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Straight duct flow in CFX 5.2

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Old   October 20, 1998, 12:11
Default Straight duct flow in CFX 5.2
  #1
Bo Jensen
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Hi!

I was just woundering if anyone have had troubles calculating on laminar flow in a striaght duct using CFX 5.2?

The results I produce using both advanced front or normal isomesh gives results which are not comparable to the ones found analytical. If you look at the maximum velcoity down through the duct the velocity is to low. Probaly because of a not fully developed boundary layer. What could be the problem? Has anyone had the same experiences?

Bo
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Old   October 21, 1998, 21:45
Default Re: Straight duct flow in CFX 5.2
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Sung-Eun Kim
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Dear CFD user,

If you want to predict a fully developed profile in a finite length of duct with a uniform velocity specified at the inlet, the duct should be sufficiently long, especially if the flow in question is laminar. The same is true in laboratories.

You may want to check if there's a "periodic" boundary condition in the code you're using. With the periodic boundary condition, you will only have to specify the mass flux such that gives the desired Reynolds number. it will give you a fully developed flow without using too long a duct.

Good luck, Sung-Eun
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Old   October 22, 1998, 09:12
Default Re: Straight duct flow in CFX 5.2
  #3
John C. Chien
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Run several cases with different Reynolds numbers, then you should be able to get some feeling. When running a laminar flow problem, it is important to know the Reynolds number of your problem, say Re=1.0, Re=10, Re=100, Re=400,Re=1000, etc. You can use either the diameter or the length of the pipe ( or duct) as the length scale. Think in terms of the non-dimensional parameters such as Reynolds number, Mach number etc, the flow likes to be treated this way. It will be more user-friendly to you, I think.
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Old   November 25, 1998, 07:15
Default Perhaps not a problem
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Bo Jensen
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Hi!

After further investigation of this problem I have found out that the problem could be overcome by using more elements in the duct.

I will return later with an better answer, as there are a couple of more testes that I would like to performe first.

Bo
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