Air permeability
Introduction
Permeability is the ability of a material to allow a fluid to pass through it without altering its composition. A material is said to be permeable if it allows an appreciable amount of fluid to pass through it in a given time, and impermeable or non-permeable if the amount of fluid is negligible.
The speed with which the fluid passes through the material depends on three basic factors:.
To be permeable, a material must be porous, that is, it must contain empty spaces or pores that allow easy passage of fluid through the material. In turn, such spaces must be interconnected so that the fluid has paths to pass through the material.
On the other hand, we must speak of an "intrinsic permeability" (also called "permeability coefficient"); as a constant linked to the own or internal characteristics of the terrain. And a "real permeability" or Darcy, as a function of the intrinsic permeability plus those of the characteristics of the fluid.
Units
The "intrinsic permeability" in SMD is measured in cm² or m². The unit derived from Darcy's Law[1] is the darcy, and the milidarcy is usually used:.
Conversion:.
Darcy permeability is instead measured in units of velocity: cm/second or m/second.
Determination of real or Darcy permeability
The intrinsic permeability of any porous material is determined by Darcy's formula:
The actual permeability, on the other hand, can be determined directly using Darcy's Law or estimated using empirical tables derived from it.
Actual permeability is a part of the proportional constant in Darcy's Law, which is related to the differences in the velocity of the fluid and its physical properties (for example, its viscosity) over a range of pressure applied to the average porosity. The specific proportional constant for water passing through a medium porosity is the hydraulic conductivity. Intrinsic permeability is a function of porosity, not fluid.