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Golf ball dimples

  1. Jun 7, 2006 #1
    Do the "dimples" on golf ball really make any difference? If so, why isn't a baseball, hockey puck, or other high speed projectile "dimpled"
     
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  3. Jun 7, 2006 #2

    russ_watters

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    An enormous difference. A golf ball without dimples would fly a fraction (1/3? 1/4?) as far as one with does.

    Other sports have different reasons. Baseball parks are big enough without people hitting 700 foot homeruns. But the stitches do a little of the same thing. Anyway, the tradition of the baseball's physical characteristics is important to the sport.

    In hockey, the puck does not travel far enough for dimples to be a factor. The sides are rough, however, for grip.

    IIRC, there are a few cars that have dimpled underbody panels.

    Other applications use similar principles - those little tabs sticking up from airplane wings are vortex generators and have a similar function.
     
  4. Jun 7, 2006 #3
    Very informative response Russ. I would like to ask a few more questions:

    You mentioned exploitation of this usefulness with airplane wing tabs and racing cars. Would there be any benefit from dimpling rotating fan blades(such as with an aircraft propeller, or wind-generator propeller)? Or do the vortex pattens and stress-relationships create a negative benefit under such circumstances?
     
  5. Jun 7, 2006 #4
    Though I'm the OP, I found this interesting comment while doing a Google search. Note, the math expresion for the critical Reynolds number did not copy/paste correctly...

    "So, why dimples? Why not use another method to achieve the same affect? The critical Reynolds number, Recr, holds the answer to this question. As you recall, Recr is the Reynolds number at which the flow transitions from a laminar to a turbulent state. For a smooth sphere, Recr is much larger than the average Reynolds number experienced by a golf ball. For a sand roughened golf ball, the reduction in drag at Recr is greater than that of the dimpled golf ball. However, as the Reyn olds number continues to increase, the drag increases. The dimpled ball, on the other hand, has a lower Recr, and the drag is fairly constant for Reynolds numbers greater than Recr.

    Therefore, the dimples cause Recr to decrease which implies that the flow becomes turbulent at a lower velocity than on a smooth sphere. This in turn causes the flow to remain attached longer on a dimpled golf ball which implies a reduction in drag. As the speed of the dimpled golf ball is increased, the drag doesn't change much. This is a good property in a sport like golf.

    Although round dimples were accepted as the standard, a variety of other shapes were experimented with as well. Among these were squares, rectangles, and hexagons. The hexagons actually result in a lower drag than the round dimples. Perhaps in the future we will see golf balls with hexagonal dimples."

    From: http://www.fi.edu/wright/again/wings.avkids.com/wings.avkids.com/Book/Sports/instructor/golf-01.html
     
  6. Jun 8, 2006 #5

    russ_watters

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    The issue here is laminar vs turbulent flow, as your wik link said, and I'll explain in my own words:

    If you've ever watched the flame/smoke from a candle, in still air, the flow sometimes becomes very straight and smooth (laminar), then a few inches above the candle, gets turbulent. But a tiny disruption will set the entire flame flickering. Laminar flow is very unstable and easily separates from an object, causing pockets of low presure behind an object: meaning pressure drag. Laminar flow, not surprisingly, has lower friction drag, but pressure drag is a bigger factor. So controlling the transition from laminar to turbulent is critical for controlling drag and lift.

    It is rare to be able to maintain laminar flow over an entire object - it requires perfect aerodynamics, of a very specific type. The P-51 had mostly laminar flow wings and something as simple as a little bird crap would destroy any benefit to be had.

    So, any application without perfect aerodynamics but where lift or drag are important might benefit from dimples/vortex generators. But there is one more issue from your link: Reynolds' number. Reynolds' number is a composite of speed, density, viscocity, and size of an object. With it, you can predict where the laminar-turbulent transition will be. Ie, higher speed means higher Reynolds' number and shorter distance before transition. So, for example, a low-speed bladed (house) fan might have no problem keeping laminar flow attached to it, while a higher-speed one might have transition issues.
     
