[MITgcm-support] Volume conservation with OBCS

[Pat Gallacher]

Dave,

Congratulations. You finally got an answer from the list, although I'm  
not sure how helpful it is. Is there a diagnostic you can use to check  
the flow through the boundaries? As a pragmatic fix what about setting  
the eastern bc the same as the western? i.e. what comes in must go out.

Pat


On Jul 1, 2008, at 2:48 AM, Martin Losch wrote:

> Hi David,
>
> not that I am an expert on the Orlanski-BCs in the code, but I don't  
> think that there is anything in them that ensures exact conservation  
> of total volume. So before the flow reaches the eastern boundary,  
> there is probably no outflow (can't you check that from the content  
> of state.*.nc (dumpFreq > 0. ) or any other diagnostics that you are  
> using?).
>
> I don't know the size of your domain, but I estimated this: if the  
> domain is (lx,ly,lz) = (50km,20km,1km), then 5h of inflow of 10cm/s  
> would give a volume increase of 3.6e10m^3, corresponding to 36m of  
> SL rise. Probably you did something similar to estimate your  
> expected SL rise in the case of no outflow.
>
> This is what I would try:
> 1. sanity check: turn off orlanski, initialize your entire domain  
> with u=0.1, v=0. and have u_west=0.1 inflow and u_east=0.1 outflow.  
> Along northern and southern boundaries specify v=0.1; (this is  
> similar to verification/exp4). With this configuration (if your  
> topography if constant along the open boundaries), you should not  
> have any SL-rise.
> 2. everything (especially the homogeneous initialization) the same  
> as 1. but turn on orlanski in the east, and see what happens.
> 3. there are flags in the code (see end of obcs_calc), that make the  
> code balance each of the 4 OB inflows indiviually (that is no net  
> inflow at each of the OBs). That's not what you want, but you could  
> modify this code to enforce a global balance, at the cost of  
> breaking the orlanski "dynamics" along the boundaries.
>
> Martin
>
>
> On 30 Jun 2008, at 21:57, David Hebert wrote:
>
>> Dear all,
>>
>> I am trying to simulate a flow over sumberged mountains. I have set  
>> up my simulation so that the western boundary is prescribed with a  
>> constant velocity (0.1 m/s), and the remaining boundaries are  
>> orlanski. I am also using the exactConserv flag for free surface.  
>> Also, I use exf to start the simulation from rest and ramp up the  
>> boundary over one hour.
>>
>> My question is that my surface height, eta, rises unrealistically  
>> to 25m after 5 hours of simulation. It seems that I have flow  
>> coming in the domain, but not out. Is there something in the  
>> orlanski boundary condition that would cause this? I have set the  
>> CMAX to 0.49 and cVelTimeScale to 40.0, which is 10 timesteps.
>>
>> Any help/advice from experienced users is greatly appreciated.
>>
>> Thanks,
>>
>> David
>>
>> _______________________________________________
>> MITgcm-support mailing list
>> MITgcm-support at mitgcm.org
>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>
> _______________________________________________
> MITgcm-support mailing list
> MITgcm-support at mitgcm.org
> http://mitgcm.org/mailman/listinfo/mitgcm-support