Permeability (fluid)
The permeability of a material (a rock, a ground, etc) is its capacity to let pass a Fluide, it is an intrinsic characteristic of material independent of the fluid. One calls also hydraulic conductivity (or coefficient of permeability) the relationship between this permeability and viscosity of the fluid (generally the fluid is water). More one medium is permeable plus the fluid will run out quickly. The permeability K is generally expressed in Darcy, which corresponds to the dimension of a surface (m ²) (1Darcy=0.98 10^ (- 12) m ²), while hydraulic conductivity is expressed in m/s.
One should not confuse the permeability with the Porosité which refers it to the volume of the Vide compared to total volume. A high permeability requires a good porosity but the reverse is not true. A very porous rock can have a very low permeability (clays for example) because this porosity is not inter-connected. It should not be confused either with the Capacity for absorption of a matter.
One can calculate the permeability of a rock thanks to the Loi of Darcy (1856), which connects the flow Q to the permeability K via the hydraulic gradient I, surface S and the time interval considered.
The impermeability , opposite notion of the permeability, employment more usually since they are usual objects. The term applies in particular to the matters in episodical but regular contact with water, for example to indicate the capacity of fabrics or the frontages of buildings to prevent the penetration of rainwater. One uses also this term to speak about the capacity to prevent the passage of the air or dust.
See too
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