Hi Xiangjun, You can use a smooth initial disturbance to avoid eigenvalue of 1. If BaseFlow_interporder is not zero, the code uses a Lagrange interpolation and it can be used for non-periodic baseflow. Additionally, you can select a near body domain to compute the eigenvalue, seting the option SelectEVCalcDomain0, since the far-field grids are usually coarse and contains large numerical errors. We have tested this feature in our previous paper, see appendix Gao A K, Cantwell C D, Son O, et al. Three-dimensional transition and force characteristics of low-Reynolds-number flows past a plunging airfoil[J]. Journal of Fluid Mechanics, 2023, 973: A43. Best Regards Ankang Gao Research Associate Department of modern mechanics University of Science and Technology of China -----Original Messages----- From:"shan xiangjun" <x_j_shan@outlook.com> Send time:Tuesday, 04/22/2025 14:17:20 To: "Sherwin, Spencer J" <s.sherwin@imperial.ac.uk>, "Chris Cantwell" <c.cantwell@imperial.ac.uk> Cc: "Nektar-users@imperial.ac.uk" <nektar-users@imperial.ac.uk> Subject: [Nektar-users] Questions Regarding Eigenvalue Computation in Stability Analysis | This email from x_j_shan@outlook.com originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list to disable email stamping for this address. | Dear Prof, I hope this message finds you well. I am currently conducting a Floquet analysis and using the ModifiedArnoldi method to compute the leading eigenvalues. When the parameter beta is small, the computation converges and produces reasonable results. However, when beta becomes large, all computed maximum eigenvalues turn out to be 1. Increasing the nvec parameter makes it difficult for the calculation to converge. I also tried using the Arpack option and obtained the same results. Could you please advise on how to resolve this issue? Additionally, I would like to inquire about the application scenarios for the BaseFlow_interporder parameter. Can it be used in stability analyses for non-periodic problems? Thank you very much for your time and support. I look forward to your guidance. Best regards, Xiangjun Shan