Measurements in supersonic wind tunnels have become almost routine in the last decade and yet little is known about the free-stream disturbances, which are present, and about the means of minimizing them. The present note outlines the problems of minimizing the mean as well as the unsteady variations of velocity and temperature and gives guidance where the trends are clear. It is found that in many wind tunnels the free-stream fluctuations are likely to be dominated by aerodynamic sound in the sense of Lighthill and by shivering Mach waves. Only conjectures can be offered on the means of minimizing the resulting sound intensity at the present time.

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