# Time-step for Implicit Unsteady solver in STARCCM+ and maximum number of steps?

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 April 24, 2020, 08:12 Time-step for Implicit Unsteady solver in STARCCM+ and maximum number of steps? #1 Senior Member   MA Join Date: Mar 2020 Posts: 163 Rep Power: 3 Hi, I am working on the CFD simulation for a wind turbine blade. I chose Implicit Unsteady Solver for this purpose. I want to associate the time step with rotational speed of the blade I am unable to figure out how should I do this and what should be the max. physical time and maximum steps for the stopping criteria? Any comments please.

 April 24, 2020, 13:42 #2 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,569 Rep Power: 53 Check the Courant number field. Make your physical time small enough to keep that Courant number in the single digits (e.g. <10) but large enough that you don't have enough time to forget about your CFD. Max steps is however many you need for your solution to converge at each time-step. This you get by creating monitors and checking for convergence at every time-step.

April 24, 2020, 13:50
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 Originally Posted by LuckyTran Check the Courant number field. Make your physical time small enough to keep that Courant number in the single digits (e.g. <10) but large enough that you don't have enough time to forget about your CFD. Max steps is however many you need for your solution to converge at each time-step. This you get by creating monitors and checking for convergence at every time-step.

How would I know that that Courant is below 10 if I choose a physical time?
Also, I want to relate the time step with the rotational speed of the blade so that steadiness of the results is obtained.

 April 24, 2020, 14:10 #4 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,569 Rep Power: 53 Well you can guess it. Either way, you pick a time. Run it for a few time-steps. Check the Courant number field. And then adjust it. I don't know what time has to do with rotational speed. Velocity and time are different concepts.

April 24, 2020, 14:16
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 Originally Posted by LuckyTran Well you can guess it. Either way, you pick a time. Run it for a few time-steps. Check the Courant number field. And then adjust it. I don't know what time has to do with rotational speed. Velocity and time are different concepts.

I am talking about "Time Step'' for the Implicit Unsteady solver not the physical time. However, I am not clear about their difference.

 April 24, 2020, 14:22 #6 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,569 Rep Power: 53 Sorry, there was a typo. I meant physical time-step size. Physical time starts at 0s and accumulates time according to how long you have run your simulation, the time-step size and number of time-steps that have been run. You set a time-step size according to Courant number. The max physical time is a stop criterion for when the simulation ends. You pick this based on how long you want the CFD to run. If you want to simulate a wind turbine spinning for 1s, set it to 1s. If you want to see it spin for a year, set it to 31,536,000 seconds.

April 24, 2020, 14:28
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 Originally Posted by LuckyTran Sorry, there was a typo. I meant physical time-step size. Physical time starts at 0s and accumulates time according to how long you have run your simulation, the time-step size and number of time-steps that have been run. You set a time-step size according to Courant number. The max physical time is a stop criterion for when the simulation ends. You pick this based on how long you want the CFD to run. If you want to simulate a wind turbine spinning for 1s, set it to 1s. If you want to see it spin for a year, set it to 31,536,000 seconds.

But what if I choose a time step size based on each degree in full turbine blade rotation i.e. 360. For instance I choose time-step based on 2, 4, and 6 degree and see in which rotating position the solution is converged. Isn't that a good way?

 April 24, 2020, 15:54 #8 Senior Member   Join Date: Jul 2009 Posts: 312 Rep Power: 16 In the turbine simulations that I have done, typically you want the time step to represent one degree or less of rotation to capture the physics. It can also depend the passage size - for example, rotor67 has 22 blades on a rotor so the passage between the blades is approximately 17 degrees. At a bare minimum you want 5 timesteps for the passage, or a max of about 3 degrees per timestep as an upper bound.

 April 24, 2020, 16:09 #9 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,569 Rep Power: 53 I mean you certainly can just put in any time-step size that you want. But good luck getting it to converge. The Courant number check just helps with the numerics.

April 24, 2020, 16:21
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 Originally Posted by LuckyTran I mean you certainly can just put in any time-step size that you want. But good luck getting it to converge. The Courant number check just helps with the numerics.

Where can I see the Courant number changing in simulation once I set the time-step?

April 24, 2020, 16:22
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 Originally Posted by agd In the turbine simulations that I have done, typically you want the time step to represent one degree or less of rotation to capture the physics. It can also depend the passage size - for example, rotor67 has 22 blades on a rotor so the passage between the blades is approximately 17 degrees. At a bare minimum you want 5 timesteps for the passage, or a max of about 3 degrees per timestep as an upper bound.

 April 24, 2020, 16:27 #12 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 4,569 Rep Power: 53 It's a field function. Just make a plot like you would for pressure or anything. Or you can make a report or whatever is your preference for looking at data.

April 24, 2020, 16:37
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 Originally Posted by LuckyTran It's a field function. Just make a plot like you would for pressure or anything. Or you can make a report or whatever is your preference for looking at data.

Thanks. One more thing:

- The reference area for the blade to create the Reports for any parameter such as drag (STARCCM) is the frontal area calculated via Report or it would be 1/3 of swept area as I have one wind turbine blade?

- In STARCCM+, Stream Edge Function for SST Transition Model is defined as \$WallDistance>0.005?1:0 which means that BL is 5 mm thin but in my case maximum BL is 0.1 m. If I replace 0.005 with 0.1 won't it create a problem in the simulation?

 April 24, 2020, 19:11 #14 New Member   Paul Hancock Join Date: Mar 2009 Location: Bellingham, WA Posts: 28 Rep Power: 14 In the CCM+ help, look up the key phrase 'Cyclic Time Unit'. I think that's the functionality that gives you a link between rotational rate and time step.