[Nek5000-users] gradm1 vs opdiv for computing velocity divergence

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

Dear all,

I am new to Nek5000 and am trying to output the velocity divergence of my 2D, unsteady N-S solution for a flat plate with elliptic leading edge using PN-PN-2 formulation.

I compute divergence using gradm1() and opdiv() and the values I get from the  two don't seem to match the DIVERGENCE  and HELMHOLTZ (p021 and p022) residuals in the .rea file (which are 1e-9 and 1e-11).  I get div ~ 1 using gradm1() and div~1e-2 using opdiv() and the divergence is close to the elliptic leading edge. Am I missing something(scaling factor maybe?) in the computation of divergence here?

      subroutine userchk
      include 'SIZE'
      include 'TOTAL'

      common /scren/   vort (lx1,ly1,lz1,lelv,3) ! x y z components
     $               , work1(lx1,ly1,lz1,lelv)
     $               , work2(lx1,ly1,lz1,lelv)
     $               , dudx(lx1,ly1,lz1,lelv)
     $               , dvdy(lx1,ly1,lz1,lelv)
     $               , div1(lx1,ly1,lz1,lelv)
     $               , div2(lx2,ly2,lz2,lelv)
     $               , div3(lx1,ly1,lz1,lelv)

      n = nx1*ny1*nz1*nelt
      ntot2 = nx2*ny2*nz2*nelt

c    Compute divergence using gradm1()
      call gradm1(dudx,work1,work2,vx)
      call gradm1(work1,dvdy,work2,vy)
      div1 = dudx + dvdy


c    Compute divergence using opdiv()
      call opdiv(div2,vx,vy,vz)
c    opdiv returns bm2*div
      call col2(div2,bm2inv,ntot2)

      call mappr(div3,div2,work1,work2)
      if(nid .eq.0)write(6,*)'Done mapping div2 to xm1'

      call outpost(div1,div3,vx,pr,vort(:,:,:,:,1),
     & 'divvort')

      return
      end

This is from the output file showing the DNORM, DIVEX

Step      2, t= 4.0340639E+00, DT= 1.0892282E-04, C=  0.500 2.1168E+01 2.1168E+01
 Solving for fluid F T T
  9.999999999999999E-012  p22            2           1
          2    Hmholtz VELX:     42   6.4400E-12   8.2479E-02   1.0000E-11
  9.999999999999999E-012  p22            2           1
          2    Hmholtz VELY:     40   6.3878E-12   9.2957E-02   1.0000E-11
         2 U-PRES gmres:    120  6.0275E-08  1.0000E-09  6.4717E-04  5.7658E+00  1.6938E+01
           2  DNORM, DIVEX  6.027476742433520E-008  6.027476742402435E-008
          2   4.0341E+00  2.0912E+01 Fluid done

Thanks
Prahladh Iyer
Postdoctoral Researcher
National Institute of Aerospace, Hampton, Virginia
Resident: NASA Langley Research Center
Email : prahladh.s.iyer at nasa.gov
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