[MITgcm-support] periodic but "closed" channel flow

[Martin Losch]

Patrick,
what are you trying to do? a zonally periodic channel that still has  
open boundaries in the zonal direction? Why?
On average, a zonal channel, unless you have a localized unbalanced  
mass sources somewhere (e.g. freshwater flux at the surface), should  
have no zonal netflow anywhere. However, if you have a time varying  
solution, there will be fluctuations in the mass flux (with zero  
mean). Removing these mass fluxes as you seem to have done will  
introduce shocks to the system at every timestep. Also I don't know  
if obcs is the best place to do that. If you have to change the  
entire flow field, I would really try to do it in the correction step  
(momentum_correction_step.F), all you need to do is include GRID.h  
into that file and then you can do what you want to do.

Does that help?

Martin

On 5 Mar 2008, at 14:53, Patrick Rosendahl wrote:

> Hello all,
>
> this is code I use for simulating a infinite channel (periodic  
> domain in EW-direction) with return flow (netflow in any section is  
> 0).
>
> It is currently hooked up in obcs_apply_uv.F via
>
>         CALL OBCS_MODIFY( bi, bj, uFld, vFld, myThid )
>
> and implemented like this in obcs_calc.F inside the IF- 
> useOBCSbalance-THEN switch
>
> C
> C calculate the mass transfer across some section
> C
>         Tr_T = 0. _d 0
>         Ar_T = 0. _d 0
>         DO K=1,Nr
>           DO J=1-Oly,sNy+Oly
>              I_obc = sNx-5
>              IF (I_obc.ne.0) THEN
>               Ar = drF(k)*hFacC(I_obc,j,k,bi,bj)*dyG(I_obc,j,bi,bj)  
> * _maskW(I_obc,J,K,bi,bj)
>               Ar_T = Ar_T + Ar
>               Tr_T = Tr_T + Ar * uVel(I_obc,J,K,bi,bj)
>             ENDIF
>           ENDDO
>         ENDDO
>         _GLOBAL_SUM_R8( Ar_T , myThid )
>         _GLOBAL_SUM_R8( Tr_T , myThid )
>         Tr_T = (0. - Tr_T)/Ar_T
> C
> C correct the entire u-field
> C
>         DO K=1,Nr
>           DO J=1-Oly,sNy+Oly
>             DO I=1-Olx,sNx+Olx
>               uVel(I,J,K,bi,bj) = uVel(I,J,K,bi,bj) + Tr_T * _maskW 
> (I_obc,J,K,bi,bj)
>             ENDDO
>           ENDDO
>         ENDDO
>
>
>
>
> I first tried the Orlanski-type BC, but it does not work very well.  
> Probably there is a proper way to hook the code.
>
> Patrick
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