Andrew,
I don't think I have time right now to dig through your code. I'll
keep it in my inbox and hopefully I'll get to it at some point. One
thing I would say is that I think the mass balance should be checked
via
var * mesh.getCellVolumes() - var.getOld() * mesh.getCellVolumes()
the value returned by that calculation should be equal to
allSourceTerms * mesh.getCellVolumes() * dt + allFluxesIn *
mesh.getFaceAreas() * dt
where you need to construct allSourceTerms and allFluxesIn as a
CellVariable and FaceVariable, respectively. You may have in fact done
this in another way. Is this basically what you did?
On Mon, May 18, 2009 at 11:56 AM, A.S.Reeve <[email protected]> wrote:
> Jonathan,
>
> Thanks for the quick response. The '_FixedBCFaceGradVariable' method was
> what I was looking for, but I still can't seem to get a reasonable mass
> (water) balance for my simulation.
>
> What I'd like to get is a list of all fluxes through sources and sinks to
> the model, a list of all the changes in water stored within each model cell,
> and then a comparision between these to verify that the amount of water lost
> (or gained) through sources and sinks is equal to the change in water stored
> within the model. I initially was doing this just to look at where water was
> coming from within the model but noticed large mass balance errors.
>
> I've tried to do this several ways with no success in closing the difference
> between the sum of sources/sinks and the sum of the changes in stored water.
> I must be doing something wrong when I set up these calculations.
>
> I've attached my script for doing this. If you run it, you'll see the
> following output:
>
> 1) change in water stored (volume of water released from storafe over
> the time step)
>
> 2) rech (source term, volume of water entering the top row of cells
> across the model.)
>
> 3) faceFluxes (fluxes across type 1 boundaries, calculated from
> divergence of cells abutting fixedValue faces;
> flux=divergence-storageChange)
>
> 4) sum of fluxes (sum of fluxes plus sum of storage changes, should
> equal zero)
>
> 5) BC discharge (sum of fluxes across faces using
> '_FixedBCFaceGradVariable' method.)
>
> I consistently get large mass balance errors between the sum of sources and
> change in storage.
>
> Andy
>
> Andrew Reeve
> Associate Professor
> Dept. of Earth Sciences
> University of Maine
> 207-581-2353
>
> On Tue, 12 May 2009, Jonathan Guyer wrote:
>
>>
>>
>> On May 12, 2009, at 11:00 AM, A.S.Reeve wrote:
>>
>>> Is there a simple way to calculate the change in mass stored in a cell
>>> over a single time step?
>>
>> Unless I'm missing something, I think this is just (mass - mass.getOld()),
>> isn't it?
>>
>>
>>> I'm trying to do this using a flux assigned
>>> to a FaceVariable with the operations (kond and head are CellVaribles):
>>>
>>> flux=-head.getFaceGrad()*kond.getHarmonicFaceValue()
>>> flux.getDivergence()
>>>
>>> This seems to work at internal cells but I'm having trouble sorting this
>>> out at cells with fixedValue boundary condtions assigned to one of their
>>> faces. How does one seperate changes in storage in these cells from
>>> influx
>>> across face with a fixedValue conditions? Does the divergence operator
>>> in FiPy lump these two terms (storage and flux across face) together in
>>> these cells?
>>
>> Neither the gradient nor divergence operators see boundary conditions by
>> default. This is on our to-do list. There is a workaround for the gradient,
>> though, which we originally wrote up for Zhiwen Liang, and which has since
>> been included in FiPy 2.0. You would write:
>>
>> > > > from fipy.variables.fixedBCFaceGradVariable import > > >
>> > > > _FixedBCFaceGradVariable
>> > > > flux=-_FixedBCFaceGradVariable(head, > > >
>> > > > boundaryConditions=???)*kond.getHarmonicFaceValue()
>>
>> where the boundaryConditions argument takes the same tuple that you pass
>> to solve. The divergence should be fine.
>>
>> That being said, I don't think you need any of this for this particular
>> task.
>
--
Daniel Wheeler
