[MITgcm-support] ALLOW_ADDFLUID option and U, V momentum equation

[Krishnakumar Rajagopalan]

Hi Angela, Jason
Angela: Thanks for your comments on the Mars GCMs. Admittedly, the practical significance of source momentum in geophysical problems is not likely to ever be great.
Jason : I talked to my post doc advisor, Gérard  Nihous (nihous at hawaii.edu) regarding your comments. As you correctly pointed out, the mass from the source changes the pressure distribution, thereby affecting the momentum equations. This ‘mass introduction’ effect comes from mass conservation and is locally isotropic (via a non-zero divergence operator for the velocity field). The source, however, can also have its own momentum (which is directional). For example, taking your analogy of a hose into a tank, we can imagine hoses with very different diameters for a given mass flow rate, that could also be pointing in different directions; in each case, the resulting flow field would be uniquely different due to this ‘jet’ effect. 
I looked into the MITgcm equations (for the hydrostatic case we have been using). Barring any misunderstanding on my part and calling uVel(i,j,k,bi,bj) and  vVel(i,j,k,bi,bj) the horizontal velocity components of the source (a vertical term would come into play in non-hydrostatic runs), it seems that the jet effect could be  implemented in mom_fluxform.F by adding two terms as follows:
a)  U momentum
 gU(i,j,k,bi,bj) = gU(i,j,k,bi,bj) -
      &    addMass(i,j,k,bi,bj)*uVel(i,j,k,bi,bj)*mass2rUnit/cellvol                  (1)
b) V momentum
gV(i,j,k,bi,bj) = gV(i,j,k,bi,bj) -
     &    addMass(i,j,k,bi,bj)*vVel(i,j,k,bi,bj)*mass2rUnit/cellvol                    (2)
Again, I am grateful to both of you for your comments.
Best regards
Krishnakumar

--- On Fri, 8/26/11, Angela Marie Zalucha <azalucha at MIT.EDU> wrote:


From: Angela Marie Zalucha <azalucha at MIT.EDU>
Subject: Re: [MITgcm-support] ALLOW_ADDFLUID option and U, V momentum equation
To: mitgcm-support at mitgcm.org
Date: Friday, August 26, 2011, 5:20 PM


For Mars the atmosphere (composed of CO2) can condense and sublimate. Usually we assume that if a parcel of atmosphere reaches the condensation temperature, it instantaneously drops to the surface.  So in that case if the parcel was moving (and therefore contained momentum), when it fell on the ground it would come to rest, transfering its momentum to the surface and removing it from the atmosphere.  The momentum effect is ignored in Mars GCMs.

  Angela




On Fri, 26 Aug 2011, Jason Goodman wrote:

> The source/sink terms affect the momentum equation through the pressure field.  If I have a tank of water with a hose running into it, how does the water already in the tank "know" to move away from the hose spigot? Pressure.  Adding fluid increases the pressure at the source, which causes a pressure gradient pushing nearby water away.
> 
> The standard MITGCM solves to find the pressure field needed to make the velocity field non-divergent.  When ALLOW_ADDFLUID is on, it solves for the pressure field needed to make the velocity field have a prescribed divergence: that pressure field then appears in the momentum equation to push water around.
> 
> I don't think there *should* be any other terms added to the momentum equations.
> 
> JCG
> 
> On 8/26/2011 12:44 PM, Angela Marie Zalucha wrote:
>> Kumar,
>> 
>> You're right, the fluid source/sink is assumed not to affect the momentum equations.  For planetary applications this term is small, I don't know about the ocean.
>> 
>>   Angela
>> 
>> 
>> 
>> On Thu, 25 Aug 2011, Krishnakumar Rajagopalan wrote:
>> 
>>> Hi All,
>>> 
>>> We have been customizing the 4x4 global ocean simulation (tutorial_global_oce_latlon) to include sources/sinks  using the ALLOW_ADDFLUID option. Checking with grep " ALLOW_ADDFLUID"  *.F,  it seems that  sources do not explicitly appear in the U or V momentum equations.  If indeed these equations are formally unchanged when the source/sink switch is on,  are we correct assuming that this version of MITgcm does not have a provision for (input) source momentum? A source could have an initial vertical momentum as well, although such cases may be more relevant for non-hydrostatic runs.
>>> Thanks a lot
>>> Kumar
>>> 
>> 
>> _-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-
>> Angela Zalucha, Ph.D.
>> Postdoctoral Associate
>> Department of Earth, Atmospheric,
>>   and Planetary Sciences
>> Massachusetts Institute of Technology
>> 
>> Office located at Southwest Research Institute
>> 1050 Walnut Street, Suite 300
>> Boulder, CO 80302
>> USA
>> (720) 208-7211
>> 
>> 
>> http://web.mit.edu/azalucha/www/index.html
>> _-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-
>> 
>> 
>> 
>> _______________________________________________
>> MITgcm-support mailing list
>> MITgcm-support at mitgcm.org
>> http://mitgcm.org/mailman/listinfo/mitgcm-support
> 
> 

_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-
Angela Zalucha, Ph.D.
Postdoctoral Associate
Department of Earth, Atmospheric,
  and Planetary Sciences
Massachusetts Institute of Technology

Office located at Southwest Research Institute
1050 Walnut Street, Suite 300
Boulder, CO 80302
USA
(720) 208-7211


http://web.mit.edu/azalucha/www/index.html
_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-_-




_______________________________________________
MITgcm-support mailing list
MITgcm-support at mitgcm.org
http://mitgcm.org/mailman/listinfo/mitgcm-support
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mitgcm.org/pipermail/mitgcm-support/attachments/20110830/8718d159/attachment.htm>