  7. Jun 8, 2006 #6

    rcgldr

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    Edit: Current articles state that dimples decrease drag and increase curvature.

    Original post, based on an old article (that was apparently wrong):
    In addition to reducing drag, the dimples also reduce how much a golf ball curves due to spin.
     
    Last edited: Jun 9, 2006
  8. Jun 8, 2006 #7

    FredGarvin

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    After all of that, do you know why the turbulent boundary layer and the increased length the BL stays attached helps to decrease drag? You've only explained part of the solution. Why would having the BL stay attached longer have any effect on drag (playing devil's advocate here).
     
  9. Jun 8, 2006 #8

    FredGarvin

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    I'm not so sure I buy that. With stitching on a baseball, curve balls are enhanced and break better, not worse. Do you have a link for this?
     
  10. Jun 8, 2006 #9

    DaveC426913

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    Aaaahh! I always wondered about this. Intuitively, I've always assumed laminar flow is desirable, and wondered why they go to great lengths to break it up.

    So you're saying that, sure, in ideal circumstances, laminar flow would be great. However it is very easy for laminar flow to be WORSE than turbulent flow.

    Cool!
     
  11. Jun 8, 2006 #10

    russ_watters

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    Yep. And a sphere does not have a teardrop-shaped tail to gradually and gently converge the airflow behind it, so if the flow starts laminar, it separates violently from the sphere. If the flow starts turbulent, it stays attached longer and thus doesn't create as big of a low pressure area behind the ball to literally pull the ball backwards.
     
  12. Jun 8, 2006 #11

    FredGarvin

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    Shhh....I was asking the OP to see if he knew that!
     
  13. Jun 9, 2006 #12

    rcgldr

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    Edit: in the current articles that I found that mentioned this, dimples cause golf balls to curve more, but rough table tennis balls curve less than smooth ones. The articles that mention this attribute it to air speed, but it may also be due to the fact that table tennis balls are very light for their volume.

    Original post:

    I'm more experienced with Table Tennis, the balls weigh the same, and have the same diameter (which keeps getting bigger as time goes on and they change the rules, 38mm, then 40mm, and proposing 44mm (this may be incorporated now), in an attempt to slow down the game for the spectators). The main variations are in the elasticity in the bounce (this has a narrow range though), and the smoothness. A smooth table tennis ball curves much more than one with a rough surface. It probably also slows down quicker, as play with a smooth ball (like a Peace) feels lighter (since it weighs the same, it must be going slower).
     
    Last edited: Jun 9, 2006
  14. Jun 9, 2006 #13

    rcgldr

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    Edit: That was wrong, based on an old article I read years ago which was incorrect. Corrected the post.

    There's a critical speed where turbulence at the leading edge is going to occur no matter what. A ping pong ball never travels fast enough for this to be an issue, but it does happen in the case of a golf ball.

    In the case of a base ball, throwing the ball so that 4 seams are involved in the spin produces more lift than a 2 seam pitch. These are protrusions, though, not dimples.

    According to the articles I can find now, in the case of a golf ball, the dimples reduce drag and increase curvature of it's path, because turbulent air will stay attached longer, and get more deflected at the rear of the golf ball than laminate air flow.
     
    Last edited: Jun 9, 2006
  15. Jun 9, 2006 #14

    rcgldr

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    Regarding laminar air flow on airplane wings, it's almost non-existant because airplanes travel too fast through the air. You end up with turbulent air flow at the seperation point in front of the wing at "cruise" speeds, well before the air even reaches the wing's surface.

    Vortex generators are for slow speed flight, reducing stall speed for more safety margin. At cruise speeds, they are useless.

    Very slow moving radio control gliders, like very light 10 ounce, 58 inch wing span, hand launch models have an issue with laminar air flow and often use "turbulator" strips across the enter wing span near the front of a wing to break up the air flow into a turbulent one.
     
