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Coana-Effect over a convex surface-causes(liquid and gas flow)

  1. Sep 9, 2014 #1
    hello
    first of all i'd like to appologise for eventual speeling mistakes i am going to make in this thread :D
    ive chosen as subject for my maturity paper the coanda-effect applied in the common experiment with water flowing along the curved-convex surface of a spoon.
    i want to make a model of forces that occur on the surface in order to explain why the spoon is being pulled inside the steam.after ive done some measurements ive discovered that even a slightly heavier spoon,but with another curveture holds longer in the water stream as i move the spoon further of the stream.so eventhough the force needed to hold the spoon increases by its weight, its is being hold at a distance where the less heavy one doesnt.
    i ve done some comparions between this experiment and the lift on a wing and i know there are a lot of differeneces, including the type of fluid, but i am pretty sure that here,like on a wing, the form and area of the spoon convex surface is very important.
    now what i wannt to do is to draw forces(velocities of water particles)on the different types of surfaces,each at some different points in order to show how the resultant forces occur, and why the other curvature doesnt produce as much lift as the other.

    i would be also glad if u could tell me exactly how van der waals forces explain the coanda effect(why water bend along the convex curvature)

    Thank you very much
     
  2. jcsd
  3. Sep 10, 2014 #2

    rcgldr

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    Wiki article on the water curving over spoon due to surface tension is noted as "dubious", so it's not a good source. What the article does get correct is that the water is flowing through the, so it's a situation dealing with two media (water and air).

    One explanation for part of Coanda effect is that as air flows over a convex surface, the surface introduces what would be a void region if the air did not somehow fill in that region. If the flow isn't too fast and/or the radius of curvature is not too small, the air will tend to follow the surface of the convex surface to fill in what would otherwise be a void. The air experiences a radial (perpendicular) acceleration towards the surface, coexistant with a pressure gradient that corresponds to the acceleration. The reduced pressure region also coexists with the acceleration of the surrounding air towards that region (so acceleration both perpendicular and parallel to the direction of relative flow).

    Another aspect to Coanda effect is surface friction and viscosity in the boundary layer. The effective profile of the convex surface could be considered to be the outer surface of the boundary layer.

    So getting back to the water flowing down the convex side of the spoon, regardless of the reason for the change in flow direction, there is acceleration of the water perpendicular to the original flow causing a change in the direction of the flow of the water, coexistant with a Newton third law pair of forces, the spoon "pulls" the water to cause the change in flow direction, and the water "pulls" the spoon with an equal and opposing force.
     
    Last edited: Sep 10, 2014
  4. Sep 11, 2014 #3
    thanks for your response.
    i would also like to represent the forces that occur on the particles and produce the "lift force" on the spoon with vectors.so at the beginning i have a force pointing down.after the impact on the surface, the direction of the water particles changes, so this force changes as well.The resultant force of the particles points in the direction of the "lift force " right? so the lift force is the sum of all these resultant forces on the surface.but what forces do i need to take in consideration?does friction force occur as well here?
     
  5. Sep 11, 2014 #4

    rcgldr

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    My last post is missing a word. The water is flowing through the air, and the air could occupy any volume of space and at ambient pressure if water flow detached from the surface spoon to prevent any "voids" with lower than ambient pressure from being formed. It's the curved flow of the water that coexists with a pressure gradient perpendicular to the flow that accelerates the water towards the receding convex surface of the spoon (from the falling water's frame of reference).

    The theory that surface tension in the water causes it to cling to the spoon was labeled "dubious" in the wiki article, but surface and internal tension is the main reason that the water stream tends to narrow as it falls as opposed to breaking up immediately into droplets, so I'm not so sure if this is really "dubious". Another factor why water could cling to the spoon is surface friction and viscosity in the water, where almost all of the flow over the surface of the spoon would be similar to a thin boundary layer (zero speed at the surface of the spoon, transitioning with distance from the spoon, to maximum speed at the outer surface of the water). It's probably a combination of both surface / internal tension, and boundary layer effects, but I suspect that surface and internal tension are the primary factors.

