# Golf ball and Tennis ball turbulence

• I
So I have read many articles on tennis ball fuzz and golf ball dimples and I am more confused than before.

Here is one that says that the dimples make the golf balls go faster:
https://entertainment.howstuffworks.com/sports/golf/basics/question37.htm

And just when it makes all sense, I read this...

Here is one that says that the fuzz on tennis balls make them go slower
https://theuijunkie.com/tennis-balls-fuzzy/

(And, oddly, that makes sense, by itself.)

But I cannot reconcile this because both dimples and fuzz are turbulence generators and the same logic should apply to both.

As I read more, I am suspecting that lazy articles are not explicitly stating that the fuzz on a tennis balls is there NOT ONLY for drag but to assist in the Magnus effect. I understand the Magnus effect is really about turning the path, and I not asking about that, but just wondering if the authors of the articles are getting lazy.

I also suspect that turbulence generators (fuzz, dimples) initially have more drag, but then the drag increases (why?). So it is possible that these articles are discussing the effect with conditions in mind.

(By the way, I cannot seem to find a qualitative statement that explains why the pressure increases in the turbulent wake -- that would be appreciated if you could discuss this as an aside.)

I do understand that the turbulence generators reduce separation. And this means a smaller turbulent wake, and this should mean (assuming there is then less turbulent pressure) that the ball goes slower (which does not explain the golf ball).

I am hoping for a qualitative explanation that covers both the golf and tennis ball, at various speeds, with NO mention of the Magnus effect (for that I understand).

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Related Classical Physics News on Phys.org
https://www.quora.com/Why-are-tennis-balls-furry

https://www.scientificamerican.com/article/how-do-dimples-in-golf-ba/

Hmm, seems like people think the fluff's primary purpose is to increase variability in the ball's movement. Anyways, since dimples are rigid while fuzz is softer, my first impression is that the dimples trap the turbulent air to reduce drag even at high speeds, since the dimples are rigid, so as to allow high speed movement, while the fuzz can do that at lower speeds, but as speed increases, its small "bubble" of trapped air gets "stripped off" as the fuzz kinda flattens out, which then produces greater drag. As such, these two balls are designed with the same principles, but for different speeds.

A.T.
As such, these two balls are designed with the same principles, but for different speeds.
For a tennis ball it's very important how it bounces off the court and the racket at different spin rates. This is less of a concern in golf ball design.

Gold Member
First, it's important to understand the sources of drag on a ball and what turbulence actually does. Drag on something like a ball can be broken down into viscous drag (due to "friction" with the air) and form drag (due to boundary-layer separation leading to a large, low-pressure wake).

Turbulence in a boundary layer is primarily characterized by mixing. In the context of drag, this means it tends to "mix" high-momentum fluid from near the edge of the boundary layer with the low-momentum fluid near the surface, creating a boundary layer that has higher momentum closer to the surface than a laminar boundary layer would. This has two consequences: an increase in skin friction drag due to the larger velocity gradient, and an increased resistance to separation due to the increased momentum near the surface where separation begins (the local momentum flux must go to zero at the wall for a boundary layer to separate). The important takeaway here is that, on a ball, inducing turbulence increases skin friction drag directly while reducing form drag due to the delayed separation and smaller wake. These two effects are competing.

All that being said, a golf ball and a tennis ball are very different in their surface character. Golf balls are rigid with dimples to induce turbulence. This does increase skin friction drag, but it reduces form drag. For a golf ball, that form drag reduction is greater than the increase in skin friction. A tennis ball has fuzz instead, which is going to do one thing that seems very important to me that rigid dimples will not: individual threads of fuzz are going to flap in the wind, which will tend to increase drag. A turbulent boundary layer will have more momentum near the surface, which means more intense interaction with the flapping fuzz. It is plausible, then, that the increase in drag due to skin friction and that flapping outweighs the decrease in form drag for a tennis ball.

256bits, russ_watters and berkeman
First, it's important to understand the sources of drag on a ball and what turbulence actually does. Drag on something like a ball can be broken down into viscous drag (due to "friction" with the air) and form drag (due to boundary-layer separation leading to a large, low-pressure wake).

Turbulence in a boundary layer is primarily characterized by mixing. In the context of drag, this means it tends to "mix" high-momentum fluid from near the edge of the boundary layer with the low-momentum fluid near the surface, creating a boundary layer that has higher momentum closer to the surface than a laminar boundary layer would. This has two consequences: an increase in skin friction drag due to the larger velocity gradient, and an increased resistance to separation due to the increased momentum near the surface where separation begins (the local momentum flux must go to zero at the wall for a boundary layer to separate). The important takeaway here is that, on a ball, inducing turbulence increases skin friction drag directly while reducing form drag due to the delayed separation and smaller wake. These two effects are competing.

All that being said, a golf ball and a tennis ball are very different in their surface character. Golf balls are rigid with dimples to induce turbulence. This does increase skin friction drag, but it reduces form drag. For a golf ball, that form drag reduction is greater than the increase in skin friction. A tennis ball has fuzz instead, which is going to do one thing that seems very important to me that rigid dimples will not: individual threads of fuzz are going to flap in the wind, which will tend to increase drag. A turbulent boundary layer will have more momentum near the surface, which means more intense interaction with the flapping fuzz. It is plausible, then, that the increase in drag due to skin friction and that flapping outweighs the decrease in form drag for a tennis ball.

Thank you so much.

I have read often how "fuzz on tennis balls and dimples on golf balls" are there to change the speed/path.
But when I read such a global sentence listing BOTH, they never say HOW it changes the path.
And I have often been led to believe that the change is the same in both cases (due to the syntax of
categorical sentences).

Then, I go to the internet to look it up and I get two competing explanations depending on the ball.
And no one states that there are two different phenomena going on.

Now I can put this in categories in my head as I reread the explanations.

Thank you very much

A.T.
It is plausible, then, that the increase in drag due to skin friction and that flapping outweighs the decrease in form drag for a tennis ball.
But is the fuzz actually added with aerodynamics in mind? Or is to achieve a certain friction coefficient, that is as consistent as possible (different court types, clean vs. dusty ball)?