Golf ball dimples for cars

[GregLocock]

thread16-262634 etc were fairly negative about the likelihood of golf ball dimples working on man rated sized vehicles.


Mythbusters built and tested a car to see what difference they made. The actual test procedure covers all the obvious criticism.

They got about a 10% improvement in mpg at 65 mph.

http://www.autoblog.com/2009/10/22/mythbusters-golf-ball-like-dimpling-mpg/

The water tank test showed that the separation bubble behind the car was smaller if the car was dimpled (not that that is necessarily the be all and end all).

It does strike me that perhaps ripples would be more effective than dimples, as the flow over a car is in a defined direction.

Cheers

Greg Locock


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[FeX32]

I agree. Ripples should be more effective.
If I get a chance I would be interested in doing some CFD to compare.




Fe

 

[IRstuff]

Maybe you can suggest it to the Mythbusters to revisit and do a side-by-side comparison

TTFN

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[GregLocock]

Well I'm actually thinking about nagging the propeller-heads at work.

Cheers

Greg Locock


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[Tmoose]

The VW fuel economy SAE paper ca late 1970s maintained full economy benefit can't be realized without a gear ratio change since less cruising power requirement means operating at a lower BMEP point and therefore almost certainly a slightly worse BSFC point as well. So, if the aero mod alone netted 10% mileage gain, it was even greater "drag" reduction.

I wonder if there aren't really just a few bands along the car where the dimples really count.

http://www.challengergary.com/images/72_rr_pass_side_4-19-08.jpg

http://i307.photobucket.com/albums/nn307/tigerstrypes/Dukes of Hazzard/dukeseps12407sl.jpg

http://www.musclecarclub.com/musclecars/dodge-superbee/images/dodge-superbee-1970c.jpg

http://www.toywonders.com/productcart/pc/catalog/33151.jpg

 

[CarbonRod]

Flow over by a frictionless fluid over any shape creates zero drag. Drag comes from skin friction and pressure distribution over the body.

A fluid flowing flowing a long a gently curved surface generally stays "attached" to the surface until the boundary later has grown large and the main flow "detaches" itself from the stagnant boundary layer and the surface. A low pressure, turbulent wake forms in the space between the surface and the detached flow. This low pressure wake creates extra pressure drag on the body.

The idea behind dimples on a golf ball is to trade an increase in skin friction drag for a decrease in pressure drag. The flow around an object the size of a golf ball at golf ball speeds is usually laminar, boundary layer is laminar. The dimples create a turbulent boundary layer. A turbulent boundary layer, although creating more skin friction drag than laminar one, is better at staying attached to the body because the turbulence puts more high energy, moving fluid closer to the body, delaying the formation of stagnant fluid and delaying the onset of flow detachment (it has a steeper velocity profile). By moving the detachment point of the flow further back on the ball the size of the low pressure wave is reduced. Dimples increase friction drag, decrease pressure drag, the new result being a decrease in overall drag coefficient.

Reynolds # for a car moving at highway speeds is generally (assuming a characteristic length on order of meters) would lead one to believe the flow is already turbulent over most of the car, leading one to believe that dimples all over the car may not be of much help. Really, the dimples would only insofar as the can keep the flow on the back side of the car attached to reduce pressure drag. If the angle between the roof and back end is very steep dimples will likely do nothing. In the case a smooth aerodynamic shape, the might help.

 

[GregLocock]

I think you have missed the main reason they use dimples on golf balls.



Cheers

Greg Locock


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[CarbonRod]

I'm pretty sure that's how it works.

 

[bkndesign]

does anybody have a suggestion on how the RIPPLES should be and in what sector of the car they can be more effective?

 

[KENAT]

Just in front of the separation point would probably be mose effective, as I believe they do on some bluff aerodynamic shapes such as engine ncacelles or something like that as I recal.

I think they called it a 'trip line' but it's been a while.

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[GregLocock]

Yup, or trip wire also used in early STOL aircraft, they ran a wire across the top of the wing to (ahem) "trip the boundary layer and reenergise it)." Their words not mine. The idea was to delay boundary layer separation.



Cheers

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[rb1957]

yeah, to make it turbulent, to delay separation

 

[KENAT]

Or as I once put in a report 'to cause turbularization of the airflow'.

Greg, didn't I use a similar phrase in that smoke stack thread?

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[btrueblood]

Glad you caught my fly-by reference to it in the strake thread, Greg

I thought that was one reason for the old "vane" on the back of station wagons, above the windows (though maybe the vane is really just to clear dust/water from the window, and not for any real decrease in aero drag). Also, at least one US auto mfgr. was putting a vane-like "spoiler" on the rear decks of their "sport" cars some years ago, angle of said spoiler was obviously going to create lift, not downforce, and the apparent intent was to turn the airflow to reduce aero seperation drag. Apparently so many people disliked the idea ("why doesn't it aim "up" like on F1 cars?") that it was quickly discontinued.

 

[starpilot22]

CarbonRod is right. The thing is, no one said otherwise in the first place I suspect because they already knew. Over-enthusiastic undergrad, maybe?

 

[tmoritz]

How to dimple your car for aerodynamic efficiency:
1) first spray the surface of your car with blackened oil. This will work better if you have a white car.
2) drive down the highway
3) note the transition area from laminar to turbulent by the change in oil film
4) locate a line about 5-10% of the car's length up stream of the transition area
5) now for the tricky part - park your car part way inside a garage when golf ball size hail or larger is forecast
6) make sure you roll the car over on it's side 1/3 through the storm
7) 2/3 of the way through the storm roll it back over to expose the other side
8) repeat as necessary for all the different speed you want to cruise down the highway at
9) or you could trust the aerodynamicists that work for the car companies that they have minimized aerodynamic drag on their vehicles for the broadest range of operating conditions.

Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[btrueblood]

"...or you could trust the aerodynamicists that work for the car companies that they have minimized aerodynamic drag on their vehicles for the broadest range of operating conditions. "

Uh...only if you include in the term "operating conditions" satisfying the marketing monkeys, focus groups, managers, car magazine writers, etc. etc. etc. who have the REAL say in how the car's external features should look...see my earlier comment regarding rear spoilers.

 

[tmoritz]

Everything I said above was tongue-in-cheek.

The best way to reduce drag on a vehicle is to reduce your speed. Aerodynamic drag is a function of velocity squared. Reduce your speed from 75mph to 65mph you will reduce aerodynamic drag by about 25%. Jimmy Carter was pretty darn smart when he set the speed limits to 55 mph in my opinion.

Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[btrueblood]

Fair nuff Tom.

 

[GregLocock]

Actually the measured optimal cruise speed for some cars is in excess of 55 mph, and in fact for very few is it below 42 mph.

Things are more complicated than just aero.



Cheers

Greg Locock


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