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Need Help Understanding Turblance / Laminar Flow

  1. Apr 5, 2009 #1
    Hi guys, first post and I hope I got the right forum.

    I have been doing some reading on fluid dynamics and there are a couple things I can't figure out.

    1) When you light a candle or something and the smoke rises. It starts of nice and smooth, but then turns all turbulent after a little bit. What is the cause of this?

    2) Golf balls. I know the dimples create a turbulent layer around the ball. This layer increase the friction drag, but it decreases the "pressure drag". I am not entirely sure how this works. I read a bunch of sites, and the best explanation i got was:
    http://wings.avkids.com/Book/Sports/Images/golf_01_sma.gif [Broken] ___ http://wings.avkids.com/Book/Sports/Images/golf_02_sma.gif [Broken]
    laminar ___________ turbulent

    "The larger separation region of the laminar case implies a larger pressure drag on the sphere." As I said before I am not sure how these increased separation slows the ball down.

    Also does laminar flow give you more control over spin and such, because ping pong balls can curve like crazy. How does this work?

    3) Finally air plane wings. Again I have done a bunch of reading on this. On normal wings the "hump" is 25% of the way in, but on a laminar flow win it is 50% of the way in. This delayed hump means that the flow will be laminar for longer. If I understand correctly this means the plane will be able to go faster and have less drag. This seems to be a good thing but my research shows that laminar airfoils are only really used in highspeed jets / planes. How is this a disadvantage?

    EDIT: if there is better suited somewhere else please let me know.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 5, 2009 #2
    I would look for the book: "Fundamentals of Fluid Mechanics", Munson, Young & OKishi, fifth edition, chapter 9. All of this is explained in there. You can ignore the equations and read the text, it will answer all your questions in more detail than I care to type on here.
  4. Apr 5, 2009 #3
    Is there an online source where I can view it?
  5. Apr 5, 2009 #4
  6. Apr 6, 2009 #5
    Ahh shucks.
  7. Apr 6, 2009 #6
    The golf ball thing is dimpled not to reduce the drag. It keeps the boundary layer attached for longer, allowing for greater lift via the Magnus effect. Its the change from laminar to turbulent flow that causes the funny effects such as swing on balls. By having the flow seperate at diffrent points on differnt sides of the ball the airflow induces spin. This is why in cricket they polish one side of the ball and let the other side get scuffed up.

    Laminar aerofoils are very rarely used, because planes travel too quickly for the flow to stay laminar. Laminar boundary layers have a tendancy to seperare (due to an adverse pressure gradient), and if you have seperation your wing stalls (produces no lift, which is obviously something you want to avoid when you are several thousand feet up). turbulent boundary layers are more robust and will stay attached in more conditions. So although there is marginally higher drag, its more than ourweighed by the extra lift generated.

    I'm unsure about the candle thing.
    Last edited: Apr 6, 2009
  8. Apr 6, 2009 #7
    This is not correct. The dimples are there to reduce drag by allowing separation farther behind the ball therefor reducing the pressure differential across it. When ever you have an object moving through a fluid you have drag the friction or from the viscosity of the fluid, and you have drag from stagnant pressure which is basically just a pressure differential across the object normal to the direction in which it is moving.
    *If you PM me your email I can send you a lab I did in my fluids class where we studied golf balls in a wind tunnel.

    The Magnus effect is what allows ping pong balls to have such a dramatic curve, and yes the dimples and golf balls do reduce this affect.

    I don't think anything still uses a laminar airfoil anymore as they aren't as efficient (higher drag) as conventional airfoils. The airfoils you might be referring to are for super-sonic aircraft which operate well within the turbulent flow regions.
  9. Apr 6, 2009 #8
    Silly me! I feel daft forgetting that (and getting it basically wrong).

    I should have clarified that as almost all shots have back spin, the dimples increase the lift effect. The reduced drag is due to the higher effective reynolds number. There is a critial Re number where the drag suddenly drops. (im assming that the laminar to turbulent boundary layer switch point).

    I had to dig out my notes for this. I'll pm you my email, because I like reading all the stuff I can.
    Last edited: Apr 6, 2009
  10. Apr 6, 2009 #9
    thanks guys.
  11. Apr 6, 2009 #10


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    Staff: Mentor

    The wiki on the Reynolds number may be helpful - it discusses the issue in the opening paragraph:
  12. Apr 6, 2009 #11


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    Science Advisor

    The laminar boundary layer can not sustain as well against an adverse pressure gradient as turbulent BL. That is due to the large amount of mixing in the turbulent BL. Because of the mixing there is a lot of momentum transfer between particles that you wouldn't normally get in a laminar BL. This, in essence, keeps the particles close to the surface moving. This is important because if the particles at the surface stop moving that is when the BL separates and you run into trouble. In the golf ball example, you have a smaller area behind the ball with a low pressure because the BL could hold on longer. This means a smaller, low pressure over a smaller area therefore a smaller retarding (drag) force created.

    Have you taken a look at this page? It may be helpful for you if you haven't seen it:
    Last edited: Apr 6, 2009
  13. Apr 6, 2009 #12
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