Dear All, I'm a new user of Nektar++. I would like to carry out some stability investigations. I have two questions in regard to the usage of the software. *1*. I wanted to compare the eigenvectors calculated by the unsteady and steady Navier-Stokes solvers, but I encountered a problem. I tried to run nektar++-4.0.1/solvers/IncNavierStokesSolver/Tests/ChanStability_Coupled.xml, but I got the following error: /"// //IncNavierStokesSolver ChanStability_Coupled.xml// //// //Fatal : Level 0 assertion violation// //Base flow must be defined for linearised forms.// //"/ Then I defined the BaseFlow as: /<FUNCTION NAME="BaseFlow">// // <E VAR="u" VALUE="-y*y+1" />// // <E VAR="v" VALUE="0" />// //</FUNCTION>/ The simulation was carried out in this case, but the results are wrong: /"IncNavierStokesSolver ChanStability_Coupled_mod.xml// // //=======================================================================// // Solver Type: Coupled Linearised NS// //=======================================================================// // Arnoldi solver type : Arpack// // Arpack problem type : LI// // Single Fourier mode : false // // Beta set to Zero : false // // Shift (Real,Imag) : 0,0// // Krylov-space dimension : 16// // Number of vectors : 2// // Max iterations : 500// // Eigenvalue tolerance : 1e-06// //======================================================// //Initial Conditions:// // - Field u: from file ChanStability_Coupled.rst// // - Field v: from file ChanStability_Coupled.rst// //Writing: "ChanStability_Coupled_mod_0.chk"// //Matrix Setup Costs: 2.37365// //Multilevel condensation: 1.29072// // Inital vector : input file // //Iteration 16, output: 0, ido=99 // //Converged in 16 iterations// //Converged Eigenvalues: 3// // 0 0 0 -nan -nan// //Writing: "ChanStability_Coupled_mod_eig_0"// // 1 0 0 -nan -nan// //Writing: "ChanStability_Coupled_mod_eig_1"// // 2 0 0 -nan -nan// //Writing: "ChanStability_Coupled_mod_eig_2"// //L 2 error (variable u) : 2.589// //L inf error (variable u) : 1// //L 2 error (variable v) : 0// //L inf error (variable v) : 0// //"//// /Maybe I misunderstood something, could you provide help with this issue? *2*. Is it somehow possible to export the pressure field of the eigenvectors or should I calculate it by the Poisson equation? In this case could you send me an example how can I calculate the derivatives of a field variable. I saw the function FldAddScalGrad but I don't know how can I use. I really appreciate any help. Best regards, Péter <mailto:nektar-users@imperial.ac.uk>