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Analysis of the necessary air suction from the test section of a transonic wind tunnel
Borivoj Blizanac
In the closed test section of a wind tunnel (with parallel walls) effective surface area of the cross section is getting smaller in the direction of the air stream. This is the result of the continuous increase in thickness of boundary layer along the walls of the test section of a wind tunnel. As the consequence of this effect the sonic velocity will be established at the end of the test section, although the velocity of the air stream in the beginning section of the test section is still significantly smaller than M=1. In order for sonic velocity in the empty test section with the closed walls to be established, condition that walls are divergent has to be met. This condition depends on parameters such as perforation of the walls, Reynolds and Mach numbers. Similar phenomena are present in the test section with partially opened walls when there is no auxiliary suction of air in the test section chamber. For the uniform air flow in the test section of a wind tunnel with perforated walls to be achieved it is necessary to perform auxiliary suction of the air from the test section to the chamber of the test section.
Key words: wind tunnel, aerodynamic testing, transonic flow
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