Time step limited by chemical time-scale

MMatt · January 16, 2020, 02:20

While running my engine simulation into Converge, I noticed that the time step gets limited by the chemical time-scale. The default time-step is 1e-8 s and chemical time-scale 0.5.

Even by reducing the time-step to 1e-9 and chemical time-scale to 0.1 while the simulation is running doesn't help getting out of this issue.

Increasing the chemical time-scale helps, but what does that mean exactly, and what should I change to avoid that happening in the first place?

Thanks!

nitesh.attal · January 16, 2020, 12:45

Hello Matt,

In CONVERGE, we limit the amount of energy added (or temperature increase) over a time step due to combustion. The parameter mult_dt_chem is used to control the time step size allowed by combustion.

dt<= (dt_previous * mult_dt_chem)/max(deltaT/T)

Generally speaking, you should not be hitting time step limit by combustion unless you are using very large cell sizes or there is something odd about the mechanism. For recommended grid sizes, please take a look at one of the example cases provided in CONVERGE STUDIO that is closest representation to your engine.

MMatt · January 17, 2020, 04:08

Hi Nitesh,

Thank you for your answer. I think the issue may be that I use the default grid size of 4mm, so I will try to reduce it and see what happens. Issue is that it already takes 8 days for a simulation to complete, but I'll see.

Thanks!

January 16, 2020, 02:20	Time step limited by chemical time-scale	#1
MMatt Member MMatt Join Date: May 2013 Posts: 59 Rep Power: 12	While running my engine simulation into Converge, I noticed that the time step gets limited by the chemical time-scale. The default time-step is 1e-8 s and chemical time-scale 0.5. Even by reducing the time-step to 1e-9 and chemical time-scale to 0.1 while the simulation is running doesn't help getting out of this issue. Increasing the chemical time-scale helps, but what does that mean exactly, and what should I change to avoid that happening in the first place? Thanks!

January 16, 2020, 12:45		#2
nitesh.attal Senior Member Nitesh Attal Join Date: Sep 2017 Location: Convergent Science, Northville MI Posts: 113 Rep Power: 8	Hello Matt, In CONVERGE, we limit the amount of energy added (or temperature increase) over a time step due to combustion. The parameter mult_dt_chem is used to control the time step size allowed by combustion. dt<= (dt_previous * mult_dt_chem)/max(deltaT/T) Generally speaking, you should not be hitting time step limit by combustion unless you are using very large cell sizes or there is something odd about the mechanism. For recommended grid sizes, please take a look at one of the example cases provided in CONVERGE STUDIO that is closest representation to your engine. __________________ Nitesh Attal Principal Engineer \| Applications CONVERGECFD

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January 17, 2020, 04:08		#3
MMatt Member MMatt Join Date: May 2013 Posts: 59 Rep Power: 12	Hi Nitesh, Thank you for your answer. I think the issue may be that I use the default grid size of 4mm, so I will try to reduce it and see what happens. Issue is that it already takes 8 days for a simulation to complete, but I'll see. Thanks!