THE EFFECT OF NOISE

In measurements of boundary layers in low turbulence wind tunnels, it has been recognised for some time that noise alone can cause a delicate boundary layer flow to change sharply. Noise generated by the fan or motor of a wind tunnel can provoke early transition of a laminar boundaiy layer, and even the sound of someone walking past the test section may cause a change. Elaborate tests have been made with artificially generated sounds of varying pitch and volume, which show that separation and stalling can to some degree be controlled by this means. Sound is, basically, a series of small compression waves in the air and this may be enough to change the microscopic turbulence. Alternatively, the air noise may cause sympathetic vibrations in the solid wing skin, which could cause the boundary layer flow to change.

In practical model flying it is quite likely that the noise and vibrations caused by the engine and propeller cause the boundary layer to make transition to turbulence sooner than would occur on a sailplane, for instance. (It is highly unlikely that shouting, screaming or even singing at a model sailplane will have any effect!)

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