[Nek5000-users] Problems in getting eigenvalues using ARNOLDI

[nek5000-users at lists.mcs.anl.gov]

Hi Guillaume,

Thanks for your reply. My reply to your queries is as below:-

1) I am getting unstable pair of Eigen values with growth rate around 0.02.
2) Solver tolerance that I used are ;
  7.00000              p020 NORDER
  0.100000E-09    p021 DIVERGENCE
  0.100000E-09.   p022 HELMHOLTZ
   0.00000             p023 NPSCAL
  0.100000E-09    p024 TOLREL
  0.100000E-04    p025 TOLABS
1.00000E-05.       p113 : ARNOLDI: arpack tolerance


3) Boundary conditions are :

I am using sponge strength at inlet and outlet with following details in
.rea file
   1.00000     p119 : SPONGE STRENGTH
   25.0000     p120 : SPONGE TOTAL WIDTH
   3.00000     p121 : SPONGE DROP WIDTH (INFLOW)
   5.00000     p122 : SPONGE RISE WIDTH (OUTFLOW)
the sponge function and its cal is detailed in userf subroutine of .usr file

Also following is defined in .usr file
c-----------------------------------------------------------------------
      subroutine userbc (ix,iy,iz,iside,eg)

      include 'SIZE'
      include 'NEKUSE'          ! UX, UY, UZ, TEMP, X, Y
      include 'PARALLEL'        ! GLLEL


      integer e,eg

c     velocity
c      e  =  GLLEL(eg)
      UX = 0.0
      UY = 0.0
      UZ = 0.0

c     t
      TEMP=0.0

      return
      end
c-----------------------------------------------------------------------
      subroutine useric (ix,iy,iz,ieg)

      include 'SIZE'
      include 'NEKUSE'          ! UX, UY, UZ, TEMP, Z

      integer idum
      save    idum
      data    idum / 0 /

      real eps

      if (idum.eq.0) idum = 99 + nid
      eps = 0.1

c     velocity
c     random distribution
      UX = eps*(ran1(idum)-0.50)
      UY = eps*(ran1(idum)-0.50)
      UZ = eps*(ran1(idum)-0.50)

c     t
      TEMP=0

      return
      end


40.0000     p109 : ARNOLDI: frequency of calling arn_solve
   4.00000     p110 : ARNOLDI: maximal number of arnoldi cycles
  90.0000     p111 : ARNOLDI: size of Krylov space
   30.0000     p112 : ARNOLDI: number of eigenvectors


If I am increasing the Krylov subspace then will I get Eigen values at
lesser restart ?

Kindly help please as I am new to this. Thanks in advance.

Alok Mishra
Research Scholar
Computational Propulsion Lab
Aerospace Department IIT Kanpur
+91-8795844555
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