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-   -   Second Order Upwind: Residuals (http://www.cfd-online.com/Forums/fluent/62856-second-order-upwind-residuals.html)

enricokr March 22, 2009 10:01

Second Order Upwind: Residuals
 
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!

paka March 22, 2009 23:33

Increase residual limit or increase number of iterations?

enricokr March 23, 2009 10:53

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.

paka March 24, 2009 05:23

Did you try solve the same problem for different mesh?

enricokr March 24, 2009 05:56

Yes, i have tried. But the result is the same. Sometimes it is worse than others.
Did you think that could be the mesh?

srjp March 24, 2009 09:29

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.

enricokr March 24, 2009 09:50

Done! I start second order after running first order...
http://www.ninarello.it/order2.jpg

srjp March 25, 2009 00:29

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?

Freeman March 25, 2009 03:27

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

enricokr March 25, 2009 04:27

I post you same images with info:
http://www.ninarello.it/grid.jpg

http://www.ninarello.it/grid2.jpg

http://www.ninarello.it/geometry.jpg

This is a rar with the case and date file:
http://www.ninarello.it/Progetto.rar

enricokr March 27, 2009 01:49

no other proposals?

xdanielx March 27, 2009 03:32

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

Freeman March 27, 2009 03:47

Quote:

Originally Posted by enricokr (Post 210994)
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!

enricokr March 27, 2009 04:53

I have to try to refine mesh along x direction, but i don't understand what about transient analisys...

Freeman March 27, 2009 05:17

---deleted---

xdanielx March 27, 2009 05:42

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...

Freeman March 27, 2009 05:58

Quote:

Originally Posted by xdanielx (Post 211017)
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?

zhaopeng March 27, 2009 06:23

Quote:

Originally Posted by xdanielx (Post 211017)
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.

enricokr March 27, 2009 10:29

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?

Freeman March 27, 2009 10:49

Quote:

Originally Posted by enricokr (Post 211050)
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 :eek:... 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 :rolleyes:...)


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