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 Giorgitseli March 14, 2012 10:29

Selection of time step

Hello everybody.

I am new user regarding the transient calculations and I would appreciate any useful information about how I can select an appropriate time step for my simulation.

Moreover does anybody know if I can "insert" in CFX a steady state flow field and then run a model for the solids that exist in the fluid domain???I hope that if this is an available option in CFX I could reduce the required total computational time. Regarding the particles I use the algebraic slip model (no deposition).

THANX:):):)

 ghorrocks March 14, 2012 17:28

By solids I assume you mean ASM of small solid particles.

Yes, you can do this, but the whole idea of ASM is it couples back to the flow. You can turn bits of the solver off with expert parameters, so you can turn off the pressure/momentum equation but solve the ASM equations. Alternately using particle tracking you could do this as a one-way coupled simulation.

 Giorgitseli March 15, 2012 07:03

"You can turn bits of the solver off with expert parameters, so you can turn off the pressure/momentum equation but solve the ASM equations. Alternately using particle tracking you could do this as a one-way coupled simulation."

Thanx for the rapid answer!
1.)How can I turn off the pressure/momentum equation and only solve the ASM equations??I ll check the particle tracking method because I have not yet worked with this!
2.)Any idea for the time step selection?

 CFD_UJI March 15, 2012 16:05

hi! from what i know, you have to select a time step so that the Courant number of your problem is approximately 1. thanx

Daniel

 Zigainer March 15, 2012 16:22

Quote:
 Originally Posted by Giorgitseli (Post 349578) 1.)How can I turn off the pressure/momentum equation and only solve the ASM equations??I ll check the particle tracking method because I have not yet worked with this!

Quote:
 Originally Posted by ghorrocks (Post 349492) You can turn bits of the solver off with expert parameters

 ghorrocks March 15, 2012 17:47

Quote:
 hi! from what i know, you have to select a time step so that the Courant number of your problem is approximately 1.
This one keeps coming up, it is wrong.

CFX is an implicit solver and does not have the CFL<1 (compressible) or Courant No<1 restriction. You should determine time step size by a sensitivity analysis, and you will probably end up with an acceptible time step size larger than 1.

This restriction applies to explicit solvers.

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