Is circulation real?

Discussion in 'Hydrodynamics and Aerodynamics' started by Mikko Brummer, Jan 25, 2013.

  1. Mikko Brummer
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    Mikko Brummer Senior Member

    Even without lift and circulation, the velocity field around a profile moving in the fluid standing still is surprising, when looked in the reference frame of the outside observer.
     

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  2. Mikko Brummer
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    Mikko Brummer Senior Member

    Here's JPGs for those who don't want to download the large videos. First, with the wing/keel moving in the fluid that is inert - like a keel slicing through water, in the reference frame of an outside observer who is standing ashore. This reveals the circulatory motion of the flow around the wing or keel profile.

    Second, with the wing stationary and the wind blowing, like in the wind tunnel, or in the sailor's reference frame moving with the boat. This shows the familiar streamlines (pathlines) around the foil, and the velocity vectors, which look so different from the first case.

    If you add the free stream (uniform flow) vectors into the vectors shown in the first case, you end up in the flow field of the second case (as far as I understand).

    The case for the sail boat and its sails is still more complicated than the wind tunnel: There, both the wind and the boat are moving, in different directions, forming the apparent wind striking on the sails.
     

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  3. Joakim
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    Joakim Senior Member

    There is no difference between moving fluid vs. moving wing. Why do you want to simulate them time dependent while the problem is totally stationary (unless you want to simulate velocity changes)? For a CFD simulation it is typically much easier to use a stationary wing in a steady flow field. Then you can view the results in any frame of reference you like. In most CFD softwares changing frame of reference for post processing is very easy.

    I didn't find anything surprising in the zero lift situation. You can see the same from streamlines as well. Of course the flow has to go around the wing with higher velocity and horizontal component in the velocity.
     
  4. philSweet
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    philSweet Senior Member

    Isn't there? Would the solution look the same without the contribution of the starting vortex? You could, of course, accelerate the flow around a fixed wing.

    I think dynamic modelling is at the heart of Mikko's interests. And I was happy to be able to look at these things from that perspective.

    Mikko, I'm a bit confused by the boundary conditions on the upper and lower surface of the model. It looks like the problem is calculated for the viewing area frame by frame, and the upper and lower boundary is sourcing and sinking the material willy nilly. Are there constraints on the boundary's ability to source and sink material? And are these applied in one dimension only (mass balance) of two dimensions (mass and energy). Just eyeballing it, there appears to be an energy balance mismatch.

    Shouldn't there be more updraft in front of the wing?
     
  5. Mikko Brummer
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    Mikko Brummer Senior Member

    I wanted to bring out the flow pattern revealing the circulatory flow. I realize changing the coordinate system is just mathematics, but my post processor does not have that feature. Also, the solver is inherently transient, there is no stationary option. And setting up motion is just as simple as static in it. To me, to see what the flow field around a moving foil in stationary fluid (like the keel of the sailboat in the water) was a revelation. So indelible in my mind is the flow pattern of the moving reference frame (wind tunnel type) - I would expect it is so to 99% of us ;-).

    You probably can read the streamlines so well, but I don't. I think the common belief would be the foil just pushes the water neatly aside as it slices through the water, not that it first shoves water forward and out, then the water is bending back on the rear part of the foil at nearly straight angles... I never anticipated it from the streamlines...
     

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  6. Mikko Brummer
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    Mikko Brummer Senior Member

    The upper & lower boundary conditions are periodic, to model a larger domain than what it really is... it's like the flow that goes out from the bottom comes back in from the top - not very realistic, but it makes for a faster simulation when the domain can be smaller. The simulations shown are very coarse, and in 2D, but they should reveal a pretty realistic flow pattern. They run almost in realtime on my system.
     
  7. Mikko Brummer
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    Mikko Brummer Senior Member

    Here's a run on 2 sails, to see if the jib really has its own circulation like Arvel Gentry professed in 1971 or so (http://www.arvelgentry.com). It's not very clear in the animation at such a small scale, to keep the file size reasonable, but zooming in the still pic clearly shows how the mainsail's and the jib's circulations oppose each other in the slot between them.
     

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  8. philSweet
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    philSweet Senior Member

    rather awkward when your purpose is to model circulation.;) Just for grins, what would happen if you reversed the y component of the flow when you pumped it back in at the top?
     
  9. Joakim
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    Joakim Senior Member

    Streamlines are quite easy to read. The principle is that the excact same volume flow goes between each streamline. It's like a set of channels. Thus the flow turns with the channel and the narrower the channel is the higher the velocity. Don't confuse streamlines with vectors. Stremlines only exist in 2D.

    Yes the foil pushes (or slows down) water in front of it and drags (or slows down) water behind it in the wake. Even a flat plate would do that due to friction, but a thick foil does it more.

    That is the reason why you should never put the log transducer close to the keel. It will read differently depending on the leeway. If the transducer is off the center, it will also show differently on different tacks. I would put the transducer at least one chord lenght in front the keel unless it becomes too close to bow.

    If you want to model foils or sails accurately, you need to have a long part of the domain after the foil and you can't use periodic boundary conditions. If you can't use steady state, just calculate long enough until the flow is steady state with constant boundary conditions (set the incoming velocity and keep the foil stationary).
     
  10. DCockey
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    DCockey Senior Member

    Streamlines exist in three dimensional flows.

    Every point in a flow, whether two dimensional flow or three dimensional flow, has a streamline passing through it. A streamline is an instantaneous curve defined as a curve which is tangent to the local velocity at every point along its length.

    Perhaps you are confusing "streamlines" with "stream function". Stream function is a 2D concept, and lines of constant stream function are 2D streamlines.

    Why can't periodic boundary conditions be used? Both periodic outer boundary conditions and constant outer boundary conditions will affect the results. The further the outer boundaries are from the foil/sail the less the influence of either type of outer boundary condition will be.
     
  11. DCockey
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    DCockey Senior Member

    The arrows in Mikko's videos and images appear to show the direction of the velocity but not the magnitude.
     
  12. Joakim
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    Joakim Senior Member

    Yes I was confusing them. I think in Fluent streamlines are called pathlines (for zero mass particles). In 2D they are the same, if equally spaced stream/pathlines are used in constant incoming velocity.

    For the periodic boundary conditions I was not reading carefully enough, since I thought Mikko was talking about left and right boundaries not upper and lower. But I still think it is not an accurate way to model. You should have all the boundaries far enough and model them as constant flow or symmetry planes.
     
  13. DCockey
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    DCockey Senior Member

    A pathline is the trajectory of a single fluid particle; ie the path a single fluid particle takes over time.

    A streakline is the instantaneous curve formed by all the particles which have passed a single point. It is analogous to the curve formed by injecting dye into a flow.

    Pathlines, streamlines and streaklines exist in all flows, both 2D and 3D, steady and unsteady. They do no coincide in general unless the flow is steady. There is a good discussion (at the time of this post) of streamlines, pathlines and streaklines and an animation showing the difference between them at http://en.wikipedia.org/wiki/Streamlines,_streaklines,_and_pathlines
     
  14. Mikko Brummer
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    Mikko Brummer Senior Member

    I expressed myself badly - the periodic wall is a completely valid boundary condition, improving results when using small domains. I don't think it would change much the flow pattern even if I put rigid walls as upper&lower boundary, but they would effect more the flow around the foil.

    I ran quickly a sim with twice as large a domain, there was little apparent change. Also added a rudder behind the keel
     

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  15. Mikko Brummer
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    Mikko Brummer Senior Member

    Yes - the arrows are uniform in length but the color indicates the magnitude of the velocity.
     
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