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Boundary layers seperating from a baseball based on the viscosity of the air

  1. Dec 23, 2007 #1
    finally in my search to understand why the boundary layer separates from the surface of a baseball I have come to understand that the reasons for the separation of the boundary layer from a baseball is incredibly similar to the separation of the boundary layer from a wing of an airplane after its angel of attack is greater then 15 degrees and I found a very interesting article below that was based on the boundary layer separating from the surface of an airplane’s wing:

    The natural question is "how does the wing divert the air down?" When a moving fluid, such as air or water, comes into contact with a curved surface it will try to follow that surface. To demonstrate this effect, hold a water glass horizontally under a faucet such that a small stream of water just touches the side of the glass. Instead of flowing straight down, the presence of the glass causes the water to wrap around the glass as is shown in figure 8. This tendency of fluids to follow a curved surface is known as the Coanda effect. From Newton’s first law we know that for the fluid to bend there must be a force acting on it. From Newton’s third law we know that the fluid must put an equal and opposite force on the object which caused the fluid to bend. (The article used this diagram )

    Why should a fluid follow a curved surface? The answer is viscosity; the resistance to flow which also gives the air a kind of "stickiness". Viscosity in air is very small but it is enough for the air molecules to want to stick to the surface. At the surface the relative velocity between the surface and the nearest air molecules is exactly zero. (That is why one cannot hose the dust off of a car and why there is dust on the backside of the fans in a wind tunnel.) Just above the surface the fluid has some small velocity. The farther one goes from the surface the faster the fluid is moving until the external velocity is reached (note that this occurs in less than an inch). Because the fluid near the surface has a change in velocity, the fluid flow is bent towards the surface. Unless the bend is too tight, the fluid will follow the surface. This volume of air around the wing that appears to be partially stuck to the wing is called the "boundary layer".
    ………………………. Typically, the lift begins to decrease at an angle of attack of about 15 degrees. The forces necessary to bend the air to such a steep angle are greater than the viscosity of the air will support, and the air begins to separate from the wing. This separation of the airflow from the top of the wing is a stall.”

    Since the reason for the air separating from the surface of a wing is the same as the reason air separates from the back of a baseball I was really interested in this article and I led me to ask a few questions

    1) So based on what was said above regarding Newton’s first law “From Newton’s first law we know that for the fluid to bend there must be a force acting on it.” In the case of a spinning baseball (like in the diagram bellow) the force being referred to in the quote would be the force of the spinning baseball on the airstream?


    2) also from the above explanation it gave some interesting information about why the air will separate from the surface of an object “Unless the bend is too tight, the fluid will follow the surface. ………………………. The forces necessary to bend the air to such a steep angle are greater than the viscosity of the air will support, and the air begins to separate from the wing.”

    a) So would the reasons for the air separating at the points of separation on the baseball (as seen in the diagram below) be because “the bend (after the half way point the ball) is to tight”?

    b) now it was the second part of the excerpt that got me a little confused “The forces necessary to bend the air to such a steep angle are greater than the viscosity of the air will support, and the air begins to separate from the wing.”

    So is the article saying that the force that the wing is exerting on the air to try to make the air bend (at such a steep angle) greater then the force the viscosity of the air can support, and that is why the air separates? Similarly in the case of a baseball as seen in this next diagram below (the ball is not spinning by the way) is the force that the ball is exerting on the airflow to try to make it bend around the ball greater then the viscosity of the air will support and that is why the airflow seperates?


    thanks again i hope someonee can help me out?
    Last edited: Dec 23, 2007
  2. jcsd
  3. Dec 27, 2007 #2
    come on can't someone please help
  4. Dec 27, 2007 #3


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    Staff: Mentor

    First of all, it is the Christmas/NewYears vacation time right now, and activity on PF is way down. Don't expect quick answers to questions posted at this time of the year.

    Second, this thread appears to be a repost of a recent thread where you did get some very helpful replies:


    If you still need additional help or information, please post your detailed follow-up questions there. This thread here is locked.

    EDIT -- upon further review, I'm re-opening this thread. Omegared -- is this a different question from your original thread? It looks to cover the same material, but is slightly different. Would it be better for you to post a follow-up question in the thread where you are already getting help for this topic?
    Last edited: Dec 27, 2007
  5. Dec 27, 2007 #4


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    Science Advisor

    How about the Reader's Digest condensed version?
  6. Dec 30, 2007 #5
    i think they are a bit different

    Well Berkeman I am glad you went back to my original post and read it and noticed the difference I am also glad you decided to reopen this topic. The problems i am having(in both threads) are with air seperating from the surface of a baseball But in this post above you will see that the the article i am trying to understand is based almost totaly on Newtons laws and viscosity of the air and "difficulties" the air has trying to flow around the wing when it is in certain positions and angles. And since the reasons for air seperating from a wing are similar to the reasons air seperates from a baseball I wanted to seee if someone can help me understand and apply this article to a baseball. This article mentiond in my post above and the questions i had about it were not mentiond in the questions from my original thread you linked to they are not based on follow up answers to questions but are based on a totaly different article. I also didn't want to post this article and my questions on it on my original thread because it would of made it too long and maybe confusing(especially since i had quite some quotes from other articles in my original thread) and i know how that would deter people from answering it. so i thaought it best to start a new thread.

    i understand its the holidays( HAPPY HLIDAYS EVERYONE :} and things slow down and i don't expect a lightening response but if someone is knowledgeable about this topic please help me out:}
  7. Jan 3, 2008 #6
    The force being referred to here is FRICTION. Viscosity is something like the friction coefficient you get when you are considering a solid surface. There is the famous Newton's law of viscosity that state Shear Stress [N/m^2]=viscosity*Normal_Velocity_Gradient. So the fluid encounters frictional force as it moves past a solid body. If this frictional force is not large enough to make the fluid follow the contours of the surface, then seperation occurs.
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