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WIND TUNNELS OF NASA

Chapter 5 - The Era of High-Speed Flight

The Scotch Tape Enigma

[54] The first supersonic tunnel built at Lewis had an 8 x 6-foot supersonic test section. Doors downstream from the sound muffler permitted either open- or closed-circuit operation.

Nozzle drag tests on the General Dynamics F- 16 inside the supersonic test section of the Lewis 8 x 6-foot tunnel.

The test sections of supersonic tunnels display a mirrorlike surface for a very good reason-the least scratch or imperfection will disturb the airflow and the accuracy of the tests. The presence of unwanted flow disturbances show up emphatically on schlieren photos of the empty tunnel. Just a few cracks or scratches will generate a cries-cross, Scotch-plaid pattern of diagonal Mach lines. Each Mach line is a small shock wave that commences at the site of the imperfection and projects at an angle dependent on the speed of the air in the tunnel. At Mach 1, the Mach lines are perpendicular to the flow; at Mach 2, the angle is 30 degrees (the angle whose sine is 1/2); and in general, at Mach M, the angle is arcsine 1/ M.

To illustrate how these curious Mach lines are produced by tiny irregularities in the tunnel walls, small pieces of Scotch tape just 0.003-inch thick placed on the tunnel wall give rise to strong artificial Mach lines on schlieren photos. The pieces of tape, however, are well within the subsonic boundary layer which extends to 0.3 inch-100 times the thickness of the tape. Yet the influence of the bits of tape is felt across the boundary layer. Disturbances like the Mach lines are undesirable in wind tunnel tests. The test section of each supersonic tunnel must be finished to a level....

diagram illustrating the effect of surface imperfectaions inside of the wind tunnel

[55]Tiny imperfections in the wall of a supersonic wind tunnel will generate Mach lines that can be seen in schlieren photos. (A) At Mach 2, the lines make 30 degree angles with the tunnel axis. (B) Scotch tape 0. 003 inch thick will create Mach lines even though the tape is well within the subsonic boundary layer of air.

Schlieren photo of several Mach lines generated by strips of Scotch tape on one wall (air velocity = Mach 2).

...of accuracy and smoothness much finer than that represented by the Scotch tape. Schlieren photos of the empty tunnel quickly reveal where reworking of the nozzle surface is required.