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Originally Posted by AeroMcAeroFace
Although that is assuming that the length and stiffness of the arm and resonant frequencies of the attaching and detaching flow don't create mirror wobble or transmitted buffeting through the arm. But I don't know if that is a realistic problem because the frequencies without design may be so far off that it is a non-issue.
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It came up with BMW. They published information from research they performed during the development of one of their passenger cars.
Only in their aero-acoustic wind tunnel did an un-wanted interaction of the side mirrors with the main body reveal itself.
From the cabin's vantage point, the mirrors produced a really undesirable flow disruption down both sides of the beltline.
Master clay modelers were able to shave and enhance the mirrors, succeeding in tuning out the unwanted turbulence.
In the United States, automakers are held to certain legal noise pollution constraints, in order to market their products. One cannot sell a vehicle if it exhibits 'whistling', 'buffeting', 'booming', as well as splash and spray hazards.