Golf ball backspin.

Shaky

Golf balls are dimpled to give the ball some grip on the air. Backspin is usually highly desirable as it increases lift and hence further distance. But why? Could it be that the friction of the ball and the air is greater on the bottom than on the top and hence heat is released under the ball causing the air to expand?

K^2

Nope. It's simple aerodynamic lift. Very similar to what happens to the wing, but it's the spin of the ball that influences different air flow above and bellow, rather than difference in shape of the wing surface. Aerodynamic lift generated by spinning projectile is known as Maguns Effect.

turbo

Golf balls are dimpled, but it's not just to try to provide lift. Depending on whether you might want to play a cut or a draw, you can get the ball to curve along its path, so you can play it around obstructions. Backspin is quite helpful, too, but generally in the short game, so when you use a wedge or a short iron to get the ball to the green, the ball will "bite" and stop short instead of shooting off the back of the green.

K^2

turbo, a component of aerodynamic force perpendicular to relative wind is called lift. It doesn't matter if it's vertical or horizontal. The trajectory of a draw or a cut shot is still caused by lift.

boneh3ad

Not to mention that the dimples also have nearly zero to do with the lift but everything to do with the drag.

K^2

Which I do not completely understand. I'm pretty sure I could construct equally convincing arguments for it increasing or decreasing drag. Nor can I say with any kind of certainty whether the dimples would strengthen or weaken Magnus Effect. I almost want to say neither, because I can't see how it would affect net circulation, but I'm fully prepared to be completely off.

So if somebody has good, solid understanding of this, I would appreciate some pointers.

meldraft

The dimples introduce turbulence in the flow along the surface of the ball. This drastically decreases drag because it moves (delays) the flow's detachment point further back, thus inducing smaller pressure drag. The phenomenon is called boundary layer effect. Effectively, the pressure profile of the air around the turbulence is like a small airfoils'.

Shaky

Interesting everyone. And sidespin, unless someone is trying to cut or draw the ball, is undesirable, as the result is an awful hook or slice, frequently into the trees.

One can read countless books on how to 'fix' a slice or hook, none of them actually work because if they did, they wouldn't need to come up with new fixes every month.

I believe science can provide a golf tip that shall stand the test of time. In order to hit the ball straight, one must close the clubface at impact such that it's perpendicular to the desired flight path. Such advice is uncontroversial. Does anyone agree or disagree?

meldraft

As long as you hit it straight (as in the velocity vector at the point of impact is in the direction you want it to go) it will go that way. Even so, I believe that it is pretty much impossible to hit it with no spin, since the club will travel at a circular arc (and it is itself curved) and it will be in contact with the ball at "different points" for some fractions of a second (thus creating some tangential force component). Moreover, because of the dimples, turbulence will be formed faster on one side of the ball (in this case probably the bottom side) which will create a Magnus effect even if it had no spin to begin with. In my opinion, giving the proper spin to the ball so that it reaches its intended destination is the part the requires the most skill in this sport.

russ_watters

Right. Dimples decrease drag.

Which also means they decrease lift.

boneh3ad

That's not necessarily true at all. Drag and lift are not coupled that was. For example, consider just a flat plate at angle of attack. To a point, as you increase angle of attack, you increase both lift and drag.

boneh3ad

Regarding golf tips for getting to of a slice, many to most of them work. Really, they are all just tricks to help one square up the club face and hit the ball in a part of the swinging motion where you are coming straight at the ball *** opposed to an angle. Slicing or hooking occurs due to the side spin, so even hitting it square can cause a slice if the club isn't also moving in a direction normal to the face, and that is more difficult to control than squaring up the face.

rcgldr

The dimples reduce drag by inducing turbulence which keeps the boundary layer attached longer, so that the boundary layer doesn't expand as much, reducing the profile drag. Note that inducing turbulence consumes energy, so the reduction in profile drag is somewhat negated by the energy required to induce turbulence.

The lift mostly occurs because the flow on the "backwards" spinning side of the ball remains attached longer, reesulting in a net diversion of air flow. I'm not sure if the dimples increase or decrease the realtive points of detachment of the flows on both "sides" of a spinning ball, and the net perpendicular diversion of air, which would increase or decrease the lift.

