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How to pause a transient simulation and resume without time step advancement? 

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January 6, 2012, 08:14 

#21  
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Ok Arjun I see your point but it depends how you proceed in your calculation procedure. Rhie and chow correction is used to improve the mass flux. Lets call this improved mass flux: m+ while m would be the mass flux without the rihie and chow correction. m+ = m + (rhie and chow correction) m+ could be used (1) only when you compute the AK, (2) only when you compute the right handside of P' equation, (3) in the both cases If you use m+ to compute AK I agree with you there is a catch 22 as you said! but if you use m to compute AK and m+ to compute the right hand side of P' equation you do not have any problem anymore. It has not been demonstrated yet as far as I know, that using m+ to compute AK was superior to only use m+ to compute the right handside of P' equation. what do you think about this ? 

January 6, 2012, 09:40 

#22  
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Now again there are two things: 1. m+ is used in momentum equation, which is the purpose of rhie chow treatment. That is to avoid checker board oscillations. So the mass flux in momentum equation and continuity has to be same. For this reason, to my knowledge all the solvers use m+ in momentum equation. That is AK is calculated from m+ and not from m. 2. Just for fun last month I did try using m and not m+ in momentum equation and used m+ in pressure equation. This method is not stable. It does not converge in more than 95% of cases. It converged for only 1 case out of few i tried. So using m in momentum equation does not work for collocated arrangement. 

January 6, 2012, 14:19 

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If you read the bible: Computational Methods for Fluid dynamics from Peric you will see that Rhie and chow correction (m+) is applied in the mass flux of the right hand side of pressure correction equation. Quote:
Because in my code, I use m+ in pressure equation only and it works fine for every cases. In the codes of Peric (comet and others ) m+ is used in pressure equation. and I could risk my hand that it is the same in StarCD 

January 6, 2012, 18:53 

#24  
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Arjun
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Interesting you say so, but Mr. Peric himself disagree with you. According to Mr. Peric in starccm+ the m+ is used in momentum equation and anyother place where mass flux is required. (This is based on my personal discussion with him). Further I do not think there is a bug in my program because it is as stable as any commercial solver out there. I have run it with very very difficult or bad meshes still able to converge with using m+ in momentum. Could you point me to any paper or study where somebody has used m only in momentum equation for collocated grid. I tried this approach because I felt it would be more stable but it turned out to be opposite of what I thought. 

January 7, 2012, 14:46 

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I just follow what is written in his book, and in his codes too... But everybody has right to change his mind... ;) Quote:
I just mean that it's not the only alternative.. In my code I use m+ only in right hand side of pressure correction equation and it works fine. But it is also possible that if I add m+ in momentum it works too and perhaps better...I'm just surprised that when you implemented m+ in pressure correction like me it didn't work. Quote:
I will check the papers of Date and send you the right references Quote:
Now I have in mind a paper where these different ways of implementing the Rhie and Chow correction have been analysed in terms of accuracy and convergence performance... As far as I remember the conclusion was that the three ways (m+ in momentum only, m+ in pressure correction only, m+ in the both) work more or less the same way. I have to check it and I will come back to you. 

January 7, 2012, 20:55 

#26  
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Arjun
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But since starccm+ is made by many people and it is not he alone that decides. Quote:
Thank you, that paper might point out what I did that was wrong or may be what mistake I made. It will be good read, ideas are always welcome. 

January 11, 2012, 11:34 

#27  
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I'm back.. here are the references of the papers I was speaking about. A.W. Date, "Fluid dynamical view of pressure chechboarding problem and smoothing pressure correction on meshes with colocated variables", International journal of heat and mass transfer 46, (2003) 48854898 A.W. Date, "Complete pressure correction algorithm for solution of incompressible NavierStokes equations on a nonstaggered grid", Numerical Heat Transfer, Part B, 29, (1996), 441458 B. Yu, WQ Tao, JJ Wei, Y. Kawaguchi, T. Tagawa, H. Ozoe, "Discussion on momentum interpolation method for collocated grids of incompressible flow", Numerical Heat Transfer, Part B, 42, (2002), 141166 Papers of Date implement Rhie and chow interpolation in the mass conservation i.e in the right hand side of pressure correction equation as I usually do. In paper 3, the authors examined 3 different practices of implementing Rhie and Chow interpolation. A refers to what you do i.e. interpolation in the mass flux of momentum equation, B refers to interpolation in the right hand side of pressure correction equation, C is the same as B including divergence of velocity in the AP coefficient while in A and B it is not. Happy reading.. I believe this discussion should be switched in Rhie and Chow specific discussion in this forum 

January 11, 2012, 14:56 

#28  
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Arjun
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I have pdf form of book from Anil Date so I will look into it as I get time.
I downloaded this paper though. Quote:
I will look into Anil Date explanation too when I get time.  Edited to add: I looked into Date's book. At least in the book , the word Rhie appears only once and that is in bibliography. The procedure that he explains in the book is some what very similar to what CFX4 used to do and later on CFX5 also does. Which is different than Fluent starCCM approach. (I read CFX4 manual in year 2000 so I do not remember it very well now but I am sure they both are same approach). This method is stable as demostrated by various CFX versions, but I am not sure it is much relevant to discussion at hand because this is not the most popular way of doing things. PS: I do have some issues how Date explained in his book but that is not much use discussing. Last edited by arjun; January 11, 2012 at 16:43. 

January 12, 2012, 08:30 

#29  
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I read this paper carefully in the evening and it seems you are correct, in approach B and C they do interpolate velocity with out rhie chow terms for momentum. I would try this approach again when i get time. 

June 20, 2014, 01:43 
Fluent Resume/Restart. How to do it?

#30 
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Sarthak Nag
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If only the initial case file and all the data files are saved and suddenly your system shuts down, can you restart or resume the program from the time step you have left, means from the time step the computer shut down.?? My computer shut down after about 80%time steps and now I have to do the simulation again.


June 20, 2014, 02:36 

#31 
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Daniele
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Open the cas, then open the last dat file and your simulation will resume from the latest time.
Daniele 

June 20, 2014, 07:50 

#32 
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Sarthak Nag
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June 27, 2014, 18:40 

#33 
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Manuel
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I got lost and I have to say I didn't read through all the posts...
But to answer the 1st question which gives title to this thread: It is possible (in FLUENT, at least), to resume calculations in the same timestep where you stopped. TUI command: solve/iterate [number_of_iterations] This, in steady state, just iterates. In transient, it adds iterations to the time step and will never step into the following timestep (it will stop when the number of iterations is reached, or upon convergence criteria). This will NOT resume the transient calculations, this just iterates in the time step you are. Therefore, if you want to resume your calculations completely, it is a 2step task: solve/iterate (to finish your last time step where you paused) solve/dti (to continue your unsteady calculation) Hope it helps 

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