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 July 22, 2011, 11:04 Adaptive Timesteps #1 New Member   Zhang Yang Join Date: Jun 2011 Location: Zürich Posts: 28 Rep Power: 9 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

 July 23, 2011, 06:19 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,991 Rep Power: 107 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.

 August 26, 2011, 06:40 #3 New Member   Join Date: Jun 2011 Posts: 15 Rep Power: 8 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] Option = Adaptive Timestep Update Frequency = 1 TIMESTEP ADAPTION: 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

 August 26, 2011, 07:05 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,991 Rep Power: 107 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.

 August 26, 2011, 09:04 #5 New Member   Join Date: Jun 2011 Posts: 15 Rep Power: 8 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?

 August 26, 2011, 13:22 #6 New Member   Join Date: Jun 2011 Posts: 15 Rep Power: 8 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]. Appreciated again your help Glenn.

 August 26, 2011, 22:07 #7 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,991 Rep Power: 107 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.

 August 30, 2011, 06:46 #8 New Member   Join Date: Jun 2011 Posts: 15 Rep Power: 8 Thanks Glenn, I have done that. Residual target of 1e-04 is ok for this case. dt has now varied around 3e-06 [s].

 August 30, 2011, 06:51 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,991 Rep Power: 107 Excellent. That sounds much more promising.

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