[MITgcm-support] problem with boundary condition on the bottom

[Martin Losch]

Camille,

The no slip condition means, that the tangent velocities AT the  
boundary are zero. For the bottom this mean, that the u/v-components  
at rG(K+1) are zero (if K is the last "wet" point above the bottom,  
in the case of exp0, this would be Nr and rG(Nr+1) is not defined,  
it's really rG(Nr)+drF(Nr)). Now, the model velocites uVel/vVel (or U/ 
V) are define at the centers of the vertical grid cells ABOVE the  
bottom (please have a look at the manual or at GRID.h for "pictures"  
of this), so that the boundary conditions could be written as U(at  
bottom boundary) = U(Nr) + U(Nr+1) = 0, that is, the average of the  
last wet-point U and the non-existing U beneath the bottom is zero,  
so that U(Nr+1)=-U(Nr), and this is actually how this boundary  
conditions implemented, similarily for V and also for the lateral  
boundaries (have a look at mom_u_bottomdrag.F if you feel  
adventurous). Keep in mind that this condition applies only to the  
vertical viscosity terms which is generally not so large (viscAr=1e-3  
or so?), so that the effect of this boundary condition in not so  
dramatic.

Martin

On 27 Nov 2006, at 13:54, camili at noos.fr wrote:

> Hello,
>
> I have a problem to understand how the 'no slip bottom'
> boundary condition is supposed to be respected.
> I have runed  the exp0 presented in your tutorial. When I
> specify the boundary condition on the bottom, it seems to be
> no difference when I specify a free slip or a no slip
> condition. The velocities are the almost the same in the two
> cases.
> I also runed a simulation quite similar with the exp0, but I
> put 5 layer instead of 1.
> Here is the data file I used:
> # Model parameters
> # Continuous equation parameters
>  &PARM01
>  tRef=20.,20.,20.,20.,20.,
>  sRef=10.,10.,10.,10.,10.,
>  viscAz=1.E-2,
>  viscAh=4.E2,
>  diffKhT=4.E2,
>  diffKzT=1.E-2,
>  no_slip_sides=.FALSE.,
>  no_slip_bottom=.FALSE.,
>  beta=1.E-11,
>  tAlpha=2.E-4,
>  sBeta =0.,
>  gravity=9.81,
>  gBaro=9.81,
>  rigidLid=.FALSE.,
>  implicitFreeSurface=.TRUE.,
>  eosType='LINEAR',
>  readBinaryPrec=64,
>  &
> # Elliptic solver parameters
>  &PARM02
>  cg2dMaxIters=1000,
>  cg2dTargetResidual=1.E-7,
>  &
>  # Time stepping parameters
>  &PARM03
>  startTime=0,
>  endTime=15552000.0,
>  deltaTmom=500.0,
>  deltaTtracer=500.0,
>  abEps=0.1,
>  pChkptFreq=604800.0,
>  chkptFreq=604800.0,
>  dumpFreq=604800.0,
>  monitorFreq=604800.,
>  &
> # Gridding parameters
>  &PARM04
>  usingCartesianGrid=.FALSE.,
>  usingSphericalPolarGrid=.TRUE.,
>  delX=100*0.2,
>  delY=100*0.2,
>  delZ=1000.,1000.,1000.,1000.,1000.,
>  phiMin=35.,
>  &
>  &PARM05
>  bathyFile='topog.box',
>  hydrogThetaFile=,
>  hydrogSaltFile=,
>  zonalWindFile='windx.sin_y',
>  meridWindFile=,
>
> And once again the velocities are not null on the bottom. And
> when I run the same simulation with a free slip condition on
> the bottom, the differences between the velocities of the 2
> experiments are really weak.
>
> So could you help me to understand how the boundary condition
> on the bottom is supposed to influence the velocity field,
> specialy in the under layer?
>
> Thanks a lot,
> Camille
>
>
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