Adaptive Timesteps
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 
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 35 coeff loops per time step (for simple models) or 610 loops per time step (for complex models, eg multiphase) is usually a good approach. 
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 = 1e06 [s] Option = Adaptive Timestep Update Frequency = 1 TIMESTEP ADAPTION: Maximum Timestep = 1e+20 [s] Minimum Timestep = 1e20 [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 
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.

Many thanks Glenn.
With max coeff loops = 10 in the control section, dt keeps decreasing with a ratio of 0.8. Current timestep is 2e10. Statistics are dropping down very sharply. RMS Co and Max Co = 0. Simulation takes forever. Is it normal? 
I have changed the residual target (convergence criteria) from 1e8 [s] to 1e6 [s]. Still costly but running definitely faster. Now, dt is approaching to 1e07 [s].
Appreciated again your help Glenn. 
Your convergence target is almost certainly far too tight. Do a residual sensitivity study so you know how tight you need to go  1e3 or 1e4 is more normal for transient flows.

Thanks Glenn, I have done that. Residual target of 1e04 is ok for this case.
dt has now varied around 3e06 [s]. 
Excellent. That sounds much more promising.

All times are GMT 4. The time now is 20:07. 