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 swiss_zhang July 22, 2011 11:04

Hello, Guys:

I am doing a transient fire simulation using CFX und have difficulties to define the timesteps.

Can anyone tell me the a rule of thumb how to define the timeteps with Adaptive method.

e.g.

initial Timestep
min Timesteps
max Timesteps
Timestep Decrease Factor
Timestep Increase Factor

Thanks a lot

Yang

 ghorrocks July 23, 2011 06:19

Set the initial timestep to be small, in general. I don't generally use max/min timesteps but set a simulation time instead. The default increase/decrease factor should be fine.

As described in the documentation, adaptive time steps aiming for 3-5 coeff loops per time step (for simple models) or 6-10 loops per time step (for complex models, eg multiphase) is usually a good approach.

 Sal August 26, 2011 06:40

Hi Glenn,

In my simulations Adaptive Timestepping has not varied with the time - direction is unchanged (ratio 1.000).
As you suggested previously, I set the 'dt range' wide enough to let the solver calculate the most suitable dt.

Could you please have a quick check to my timestep set up:

TIME DURATION:
Option = Total Time
Total Time = 0.006 [s]
END
TIME STEPS:
First Update Time = 0.0 [s]
Initial Timestep = 1e-06 [s]
Timestep Update Frequency = 1
Maximum Timestep = 1e+20 [s]
Minimum Timestep = 1e-20 [s]
Option = Number of Coefficient Loops
Target Maximum Coefficient Loops = 5
Target Minimum Coefficient Loops = 3
Timestep Decrease Factor = 0.8
Timestep Increase Factor = 1.06

CONVERGENCE CONTROL:
Maximum Number of Coefficient Loops = 4
Minimum Number of Coefficient Loops = 3
Timescale Control = Coefficient Loops

Regards,

Sal

 ghorrocks August 26, 2011 07:05

Remove the max and min number of coeff loops in the convergence control section. This is forcing it to do either 3 or 4 loops per time step and that means your min3 max5 adaptive timestep critereon is never reached. Instead make the max number of coeff loops a number much larger, say about 10.

 Sal August 26, 2011 09:04

Many thanks Glenn.
With max coeff loops = 10 in the control section, dt keeps decreasing with a ratio of 0.8.
Current timestep is 2e-10. Statistics are dropping down very sharply. RMS Co and Max Co = 0. Simulation takes forever.
Is it normal?

 Sal August 26, 2011 13:22

I have changed the residual target (convergence criteria) from 1e-8 [s] to 1e-6 [s]. Still costly but running definitely faster. Now, dt is approaching to 1e-07 [s].

 ghorrocks August 26, 2011 22:07

Your convergence target is almost certainly far too tight. Do a residual sensitivity study so you know how tight you need to go - 1e-3 or 1e-4 is more normal for transient flows.

 Sal August 30, 2011 06:46

Thanks Glenn, I have done that. Residual target of 1e-04 is ok for this case.
dt has now varied around 3e-06 [s].

 ghorrocks August 30, 2011 06:51

Excellent. That sounds much more promising.

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