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