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Second Order Upwind: Residuals

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Old   March 22, 2009, 10:01
Default Second Order Upwind: Residuals
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Enry Lorna Neil
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In a pipe sudden expansion simulation, with first order upwind i have no problems with residuals, but when i switch to second order upwind the residuals remain constants at 10e-1. Why? What i have to do?
Thanks!
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Old   March 22, 2009, 23:33
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Increase residual limit or increase number of iterations?
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Old   March 23, 2009, 10:53
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Residual limits are at 10e-12
Increasing number of iterations is not useful because the residuals remains constant after a certain number of iterations. Constant at 10e-1.
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Old   March 24, 2009, 05:23
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Did you try solve the same problem for different mesh?
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Old   March 24, 2009, 05:56
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Yes, i have tried. But the result is the same. Sometimes it is worse than others.
Did you think that could be the mesh?
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Old   March 24, 2009, 09:29
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Did you start the second order case from the first order solution?
This might have happened if you directly run a second order simulation and the solution is not able to converge.
Try using the 1st order solution as the starting point for the 2nd order.
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Old   March 24, 2009, 09:50
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Done! I start second order after running first order...
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Old   March 25, 2009, 00:29
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Hi,
That's unfortunate.
Did you try changing the under-relaxation factors?
Looks like the solution is not able to converge by the sudden change in descretization.
Try reducing the URFs to get a delayed but better convergence. After the residuals go down, you may set them back to default.

If this doesn't work, may be you should not let the first order solution converge to the low residual limit. Say, set the limits to 10e-2, run first order, and stop after converging, and then run in second order.

BTW, is this not turbulent? You are tracking only momentum and velocities?
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Old   March 25, 2009, 03:27
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Can we have a look on your mesh? Could you post a pic of it? I guess the problem might be in the mesh or boundary conditions...

regards
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Old   March 25, 2009, 04:27
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I post you same images with info:






This is a rar with the case and date file:
http://www.ninarello.it/Progetto.rar
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Old   March 27, 2009, 01:49
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no other proposals?
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Old   March 27, 2009, 03:32
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I often encountered such a behavior. Usually, a transient analysis is necessary, as the transient effects are "washed out" due to the diffusive upwind scheme....

monitor a value of interest to see when your simulaiton converges...

good luck
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Old   March 27, 2009, 03:47
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Quote:
Originally Posted by enricokr View Post
no other proposals?
I see those cells quite stretched. Have you tried to make pore divisions of your domain in X-direction? I remember that in a project I made a boundary layer in a wind tunnel floor with cells too streched in the flow direction and simulation diverged all the time: when I made cells more "square-shaped" simulation absolutely converged.

By the way, your Y-direction cell divisions seems to be all right.

I suggest you to try this. Good luck!
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Old   March 27, 2009, 04:53
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I have to try to refine mesh along x direction, but i don't understand what about transient analisys...
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Old   March 27, 2009, 05:17
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Old   March 27, 2009, 05:42
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strange...when i run the simulation it converges very well with your settings...

only thing i noticed is that you have reversed flow on your pressure outlet, but residuals look fine...
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Old   March 27, 2009, 05:58
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Quote:
Originally Posted by xdanielx View Post
strange...when i run the simulation it converges very well with your settings...

only thing i noticed is that you have reversed flow on your pressure outlet, but residuals look fine...
Hi xdanielx,

At which iteration has you switched to 2nd order?
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Old   March 27, 2009, 06:23
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Quote:
Originally Posted by xdanielx View Post
strange...when i run the simulation it converges very well with your settings...

only thing i noticed is that you have reversed flow on your pressure outlet, but residuals look fine...
agree with xdanielx,at the 539 iterations,residuals reach convergence.then adjust pressure discretization to 2nd oreder,it reach convergence at the 772 iterations.
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Old   March 27, 2009, 10:29
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Can i change under relaxed factor for pressure from 0.3 to 0.1? With this value residuals decrease...
I don't know why the simulation converge with my settings to you but not to me ... Can it be the PC?
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Old   March 27, 2009, 10:49
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Quote:
Originally Posted by enricokr View Post
Can i change under relaxed factor for pressure from 0.3 to 0.1? With this value residuals decrease...
I don't know why the simulation converge with my settings to you but not to me ... Can it be the PC?
Stupid question but, are you using double precision when you start Fluent?

Have you tried also to switch to 2nd order more before those 1200it? I don't think this may be the problem, but who knows ... When you reach 1e-3 in 1st order (I guess at 200it) you are ready to change to a higher order. I have changed to 2nd order even at the 50th iteration (when you're in a hurry ...)
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