June 29, 2021 user

If golf balls are made more aerodynamic by having a dimpled surface, then why don’t we use this design principle for other things that need low drag such as cars, aircraft, boats, etc?

This may get a bit technical, but the very hand-wavy stuff is: the dimples are really ever only useful for spheres, and for spheres that move at very specific speeds (or of very specific sizes).

The reason why is that a sphere is a bluff body with some very weird properties when it moves at relatively high speeds. We add dimples on top of spheres to get a more regular flow around the back of the sphere, which is the critical part, but in doing so you are adding some drag. It turns out that the drag you get with dimples is less than the drag you lose by improving the flow on the back of the sphere, so it works there.

On the other hand, the fuselage of an airplane doesn’t really have this kind of problems. The back of the fuselage usually is pointed, making for an already decent enough flow on the back. What’s more, for very complex reasons I won’t explain, the dimples make the flow turbulent for the golf ball, but the air is already turbulent near the airplane, so it wouldn’t have any effect if not increasing drag the same way it did for spheres.

A friend studied this in one of our fluids classes. Spin will only affect the speed of the air around the ball, but the dimples will still have the same effect. This is because the speed of the air near the dimples will generally be very small. So the only thing that changes is where the flow separates from the surface. The greatest effect of spin will be creating pressure differences on the surface which creates “lift” on the ball. Making it stay in the air longer than if it wasn’t spinning. This is called the Magnus effect. Here’s a cool YouTube video of it in action: https://youtu.be/2OSrvzNW9FE.

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