COUPLED Solver Stability Issues

abhi.jam22 · July 5, 2017, 19:23

Hey guys,

I have been performing 3D URANS simulations for Re 3 million on NACA 4415 AR 12 Wing using COUPLED Solver on structured hexahedral grid. I started off by running steady state simulations first using the same solver and got satisfactory results. But when i try to run Unsteady simulations, the result diverges in the first step even though the grid is the same. My momentum urf is 0.2, pressure 0.3 for both steady and unsteady. I have been able to get the solver to run for momentum urf to be 0.05 but the time-variation of CL (lift coefficient) is not realistic.

Can anyone suggest a solution to this issue?

Any help is highly appreciated.

LuckyTran · July 5, 2017, 22:45

You shouldn't be using that small of a urf for transient simulations. Run small time-steps with large urf's (all 1's). Set the courant number to super large value, i.e. 1e7.

Is this compressible or incompressible? Check your courant numbers (acoustic and convective respectively)..

abhi.jam22 · July 6, 2017, 00:43

Incompressible.

LuckyTran · July 6, 2017, 16:22

Quote:

Originally Posted by abhi.jam22

Incompressible.

Then check the convective courant number and make sure it's <2, preferably <0.5

July 5, 2017, 19:23	COUPLED Solver Stability Issues	#1
abhi.jam22 New Member Abhimanyu Join Date: Jul 2014 Posts: 19 Rep Power: 12	Hey guys, I have been performing 3D URANS simulations for Re 3 million on NACA 4415 AR 12 Wing using COUPLED Solver on structured hexahedral grid. I started off by running steady state simulations first using the same solver and got satisfactory results. But when i try to run Unsteady simulations, the result diverges in the first step even though the grid is the same. My momentum urf is 0.2, pressure 0.3 for both steady and unsteady. I have been able to get the solver to run for momentum urf to be 0.05 but the time-variation of CL (lift coefficient) is not realistic. Can anyone suggest a solution to this issue? Any help is highly appreciated.

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July 5, 2017, 22:45		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,751 Rep Power: 66	You shouldn't be using that small of a urf for transient simulations. Run small time-steps with large urf's (all 1's). Set the courant number to super large value, i.e. 1e7. Is this compressible or incompressible? Check your courant numbers (acoustic and convective respectively)..

July 6, 2017, 00:43		#3
abhi.jam22 New Member Abhimanyu Join Date: Jul 2014 Posts: 19 Rep Power: 12	Incompressible.