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Using golf ball dimples to decrease vaccuum drag on motor vehicles and aircraft.

  1. Jul 23, 2010 #1
    A golf ball without dimples experiences more drag because the deflection area of the airflow around the ball is larger which means that the pressure drop behind the golf ball is also higher. The increased drop in pressure creates a suction vacuum behind the golf ball which pulls it back and decreases its velocity and range. The dimpled golf ball on the other hand experiences less drag because the dimples create a turbulence that causes a larger surface area of the golf ball to be covered by the flowing air which decreases the size of the vacuum behind the golf ball and subsequently its drag. So if the drag reduction caused by the dimples can be used on golf balls, then why not use this technique on the bodies of motor vehicles and aircraft to increase their range and speed?

    Sources: http://wings.avkids.com/Book/Sports/instructor/golf-01.html
    http://www.youtube.com/watch?v=nufXpDBELdQ&feature=related
     
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  3. Jul 23, 2010 #2

    russ_watters

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    A lot of airplanes use vortex generators to do something similar: making a controlled transition from laminar to turbulent airflow, which is much less violent and delays flow separation. http://en.wikipedia.org/wiki/Vortex_generator

    For cars, I suspect the answer is two-fold:
    1. Aesthetics.
    2. They aren't aerodynamic enough in the first place for it to matter.
     
  4. Jul 24, 2010 #3
    Actually, the youtube link included in my first post shows a Mythbusters experiment with a dimpled automobile and proves that the fuel economy of the automobile covered in a dimpled body rises from 26mpg to 29.65mpg which is a 14% increase in fuel economy. With such a substantial increase in fuel savings, why not modify the body of the car but using the easier to machine vortex generators since they produce the same effect as the dimples?
     
  5. Jul 24, 2010 #4
    Airplanes don't want turbulent flow, otherwise they can stall. The dimples on a gold ball work to increase the Reynolds number of a flow that will become turbulent, effectively triggering the laminar to turbulent transition sooner. For a golf ball, this leads to a smaller effective space where turbulent flow occurs behind the ball, reducing the drag. That's my understanding, at least.
     
  6. Jul 24, 2010 #5

    russ_watters

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    Wow, I stand corrected. I would have thought the front end of a car was so bad aerodynamically that the air would be turbulent right away.
    Aesthetics, I'm sure.
     
  7. Jul 24, 2010 #6

    russ_watters

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    Anything moving through the air is going to get turbulent flow whether it wants it or not, with only a few rare exceptions. So the key is controlling it.

    The P-51 is one of the few that was designed to have completely laminar flow, but it is extrordinarily difficult to keep the surfaces smooth enough to actually keep it laminar. Plus, laminar flow is unstable, so stalls are very violent in such aircraft:
    http://www.aviation-history.com/theory/lam-flow.htm
    Well, partly right. Since the transition is sooner, there is more turbulent air in a dimpled ball than a smooth one. But tubulent air stays attached to the ball better, which makes the flow separation region behind the ball smaller.
     
  8. Jul 24, 2010 #7
    In the case of a sphere, there is a sudden and dramatic drop in drag with the use of dimples for a certain Reynolds number. For low Re, dimples are worse, for high speed, dimples are better. In the case of an aerodynamic shape such as an aircraft or car, dimples do you no good - hence why they are not employed.
     
  9. Jul 24, 2010 #8
    Im afraid this just isnt quite right - turbulence and separation (stall) are not one in the same.
     
  10. Jul 25, 2010 #9
    Actually, the youtube link included in my first post shows a Mythbusters experiment with a dimpled automobile and proves that the fuel economy of the automobile covered in a dimpled body rises from 26mpg to 29.65mpg which is a 14% increase in fuel economy. With such a substantial increase in fuel savings, why not modify the body of the car but using the easier to machine vortex generators since they produce the same effect as the dimples?
     
  11. Jul 25, 2010 #10
    Again, this is generally not true and does not work on cars. They got lucky with a freak case, I'll check out your link to see their setup better and report back.
     
  12. Jul 25, 2010 #11

    Ivan Seeking

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    On a related note, plasma fields for reducing drag, seems to be an active area of interest.
    https://www.physicsforums.com/showthread.php?t=116408
    http://defensetech.org/2006/03/30/stealths-radioactive-secret/

    I haven't read up on this in quite awhile, but, IIRC, it was dismissed for stealth applications because of the associated optical effects - early testing of this concept may account for some UFO reports. But what about commercial applications? Is that still a possibility? Anyway, another thread perhaps, but it came to mind.
     
    Last edited: Jul 25, 2010
  13. Jul 26, 2010 #12
    I just watched the clip they did with the car, wow. Are you kidding me? That is not a valid test by any stretch of the imagination. They have so much crap that the basic aerodynamic shape of the car is altered to a significant degree. All they had to do was make a two models of a car with and without dimples and stick it into a wind tunnel. Bogus test, sorry.
     
  14. Jul 26, 2010 #13
    Why did they allow this to air on discovery channel if it is bogus? Should they not get sued for misinformation?
     
  15. Jul 26, 2010 #14

    BobG

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    Mythbusters is designed for entertainment; not scientific rigor. None the less, their experiments are often designed well providing both goals can be met.

    Popular Mechanics conducted a similar test with different results and explained some of the problems with the entire idea of dimples: http://www.popularmechanics.com/cars/news/4316702
     
  16. Jul 26, 2010 #15
    It's a TV show - it's fun to watch. Based on their personalities on the show, I don't doubt their results are accuate; however, the problem is correlation does not eqaul causation (It is a poorly designed experiment with no definitive result that it was the dimples that reduced the gas milage, and not due to some other phenonemnon).

    As an aside, you can't sue someone for 'misinformation.'
     
  17. Jul 26, 2010 #16
    Another factor that I feel was over looked is body shape. A gold ball is a sphere and an airplane/car is not. The "dimple effect" ultimately comes from flow separation and where the flow separates. On a golf ball, you want flow separation as late as possible due to its circular cross section in order to reduce drag created from a difference in pressure. However, aircraft don't really much flow separation except maybe cases with large angles of attack. So the only real effect dimple will have is if they reduce the influence of surface friction which as far as I know, they don't.
     
  18. Jul 26, 2010 #17
    Precisely.
     
  19. Jul 27, 2010 #18
    Incidentally the study by Popular Mechanics looks at dimples that are ~5mm, while the Myth Busters looked at dimples ~75mm (just guessing, estimating 3 inches as approximately the the size of the tennis ball they used). I googled a picture of what the Fastskin's dimples look like and found this. http://www.google.com/imgres?imgurl...tx=64&ty=88&page=1&ndsp=20&ved=1t:429,r:2,s:0

    I'm not saying the MythBusters got it right, although it looked reasonably convincing, but its not clear to me that the Popular Mechanics example disproves MythBusters results.
     
  20. Aug 2, 2010 #19

    Mech_Engineer

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    On a side note, I've seen at least one production car with vortex generators very similar to ones used on airplanes; the Mitsubishi Lancer Evolution VIII. Whether they're helpful in terms of mileage, no idea.

    mitsubishi-lancer-evolution-viii-mr-fq-400-top-3_124.jpg
    2005-Lancer-Evolution-VIII-MR-FQ-400-RA-1024x768.jpg

    turp_0901_09_z+project_mitsubishi_lancer_evolution_ix_interior_exterior+vortex_generators.jpg
     
  21. Aug 3, 2010 #20
    It is a good start.
     
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