Golf ball dimples
 

Those golf ball dimples, you know the ones that supposedly make the ball fly furthur. Anyone the theory behind them? Just been watching 'Myth busters' they were testing the theory that a clean car gets better gas mileage. They had three cars, one dirty, one clean, and one with tennis ball sized dimples similar to the ones on a golf ball. mileage on the first two was identical but the one with the dimples got 12% better mileage. Anyone know why.

RE: Golf ball dimples
 

Inside the dimples are stagnant vortices ( air that just sits in the hole, spinning around in circles ). This basically gives the object a cushion of air surrounding it and allows energy to transfer from the vortices to the streamlined airflow as needed. The streamlined air doesn't stick to the vortex air as easily as it does to the surface of the object and so drag is reduced. At least, that's how I understand it. I'm sure there's a lot more to it than that. Maybe think of it as an invisible ball bearing inside the dimple ?

RE: Golf ball dimples
 

The reduced surface drag due to dimples is minimal. The real reduction comes in the reduction of pressure drag due to separation of the boundary layer.

When the airflow over a surface is laminar, the boundary layer separates earlier creating more drag. When the airflow over a surface is turbulent, the boundary layer separates later so there's less drag. The bottom line is turbulent airflow over a golf ball's surface creates less drag.

The dimples act as turbulators, they create turbulence near the surface and thus reduce the pressure drag due to separation.

But 12% better mileage? I'd question their results. If it worked that well on cars we'd all be driving dimpled cars.

Dave

RE: Golf ball dimples
 

Yeh buddy. Oh that's right, it's just TV land, they don't have to be accurate.

RE: Golf ball dimples
 

The Navy does the same thing to the skins of nuclear submarine. I woudn't think 12 percent is too big a stretch.

RE: Golf ball dimples
 

I chewed on this for a while, and can not completely agree with it. Laminar flow is a good thing, but when the surface loses laminar, then there is separation of the flow. That separation is draggy, but you do not improve the total drag by having turbulent airflow over the entire surface. Rather what is desired is to keep the flow laminar for as long as possible, then trip the flow to induce the turbulence to keep the boundary layer as thin as possible.

Not much clearer, but I feel better.

As to the original question, since golf balls spin and are free to rotate during flight, you dimple the entire surface for the reasons already given. However on a vehicle, keep the flow laminear until the flow separates, then take corrective action. Tuft the vehicle and see what the air is doing.

Recently, Damler Benz developed a semi truck that gets 12 mpg @ 50 mph, about 50% better than semi trucks on the roads today. Yet a train still moves the same payload 50% further with the same fuel. A recent article on drag reduction of coal cars showed that having a completely empty car has more drag than one with two bulkheads at the 1/3 and 2/3 point inside of the car.

RE: Golf ball dimples
 

You don't have to agree with it but it's true and it's basic aerodynamics. Just look it up. Do a search on golf ball dimples. Here's a site you can start at http://www.fi.edu/wright/again/wings...r/golf-01.html

Or this one http://www.aerospaceweb.org/question...cs/q0215.shtml half way down the page.

As a matter of fact, the Mythbusters epsode that started this thread showed a smooth ball and a dimpled ball in a wind tunnel and the image was exactly that shown in the figures.

The error you're making is in assuming that turbulent flow equals boundary layer separation. Check the diagrams referenced above, the turbulent flow causes the boundary layer to stay attached longer, creating a smaller wake. That's where the drag is saved.

Dave

RE: Golf ball dimples
 

No, what I said is that the overall drag is lower if it stays laminar longer, then has the boundary layer tripped to avoid separation. Much lower drag than having the entire surface turbulent.

RE: Golf ball dimples
 

I reckon that's more to do with reflecting sound waves to become harder to detect, rather than a decrease in drag ? But, if it serves both purposes...I suppose it's a good thing.

RE: Golf ball dimples
 

dbacque,

Your general statement:

isn't generally true.

For the reasons you mention, turbulent airflow over a surface CAN create less drag, but this is typically only true when laminar flow over the surface would have separated.

