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Flow over a hole

  1. Jan 17, 2005 #1
    Hi, I have a problem concerning a flow (water) over a plate with hole in it.
    Under the hole, there is an air bubble desperately wanting to get out. What I would like to know is the following: How to shape the hole in order to minimize the drag.

    the problem is as follows:
    There is a torpedo-shaped hull (submarine) sailing at a few meters below the water surface. There are a few holes in the hull, so the pressure on the inside is approximately equal to the pressure outside.
    Inside the hull is a (large, about a dm^3) airbubble. We need to get the air out, so we drill a hole in the hull just above the airbubble. The air will pour out and mix with the flow outside. This way, we want to loose all the air in a few seconds.
    Since we want the drag to be as low as possible, we do not want the flow to be disturbed to much (not to trigger flow separation).
    There are a few options.

    1. drill one big hole, creating a big, but local, disturbance
    2. drill a few smaller holes, creating a smaller disturbance, but over a larger area
    3. drill holes of some special shape (not just circular) or under some strange angle
    4. use some sort of 'filter' of 'canvas' in order to create microbubbles.

    I've been searching for some theory about this kind of problems, but I can't really find anything. I was hoping you could help me a bit ?
  2. jcsd
  3. Jan 28, 2005 #2
    I would drill the holes at a 45 degree angle so that the drill would push out with a component directed towards the rear of the submarine. this would help to get the bubble headed in the same direction as the water streaming by, helping minimize turbulance. The wake of bubbles skimming the outside hull might help to create some kind of pressure differential effects, but I wouldn't know off hand where to drill more holes to take advantage of these effects. I would maybe try drilling another (but smaller) hole just downstream of the first, or maybe two holes equadistant from the longitudinal line down the hull from the first hole and on a line perpendicular to it. But yeah, I'd start with a big hole.
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