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

[Patrick Rosendahl]

Martin,

I forgot to mention that I closed the N+S boundary. Yes, I am expecting 
a static stationary solution.
The E+W boundaries are open, and windstresss will generate some profile 
into the wind direction, causing a netflow in that direction.
This patch will act as if there was a constant pressure pushing as much 
water back until the netflow over the boundary is zero (in this case 5 
pixels left of the E boundary), e.g. the channel is "closed".

I dont expect this to work with time-varying solutions. In this case you 
should average over 1 waveperiod, if that length can be determined at all.

Thank you for the answer, and yes, it helps. So I will apply the 
correction in the momentum-correction-step routine only (also the 
obcs_apply_uv is called from there in line 104).

Patrick

Martin Losch wrote:
> 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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> 
> 
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