[citation][nom]cjl[/nom]Except that dimples do not always reduce profile drag.The reason that the dimples work on a golf ball is that turbulent boundary layers are more resistant to flow separation. This means that the flow stays attached to the surface of the ball farther around the back of it, causing it to have a smaller region of stagnant air behind it. Less stagnant air = less drag.With an airfoil or streamlined object, such as a fan blade, a turbulent boundary layer is not beneficial. Although it is still resistant to flow separation, an airfoil or streamlined shape is not prone to flow separation regardless, even with laminar flow. Because flow separation is not a problem, skin friction drag is the majority of the profile drag (rather than pressure drag, as with a sphere). With a turbulent boundary layer, which is what the dimples are meant to induce, the skin friction drag actually increases. There's a reason that airplanes aren't dimpled. In fact, on the Boeing 787 (now in testing), a tremendous amount of effort went towards decreasing the amount of turbulence on the front part of the wings (especially on the engines), maintaining laminar flow as long as possible to reduce drag. (Oh, and induced drag is effectively unchanged)[/citation]
Everything you said makes sense, but if we assume that the dimples on the fan do have beneficial effects (as the mythbusters seem to have proven), then how do the benefits come about? I was wrong to compare a sphere to a streamlined shape, so I agree with your statement that skin friction drag (profile drag) increases. More drag is generally worse, so how then do the dimples give this fan an advantage? It's starting to look like a marketing gimmick to me... can you explain how dimples make it better?
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