Is it practically feasible to know or measure the actual velocity of fluid motion or the actual cross-sectional area of fluid flow in a porous medium precisely?
Is this the reason why - Are we interested in using the quantity "Specific Flux" (Or Specific Discharge) rather than focusing on actual velocity of fluid motion?
I would say there are experiments that can determine such quantities (but for practical applications, the actual velocity would be highly distributed in the material, so a specific flux may be a more sound quantifier).
Anyway, velocity measurements would be easiest using MRI - there's several groups using that technique. Perhaps ultrasound imaging or electrical resistance tomography may give insights too.
Dear Dr. Haringa,
Thanks for your reply.
For velocity measurements, we need to know the " actual distance " that a particle of fluid has traveled in moving through a section of a porous material.
But, I am afraid that it is feasible.
In case, if it so, then, do we really need the concept of "Specific Flux" as such?
We can very well use the concept of "mean fluid velocity" directly without bringing the very concept of porosity at all, I suppose.
You can use a superficial velocity indeed (flux divided by cross-sectional area of the material). That is quite often done for monoliths I think, or flows through pipes and so on. But that does not take the length (resistance) of the material into account in itself, so I can imagine for some applications a specific flux may be more instructive. In principle both can be used though.
For any flow condition if you are interested in micro scale magnitude of flow properties hot wire measurement is preferred than other tools, e.g. PIV or LDV devices which are suitable for macro scale measurement like flow flux or flow vector field.
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