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April 19, 2013, 19:25 
Steady and Unsteady nature

#1 
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Natali
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Hello,
I am very beginner in CFD, sorry that I ask this question. I am wondering how to compare a problem in steady and unsteady. Because I think any problem that is steady can be solved unsteady as well. However, I am running a case, but heat transfer coefficient in steady state is much much larger than that of unsteady. The problems were runned for some hours and when I see the transient solutions, it seems they reached to a steady state. So, if the transient solution reached to steady state, why the results of steady and unsteady heat transfer coefficients are not the same? Again, sorry if my question was childish but I really need your experts. Natali 

April 19, 2013, 19:46 

#2 
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Agustín Villa
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Hi Natali;
I'm not an expert, but the difference between unsteady and steady is that you are not studying the flow at the same time: steady, the flow becomes "fix"; meanwhile unsteady, the flow changes along the time, until it arrives the time unsteady flow approachs the steady conditions. For this reason, unsteady and steady heat transfer coefficients are not the same: you're not studying the same flow conditions. I hope you understood my explanation. 

April 19, 2013, 20:16 

#3 
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Natali
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Thanks for your answer,
But, after some times, the unsteady flow should be steady. Am I right? So, why at that time the characteristics are not the same? like heat transfer coefficient? 

April 20, 2013, 01:17 

#4 
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OJ
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Not all the flows are steady "after some time". eg formation of vortices behind the bluff body. These vortices will keep changing their positions and hence this flow can never be steady.
Even unsteady flow can be categorized into, periodic and chaotic. Periodic flows have a pattern of repetition after certain period and thus has a certain dominant frequency. In chaotic flow, there is no single dominant frequency but there are set of frequencies. Then you need to use statistical techniques like FFT to figure out the relatively dominant frequencies from the flow characteristics. OJ 

April 20, 2013, 10:03 

#5  
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I agree with comments of OJ.
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Last edited by Far; April 20, 2013 at 10:44. 

April 20, 2013, 17:15 

#6  
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Natali
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Quote:
Yes, all the conditions are the same. I did grid study as well. So, there is no place to consider difference between the results. I saw that the flow characteristics remain unchanged for a long time in a transient solution. Then I decided to call the flow steady. So, I expected to see the same results from the transient simulation at this time and the steady sate simulation. But, the results are much different. 

April 20, 2013, 23:38 

#7 
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Maybe your steady case has not converged yet. Have you tried to switch from unsteady to steady simulation with the case&data you obtained from longtime transient simulation?


April 21, 2013, 00:23 

#8 
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Natali
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I didn't try that but for sure the steady state solution is converged. I checked that by observing several flow characteristics, as well as checking the changes in mass flow rates, friction coeff, heat transfer coeff, etc.


April 21, 2013, 11:33 

#10  
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Natali
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Quote:
Regarding the convergency, I just check the changes in flow parameters and since for a long time no changes happen for their values, I decided that the flow became steady. Is that enough to check the convergency? Or do I have to check anything else? Thanks for all your times and care, Natali 

April 21, 2013, 11:47 

#12 
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Natali
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April 21, 2013, 11:51 

#13 
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I am saying if your case is steady state then it should give same results with steady, unsteady with 0.1 s time step, unsteady with 0.005 s time step and 1e06 s time step. With 1e06 time step you may need longer time to reach steady state while with larger time step you may get convergence in less time.


April 21, 2013, 11:56 

#14  
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Natali
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Quote:
I really appreciate you correct me wherever I am wrong. Thanks, 

April 21, 2013, 12:40 

#15  
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April 21, 2013, 12:43 

#16 
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Natali
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