  16. Jun 9, 2006 #15

    rcgldr

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    Regarding dimples on a race car, it's not done on any that I know of. In the case of F1 cars, underbody tunneling is banned, and they have to use skid boards. It's pretty obvious that the upper surfaces are extremely smooth. Indy Racing League, and Champ cars do allow underbody tunneling, and these are not dimpled. The monoque bodywork, mostly carbon-fiber and resin, (there's no overall frame in these cars, just front and rear sub-frames, and a driver protection cage), is extremely smooth below and above.

    There were some older aircraft that appeared to have dimpled bodies, but you never see this on modern aircraft.

    My guess is that dimples help on spinning objects like golf balls, but not non-spinning object like wings and fuselages.
     
    Last edited: Jun 9, 2006
  17. Jun 9, 2006 #16

    russ_watters

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    Expand: By keeping the air attached to the wing, they increase the maximum angle of attack. At some point, "stall speed" becomes a meaningless concept because whether the wing has actually stalled or not, if the plane is going too slow, it isn't producing enough lift to keep the plane airborne. In practice, the effect is that the planes never stall - even if they are flying too slow to stay airborne, they are still controllable, wherease if the plane stalls, the control surfaces become useless and the plane may even "depart" from controlled flight - go into a spin or something similar.

    Planes like the F-18 and Mig-29 use wing root extensions to create powerful vortices over the wing that make them virtually impossible to stall. Combined with high thrust to weight ratios and on newer planes, thrust vectoring, a plane can be controllable at virtually any speed and pitch attitude.
     
  18. Jun 9, 2006 #17

    rcgldr

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    But do these vortex generators have any significant effect at high speed high AOA (high g manuevers)? The air is already turbulent at high speed, so laminar flow issues shouldn't exist.

    Getting a bit off topic here, but since aircraft was mentioned ealier in this thread:

    In the case of some fighter planes, the leading edge and trailing edges are rotated (computer controlled) to adjust camber depending on the AOA (g-loading), in addition to thrust deflection.

    Swept back wings also allow vortices to flow back and across a wing, increasing maximum AOA. Delta wing designs generally have the highest maximum AOA, over 20 degrees in some cases.

    As previously posted, I'm not aware of modern aircraft of racing cars that use dimples to reduce drag.
     
  19. Jun 11, 2011 #18
    Dimples cause the relative airflow around the ball to be stickier thus increasing surface or friction drag. It causes the relative airflow to stay attached longer leaving less wake reducing pressure drag. Because pressure drag is the most prevalent drag on the ball reducing it reduces over all drag even though dimples increase friction drag.

    When the ball spins while going through the air this friction drag causes the ball to have a little different path (curve) than it would have if it did not spin. If you were to drive a nail through a golf ball and chuck it up in a fast spinning drill and pull the trigger the drag on the ball is reflected in a toque force that opposes rotation. If you were to squeeze the ball with your thumb and index finger the friction drag will go up but will not result in a linear force because the increase drag from your thumb is not only equal to the friction drag from your index finger it is in the opposite direction counseling each other out. If the friction drag were to become unequal on one side of the ball it will result in a more linear motion as when you were to push the spinning ball into a surface like a tabletop. This non aerodynamic friction drag between the ball and the tabletop causes the ball to takeoff down the tabletop.

    The uneven friction drag around a spinning ball being pushed into the air by its motion through it is not as dramatic but does create an aerodynamic phenomenon known as the Magnus effect. It is no coincidence that surface preperation that increase friction drag (dimples, fuzz, threads) also increase the Magnus effect. Drag can cause the motion of an object in any direction but even when drag opposes one motion it can cause another as is the case for the Magnus effect, canoe, paddle boat, bird flight, etc. Calling this friction force lift is based on some intentional ignorance. Text tell you the more the ball spins the more Magnus effect but when it comes time to determin what aerodynamic force causes it, the large and obvious fact that the ball is spinning is totally ignored.
     
  20. Jun 11, 2011 #19

    Ryan_m_b

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    Roy if you note the date above the posters names you'll see that this thread is five years old.
     
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