    I'm wondering if soapy water (with surface tension about 1/3rd of plain water suface tension) would produce the same results.

    In my opinion, the reason the spoon is drawn into the stream is because of surface and internal tension in the water as it flows around the spoon pulls on the spoon (while the spoon pulls on the water to cause the flow direction to change).

    You can read discussions about this in the wik talk page for Coanda effect:

    Talk:Coanda effect.htm
     
    Last edited: Sep 11, 2014
  6. Sep 12, 2014 #5
    i thought the boundary layer effects occur only if the flow and the ambient consist both in the same type of fluid?am i wrong?i already included that in the chapter about the "air-air situation",can i use it also for "air-water situation"?

    also, with regards to the resultant force, how can i explain that a type of convex surface holds longer in the fluid stream as it is being pushed further away from the stream,eventhough is less heavy than the other one with a different(extremer ) curvature on microscopic level?
     
    Last edited: Sep 12, 2014
  7. Sep 12, 2014 #6
    There is a nice Report on the first European mechanics colloquium, on the Coanda effect, described in a free issue of the Journal of Fluid Mechanics:
    http://journals.cambridge.org/download.php?file=/FLM/FLM23_04/S0022112065001702a.pdf

    Basically the Coanda effect is due to one or two of the following contributions: pressure differences across the jet and air entrainment. Obviously, for water flowing over a spoon, there is no air entrainment and the flow is laminar. In this case there is only pressure gradients and viscosity that play a role. You can show mathematically for simple cases that for convex surfaces, the pressure gradient in the boundary layer leads to a force in the direction of the surface.

    There is no need to add surface tension to it to explain the phenomena. We know from the analysis of liquid jets impinging on a straight plate that including the surface tension effects of the liquid does not significantly alter the shape of the jet, see e.g. the work of Lienhard for an inviscid analysis including surface tension
    http://web.mit.edu/lienhard/www/stagnation_point_heat_transfer_liquid_jets.pdf
     
  8. Sep 12, 2014 #7

    rcgldr

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    There needs be some form of internal tension, otherwise a falling stream of water would just break up into droplets instead of converging into a narrower stream.
     
  9. Sep 13, 2014 #8
    All I am saying is that surface tension is not necessary to explain the Coanda effect. The Coanda effect is only about the observation that a jet or sheet of fluid (liquid or gas) follows a convex surface when this surface is placed in the stream. The surface tension of a liquid jet would modify the shape of the jet compared to a gas jet but both the liquid and the gas jet would follow the convex surface.

    Also, if the surface tension of water would be zero, then water would not break up into droplets but behave like a gas. Actually, it does exactly this when liquid water reaches the boiling temperature and the surface tension becomes zero.

    What surprises me is the large amount of people saying that liquid water flowing over the back of a spoon is not a demonstration of the Coanda effect. These people somehow think that it is different just because water has surface tension. But if you do the same with a jet of air (blowing through a straw) then you would get the same outcome. Also, Coanda never made a difference between liquid and gas jets, even if it would be due to different physical phenomena.
     
  10. Sep 13, 2014 #9

    rcgldr

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    It's falling water that would break up into droplets instead of converging into a narrower stream if there was no suface or internal tension. This is different than water in a steady flow.
     
  11. Sep 13, 2014 #10
    i still dont get why a certain type of convex curvature holds longer in the stream as it is being pushed further away than others
    http://ss.kwix.eu/knus94u2js [Broken] this spoon weighs 3,7 gram
    http://ss.kwix.eu/mm47alyqj7 [Broken] this one is heavier(like 5 gram)
    the heavier one can be pushed further at a greater angle before it separates from the flow, so the force required to lift it is greater.why?i thought this happens due to different(less extreme)type of curvature which leads to different forces/pressure distribution
    note: as they are getting pushed the surface that s touching the stream decreases
     
    Last edited by a moderator: May 6, 2017
  12. Sep 14, 2014 #11
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