[Nek5000-users] Selective frequency damping

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

Hi Nek's,

I've been trying to implement the selective frequency damping (Akervik E.,
Brandt L., Henningson D.S., Hoepffner J., Marxen O., Schlatter P. 2006. *Steady
solutions of the Navier-Stokes equations by selective frequency
damping.*Physics of fluids
*18*) which enables to compute stationnary solution. Here is what I have
written in my .usr file:

In userchk:

       common /vxold/ vxold(size(vx,1),size(vx,2),size(vx,3),size(vx,4))
       common /vyold/ vyold(size(vy,1),size(vy,2),size(vy,3),size(vy,4))
       common /vzold/ vzold(size(vz,1),size(vz,2),size(vz,3),size(vz,4))

       common /qxold/ qxold(size(vx,1),size(vx,2),size(vx,3),size(vx,4))
       common /qyold/ qyold(size(vy,1),size(vy,2),size(vy,3),size(vy,4))
       common /qzold/ qzold(size(vz,1),size(vz,2),size(vz,3),size(vz,4))

       common /for_x/ f_x(size(vx,1),size(vx,2),size(vx,3),size(vx,4))
       common /for_y/ f_y(size(vy,1),size(vy,2),size(vy,3),size(vy,4))
       common /for_z/ f_z(size(vz,1),size(vz,2),size(vz,3),size(vz,4))

       real chi, omega_c
       integer step_filtr


       chi = .25
       omega_c = .1
       step_filtr = 020000

       if(istep.EQ.step_filtr.AND.nid.EQ.0) write(*,*) 'Filtrage'

       if(istep.EQ.0) then
       vxold = 0.0D+00
       vyold = 0.0D+00
       vzold = 0.0D+00
       endif

       if(istep.LT.step_filtr) then
       qxold = 0.0D+00
       qyold = 0.0D+00
       qzold = 0.0D+00

       f_x = 0.0D+00
       f_y = 0.0D+00
       f_z = 0.0D+00

       call opcopy(vxold,vyold,vzold,vx,vy,vz)
       endif

       if(istep.GE.step_filtr) then
       qxold = qxold + dt*omega_c * (vxold - qxold)
       qyold = qyold + dt*omega_c * (vyold - qyold)
       qzold = qzold + dt*omega_c * (vzold - qzold)

       f_x = -chi * (vxold - qxold)
       f_y = -chi * (vyold - qyold)
       f_z = -chi * (vzold - qzold)

       call opcopy(vxxold,vyyold,vzzold,vxold,vyold,vzold)
       call opcopy(vxold,vyold,vzold,vx,vy,vz)
       endif

and in userf:

       common /for_x/ f_x(size(vx,1),size(vx,2),size(vx,3),size(vx,4))
       common /for_y/ f_y(size(vx,1),size(vx,2),size(vx,3),size(vx,4))
       common /for_z/ f_z(size(vx,1),size(vx,2),size(vx,3),size(vx,4))


      integer e,f,eg
c     e = gllel(eg)


c     Note: this is an acceleration term, NOT a force!
c     Thus, ffx will subsequently be multiplied by rho(x,t).


     ffx = f_x(ix,iy,iz,eg)
     ffy = f_y(ix,iy,iz,eg)
     ffz = f_z(ix,iy,iz,eg)


I have tried to compute with such routines the base flow for a 2D lid-driven
cavity flow at Re = 8500, however depending on the number of spectral
elements I use, I sometimes get a stationnary solution that looks slightly
as the one I'm looking for (but some differences near the lid) and sometimes
I get one which is actually not stationnary. I applied the same algorithm
for a finite-difference code and encountered no problem, so I was wondering
then if I missed something specific to spectral elements or Nek 5000 (such
as a mass matrix multplication for instance) or to F77 (which I'm not at
ease with, see my definition of the common for instance)?

Sincerely yours,

-- 
Jean-Christophe
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