- #1
pallidin
- 2,209
- 2
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"
The issue here is laminar vs turbulent flow, as your wik link said, and I'll explain in my own words:pallidin said: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?
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).pallidin said: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
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?Jeff Reid said:In addition to reducing drag, the dimples also reduce how much a golf ball curves due to spin.
russ_watters said: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.
Shhh...I was asking the OP to see if he knew that!russ_watters said: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.
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.FredGarvin said: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?dimples reduce the curve
Edit: That was wrong, based on an old article I read years ago which was incorrect. Corrected the post.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?dimples reduce the curve
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.Jeff Reid said:Vortex generators are for slow speed flight, reducing stall speed for more safety margin. At cruise speeds, they are useless.
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.Vortex generators on wings ... By keeping the air attached to the wing, they increase the maximum angle of attack
pallidin said: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"
The dimples on a golf ball are specifically designed to reduce drag and increase lift, allowing the ball to travel further and more accurately through the air. This is due to the dimples creating a thin layer of air around the ball, reducing the overall drag force and creating a lift force that helps the ball stay in the air longer.
Dimples on a golf ball can significantly impact the distance it travels. The dimples reduce the drag force, which allows the ball to fly further and stay in the air for a longer amount of time. This results in longer drives and better overall distance on shots.
No, there are a variety of different dimple designs on golf balls. The size, depth, and shape of dimples can vary, and each design can affect the ball's performance differently. Golf ball manufacturers spend a lot of time and resources researching and testing different dimple designs to find the most optimal one for their specific ball.
Yes, dimples can affect the spin of a golf ball. The dimples can create turbulence around the ball, which can increase the spin rate. This can be beneficial for some shots, such as chips and pitches, but can also cause the ball to spin too much on longer shots, resulting in less accuracy.
In addition to reducing drag and increasing lift, dimples also help the ball fly in a more stable and consistent trajectory. This can improve the overall accuracy of shots as well as reduce the effect of external factors, such as wind, on the ball's flight. Dimples also give golf balls their iconic appearance and help players easily identify their ball on the course.