[Nek5000-users] Robin boundary conditions for the velocity

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

I actually miswrote my boundary conditions. What I want to impose is:

(U.n) u + (p * IdentityMatrix + inv(Re) * grad(u)) . n = 0

that is, if n = (0,0,1)^T = ez:

p = 0
dux/dz = Re * W * ux
duy/dz = Re * W * uy
duz/dz = Re * W * uz

Concerning the velocity bc, I assume once again that the following code
would do the trick:

subroutine usrchk

[...]
common /mygrad/ gradux(lx1,ly1,lz1,lelt), ... , gradwz(lx1,ly1,lz1,lelt)

call gradm1(graduv,graduy,graduz,vx)
call gradm1(gradvx,gradvy,gradvz,vy)
call gradm1(gradwx,gradwy,gradz,vz)

[...]

and then in userbc

 subroutine userbc (ix,iy,iz,iside,eg)
     include 'SIZE'
     include 'TOTAL'
     include 'NEKUSE'

     common /mygrad/ ...

     integer e,eg

     e = gllel(eg) ! global element number to processor-local el. #

c Assuming param(2)<0 in .rea, Re = 1./param(2) since in connect2 or subs1 I
do not remember there is first param(2) = -1/param(2)

     ux = (1./param(2)) * vz(ix,iy,iz,ie) * graduz(ix,iy,iz,ie)
     uy = (1./param(2)) * vz(ix,iy,iz,ie) * gradvz(ix,iy,iz,ie)
     uz = (1./param(2)) * vz(ix,iy,iz,ie) * gradwz(ix,iy,iz,ie)


However, how do I make the code take this into account along with keeping
the p = 0 imposed at the outflow? I presume I should use the 'o' bc
character instead of 'O' in my .rea boundary conditions definition section.
Is that correct?

Best regards,
JC

On 19 April 2011 19:34, Jean-Christophe Loiseau <loiseau.jc at gmail.com>wrote:

> Hi Nek's.
>
> I would like to use the following boundary condition as my outflow
> condition for the adjoint perturbation mode:
>
> p = du/dz = dv/dz = 0
> dw/dz = Re * Uz * uz
>
> where Uz is the z-component of the base flow. I would be tempted to do
> something like:
>
> subroutine usrchk
>
> [...]
> common /mygrad/ gradwx(lx1,ly1,lz1,lelt)
>              $             gradwy(lx1,ly1,lz1,lelt)
>              $             gradw(zlx1,ly1,lz1,lelt)
>
> call gradm1(gradwx,gradwy,gradz,vz)
>
> [...]
>
> and then in userbc
>
>  subroutine userbc (ix,iy,iz,iside,eg)
>      include 'SIZE'
>      include 'TOTAL'
>      include 'NEKUSE'
>
>      common /mygrad/ gradwx(lx1,ly1,lz1,lelt)
>     $              , gradwy(lx1,ly1,lz1,lelt)
>     $              , gradwz(lx1,ly1,lz1,lelt)
>
>      integer e,eg
>
>      e = gllel(eg) ! global element number to processor-local el. #
>
> c Assuming param(2)<0 in .rea, Re = 1./param(2) since in connect2 or subs1
> I do not remember there is first param(2) = -1/param(2)
>
>      uz = (1./param(2)) * vz(ix,iy,iz,ie) * gradwz(ix,iy,iz,e)
>
> However I am not sure how to handle the other boundary conditions on u, v
> and p.
>
> Regards,
> JC
>
> --
> Jean-Christophe
>



-- 
Jean-Christophe
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