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Transient Inlet boundary condition - fully develop

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Old   November 27, 2006, 13:15
Default Transient Inlet boundary condition - fully develop
  #1
Omer
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I am working on a pipe junction, with the main header of 54 inch diameter and 551 inches length. I need to run a transient turbulent case. Since my domain is huge and I would also need to resolve the boundary layer, the number of mesh elements are huge.

My questions are

1.What do I understand by fully developed condition in a turbulent transient problem?

2. My mesh is already huge, if I add an entrance length to the pipe, it becomes impractical for me, doesn't it? Considering that I would need more elements.

3. Can I solve for a steady case problem seperately with another model, and use the solution from this as a profile for my header inlet condition? But then again, will this be ok? Because my problem is transient. Back to question one, what is fully developed condition in transient problem?

Thanking the forum in anticipation

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Old   November 27, 2006, 17:19
Default Re: Transient Inlet boundary condition - fully dev
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Glenn Horrocks
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Hi,

All turbulent CFD simulations need to resolve the boundary layer, you have nothing unique there. The question should rather be do you need to resolve down to y+=1 or is a wall function approach acceptable.

For flows in large structures such as this usually the Reynolds number is very large, meaning that the wall function approach is usually good. In that case putting the first node in the range 20<y+<200 should be OK, and 10 nodes in the boundary layer. This is described in the documentation.

To answer your direct questions: 1) Fully developed flow is flow which does not change in the flow direction.

2) Possibly, but we will need more details on your simulation to confirm this. Serious CFD does require huge computer resources.

3) Yes, that sounds like a sensible approach, however it depends on exactly what you are trying to model. Again more details would be useful.

Glenn Horrocks
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Old   November 28, 2006, 06:20
Default Re: Transient Inlet boundary condition - fully dev
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Omer
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Thanks Glenn.

Yes, I will be using the automatic wall function approach , with an SST model, with compressible methane in the pipe . I am thinking about using the power law approach for the boundary condition. Would that be a good way to do it?

Thanking You Again
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Old   November 28, 2006, 17:22
Default Re: Transient Inlet boundary condition - fully dev
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Glenn Horrocks
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Hi,

Do you mean use the power law to describe your inlet flow profile? Whether that is a good idea or not depends on whether it accurately describes your inlet flow. If it accurately describes it then it sounds like a good idea.

Don't forget you will also need to set values for turbulence parameters and temperature. You will to set accurate values for these parameters as well.

Glenn Horrocks
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