maximum flow time. The flow times are dependent on the Mach number of the transonic flow
being created, with the longest realistic flow time being approximately sixty milliseconds. The
majority of the shock tube was built from commercially available steel construction tubing which
was then attached to a pressure vessel of similar cross sectional dimension. A test section
containing windows was constructed and placed in a position along the length of the tube to
maximise the available test flow time. The position optimisation was calculated based on standard
shock wave theory. The incident shock wave, as well as any resulting flow features, were
visualised using schlieren photography. The test piece was designed to be set at angles of attack
of up to ten degrees, both positive and negative. The main purpose of the testing carried out was
to validate the functioning of the wind tunnel rather than obtaining more data on the test piece.
An RAE2822 aerofoil was used as the test piece due to the large amount of aerodynamic data
available on it, especially in the transonic flow region, thus making it an excellent tool for
validation. In addition, the Fluent computational fluid dynamics package made use of the same
aerofoil to validate their numerical results when the package was under development. This meant
that for any numerical result obtained for the RAE2822 aerofoil using the Fluent package, there
was a high degree of confidence. This fact provided a great tool for comparing results obtained
experimentally in the wind tunnel with results obtained numerically. The short duration testing
time was found to be adequate for establishing semi-steady state flow at any transonic flow Mach
number. The bursting of the weak diaphragm at the end of the driven section of the shock tube
resulted in the upstream propagation of a disturbance with a much lower velocity than would be
seen if the incident shock wave reflected off a solid boundary and thus its arrival at the test
section was delayed, resulting in a significant increase in testing time.
The results obtained experimentally compared well to results obtained numerically. Transonic
shock waves that were set up on the test piece had very similar shapes, features and chord-wise
positions in both experimental and numerical results, showing that the geometric layout of the test
section was correct. Furthermore, it was shown that a short duration flow time wind tunnel could
be constructed using a shock tube and that accurate results could be obtained through its use.