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Aerospace Flow seperation doubt

  1. May 13, 2010 #1
    Hi PFians

    I saw a fluid dynamics video (classic one) that when a body moves within a fluid then flow separation occurs at or near the rear of a body (like sphere or foil) but when fluid flows over the body i.e body at rest , then flow separation doesn't occurs! Why?
    Last edited: May 13, 2010
  2. jcsd
  3. May 13, 2010 #2


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    Gold Member

    It has to do with displacement in the medium. In truth, it actually depends upon the fluid, "angle of attack", and the body geometry.

    The front of displacement in a moving body expands based on the geometry of the body. Think of pushing a traffic cone through a pile of M&Ms. The M&Ms will act on each other and push the front of displacement beyond the bounds of the body itself. This leaves a cavity in the wake of the body until the stationary fluid fills the cavity (based on either Brownian motion or some other fluid dynamic mechanic).

    However, the same effect (M&Ms against M&Ms) actually prevent the cavity from forming in the presence of a stationary body. Since there is no front of displacement, the M&Ms fill the cavity much easier. If your body were shaped like a "C" with the opening facing rear, you would still see that cavity even with a stationary body and moving medium.

    Wow... I just realized how hard this is to explain. I'm sorry, this is probably totally useless.
  4. May 15, 2010 #3
    This is interesting and perplexing to me. Ideally, there should be absolutely no difference between the two cases. Would you have a link to said video, I'd really like to see this. My best rational is for the following reason. When you have the body moving through stationary air, then the air truly is quiescent. On the other hand, if the air is moving over the body it is very important that the moving air is of "high quality." By "high quality" we mean a low turbulence factor. This is usually what wind good wind tunnels boast about - having low turbulence in their flow. So if the experiment in the video was not very good, i.e. the turbulence factor was high, its quite possible that the turbulence was just enough to energize the boundary layer to prevent separation (this effect is much like dimples on a golf ball, which prevent separation due to the added turbulence).
  5. May 15, 2010 #4
    in the experiment some colored liquid fluid was used instead of air......
  6. May 15, 2010 #5
    i don't remmber from where i downloaded the video....... i have saved it though!
  7. May 15, 2010 #6
    A fluid is a fluid, so it doesn't really matter that it is not air. Hmmm, you should upload your saved video file to Youtube or something.
  8. May 17, 2010 #7
    I found the answer i think:
    When fluid moves over body consider the case of steady flow then flow separations donot occur or fluid remains in contact with the body from front to rear. But when body moves in fluid then from the viewpoint of an observer sitting on body the flow becomes unsteady, that's why then a different pattern is observed.
  9. May 17, 2010 #8
    It doesn't make any difference if the body is moving or the air is moving. As far as the body is concerned the effect is the same. This is why windtunnels work, they flow the air at the same (non dimensional) conditions as the real thing, without the real object moving.

    I'd tend to agree with Cyrus, it's likely this was a poor windtunnel causing turbulence.
  10. May 17, 2010 #9
    download the 2nd video on this page and watch at 00:59, the particles create whirls at the rear of the body when body moves within the fluid which is similar to flow separation:
    http://www.iihr.uiowa.edu/products/dhrm.html [Broken]
    Last edited by a moderator: May 4, 2017
  11. May 27, 2010 #10
    Hi, I just watched your video. Here's a short response. The velocities were different. Case where the shape was still and the fluid moved: it took a ~3 count for a particle to traverse the shape. Case where the shape was moved and the fluid still: it took a ~2 count for a particle to traverse the shape. In both cases there were vortices and backflow, it was just more pronounced in the shape moving / fluid still case because the shape was travelling at a higher velocity.

    Here's the longer explanation...

    Flow (boundary layer) separates from a body when it experiences an adverse pressure gradient. What the heck does that mean? Let’s explain:

    When an object is subjected to a fluid flow the fluid has to traverse around the object. Two important things occur when this happens. 1) First, a boundary layer of flow is formed around the object because of viscous forces or the friction between the fluid and the surface of the object. 2) Second, thanks to the laws of conservation of mass and energy when the fluid traverses the object the fluid speeds up to go around the shape and then slows down as it leaves the object. (It’s actually more complicated but let’s just keep it simple.)

    This acceleration and deceleration of the fluid means a pressure gradient exists where the fluid speeds up and slow downs. Where the fluid speeds up the pressure is decreasing. Where the fluid slows down the pressure is increasing. Where the fluid is decelerating, if this slowdown is dramatic enough the pressure pushes back against the boundary layer flow and is strong enough to cause it to detach from the object. The zone of increasing pressure that causes this is called an adverse pressure gradient. Adverse pressure gradients can also be strong enough to cause backflow in the boundary layer which then in turns leads to vortices and swirls like we saw in the video.

    Here's why I think we saw more separation in the shape towed case. The higher velocity in that situation means that their's a more dramatic speed up and slow down of the fluid. A more dramatic slow down means a more pronounced adverse pressure gradient exists and thus more boundary layer separation.
  12. May 27, 2010 #11
    Hi dtango,

    Great post and observation about the video, it was both concise and precise -Welcome to PF.
  13. May 27, 2010 #12
    Thanks for the kind comments Cyrus. Glad to be here :).
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