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Time marching in incomprerssible flow

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Old   May 25, 2011, 13:47
Default Time marching in incomprerssible flow
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al mazdeh
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There was a discussion a while back about how CFX treats unsteady incompressible Navier-Stokes equations.
The question was that as the Time marching methods cannot be applied directly to incompressible flows because the governing equations are not hyperbolic so whether CFX uses an artificial compressibility method in order to treat the inviscid fluxes using the traditional characteristics-based schemes.
I have searched CFX manual I had no luck finding any info regarding time marching for incompressible flow
I wonder if u can shed some light on this, I am interested in mathematics because if we know how this is being dealt with CFX then we can make better judgment about our results
Thanks
Al
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Old   May 25, 2011, 19:04
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Glenn Horrocks
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CFX uses backward eular scheme for the transient term, either first or second order. You can march this out in time just fine. CFX does not use artificial compressibility.

This is all described in the theory documentation.
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Old   May 26, 2011, 10:07
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al mazdeh
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GHORROCKS,
I know u from your previous posts, that u have a vast knowledge of the tool-so please help me understand this

u are saying that the solution is not done differently in case of compressible and incompressible

how the time marching is applied on a continiuity equation that has only space variables-

also if u can please point me to the some reference that CFX uses for its solver


in other posts, I saw that u mentioned pressure and velocity are fully coupled in CFX is that why there is no need for artificail compressibility-

If so please tell me how pressure and velocity are coupled for the incompressible case-thanks
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Old   May 26, 2011, 17:22
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Obviously there are differences when compressibility is activated, but that is just to allow the density to vary. See the doco for details.

Everything you are asking is discussed in the CFX Theory manual.
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Old   June 24, 2011, 15:20
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al mazdeh
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Quote:
Originally Posted by ghorrocks View Post
Obviously there are differences when compressibility is activated, but that is just to allow the density to vary. See the doco for details.

Everything you are asking is discussed in the CFX Theory manual.
Hi Glen,

I have spent a month reviewing literature and cfx manual along with fluent documentation-

Finally I know some basics from all of these

I can tell that CFX uses pressure-based coupled solvers but I can not find much about the math on any of these manual-fluent offers more info


Now I like to go deeper and would like to know some kind of clasical ref where CFX was developed based upon -

to be honest with in my search I came up with so many articles on this topic but I like to get closer to the one that CFX uses

this is very critical for me since my dissertation will not go forward if I can not answer question on CFX solver theory
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Old   June 25, 2011, 06:53
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The CFX solver is not as well documented as Fluent. Also, as it is a little unusual it is hard to find information about how it works. The documentation is pretty good and the references it links to are important too. Also the CFX solver is based on one developed for an old CFD code called TASCFlow. You might find more information on documentation for tascflow.
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