K^2

So why are many smaller vortices energetically favorable to few large ones? Is there a simple statement that covers that, perhaps something to do with Reynolds numbers, or is this a complicated question in itself?

Well, I'm glad I'm not the only one.

rcgldr

There's also the issue that the transition from laminar to turbulent flow occurs later without the dimples. The delayed transtion (or larger vortice) creates a larger effective cross sectional area, which in turn creates more profile drag. Link to an article about drag, but not how this changes Magnus Effect:

golf_ball_aerodynamics.htm

At least a diagram of Magnus Effect with spin, but then the article text mentions relative surface speeds and Bernoulli effect, which considering how thin the boundary layer is, would not explain Magnus Effect. The diagram does properly show the diverted airflow at the back side of a spinning ball.

golf_ball_magnus_effect.htm

Wiki article on Magnus Effect:

it is more likely that most of the Magnus effect is due to the earlier detachment of the air flow on the forward-moving side

http://en.wikipedia.org/wiki/Magnus_effect

boneh3ad

Well, this is almost right. The dimples do trip the flow to turbulence, and that does keep the flow attached, mainly because the greatly increased diffusion in a turbulent boundary brings momentum lower into the boundary layer more effectively which helps it resist the adverse pressure gradients that lead to separation.

It isn't the size of the boundary layer that lowers the profile drag, however. It is the presence (or lack thereof) and location of a separation bubble and thus the size of the wake. When the boundary layers separate earlier, they create the giant low-pressure regions where the flow is separated, which lead to very high drag. Keeping the flow attached simply minimizes the size of these regions.

It is very true, however, that the transition to turbulence does carry with it a drag penalty in the form of viscous drag, but that is nowhere near as large as the drag saved by reducing the separation bubble size, so you have a net gain. In terms of forces, this viscous drag penalty is because of the diffusivity of the turbulent boundary layer pulls momentum lower into the boundary layer, meaning the profiles are much more sharply curved by the surface and therefore leading to a much higher shear stress (up to an order of magnitude greater than in the laminar case).

This method of drag reduction would work just as well on a non-spinning ball as it does on a spinning ball. The flow would simply be symmetric if the ball wasn't spinning and the dimples would delay separation. For the spinning ball, the separation occurs at different points but the dimples will delay them both.

Lift on a golf ball is a result of the Magnus effect and would occur even without separation. The backspin makes it so that, when the ball is moving, the air is essentially moving faster over the top of the ball compared to that over the bottom of the ball (relative to the ball) just like an airfoil. The flow deflection would occur, therefore, regardless of separation.

You do bring up a good point about the rotation serving to delay separation on the top and encourage it on the bottom, however. The top is spinning in the direction of the overall airflow, meaning it adds momentum to the fluid low in the boundary layer and helps it resist separation. Conversely, the bottom side rotates against the fluid motion, which removes momentum from the lower portion of the boundary layer and encourages transition.

Not sure what you mean by this, to be honest. It is an interesting thought but what do you mean by "energetically favorable"?

Generally speaking, flow separation tends to decrease lift, often quite dramatically (leading to stall on an airfoil). I need to think a little bit to try and make sense of what that means in terms of lift and the Magnus effect, however. It is an interesting question.

K^2

That was awfully vague of me. I'm talking about energy loss due to generation of the vortices. The mechanical energy of the golf ball becomes, mostly, mechanical energy of the vortices. What isn't immediately apparent is why few large vortices generated by a smooth ball would require more energy to produce than many small vortices due to a dimpled ball.

It just seems to me that, for a qualitative assessment, considering energy required to generate a vortex might be easier than trying to figure out the drag directly.

Right. That argument mostly makes sense. What isn't clear to me is why the turbulence in the boundary layer does not contribute to the profile drag, or at least, not as strongly as the turbulence in the wake. What is so fundamentally different between the turbulence in the two regions? After all, the flow isn't strictly laminar in either.

Edit: Or is it just about the cross-section? While the turbulent boundary layer covers a significant fraction of the golf ball's surface area, the cross-section area of the turbulent boundary layer at any given slice is rather small.