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Old   February 18, 2005, 08:58
Default boundary layer
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I am reading something on turbulent boundary layer. What confuses me is about zero-gradient pressure boundary layer or constant-pressure boundary layer. I am wondering how this is realised in both physical and numerical experiments, and why this concept is so important that it is referred all the times.

Many thanks, Best wishes, -thomas
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Old   February 18, 2005, 15:19
Default Re: boundary layer
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Imagine a flow from left to right. Place a flat plate in the direction of the flow.

The pressure is the same from the beginning to the end of the plate, in free field. This is the portion of the flow that isn't influenced by the presence of the plate.

There is no pressure gradient in the direction of the flow and a boundary layer (BDL) will exist above (and below) the plate. This is the type of BDL you are referring to.

For this specific case, the Navier-Stokes equations can be solved if some assumptions are made. Google on "Blasius profile" for more info an this type of BDL.

Flow over a profile is subjected to a pressure gradient and the layer can separate from the wing because of adverse pressure gradient. Look for "Falkner-Skan equations" for a first solution to this problem.
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Old   February 26, 2005, 13:53
Default Re: boundary layer
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Hi Yves, Thank you very much for making effort to explain with plain words that I am able to understand.

I can imagine the free stream at some point high above the plate is not affected by the plate, and therefore flow without pressure gradient present, pretty much similar to a physical object moving freely without any force present. The free stream drag the flow below to move forward, which is something like Couet flow (if it is wrong, please point it out). This is an idealised picture, I guess.

Could you please give me a picture of a flow with pressure gradient, preferably in plain words that I can be able to understand.

Many thanks, Best wishes, Thomas
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