
[Sponsors] 
April 19, 2013, 19:25 
Steady and Unsteady nature

#1 
New Member
Natali
Join Date: Apr 2013
Posts: 21
Rep Power: 4 
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 
Member
Agustín Villa
Join Date: Apr 2013
Location: Brussels
Posts: 42
Rep Power: 4 
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 
New Member
Natali
Join Date: Apr 2013
Posts: 21
Rep Power: 4 
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 
Senior Member
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 475
Rep Power: 11 
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  
Super Moderator
Sijal Ahmed Memon (turboenginner@gmail.com)
Join Date: Mar 2009
Location: Islamabad Pakistan
Posts: 3,905
Blog Entries: 6
Rep Power: 38 
I agree with comments of OJ.
Quote:
Quote:
Quote:
Last edited by Far; April 20, 2013 at 10:44. 

April 20, 2013, 17:15 

#6  
New Member
Natali
Join Date: Apr 2013
Posts: 21
Rep Power: 4 
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 
Senior Member
Join Date: Aug 2011
Posts: 315
Rep Power: 11 
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 
New Member
Natali
Join Date: Apr 2013
Posts: 21
Rep Power: 4 
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  
New Member
Natali
Join Date: Apr 2013
Posts: 21
Rep Power: 4 
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 
New Member
Natali
Join Date: Apr 2013
Posts: 21
Rep Power: 4 

April 21, 2013, 11:51 

#13 
Super Moderator
Sijal Ahmed Memon (turboenginner@gmail.com)
Join Date: Mar 2009
Location: Islamabad Pakistan
Posts: 3,905
Blog Entries: 6
Rep Power: 38 
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  
New Member
Natali
Join Date: Apr 2013
Posts: 21
Rep Power: 4 
Quote:
I really appreciate you correct me wherever I am wrong. Thanks, 

April 21, 2013, 12:40 

#15  
Super Moderator
Sijal Ahmed Memon (turboenginner@gmail.com)
Join Date: Mar 2009
Location: Islamabad Pakistan
Posts: 3,905
Blog Entries: 6
Rep Power: 38 
Quote:
Quote:


April 21, 2013, 12:43 

#16 
New Member
Natali
Join Date: Apr 2013
Posts: 21
Rep Power: 4 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Unsteady DPM with steady solver  elobb  FLUENT  3  May 31, 2015 04:11 
conversion steady to unsteady  Clementhuon  CDadapco  7  January 26, 2012 05:33 
Steady needs unsteady.  nico  Main CFD Forum  0  September 21, 2007 04:50 
Turbulent: Steady or Unsteady: confusion  prem  FLUENT  0  March 30, 2006 10:40 
steady or unsteady? (in dpm)  winnie  FLUENT  1  April 28, 2003 11:30 