Difficulties in convergence

mahdidelfan · July 26, 2014, 03:24

Hello to all
my Domain is rectangular duct whit 1.36m length and 40*40 Cross section.Average Scale Velocity = 2.5920E+02
Shock wave boundary layer interaction is simulated.turbulence model is k-e.
1-What is the amount of my simulation physical time scale ?
2-What is the amount of my simulation Length scale ?
3-Which is to apply the appropriate time scale?Auto, physical, or local time scale?
4-My solution does not converge. Way to solve what?
5-How is to calculated Length scale for physical time scale in cfx?

Thanks to all

mahdidelfan · July 26, 2014, 03:27

Placing Photo

ghorrocks · July 26, 2014, 06:13

Many of these questions are covered in the CFX documentation and the FAQs: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

In a steady state simulation the time step size is simply the speed at which you march the equations. It behaves the same as any similar factor in any numerical calculation - if it is too small you will get consistent but slow convergence, if it is too large you get divergence. So you use change command file while running to adjust it up and down to find the best compromise between convergence and numerical stability.

July 26, 2014, 03:24	Difficulties in convergence	#1
mahdidelfan New Member mahdi Join Date: May 2014 Posts: 14 Rep Power: 11	Hello to all my Domain is rectangular duct whit 1.36m length and 40*40 Cross section.Average Scale Velocity = 2.5920E+02 Shock wave boundary layer interaction is simulated.turbulence model is k-e. 1-What is the amount of my simulation physical time scale ? 2-What is the amount of my simulation Length scale ? 3-Which is to apply the appropriate time scale?Auto, physical, or local time scale? 4-My solution does not converge. Way to solve what? 5-How is to calculated Length scale for physical time scale in cfx? Thanks to all

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July 26, 2014, 06:13		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Many of these questions are covered in the CFX documentation and the FAQs: http://www.cfd-online.com/Wiki/Ansys...gence_criteria In a steady state simulation the time step size is simply the speed at which you march the equations. It behaves the same as any similar factor in any numerical calculation - if it is too small you will get consistent but slow convergence, if it is too large you get divergence. So you use change command file while running to adjust it up and down to find the best compromise between convergence and numerical stability.