Golf ball backspin.

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  • #26
boneh3ad
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The turbulent boundary layer would be "dragged" exactly as much as as the laminar one. The surface is spinning the same way in either case, so the fluid at the surface would be moving the same in either case. I would imagine then that with the spinning, that would lead to a thinner boundary layer on the top since the Reynolds numbers would be lower up there and the bottom would be slightly thicker than the no spin case since Reynolds numbers would effectively be higher. Here, I am referring to a Reynolds number relative to the distance along the surface and referencing free-stream velocity relative to the moving surface so that it approximates the x-Reynolds number used over a flat plate.

I don't imagine that this would have any really noticeable effect on the magnitude of the Magnus-induced lift because ultimately the effect is as a result of a shifting of the rear stagnation point, and that is governed by the inviscid flowfield around the object, not the state of the boundary layers. Of course, this statement assumes no separation. When separation occurs (as it does here) then the effect this has on the lift is more nebulous other than trying to reason it out based on a wing, which probably is not a very good analogy since both sides are separated here. The separation will modify the predicted potential flow outside the region so it could modify the rear separation point, and I can't think of a good way to analytically determine how it would modify the rear separation point and overall lift quantitatively or really even qualitatively.

My gut instinct tells me that it isn't that much since experience shows that golf balls fly farther with dimples so the drag reduction must be more than enough to make up for any loss of lift. I would even bet that the lift is slightly increased since it isn't as massively separated, but again I can't think of a good way to prove that analytically.
 
  • #27
rcgldr
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Turbulent flow is THICKER than laminar flow, it just doesn't separate as easily.
I meant the net effect assuming that lamimar flow is going to eventually transition into turbulent flow, probably along with a seperation bubble on a curved surface such as a golf ball. In the articles that mention separation points, for smooth golf balls, seperation occurs at about 80° from the center of the leading edge, which would end up diverting air outwards (and I assume creation of a separation bubble, probably turbulent or at least vortice like), increasing profile drag (and wake). The same articles metion that the dimples delay the seperation to about 110° from the center of the leading edge, reducing profile drag (and wake).

In the case of some glider wings, roughing up the leading edge, using turbulator strips, or using vortex generators, can force the transtion from laminar to turbulent sooner, reducing the seperation bubble, resulting in a smaller amount of form (profile) drag with a net reduction in overall drag.

This web page article implies that the dimples increase lift, but without much explanation in the Flight Conditions section. I haven't found any other articles that mention if the net effect of dimples increases or decreass lift (only that it decreases drag) to back up this one particular article.

http://www.franklygolf.com/golf-ball-aerodynamics.aspx
 
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  • #28
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I examined the Wikipedia article called "Magnus Effect", but I noticed the ball in the diagram is showing front spin as opposed to back spin. The theory, seems to argue the ball goes up because air is drawn below the ball, causing the air to compress under the ball relative to the top; the body will experience a force towards the low pressure area (i.e. up). That makes more sense than my theory that backspin causes the air under the ball to "heat up" causing the air to expand. So, the questions become, how do we determine if it's back spin, as opposed to front spin, that lifts the ball up? Then, where must the club face strike the ball such that the desired spin is obtained? I think the pros would find this kind of knowledge valuable.
 
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  • #29
K^2
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It has nothing to do with heat. Back spin will result in lower pressure over the ball and higher pressure bellow the ball, providing positive lift.
 
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The diagram on Wikipedia entitled "Magnus Effect" is correct after all. I thought the contour lines, when they are closer together as opposed to apart, indicate a high pressure area. I had it the other way around. No wonder I had trouble understanding the phenomena.
 
  • #31
boneh3ad
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On the Wikipedia diagram, just look at the direction the streamlines are moving after the ball. Don't worry about pressures at the moment. The streamlines are bent upward as a result of the spin. This required a change in momentum, i.e. a force. The vertical opposite and equal reaction force to the bending of the streamlines is the lift. In the case of the Wikipedia diagram, the streamlines are bent up, so the lift points down.
 

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