Steady flow in steady and unstedy simulation

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August 29, 2012, 04:05
Steady flow in steady and unstedy simulation
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Robert
Join Date: Mar 2011
Location: Warsaw
Posts: 62
Rep Power: 6
I have conducted two simulations of the steady flow in a pipe with some obstacles inside.
First simulation was in steady state and second was in unsteady state with 0.05 second step and long simulation time was used. Flow generally is laminar but gets turbulent in vicinity of obstacles.
The problem is that I get two different solutions. Contours of velocity magnitude are in attachments.

Does anyone know why these solutions are different?
Attached Images
 u_v.jpg (83.0 KB, 17 views) velocity_magn.jpg (79.7 KB, 13 views)

Last edited by Kwiaci; August 29, 2012 at 04:07. Reason: attachements

 August 29, 2012, 04:18 #2 Senior Member     siamak rahimi ardkapan Join Date: Jul 2010 Location: Copenhagen, Denmark Posts: 218 Rep Power: 9 It is hard to decide, but I think the right picture seems that it is not converged very well, since there is high velocity at top left of the picture. I think you need to let it to converge more. What is the reason that you are comparing the steady state and unsteady case?

 August 29, 2012, 05:14 #3 Member   Robert Join Date: Mar 2011 Location: Warsaw Posts: 62 Rep Power: 6 Thanks, I will converge it more. I wanted to check if there are any vortices separating from the edges which disappear before the outlet - but there are not there. This was just an additional analysis.

 August 29, 2012, 05:41 #4 Super Moderator     Sijal Ahmed Memon (turboenginner@gmail.com) Join Date: Mar 2009 Location: Islamabad Pakistan Posts: 3,909 Blog Entries: 6 Rep Power: 38 you cannot compare two solutions directly. Take some average for the transient solution and then compare it to steady state.

 August 29, 2012, 12:46 #5 Member   Robert Join Date: Mar 2011 Location: Warsaw Posts: 62 Rep Power: 6 OK, after it converged more I got almost identical results. Thx Far, you have mentioned about taking some average for the transient solution. How it can be done in Fluent to obtain, for example, time-averaged heat flux on a wall?

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