Divergence with LU-SGS

Joachim · June 24, 2014, 10:06

Hey everyone!

I am trying to verify my implementation of the LU-SGS scheme and I am having some issues. First, I am checking the free-stream capture property of the code, which apparently is not working.

I used a rectangular box with uniform spacing. As soon as I have a small perturbation at the boundaries, it is slowly amplified until a pattern as shown in the attached figure starts to appear. Has anyone already seen something like this before? Where could it come from?? I believe that my calculation of the residual is ok since I previously used it successfully with my explicit solver. I am using central differencing, so that the jacobian is computed analytically.

Ant idea would be greatly welcome!!

Thank you for your help!

Joachim

Joachim · June 24, 2014, 10:36

and here is the picture...

Joachim · June 24, 2014, 14:02

If I use a low CFL number (~0.1), the free-stream capture works just fine and the full simulation converges (checked with Blasius and a naca0012 airfoil). I guess that confirms that my residual is ok but there is an issue with my implicit operator?

How could I identify the precise source of the bug??

Thanks!!

Joachim

FMDenaro · June 24, 2014, 17:08

without detalis about the formulation and the code is not possible to help you...

June 24, 2014, 10:06	Divergence with LU-SGS	#1
Joachim Senior Member Joachim Join Date: Mar 2012 Location: Paris, France Posts: 145 Rep Power: 15	Hey everyone! I am trying to verify my implementation of the LU-SGS scheme and I am having some issues. First, I am checking the free-stream capture property of the code, which apparently is not working. I used a rectangular box with uniform spacing. As soon as I have a small perturbation at the boundaries, it is slowly amplified until a pattern as shown in the attached figure starts to appear. Has anyone already seen something like this before? Where could it come from?? I believe that my calculation of the residual is ok since I previously used it successfully with my explicit solver. I am using central differencing, so that the jacobian is computed analytically. Ant idea would be greatly welcome!! Thank you for your help! Joachim

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June 24, 2014, 14:02		#3
Joachim Senior Member Joachim Join Date: Mar 2012 Location: Paris, France Posts: 145 Rep Power: 15	If I use a low CFL number (~0.1), the free-stream capture works just fine and the full simulation converges (checked with Blasius and a naca0012 airfoil). I guess that confirms that my residual is ok but there is an issue with my implicit operator? How could I identify the precise source of the bug?? Thanks!! Joachim

June 24, 2014, 17:08		#4
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,771 Rep Power: 71	without detalis about the formulation and the code is not possible to help you...