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Dear Gaurav,

Thank you for your kind email.

I would suggest reducing the Krylov dimension to 16. Since your base flow is 2D2C, you can use HalfMode as the ModeType, which will reduce your computation time. Furthermore, I noticed that for β = 1, you have set Lz = 1, which is incorrect. It should be Lz = (2π)/β.

I have also found that ModifiedArnoldi sometimes works better than Arpack:
<I PROPERTY="Driver" VALUE="ModifiedArnoldi" />

So I suggest the following solver condition:

      <I PROPERTY="SOLVERTYPE"              VALUE="VelocityCorrectionScheme"/>

      <I PROPERTY="EQTYPE"                  VALUE="UnsteadyNavierStokes"    />

      <I PROPERTY="Projection"              VALUE="Continuous"              />

      <I PROPERTY="EvolutionOperator"       VALUE="Direct"               />

      <I PROPERTY="TimeIntegrationMethod"   VALUE="IMEXOrder3"              />

      <I PROPERTY="Driver"                  VALUE="ModifiedArnoldi"                />

      <I PROPERTY="HOMOGENEOUS" VALUE="1D"/>

      <I PROPERTY="ModeType"          VALUE="HalfMode"/>


and the following parameter condition:

      <P> kdim          = 16        </P>

      <P> nvec          = 1         </P>

      <P> nits          = 500       </P>

      <P> evtol         = 1e-06     </P>

      <P> HomModesZ     = 1          </P>

      <P> BETA = 1.0 </P>

      <P> LZ            = (2*PI)/BETA     </P>


Best regards,
Abhishek



On Sun, Aug 31, 2025 at 6:15 PM Gaurav Gupta <gauravxpgupta@gmail.com> wrote:
Hello Prof. Abhishek,
I saw your answers to multiple queries in the NEKTAR++ community and believe would be help me with stability analysis using NEKTAR++. I am trying to validate the results of biglobal LSA of NACA0012 at AOA=16 with ARPACK. I was able to recreate the base-flow and tried to do the direct analysis with shift invert transformation at 0i, 2i, 4i, 6i, 8i and 10i and beta=1 but the eigenvalues are not correct. I have attached the case files as well as the reference paper. I will be highly grateful, if you could check my implementation and highlight any possible issues. 

Thanks!

Regards,
Gaurav Gupta
Scientist/Engineer-SC

Space Applications Centre, ISRO

Ahmedabad, GJ, India