Why golf balls are not smooth?

[Gonzolo]

Hi,

I think I understand so so why golf balls aren't made smooth (is there a name for those circular dips). I assume it allows a better (laminar?) airflow, with less turbulence and higher pressure on the backside. I would appreciate if someone could confirm this or give a clear and concise, and perhaps rigorous (mathematical) explanation if its possible.

Now are there any other systems where similar dips are used to improve an airflow? Would it work on airplane wings? If so, would ellipses be better than circles? How would the optimum dip specifications be determined? Thanks.

 

[Clausius2]

Hi,

I think I understand so so why golf balls aren't made smooth (is there a name for those circular dips). I assume it allows a better (laminar?) airflow, with less turbulence and higher pressure on the backside. I would appreciate if someone could confirm this or give a clear and concise, and perhaps rigorous (mathematical) explanation if its possible.

Congratulations Gonzolo, you're always living up over here.

Well, as a cautionary note I must tell you the flow over a golf ball is not laminar at all. In fact, aerdynamicists intends to provoke a turbulent boundary layer by means of those surface holes. A turbulent boundary layer enhances less drag (in the below link it is said the dimples increase slightly the drag, question I think will depend on the aerodynamic body itself) because the point of layer separation is translated towards the rear part of the ball. Moreover, a turbulent boundary layer is more pressurized than a laminar boundary layer because of the external eddies rotating over it, and also due to a change in turbulent viscosity(it can cause a greater drag) and additional turbulent stresses. As a result, the layer remains more length being attached to the golf ball, so further separation and decayment of pressure at the rear is avoided.

What do you think about it? Well, that's my interpretation from my knowledge, if any aerodynamicist don't agree I will listen to him and learn something about his words.

 

[imabug]

Why are golf balls dimpled
Physics of Golf
Golf balls and dimples

 

[airbuzz]

Well, as a cautionary note I must tell you the flow over a golf ball is not laminar at all. In fact, aerdynamicists intends to provoke a turbulent boundary layer by means of those surface holes. A turbulent boundary layer enhances less drag (in the below link it is said the dimples increase slightly the drag, question I think will depend on the aerodynamic body itself) because the point of layer separation is translated towards the rear part of the ball. Moreover, a turbulent boundary layer is more pressurized than a laminar boundary layer because of the external eddies rotating over it, and also due to a change in turbulent viscosity(it can cause a greater drag) and additional turbulent stresses. As a result, the layer remains more length being attached to the golf ball, so further separation and decayment of pressure at the rear is avoided.

I think you´re right...

 

[russ_watters]

Clausius is right. And as for working on airplane wings, it does. Most airliners have tiny (1"x2" at most) metal protrusions lined up on the top of the wing at the point of greatest thickness. These are vortex generators and work the same way: the turbulence of the vortex breaks up the laminar flow in a controlled manner, keeping the flow attached to the wing for longer, greatly enhancing low-speed performance.

http://www.microaero.com/

 

[Clausius2]

Clausius is right. And as for working on airplane wings, it does. Most airliners have tiny (1"x2" at most) metal protrusions lined up on the top of the wing at the point of greatest thickness. These are vortex generators and work the same way: the turbulence of the vortex breaks up the laminar flow in a controlled manner, keeping the flow attached to the wing for longer, greatly enhancing low-speed performance.

http://www.microaero.com/

I've never heard about that. One is prone to think in smoothed surfaces at wings. But one always learn something here.

 

[russ_watters]

I've never heard about that. One is prone to think in smoothed surfaces at wings. But one always learn something here.

There were a lot of early planes (the P-51) designed with laminar flow airfoils. The main design difference is the bulge in the wing is further back. The problem is that laminar flow is very unstable, so a little bit of battle damage (or even some bird crap) ruins the effect. Plus, when you stall a laminar flow airfoil, its pretty violent.

edit: more.

DARPA has used an F-18 as a High Alpha (angle of attack) Research Vehicle. Its ability to fly at angles of attack that would stall other planes is one of the main reasons its so maneuverable. HERE is the index to the photo gallery (btw - browse the main site for some outstanding pics of X-planes). HERE and HERE are photos of the HARV in high-alpha flight (well, the 2nd one is only 20 degrees - not terribly high). Notice that the skirt (not sure if that's what its reallly called) that extends forward from the wing to under the cockpit produces a powerful vortex that keeps the air attached to the wing. Another problem addressed here is that at high angles of attack, the vertical stabilizers are shadowed by the wing, reducing yaw control.

 

[GENIERE]

Russ-Watters

Are these planes only in horizontal flight, or are they gaining altitude? If only horizontal, how are the wings producing lift?

 

[Gonzolo]

They are flying horizontally, and if it's anything like a regular single-prop plane, the thrust from the propulsion (vertical vector) provides the extra lift to compensate for the loss of lift from the wings.

I suppose the sewings on a baseball have a similar effect than golf ball dimples. Although it is perhaps not the original purpose. Anyone cares to predict how a ping-pong ball would behave if it had dimples? If I understand correctly, a ping-pong ball with dimples would (keeping the same weight and volume):

- go faster, hit harder, travel further
- curve less because the turbulent layer would prevent the ball from "gripping" the air on the spin direction side (underside for a topspin etc.).

... right?

 

[russ_watters]

They are flying horizontally, and if it's anything like a regular single-prop plane, the thrust from the propulsion (vertical vector) provides the extra lift to compensate for the loss of lift from the wings.

That's only a small part of it - an F-18 really doesn't stall until the AoA gets pretty high and is controllable even without thrust vectoring up untl maybe 35 degrees. An F-14 or F-15 couldn't do what they did with this F-18, though thrust vectoring is a brute-force approach that can maintain controllability in almost any part of the envelope (yaw control was the big challenge).

http://www.dfrc.nasa.gov/Newsroom/FactSheets/FS-002-DFRC.html is some more info.

http://http://www.aoe.vt.edu/~mason/Mason_f/LEXS04.ppt is a great ppt presentation. Slide 5 has lift coefficient vs angle of attack graphs for what looks like an F-18 wing and a similar wing without the leading-edge extension. The wing without the leading edge extension starts to lose lift at about 12 degrees (the graph is linear until then), and stalls at about 18. The F-18 wing stalls at about 28 degrees. But just as important, the stall is much less violent - making the plane controllable even after it stalls.

 

[Gonzolo]

Interesting. Is there any info on how this relates to the Mig-29 (I think ...), that can bring its engines underneath and beyond the nose (>90 degree AoA) and recover while flying horizontally in a controlled (albeit unstable) manner?

 

[russ_watters]

Interesting. Is there any info on how this relates to the Mig-29 (I think ...), that can bring its engines underneath and beyond the nose (>90 degree AoA) and recover while flying horizontally in a controlled (albeit unstable) manner?

That's part of why it can do that. Obviously, at 90 degrees, the plane is pretty much sitting on its engine. Supposedly, the F-18 can do something similar - and rumor has it that the leading edge extensions were added to the Mig-29 after they saw it on the F-18.