# time step in CFX pre

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

 April 11, 2021, 22:07 time step in CFX pre #1 Member   phd Join Date: Oct 2013 Posts: 76 Rep Power: 12 Hi, there May I confirm two things regarding to the time step set-up in CFX pre? 1, in the steady simulation, I’d like to set up my time scale depending on the number of iterations. When the number of iterations is smaller than 200, I will use 2e-6 [s] as the physical time scale. Then the time scale slides gradually to 2e-4 [s] in the following 100 iterations. After that, the time scale keeps constant as 2e-4 [s] in the last 700 iterations. can I use the following CEL: step(200-citern)*2e-6 + step(citern-200)* step(300-citern)*((citern-200)*1.98e-6+2e-6) + step(citern-300)* 2e-4 to achieve my goal? Is there any other better way of doing that? Also what is the difference between aitern and citern? Does aitern also count the iteration from the initial guess? 2, In the transient simulation, what is the difference between citern and ctstep? Does citern refer to the number of iteration in the inner loop and ctstep refer to the number of time step in the outer loop? If I’d love like to have a sinusoid inlet velocity depending on the time, could I use the following CEL 11.07[m/s]+1.63[m/s]*sin(41.9[s^-1]*t ) ? Thank you very much!

 April 12, 2021, 03:00 #2 Senior Member   Gert-Jan Join Date: Oct 2012 Location: Europe Posts: 1,827 Rep Power: 27 Search the CFX forum to find the difference between aitern and citern, e.g. aitern vs citern Regarding the functions, I think these are completely valid for the purpose. I would recommend to plot the CEL expressions in CFX-Pre to see if it provides what your expect. If so, then you can trust that CFX will use the time step increase or inlet velocity as you proposed. lostking18 likes this.

 April 12, 2021, 05:46 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143 And don't forget all variables are defined in the CFX documentation, in the reference guide. A further comment: While your function with lots of step() functions is valid, if you use a 1D interpolation function it is much easier to set up and understand. You just plot out the points to define the function and CFX interpolates along it. lostking18 likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 April 12, 2021, 16:53 #4 Member   phd Join Date: Oct 2013 Posts: 76 Rep Power: 12 Hi, Gert-Jan, Thank you very much for your reply. Drawing the CEL expression is really a good idea.

April 12, 2021, 16:55
#5
Member

phd
Join Date: Oct 2013
Posts: 76
Rep Power: 12
Quote:
 Originally Posted by ghorrocks And don't forget all variables are defined in the CFX documentation, in the reference guide. A further comment: While your function with lots of step() functions is valid, if you use a 1D interpolation function it is much easier to set up and understand. You just plot out the points to define the function and CFX interpolates along it.

 Tags time step