[Nek5000-users] Robin boundary condition on velocity

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

Hi Neks
I'm trying to implement a slip-length (Robin) as boundary condition for a
plane channel flow, namely:
(u+du/dy)_{wall}=0
where x and y are the streamwise and wall normal directions respectively.

Following the latter post:
http://lists.mcs.anl.gov/pipermail/nek5000-users/2011-April/001313.html

here there's what I came up with:

c-----------------------------------------------------------------------
      subroutine userchk
      include 'SIZE'
      include 'TOTAL'
      common /my_DEBUG/ rmodulate
      common /my_ROBIN/ ROBIN_mask(lx1,ly1,lz1,lelt)
     $                ,      work1(lx1,ly1,lz1,lelt)
     $                ,      work2(lx1,ly1,lz1,lelt)
     $                ,     graduy(lx1,ly1,lz1,lelt)
      n = nx1*ny1*nz1*nelv
!!!Build Up MASKS!
      if (istep.eq.0) then
        do i=1,n
          x=xm1(i,1,1,1)
          y=ym1(i,1,1,1)
          if(x.gt.3.and.x.lt.4)then
            ROBIN_mask(i,1,1,1)=1.0
          else
            ROBIN_mask(i,1,1,1)=0.0
          endif
        enddo
      endif
!Compute velocity gradient
      call rzero(graduy,n)
      call gradm1(work1,graduy,work2,vx)
      call dsavg(graduy)
!      call outpost(work1,graduy,work2,pr,t,'GRD')
!Da aggiungere l'eventualità del 3D...
       rmodulate = 0.5*(tanh(0.25*time-10)+1)
      if (nid.eq.0) write(6,*)"MODULATE", rmodulate,time
      return
      end
c-----------------------------------------------------------------------
c-----------------------------------------------------------------------
      subroutine userbc (ix,iy,iz,iside,eg)
c     NOTE ::: This subroutine MAY NOT be called by every process
      include 'SIZE'
      include 'TOTAL'
      include 'NEKUSE'
      integer e, eg
!Imposing Fancy Boundary Conditions...
![Nek5000-users] Robin boundary conditions for the velocity
      common /my_ROBIN/ ROBIN_mask(lx1,ly1,lz1,lelt)
     $                ,      work1(lx1,ly1,lz1,lelt)
     $                ,      work2(lx1,ly1,lz1,lelt)
     $                ,     graduy(lx1,ly1,lz1,lelt)
      common /my_DEBUG/ rmodulate
      e=gllel(eg) ! global element number to processor-local el. #
      ux=1*graduy(ix,iy,iz,e)*rmodulate
      uy=0.0
      uz=0.0
      return
      end
c-----------------------------------------------------------------------
      subroutine useric (ix,iy,iz,ieg)
      include 'SIZE'
      include 'TOTAL'
      include 'NEKUSE'
      ux=1.0*y**2
      uy=0.0
      uz=0.0
      temp=0
      return
      end
c-----------------------------------------------------------------------

The code becomes unstable in the gradient computation due to some
point-to-point oscillations.
As you can see I've also tried to use a ramp function in order to make the
onset of Robin condition more gentle,
but at a given "slip-length amplitude" oscillations deteriorates the
solution.
I use ROBIN_mask in order to chose where to have such BC, but it seems that
I'm still missing something in computing the velocity gradient GRD.

By the way, this is my box.BOX file:

base.rea
2                      spatial dimension  ( < 0 --> generate .rea/.re2 pair)
1                      number of fields
#=======================================================================
#
#    Example of .box file for channel flow
#
#    If nelx (y or z) < 0, then genbox automatically generates the
#                          grid spacing in the x (y or z) direction
#                          with a geometric ratio given by "ratio".
#                          ( ratio=1 implies uniform spacing )
#
#    Note that the character bcs _must_ have 3 spaces.
#
#=======================================================================
#
Box
-32  -16                                          nelx,nely,nelz for Box
0  6.2832 1.                                       x0,x1,gain
-1  1        1.                                       y0,y1,gain
P  ,P  ,v  ,v  ,   ,                                bc's  (3 chars each!)


I was wondering if you might be able to give me some advice.

Best regards,

Francesco
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20170421/fcdeceb5/attachment.html>