QotW: Why don't we have dimpled cars?

16 June 2020

GOLF_BALL

A golf ball on a green.

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Question

Darren says: "Gold balls are dimpled to disrupt the air around the ball. As far as I can gather, this reduces the drag and allows them to fly further than they would if they were perfectly round. Why do we not see dimpled cars, aircraft, and trains? If this effect is so effective for golf balls, why not use it on Formula 1 cars for instance?" 

Answer

Adam Murphy has been finding out...

Darren - Golf balls are dimpled to disrupt the air around the ball. As far as I can gather, this reduces their drag and allows them to fly further than they would if they were perfectly round. Why do we not see dimpled cars, aircraft, and trains? If this effect is so effective for golf balls, why not use it on Formula 1 cars, for instance?

Adam - And why not, you can drive a golf ball and a car after all. So, what makes one good for dimpling and the other not. Sam Grimshaw, from the Whittle Jet Engine lab at the University of Cambridge is here to take a swing at the answer

Sam G - To answer this question I need you to picture a stream of air flowing past an object. We call the messy flow behind the object a wake, just like you see behind a boat. The drag on the object is related to the size and shape of this wake.

Now if we zoom in close to the object’s surface, we see that friction slows the air; this region of slower flow is called a Boundary Layer.

If the overall flow is relatively slow and the object small, then the viscous nature of the air makes the boundary layer flow smooth. This flow struggles to follow a curved surface, so for a sphere, the air leaves the surface about half way around, producing a large wake and lots of drag.

Adam - Imagine dragging your hand through some water. If you go with your palm first, you’re going to make a big wake, and you’re going to feel a lot of push back. But if you use the thinner edge of your hand you’re going to chop through the water a lot easier, and make a much smaller wake. So because a ball isn’t a sleek, streamlined shape, the air can’t follow the ball all the way around and it makes a big giant wake behind it, pulling it back...if only there was some way to mess that big wake up...

Sam G - For fast flow past a large object, the boundary layer becomes churned up, or turbulent. This type of flow follows the surface of the sphere further around, giving a smaller wake and reduced drag. 

A golf ball, which is small but fast, is delicately balanced between these two behaviours. A smooth golf ball tends to have that smooth kind of boundary layer which gives high drag. However, dimples disturb the flow enough to make the boundary layer turbulent, reducing the drag and allowing you to hit the ball further. For a car or train, which are quite fast and very large compared to a golf ball, the boundary layer is turbulent anyway so dimples have no effect.

Adam - Shame, I’d like to see a dimpled F1 car, personally.

Thanks to Sam for that answer, and also to evan_au, and Janus on the forum, who came to similar conclusions. Join us next time, when we answer this question, from a different Sam...

Sam - If identical twin brothers marry identical twin sisters, and each of those couples has a child, will those two children be like twins?

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