Urgent: How do the dimples on a golf ball work?

[Luke Skywalker]

Well, I have a science fair project I'm to present tomorrow (unrelated to school) involving the application of a golf ball's dimples to a motorized model car to see if it increases its velocity (which it did).

Honestly, I have next to no knowledge on aerodynamics, and trying to do research has only confused me. Could someone please clarify exactly how a golf ball's dimples work? My understanding is that it magnifies the magnus effect (although this presumably wouldn't apply to an automobile) and actually increases turbulence by creating air pockets and to reduce drag...the problem is, I really don't understand what any of this means.

[Borgholio]

As per the Mythbusters, it actually DOES work with a car. Just not as well.

[MrDakka]

If I remember my aerodynamics course correctly, the whole point of the dimples is to reduce the total drag of the ball by reducing pressure drag (a component of total drag), and not to magnify the Magnus effect (which is a separate phenomenon altogether and relies on the object in question rotating while in flight).

The dimples cause the boundary layer (literally a layer of air right above the surface; in this case the surface of the ball) to change from laminar to turbulent (these terms refer to airflow and are demarcated by something called Reynold's number; basically if airflow is above a certain Reynold's number, it is turbulent, if not, it is laminar).

This boundary layer adheres to the surface because air is viscous. Now when the boundary layer no longer adheres to the surface (called flow separation), eddies and vortices form downstream causing a huge spike in drag especially pressure drag. Thus you wan to delay this flow separation as much as possible.

Now basically a laminar boundary layer is much more prone to flow separation than a turbulent one. Thus a turbulent boundary layer sticks to the surface longer, which reduces the amount eddies and vortices formed, thus reducing pressure drag and thus total drag.

Therefore with this reduction of total drag, the golf ball can now go further. Similar contraptions called vortex generators and turbulators can be seen on aircraft and gliders for this drag reduction.

Now the Magnus effect is a separate phenomenon that generates lift for the golf ball when it is undergoing backspin. This additional lift helps increase the range of a golf ball in addition to its' dimples.

Hope that helps explain the effect of the dimples of the golf ball.

If anyone sees an error in my explanation, feel free to correct me. I'm eating dinner while writing this and don't have my books on hand to consult

[Simon_Jester]

Stand back! I'm theoretically qualified to teach high school physics! Or will be soon...

Dakka's explanation looks pretty classy; I'm going to try to explain it with fewer words for the people who didn't take a few years or more of STEM subjects in college.



OK. Short form, spheres have really shitty aerodynamics. Surprisingly so. The problem is that they're big and fat, so air piles up against the front of the sphere, slides a short distance along the sides, and then spreads out in a big fat wake. The wake is full of turbulence and vortexes, and churning up that air takes a lot of energy, which acts as a drag on the moving sphere.

Good aerodynamics come from having a steady flow of air parallel to the surface of the object. Once the airflow isn't being sucked along the surface of the object, you get turbulence and drag.

One of the big ways we try to reduce drag is by making the sphere narrower, into a cigar or teardrop shape. That has obvious benefits, because the thinner object pushes aside less air, and the streamlined shape means air sticks to the back side of the object instead of creating lots of wake problems.

But you can't do that on a golf ball. What you CAN do is create controlled, small-scale turbulence along the surface- whirling, wobbling airflow. But with the right dimple depth, the slightly turbulent air will actually 'stick' to the dimpled surface better than a smooth airflow would stick to a smooth surface. Therefore, the ball creates less wake and less drag.



Note: Dimpling would not be helpful on a highly streamlined surface. It's most useful right around where you expect the airflow to 'peel off' a surface and turn into a wake.

[Magis]

The previous comments provide a pretty good explanation of the phenomena, although things like boundary layer separation (and even boundary layers themselves) are not very intuitive and can be difficult to understand on their own.

This video (youtube link) gives a decent primer on laminar flow, turbulence, boundary layers and boundary layer separation.

[Simon_Jester]

Someone actually makes golf balls that have a band of dimples around the 'equator' (where they do the most good for the airflow, and act to stabilize the ball's flight through the air). However, they are illegal for tournament play.

[MrDakka]

Sorry for sounding like a dry old fart, I was channeling my inner professor.

The replies given by the other posters are much easier to understand especially with the pics and video.

BTW Skywalker, how did the presentation turn out?

[Hawkwings]

Someone actually makes golf balls that have a band of dimples around the 'equator' (where they do the most good for the airflow, and act to stabilize the ball's flight through the air). However, they are illegal for tournament play.

How do they keep the golf ball from spinning and therefore not having the dimples stay at the equator?

[Beowulf]

Someone actually makes golf balls that have a band of dimples around the 'equator' (where they do the most good for the airflow, and act to stabilize the ball's flight through the air). However, they are illegal for tournament play.

How do they keep the golf ball from spinning and therefore not having the dimples stay at the equator?