RE: Golf ball dimples
 

Very interesting reading reading, particularly the one on the aerospaceweb. In the case ov the car which is not rotating do you not think that it would only be necessary to put the dimples in say the last third of the vehicle? Or perhaps some simpler way of creating turbulance, as on the Gloster Javelin.

RE: Golf ball dimples
 

In the case of a very non aerodynamic shape such as a car there's a whole heap of other issues that would control where to place any sort of tubulating device. And in a lot of cases unless the body shape is designed to be decently aerodynamic in the first place it's highly questionable if the shape will even benefit from using turbulation. Cars, even the sexy "aerodynamic" ones, are so often designed first for style and only second for drag reduction that you really need to look at each case and work with wind tunnels or some pretty complex computational flow software.

For example, does anyone remember the old Dodge that had the rear air pickup scoop on the hood? Common sense would indicate that a hood scoop should be pointed forward, right? But some bright wig finally got the idea that common sense applied to car aerodynamics isn't always so common and tested the hood area for signs of a high pressure zone. Yep, there was a good one right at the base of the windscreen at very high speeds. So the next season their NASCAR model had a reversed hood scoop. And of course to homologate it the same hood came out for the street model.

So when you're talking about cars you just can't in any way even think about using a blanket solution except MAYBE in the case of some of the special one off development projects that look like airfoils with wheels.

RE: Golf ball dimples
 

I used to play a lot of golf spending five years on a golf team while in school.

If you have ever stood next to a green with someone hitting up to play through, the golf ball because its compressed upon contact does not necessarily fly round, it flies egg shaped. It flies egg shaped (essentially end over end however slightly) until it looses contact energy then recovers to round at which time it slows and descends. That is why the ball appears to kind of shake or wobble at the apex of flight as it nears the green while recovering to round. Golf balls are made in different degrees of hardness or compression valves. The higher the compression the greater the club head speed has to be to compress the ball to knock it out of round. Once compressed the harder ball takes longer to come back to round so it flies further if the golfer can compress it. Pro golf ball compressions are valued in the 100+ range with average golfers ball generally below 90 compression, women's golf balls generally below 80. Range balls are really soft almost marshmellow like shortening flight distance significantly on the practice range.

The number and shape of dimples do effect the flight characteristics of the balls the number being controlled by golf's governing body. The ball dimples control to some extent the trajectory of the ball and the amount of spin that can be placed on the ball. The same is true of the design golf clubs its controlled as to what materials, length of club, and so forth. Today's golf clubs and ball technology is out distancing many existing golf courses. In the hay day of Arnold Palmer a long drive would have been 265-280 yards with a wooden headed club, today its 325-350yds + with metal woods. Pro golfers are hitting a five iron 200yds with the improved equipment and balls.

So there is a lot to the technology, with all the major manufacturers making excellent equipment and balls.

RE: Golf ball dimples
 

Do the younger golf balls have pimples instead of dimples?

RE: Golf ball dimples
 

Well, the dimpled area is intended to reduce drag in the water. The "dimples" are much smaller and do little to actually "reflect sonar", however the reduced drag does enable the sub to slip through the water more quietly, thus making it harder to "hear" and its wake less "visible" to sonar. These smaller "dimples" create almost microscopic areas of turbulance, but the concept is the same as the golf ball. There are also other things done with the shape and construction of the hull that help reduce sonar detection, but that's another topic.;)

RE: Golf ball dimples
 

Golf balls are dimpled simply to keep them from rolling off gently sloped greens. However they do make the ball travel further. But because of increased lift not reduced drag. The mythbustes increased milage may be due to an error in measurment, differance in engines (better ring seal, or larger or smaller bearing clearances, etc), or dumb luck that the dimples were placed at just the right spot to gain some decreased drag from vortex which caused stagnet air near high drag areas. However with a decrease of 12% I suspect its a measurment error.

