Darcy's Law

It is often required to be able to describe the flow of fluid through porous media, for example in a fish tank filter. To do this we use Darcy's law. 

q= Volumetric Flow Rate (m3/s)

A= Cross-Section Area (m2)

k= coefficient of permeability(m/s)

dh/dx = hydraulic gradient (no units)

In more detail

        Porosity                 The volume of empty space within the specimen, which is expressed as a fraction.

        Permeability         This is abliity of a medium to alow a flow through it. It is also known as the hydraulic conductivity. It's coefficient is denoted as k.

        dh                           Known as rhe difference in the head level in civil, however also known as pressudifference in other disaplins. In other words it is the difference in height of the fluid in the piesometers either side of the specimen. 

        dx                            The length of the porous material the fluid flows though.

        dh/dx                      The change of head over the distance in which the head occurs, this is also known as the hydraulic gradient.

        Darcy's Velocity     q/A is "Darcy's velocity" and is the velocity of the fluid if you assume it had no porous material obstructing its flow.

        True Velocity          This is Darcy's velocity/ porosity. The true velocity of the fluid is larger than Darcy's velocity due to it moving though porous material.