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February 14, 2012, 18:08 

#21  
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Amin Shariat KHah
Join Date: Apr 2011
Location: Shiraz
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Quote:
I think transient flows are not easy to understand Thanks Very Very much 

February 14, 2012, 18:12 

#22  
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Filippo Maria Denaro
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Quote:
as a matter of fact, there is not much difference between steady or unsteady flows, the continuity equation is a constraint that does not have an unsteady term. I hope that the reading of those papers can help you 

February 15, 2012, 12:01 

#23  
Member
Amin Shariat KHah
Join Date: Apr 2011
Location: Shiraz
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Quote:
A basically maybe very simple question: If I discretize my equation like this: (U_n+1  U_n)/dt = f( U_n+1 ) And solve my equation with a iterative equation system solver, in first iterate I calculate a U that named U_n+1 but it’s not actually U_n+1 And in next iteration I replace U_n with U_n+1 and continue to solving problem, Which means marching in TIME !!!!! and is not suitable because I don’t want to march in time till my iteration reach a true answer for U_n+1 This is a conceptual subject which confusing me. I think the problem is in discretizing and I should change it. Is my question foolish and I’m wrong in a simple thing or it is a usual question? What is the solution of this problem in CFD? 

February 15, 2012, 13:38 

#24 
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Filippo Maria Denaro
Join Date: Jul 2010
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As you can see, your way to integrate needs to face with the continuity constraint. I think you need to clarify the ideas about the existing methods such as simple,simplec, piso, fractional, etc. You will find some help starting the reading in the book of Ferziger & Peric. Once assessed the method you want to explore you can go more indeep the topic.


February 22, 2012, 05:00 

#25 
Senior Member

Dear amin,
i really reccomend to follow the suggestions from Filippo (Hi Filippo). However,to quote you, "you are wrong in a simple thing". That is, when you need to iterate at FIXED time in: U_n+1 = U_n + dt * f(U_n+1) because of the implicitness of f (and probably the non linearity), you don't have to touch U_n, the first term in the second member or, as you said, you would be advancing in time (which is not what you want). At the end of the iterations your U_n+1 will satisfy the above relation with respect to U_n, which is what you want to achieve in a single time step dt. This is general and even apply to linear parabolic equations. 

February 22, 2012, 05:04 

#26  
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Filippo Maria Denaro
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Quote:
Ciao .... 

February 22, 2012, 09:03 

#27  
Member
Amin Shariat KHah
Join Date: Apr 2011
Location: Shiraz
Posts: 86
Rep Power: 8 
Quote:
You would be a good teacher you are the first one that understood what i'm saying my mistake was too simple as you said I found it after reading Ferziger CFD book and I became sure after reading your post Thanks very very much guys 

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