RE: Golf ball dimples
 

I watched the Mythbuster's episode too, and they did try to account for the differences, using the same car each time, and also controlling for the added weight of the clay they used to create the dimples. I believe they were using an electronic fuel flow meter to get exact quantities of fuel used, so it should be accurate. However, I too thought the improvement sounded high, and, as mentioned above, if it were really that great, we would all be driving dimpled cars now, because the manufacturer's would love a 12% increase in mileage to meet new CAFE regs...

RE: Golf ball dimples
 

They probably forgot to account for something that changed the fuel flow readings. Temperature, humidity, pressure, or strange vapors emitting from the clay.

RE: Golf ball dimples
 

LMAO at that one! In that case I need some golf balls with bigger dimples! I have to nit pick with you about the lift though. A round ball would create "lift" if the ball is spinning in the right direction, mine usually create lift on the right side (otherwise called a slice!) but the effect on distance is due to reduced drag, not lift. As far as cars with dimpled surfaces, you won't see it until auto mfgs are forced to do it or they figure out a cost effective way to mfg a textured surface car body. I'm not sure even a 12% increase would offset the r&d,tooling,equipment, costs etc. that would be involved. Even if the Mythbuster's measurements are accurate, the conditions are not all inclusive. They're test conditions were probably about as ideal as could be. Different vehicle, driving conditions, etc. would still create a wide range of actual results.

RE: Golf ball dimples
 

I may be wrong, but I believe that the way most clubs are angled to the back to hit the ball up in the air creates backspin, thus producing lift. I know people have measured the drag of the ball and claim less drag with a dimpled ball, but its very small, the dimples do however help the ball to push the air past on the top which is spining back and slows it down on the bottem which is spining to the front. Since the air is now moving faster on the top, and slower than on the bottom it puts more pressure on the bottom than the top which creates lift. Or if you prefer Newton it means that the air on the top is spun in a downward direction and thus the ball reacts in an upwards reaction.


12% increase for a car getting 25 MPG would be an increase of 3 MPG, I think that would be enough to get them moving. I would like to see the same experment done on several types of cars and several of each type, just to be sure it wasn't some kind of dumb luck.

RE: Golf ball dimples
 

Sorry it was a dupe.

RE: Golf ball dimples
 

Dbacque's first post is right on, apart from the general assertion that "turbulent airflow over a surface creates less drag". For a sphere, or other 'blunt' shapes, the drag is strongly affected by the location of the separation. The turbulence created by the dimples does keep the flow attached longer, and thus reduces drag. As Highplains points out, if it were possible to keep the flow laminar as long as possible, and then force it to transition to turbulent flow just in time, the drag would be even lower. This isn't feasible because the golf ball spins. It is possible with wings, and is exactly what 'laminar flow' wings do; the shape of the wing is such that the flow tends to remain laminar. Often, wings like this have some type of 'trip' device added to force the flow to become tubulent rather than experiencing a laminar separation bubble, which also increases drag.

In general, dimples or other 'turbulators' can reduce the drag of an object if the drag is strongly dependent on where the separation occurs and it's feasible to shift the separation point back. For some bodies, the separation point is determined by the shape, such as a sharp corner toward the back, and forcing the flow to be turbulent can't do much about that. On the other hand, if a shape is so smooth that laminar flow could be maintained everywhere, with no separation before the trailing edge, then turbulence would only increase the drag.

banktoturn

RE: Golf ball dimples
 

Now that you have read a few web pages on this. Tell me how much the drag is, how much that slows the ball down, and how much lift is created from the spinning ball.

RE: Golf ball dimples
 

Yes, it is better if you can keep the airflow laminar and have a turbulator in just the right location to delay boundary layer separation. But the spin of a golf ball makes that pretty difficult.

Yes, I left a couple of words out of a sentence that seems to be getting some panties in a knot but I really thought we were discussing golf ball flight, not the general case of boundary layer dynamics. It was really meant to be a summarizing statement and not a general case.

I have corrected the offending statement in post 3 by adding the words "golf ball's". It now reads "The bottom line is turbulent airflow over a golf ball's surface creates less drag."

We all pals again?

Dave

RE: Golf ball dimples
 

If I wanted to read generally meaningless statements, I would read the AMA forum.