converges first then starts to diverges! why?

nenazarian · April 19, 2012, 04:51

Hi all

I trying to simulate the urban environment and for now Im doing that on a uniformly spaced rows of buildings with same height with periodic boundary condition in both stream-wise and span-wise direction. ( so that it would form an unlimited rows and columns of buildings)

right now Im only concerned with the flow as the 1st step of simulation.
I tried both RANS and LES methods. Though LES seems good but unfortunately, it take forever to run one case (even without heat transfer simulation) and RANS methods have very strange behavior.

What I see from RANS is that it converges for the first 1000 iteration then it starts diverging ... then all the symmetry that I expect from the flow goes away ... I changed turbulence methods, checked meshing and Yplus range, used 1st order discretization (to make sure divergence doesnt come from higher order scheme instability) almost everything I could think of ...

So please let me know if there is anything I should check to find out what this divergence is coming from ...

I should have moved to heat transfer part month ago but Im still struggling with how to solve the flow

(

Thanks in advance
Cheers

Negin

nenazarian · April 19, 2012, 05:07

Well by checking the meshing I mean I tried Boundary layer meshing and uniform and tried to use a fine mesh, but due to the limitation of my computer I dont know if the resolution is enough to capture wall effect or not... anyway I dont think that would be the problem

April 19, 2012, 04:51	converges first then starts to diverges! why?	#1
nenazarian Member Negin Nazarian Join Date: Jan 2012 Location: San Diego Posts: 61 Rep Power: 14	Hi all I trying to simulate the urban environment and for now Im doing that on a uniformly spaced rows of buildings with same height with periodic boundary condition in both stream-wise and span-wise direction. ( so that it would form an unlimited rows and columns of buildings) right now Im only concerned with the flow as the 1st step of simulation. I tried both RANS and LES methods. Though LES seems good but unfortunately, it take forever to run one case (even without heat transfer simulation) and RANS methods have very strange behavior. What I see from RANS is that it converges for the first 1000 iteration then it starts diverging ... then all the symmetry that I expect from the flow goes away ... I changed turbulence methods, checked meshing and Yplus range, used 1st order discretization (to make sure divergence doesnt come from higher order scheme instability) almost everything I could think of ... So please let me know if there is anything I should check to find out what this divergence is coming from ... I should have moved to heat transfer part month ago but Im still struggling with how to solve the flow ( Thanks in advance Cheers Negin

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April 19, 2012, 05:07		#2
nenazarian Member Negin Nazarian Join Date: Jan 2012 Location: San Diego Posts: 61 Rep Power: 14	Well by checking the meshing I mean I tried Boundary layer meshing and uniform and tried to use a fine mesh, but due to the limitation of my computer I dont know if the resolution is enough to capture wall effect or not... anyway I dont think that would be the problem