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cannot set exact 'endtime' in controlDict when using a steady state solver 

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July 23, 2018, 18:16 
cannot set exact 'endtime' in controlDict when using a steady state solver

#1 
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Pu Gong
Join Date: Jul 2018
Location: London
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Hi everyone
I am new to openFoam and I am doing simulation on CO2 diffusion in a room. I want to know the distribution of CO2 concentration in the room after 1 hour and For time saving I used a steady state solver. I firstly set 3600 for the 'endtime' and 10 for 'delTaT' in controlDict for the simulation. Then I doubled both the 'endtime' and 'delTaT' and do the simulation again and I got the exact same results for the two modeling! It seems that in the steady state simulation only endTime/delTaT make sense since it control the iterations of the modeling. And I guess set 3600 for the 'endtime' doesnt mean the endtime in modeling is 1 hour? What should I do if I want to set the exact endtime using a steady state solver? 

July 23, 2018, 21:46 

#2 
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Peter Baskovich
Join Date: Jul 2014
Posts: 127
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It's a steady state solver mate, the time means nothing.
Set your deltaT to 1 so at least you can use it as an iteration counter. 

July 23, 2018, 22:24 

#3  
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Pu Gong
Join Date: Jul 2018
Location: London
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Quote:
I do set the deltaT to 1 for the simulation but I just have no clue about the result. I think what I get for each folder (1,2,3...) is the room condition after (1,2,3...) iterations, like you said, an iteration counter. But what i really want is to know the time interval for each iteration, or the room condition after xxx seconds. Basicly you are saying that the concept of time is not exist in steady state modeling? So does it mean that we could only use a trasient solver to implement exact time control? I tried trasient solver but it is really time consuming since I need to set a very small delTaT to limit courant number under 1. So.. are there any other ways to do that? 

July 23, 2018, 22:51 

#4  
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Peter Baskovich
Join Date: Jul 2014
Posts: 127
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Quote:
A transient solver is what you need if you want to watch the fields develop over time. There are some things you can do to speed it up but it will always be more time consuming than steady state. 1. Use geometric simplification Can you run in 2d and get a meaningful result? Is the case axisymmetric? Is the case mirrored? 2. Coarsen you mesh and refine only where needed. 3. Use wall functions 4. You can run CFL > 1 with implicit schemes at the cost of accuracy 

July 24, 2018, 05:13 

#5  
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Robert
Join Date: May 2015
Location: Bremen, GER
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Quote:
We did not talk about the solver in use here. From my experience, if you only want to solve the diffusion of a species, try using scalarTransportFoam. It usually runs pretty quickly.
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