CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

Using CEL-expressions to automatically set the physical timestep

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 29, 2017, 10:49
Lightbulb Using CEL-expressions to automatically set the physical timestep
  #1
New Member
 
Join Date: Mar 2017
Posts: 2
Rep Power: 0
.bastian is on a distinguished road
Actually im searching for a way to automatically change the physical timestep in dependency of the actual iteration step.

It should be something like:
its <= 10: 1e-3 s
10 < its 100: 1e-4 s
100 < its: 1e-5 s


Struggeling the forum search i found something which seems to be promising:
Quote:
Originally Posted by ghorrocks View Post
Have you tried setting the autotimescale factor to a CEL expression then using a CEL expression like if(aitern>15,10,1)?
My solution based on this hint is: if(aitern<=10,1e-3 [s],if(aitern>100,1e-5 [s],1e-4 [s]))

but unfortunately it doesnt work. Setting this as an expression for the physical timestep in CFX Pre the solver starts normally using the set timestep 1e-3 [s] but it doesnt change the timestep when the iteration levels are reached.

Any idea what i did wrong?

thanks in advance

bastian
.bastian is offline   Reply With Quote

Old   April 30, 2017, 20:10
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 13,738
Rep Power: 106
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
You can also do this with a 1D interpolation function. I am a bit puzzled about how it did not work, however.

But looking at the bigger picture: I assume this is for a steady state simulation. Decreasing time scale size as you progress to convergence is not recommended. Usually you want to increase the time scale size. Why do you want to decrease it?
ghorrocks is offline   Reply With Quote

Old   May 1, 2017, 09:11
Default
  #3
New Member
 
Join Date: Mar 2017
Posts: 2
Rep Power: 0
.bastian is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
[...]
I am a bit puzzled about how it did not work, however.
[...]
Why do you want to decrease it?
Thanks for your Answer.

I also found the variable citern in place of aitern but it doest work either. For the the fact that the expression itself seems to work until it starts with the correct value set for the first ten iterations.
Maybe during the run the expression is not evaluated correctly?!
And yes you`re right, its steady state.

To the more common things.
The simulation itself is a double choking supersonic jet-pump, just to give you a little insight into the model itself.

I'm not sure if its the right explanation until its one of my first bigger projects in cfx but i think you'll know what i mean.
The first few steps, whilst CFX "fills in" the fluid into the domain i want to use bigger timesteps until i see no need to spend more time doing this.

But as soon as the first shock regions occuring it becomes heavily unstable. But all in all it converges using a timestep small enough.

So all in all i want to start with bigger timesteps to get quicker to the point i have to resolve finer.
.bastian is offline   Reply With Quote

Reply

Tags
cel, cfx, physical, timestep

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Timestep control via CEL? Claudia CFX 7 September 14, 2007 05:00
Physical timestep Calculation Joe CFX 3 July 4, 2006 15:07
Different Physical timestep for different domain new@cfx CFX 0 September 6, 2005 10:53
Physical Timestep Ted Crilly Siemens 2 February 18, 2003 05:05
CFX 5.5 Roued CFX 1 October 2, 2001 16:49


All times are GMT -4. The time now is 10:07.