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Old   November 16, 2000, 10:59
Default Help: Student in over his head...
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Hi, I could really do with some help here!! I'm using fluent for an assignment at university, we're modelling a sudden expansion problem with a heated lower wall (we're comparing our results to JC Vogels experimental paper).

Anyway, i've got a nice mesh (i think) and have calculated some results but i can't for the life of me calculate the re-attachment point of the boundary layer. Am i right in thinking that the re-attachment point is when the wall shear along the lower wall is zero? if so, how do i get fluent to give me this info?

I've tried plotting wall shear stress along the lower wall, but i've got the same number for 2 different designs. One with an upstream length of 500mm and one with 1m upstream, i've been told that changing this length should move the re-attachment point.

And don't even mention my lecturer... i asked him this and he just asked me 3 questions in reply?!?!

Cheers for any help! Ben,
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Old   November 19, 2000, 11:48
Default Re: Help: Student in over his head...
John C. Chien
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(1). Even with a PhD and many years of experience, I don't think, there is a simple solution to this problem with flow separation. (2). Any time, you have flow separation, the current state of the art simply can't handle the problem accurately. (yet) (3). And this problem has been used as a standard problem to develop improved turbulence models. (4). You are not going to get the right answer, regardless of what you do, changing the inlet length (thus changing boundary layer thickness approaching the step), changing the mesh distribution and density, changing the turbulence models. (not to mention the code limitations in handleing the nemerical solutions and algorithms.)(5). If someone is serious about the CFD, then he should try very hard to solve this problem first, with heat transfer. There is nothing wrong with you and your teacher. We simply don't know how to solve this type of flow separation problem accurately,yet. So, keep trying.
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