[Nek5000-users] Robin boundary conditions for the velocity

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

Hi Jean-Christophe,

Sorry for the delay with the reply to your emails -- we are looking into them and will give a detailed answer next week.

Thanks,
Aleks


----- Original Message -----
From: nek5000-users at lists.mcs.anl.gov
To: "Nek 5000" <nek5000-users at lists.mcs.anl.gov>
Sent: Wednesday, April 20, 2011 9:03:03 AM
Subject: Re: [Nek5000-users] Robin boundary conditions for the velocity


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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