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Species transport:segragated vs.coupling

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Old   January 18, 2006, 13:34
Default Species transport:segragated vs.coupling
  #1
Jack
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Hi, all

There are two ways to model reactions using species transport equations.One is to obtain the flow field first, then disable flow equations and solve transport equations in a fixed flow field. Another way is to solve flow and transport equations simulatously. Can someone please explain the difference between the two ways. Will we get a same result via these two methods?

Thanks

Jack
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Old   January 18, 2006, 19:34
Default Re: Species transport:segragated vs.coupling
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Renato.
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It depends... you'll need to ask yourself (and be able to answer) some questions:

1) Can the transport field interfere in your flow? I mean, if you're transporting temperature you'll probably have buoyancy driven forces in your flow, and if these forces can't be neglected you'll have to solve both problems coupled.

2) Do you have a steady state solution of your flow field?

3) Is your flow turbulent?

4) Do the species have similar properties? Imagine if you're transporting 2 gases, one heavy and the other very light, in a low velocity flow field the flow could be driven by the two gases interaction instead by the flow field.

Certainly other guys here could improve this list but, first of all, use your sense to ask yourself about the problem you're solving and what assumptions should be done.

Regards

Renato.

NOTE: Let me extend your question...

We could couple or segregate problems in several ways... we could solve flow and transport degrees of freedom at the same time (only one system of equations), we could solve firstly the flow and secondly the transport within the same time step, we could do sub cycles between flow and transport considering the solution changes of the solvers, we could solve each degree of freedom separately, etc, etc, etc...

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Old   January 19, 2006, 01:20
Default Re: Species transport:segragated vs.coupling
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Bak_Flow
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Hi Jack,

If the physics is segregated then solving one field like velocity and pressure and "freezing" it to solve the other "passive scalar" fields can be very economical.

For coupled physics segregated or coupled methods should be able to get the same solution but the efficiency depends on how close the coupling is or more appropriately how close the "numerical coupling" is to the "physical coupling".

Regards,

Bak_Flow
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Old   January 19, 2006, 11:40
Default Re: Species transport:segragated vs.coupling
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Jack
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Hi, Renato and Bak_flow.

Thanks a lot for your replies, which are very helpful for me to understand the question.

Renato: Could you please explain if(and how) the turbulence can significantly affect the results from segregated and coupled approaches?

Bak_Flow: I am not quit clear of the two terms you used: physical segregated and physical coupling. Could you please clarify them with simple examples?

Thanks again!

Regards!

Jack

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Old   January 19, 2006, 12:18
Default Re: Species transport:segragated vs.coupling
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Renato.
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Well, let's say that in turbulent regime you'll have only an "averaged steady state" field. In this case it's hard to be sure about if your transported field would interfere (or be interfered) by the flow.

regards

Renato.

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