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Permeability of Wood Species

I can see that a Darcy's law permeability equation exists and is as follows:

k = (volume of liquid x length of specimen)/(time of flow x cross-sectional area of specimen x deltaP)

In this equation, there are 2 parameters that I do not understand
1) When the equation refers to the "time of flow" (t) what is meant by that? Is that something like the amount of time it might rain on a piece of wood? Or, the amount of time water might be in contact with the wood?
2), I do not understand deltaP in this case - why would there be any pressure difference from end to end in a specimen.

So, I would like to better understand this computation so that I might empirically be able to derive various permeabilities of wood species.

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As you requested in the message, I shall explain the Darcy's law permeability equation in substantial detail - what are the quantities involved in it and how do they come to be involved.

Darcy's law is fairly well presented at web page http://en.wikipedia.org/wiki/Darcy's_law

Here we review the elements involved in the very first formula at this page:

(Volume Flow Q) = - (Permeability K) * (cross-section area A)
*[ pressure drop (P_{end}- P_{start}) ]
/ [ (viscosity Mu) * (Length L) ] (1)

for a uniform steady flow through some flow tube of cross-section area A and length L.

Volume Flow Q in m^3/sec is the volume of the liquid that crosses the cross-section of some ...

Solution Summary

This solution provides answers to various questions regarding permeability of wood species.