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Old   April 29, 2008, 04:04
Default pipe turbulent flow
  #1
Hao
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i'm using RSM to simulate a 5 m/s turbulent flow through a pipe. The geometry is 0.02*0.4 (width*length). I couldn't get a fully developed flow by one time. So I ran the simulation first time and got the profiles at the outlet. Then i imported the profiles as the inlet conditions in the second simulation. Is this method right? It seems that all the turbulence properties varies little along the pipe after the second one. However, one thing is just weird. The wall shear stress has a sudden rise at the very outlet when the outlet type is pressure outlet. With a outflow-oulet, the wall shear stress has a sudden drop. could anyone give me some suggestion? Thanks in advance.
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Old   April 29, 2008, 05:11
Default Re: pipe turbulent flow
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Ebrahim
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you can only use outflow boundary when you have a fully developed velocity profile at the outlet (see manual of fluent) may be thats why you have a drop?
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Old   April 29, 2008, 21:20
Default Re: pipe turbulent flow
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AAA
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Hi

You can use this method to acheive fully developed flow in your case. Regarding the unstable wall shear stress at the outlet, it seems that you have back flow there. The Pressure-outlet BC is always more stable than the outflow BC.

Experiments show that the mean velocity profile becomes fully developed within 25 to 40 pipe diameters from the entrance, so you are on the borderline (0.4/0.02=20). The details of the turbulent motion however may not be fully developed for 80 or more pipe diameters. In your case, this might be a large distance, so stop the process whenever you get a fully developed profile.

what profile values are you writing? velocity only? Turbulence intensity?

Regards

AAA
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Old   April 29, 2008, 22:19
Default Re: pipe turbulent flow
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Hao
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i use velocity and kinetic energy and dissipation profiles at the inlet. i still cannot get the fully develpoped flow. though the velocity remains stable down the stretch, the turbulence properties are still changing (rising or droping, i cannot remember).
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Old   April 29, 2008, 22:30
Default Re: pipe turbulent flow
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Hao
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i also think i have back flow there.
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