CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Divergence of CD Nozzle

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree2Likes
  • 1 Post By Indra
  • 1 Post By Indra

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 20, 2022, 00:46
Lightbulb Divergence of CD Nozzle
  #1
New Member
 
John
Join Date: Jul 2022
Posts: 2
Rep Power: 0
Kepler12 is on a distinguished road
I am trying to simulate the flow through a simple converging-diverging nozzle with known inlet and outlet pressure and temperature.

Inlet
Pressure: 700000 psi
Temperature: 3400K

Outlet
Pressure: 101325 psi
Temperature: 300K

It is 2-D density based and axisymmetric using air as the fluid. Energy is on and I am using the k-omega SST viscous model.

I am not sure what I am doing wrong but the simulation consistently diverges. Below I have attached 1) Scaled residuals chart, 2) Best Mach Contour number produced, and 3) Mesh Quality Stats, all for ~225 iterations.

I am fairly new to CFD, so feedback would be greatly appreciated.
Attached Images
File Type: jpg scaled-residuals.jpg (45.1 KB, 23 views)
File Type: jpg mach-number-contour.jpg (18.7 KB, 20 views)
File Type: jpg mesh-quality.jpg (18.2 KB, 13 views)
Kepler12 is offline   Reply With Quote

Old   July 20, 2022, 10:46
Default
  #2
Member
 
Void_CFD-user
Join Date: Feb 2022
Posts: 67
Rep Power: 4
Indra is on a distinguished road
Quote:
Originally Posted by Kepler12 View Post
I am trying to simulate the flow through a simple converging-diverging nozzle with known inlet and outlet pressure and temperature.

Inlet
Pressure: 700000 psi
Temperature: 3400K

Outlet
Pressure: 101325 psi
Temperature: 300K

It is 2-D density based and axisymmetric using air as the fluid. Energy is on and I am using the k-omega SST viscous model.

I am not sure what I am doing wrong but the simulation consistently diverges. Below I have attached 1) Scaled residuals chart, 2) Best Mach Contour number produced, and 3) Mesh Quality Stats, all for ~225 iterations.

I am fairly new to CFD, so feedback would be greatly appreciated.
Can you send a snapshot of solution methods. There's a lot of things that can be don eher to imporve convergence. the first thing i would do is change to first order schemes. Also, are you getting any messages in console durng the run? That could tell us what relaxation factirs to reduce.
Kepler12 likes this.
Indra is offline   Reply With Quote

Old   July 20, 2022, 11:38
Post Solution Methods and Console
  #3
New Member
 
John
Join Date: Jul 2022
Posts: 2
Rep Power: 0
Kepler12 is on a distinguished road
Thanks for the help Indra.

I attached a screenshot of my solution methods. For the next iteration I will change them all to "first order upwind". Just for future understanding, how might changing the solution method to first order allow for convergence?

In the console the message "time step reduced in X cells due to excessive temperature change" after every few iterations. X being some number usually between 2-15. I did not want to attach a screenshot of my console because it has some sensitive information. I've also got a message saying "temperature limited in X cells.

Edit: still divergence while using first order equations.
Attached Images
File Type: jpg solution methods.jpg (59.8 KB, 16 views)
Kepler12 is offline   Reply With Quote

Old   July 20, 2022, 12:41
Default
  #4
Member
 
Void_CFD-user
Join Date: Feb 2022
Posts: 67
Rep Power: 4
Indra is on a distinguished road
Quote:
Originally Posted by Kepler12 View Post
Thanks for the help Indra.

I attached a screenshot of my solution methods. For the next iteration I will change them all to "first order upwind". Just for future understanding, how might changing the solution method to first order allow for convergence?

In the console the message "time step reduced in X cells due to excessive temperature change" after every few iterations. X being some number usually between 2-15. I did not want to attach a screenshot of my console because it has some sensitive information. I've also got a message saying "temperature limited in X cells.

Edit: still divergence while using first order equations.
Right, there we go, so you have to go to solutions>controls>limits and reduce the positivity rate. This will make sure that the rapid temperature change is controlled and you shouldn't get any report sayig temp. limited.

Also, I have seen that AUSM scheme in under flux type is more stable than ROE-Fds for mach number>1.5.

As for your questions regarding schemes, first order increases the convergence but it is not as accuracte as second order. So we usually run enough iterations in first order to make sure our residuals drop and then swith to second order. There's also a method to use blended first and second order approach which you can access through the TUI but again, don't worry about it for now. Always change oe thing at a time. For now try changing positivity rate and switch to first order for residuals to drop.
Kepler12 likes this.
Indra is offline   Reply With Quote

Old   December 1, 2024, 11:14
Default
  #5
New Member
 
Tariq Mahbub
Join Date: Mar 2021
Posts: 1
Rep Power: 0
MahbubTariq is on a distinguished road
Hi Kepler 12,
Have you been able to sovle the convergence issue? I am facing similar issue while modeling a convergent nozzle. I am having stuck residuals in the order of 10e-1.

Thanks.
MahbubTariq is offline   Reply With Quote

Reply

Tags
ansys, fluent, nozzle, rocket, supersonic

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Divergence detected in AMG solver. VOF. Mr.Mister Fluent Multiphase 5 November 22, 2024 07:32
Error SIGSEGV using VOF and UDF JERC_UTFSM Fluent UDF and Scheme Programming 14 November 8, 2021 00:17
Fail to converge when solving with a fabricated solution zizhou FLUENT 0 March 22, 2021 07:33
[ANSYS Meshing] Help with element size sandri_92 ANSYS Meshing & Geometry 14 November 14, 2018 08:54
fluent divergence for no reason sufjanst FLUENT 2 March 23, 2016 17:08


All times are GMT -4. The time now is 12:46.