Second Order Upwind: Residuals

enricokr · March 22, 2009, 10:01

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...

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:

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

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

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

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

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

...)

March 22, 2009, 10:01	Second Order Upwind: Residuals	#1
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	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!

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
weno upwind 5th order 1 d code	Chi	Main CFD Forum	1	March 11, 2007 23:44
First order upwind	leung	FLUENT	2	June 13, 2004 09:09
second order upwind	muslum arici	Main CFD Forum	6	July 28, 2003 10:25
First Order Upwind	Giovanni Ieria	FLUENT	3	November 30, 1999 19:43
second order FD upwind scheme	Heinz Wilkening	Main CFD Forum	2	November 3, 1998 15:33

March 22, 2009, 23:33		#2
paka Senior Member Join Date: Mar 2009 Posts: 225 Rep Power: 18	Increase residual limit or increase number of iterations?

March 23, 2009, 10:53		#3
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	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.

March 24, 2009, 05:23		#4
paka Senior Member Join Date: Mar 2009 Posts: 225 Rep Power: 18	Did you try solve the same problem for different mesh?

March 24, 2009, 05:56		#5
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	Yes, i have tried. But the result is the same. Sometimes it is worse than others. Did you think that could be the mesh?

March 24, 2009, 09:29		#6
srjp Member JP Join Date: Mar 2009 Posts: 57 Rep Power: 18	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.

March 24, 2009, 09:50		#7
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	Done! I start second order after running first order...

March 25, 2009, 00:29		#8
srjp Member JP Join Date: Mar 2009 Posts: 57 Rep Power: 18	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?

March 25, 2009, 03:27		#9
Freeman New Member Join Date: Mar 2009 Posts: 29 Rep Power: 17	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

March 25, 2009, 04:27		#10
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	I post you same images with info: This is a rar with the case and date file: http://www.ninarello.it/Progetto.rar

March 27, 2009, 01:49		#11
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	no other proposals?

March 27, 2009, 03:32		#12
xdanielx New Member Daniel Langmayr Join Date: Mar 2009 Posts: 15 Rep Power: 17	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

March 27, 2009, 04:53		#14
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	I have to try to refine mesh along x direction, but i don't understand what about transient analisys...

March 27, 2009, 05:17		#15
Freeman New Member Join Date: Mar 2009 Posts: 29 Rep Power: 17	---deleted---

March 27, 2009, 05:42		#16
xdanielx New Member Daniel Langmayr Join Date: Mar 2009 Posts: 15 Rep Power: 17	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...

March 27, 2009, 10:29		#19
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	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?