The equator isn't horizontal when you hit it, but rather vertical. The plane of the equator is lined up with the direction you want the ball to go.

[His Divine Shadow]

Stand back! I'm theoretically qualified to teach high school physics! Or will be soon...

Dakka's explanation looks pretty classy; I'm going to try to explain it with fewer words for the people who didn't take a few years or more of STEM subjects in college.



OK. Short form, spheres have really shitty aerodynamics. Surprisingly so. The problem is that they're big and fat, so air piles up against the front of the sphere, slides a short distance along the sides, and then spreads out in a big fat wake. The wake is full of turbulence and vortexes, and churning up that air takes a lot of energy, which acts as a drag on the moving sphere.

Good aerodynamics come from having a steady flow of air parallel to the surface of the object. Once the airflow isn't being sucked along the surface of the object, you get turbulence and drag.

One of the big ways we try to reduce drag is by making the sphere narrower, into a cigar or teardrop shape. That has obvious benefits, because the thinner object pushes aside less air, and the streamlined shape means air sticks to the back side of the object instead of creating lots of wake problems.

But you can't do that on a golf ball. What you CAN do is create controlled, small-scale turbulence along the surface- whirling, wobbling airflow. But with the right dimple depth, the slightly turbulent air will actually 'stick' to the dimpled surface better than a smooth airflow would stick to a smooth surface. Therefore, the ball creates less wake and less drag.



Note: Dimpling would not be helpful on a highly streamlined surface. It's most useful right around where you expect the airflow to 'peel off' a surface and turn into a wake.

I dunno if this is the same phenomenon or not (or even slightly related), but it reminded me of those old 1000 yard shooters in the 1800's used when shooting large BP projectiles for such a large distance that the projectile would go from supersonic to subsonic mid-flight. With a modern smooth bullet that means destabilization for sure and accuracy is lost, it'll most likely land on its side. But the lead slugs they used back then had many grooves along the length (for lube) that helped reduce the collapse of the bow wave into many small ones rather than one large one, which meant they rode through the transition more or less unaffected.

[Hawkwings]

Someone actually makes golf balls that have a band of dimples around the 'equator' (where they do the most good for the airflow, and act to stabilize the ball's flight through the air). However, they are illegal for tournament play.

How do they keep the golf ball from spinning and therefore not having the dimples stay at the equator?

The equator isn't horizontal when you hit it, but rather vertical. The plane of the equator is lined up with the direction you want the ball to go.

Same question then: how does the equator stay perpendicular to the axis of motion?

I dunno if this is the same phenomenon or not (or even slightly related), but it reminded me of those old 1000 yard shooters in the 1800's used when shooting large BP projectiles for such a large distance that the projectile would go from supersonic to subsonic mid-flight. With a modern smooth bullet that means destabilization for sure and accuracy is lost, it'll most likely land on its side. But the lead slugs they used back then had many grooves along the length (for lube) that helped reduce the collapse of the bow wave into many small ones rather than one large one, which meant they rode through the transition more or less unaffected.

It's not the same phenomenon, but it is definitely interesting. Supersonic shock waves only appear near mach 1 (obviously) and it certainly is possible to lessen the shock by having a series of small ones instead of one large one. That is in fact how sonic booms are quieted. This sort of research was done for the supersonic jetliner projects, as a noise-reduction measure. Still, you get a fairly substantial shock wave at the front and back of the bullet, unless it is ridiculously sharp at the front and tapered at the back (which I doubt).

Here's some good photos of what is happening:





You can see that shock waves emanate from many places along the bullet's body, but the main shocks are at the front and the back. There is actually a shock every time the slope changes, with faster change resulting in stronger shock. For the nice tapered rifle bullet's it's fairly clean, but as you can see the last bullet has a ton of shocks coming off it.

[His Divine Shadow]

This is the most detailed explanation I have seen on the subject:
http://www.lasc.us/Fryxell_Book_Chapter ... Design.htm

A bit beyond the middle of page, the 2nd paragraph named Aerodynamics.

[Beowulf]

The equator isn't horizontal when you hit it, but rather vertical. The plane of the equator is lined up with the direction you want the ball to go.

Same question then: how does the equator stay perpendicular to the axis of motion?

Rotational inertia? You hit the ball, and it starts spinning. And the equator is parallel to the axis of motion. The axis of rotation is perpendicular to the axis of motion.

[Hawkwings]

Wait, hold on. I interpreted it as the equator being a plane on the ball such that the normal vector is the direction of motion. What I'm hearing instead is that the direction of motion is a vector that lies in the plane of the equator?

[Beowulf]

Wait, hold on. I interpreted it as the equator being a plane on the ball such that the normal vector is the direction of motion. What I'm hearing instead is that the direction of motion is a vector that lies in the plane of the equator?

The plane of the equator is lined up with the direction you want the ball to go.

Yes.