Re SD0250: fermentation processes
Jaime -
At the risk of providing just a me too comment here, I would agree with
Andy Ford that a model of eutrophication of lakes would have a great deal
of similarities with your problem. This illustrates one of the great
under-utilized advantages of system dynamics. Once a system is thought out
and modeled, the pieces can be plagiarized (!) for some entirely different
use.
I am injecting drug through a guide cannula into the brain of a mouse
(0.5-1.0 microliters) to determine how the drug in that -specific area-
affects behavior. Despite the fact that many people use this technique
with a variety of drugs, behaviors, species, etc., there is almost complete
ignorance about how the drug diffuses (spherically (mostly)) over time. We
modeled this as drug moving from one layer to another through spherical
lamina, like the layers of an onion.
As simple as this sounds, the mathematics of spherical diffusion become
quite complex. In addition, we have found that diffusion depends upon the
lipid solubility, capillary permeability, and so on, complicating the diff
eq solution immensely.
In walking around the last Neuroscience Meeting and asking these questions,
I found that even those people -doing- this work had only the sketchiest
understanding of where the drug was going, and over what time course. And
as usual, their understanding was diametrically opposed to the behavior
of the model. We are currently testing this model against experimental
work in the lab, with qualitative agreement with these non-intuitive
predictions.
Now comes the good part. I was describing this model to a former grad
school buddy, who is now an environmental toxicologist in a govt agency.
He listened carefully and then said, "Wow, I bet that model would work to
describe pollution in a well site. The contaminant enters the well (the
guide cannula), and then diffuses spherically through the surrounding
soil (brain tissue).
There will be important differences, of course, but he is absolutely right.
My model could serve as an excellent starting point, and all the spherical
diffusion math is done.
This brings us back to your situation. Several weeks ago it was suggested
by several wise correspondents that one can benefit from -building- (as
opposed to reading about) some of the classic SD models. The
eutrophication model sounds excellent, but dont just read about it! Build
it, from the simplest core, to the more complex complete model. Study the
behavior of the model at each stage in some detail before adding the next
piece.
Then start all over again, and assemble your fermentation model from simple
pieces. Dont build faster than you can contemplate the new behaviors.
(The building of classic models was referred to in SD0223 by Jim Thompson,
and attributed to John Sterman; Ill forward this message to you, but I
couldnt find any more specific reference.)
Good luck! Keep us (or me) posted.
ed gallaher
gallaher@teleport.com