Naked Science Forum

General Science => Question of the Week => Topic started by: EvaH on 10/06/2020 14:38:34

Title: QotW - 20.06.10 - Why don't we make dimpled cars like dimpled golf balls?
Post by: EvaH on 10/06/2020 14:38:34
Darren wants to know:

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?

We're going to be answering this question on next week's show, but what do you think?
Title: Re: QotW - 20.06.10 - Why don't we make dimpled cars like dimpled golf balls?
Post by: Janus on 10/06/2020 16:36:58
My best guess is that a golf ball has to maintain a spherical shape, so the only effective manner of reducing its drag coefficient is to add dimples.
A car or plane doesn't have this shape restriction and the drag coefficient can be reduced by adjusting the shape of the car or plane.
The drag coefficient for a sphere is 0.47, while that for a streamlined body like a plane(like a plane) is 0.04, and for a half-streamline shape (like a car) is 0.09

Adding dimples to a golf ball can reduce its drag coefficient by about 1/2,  but this is still much more than what you can achieve through streamlining.   

I not even that sure that adding dimples to an already streamlined body would end up in any further reduction of the drag coefficient, as the extra turbulence might actually increase drag.  At the very least, any decrease would be too small to bother about.
Title: Re: QotW - 20.06.10 - Why don't we make dimpled cars like dimpled golf balls?
Post by: evan_au on 10/06/2020 22:22:24
The reason golf balls can fly so far is because of two effects, both of which are enhanced by the dimples:
- The golf ball spins in the air, and this gives it lift (longer flight time), via the Magnus effect. The dimples help it "grab" the air better.
- The dimpled surface makes the turbulent air hug the surface of the ball for longer, resulting in less drag

Airplanes have wings for lift, with much less drag than a round ball.
- Cars don't need lift - they need to stay on the road; in fact, racing cars which lift off the road and spin in the air are a real hazard to driver and bystanders...

Airplanes with a smooth surface have laminar flow over the entire surface of the plane, and try to minimize turbulent flow; low drag is achieved by the streamlined shape.
- Cars often intentionally disrupt smooth airflow with spoilers, in an attempt to keep the car on the road at high speeds...