The characteristic flow equation: A tool for engineers and scientists

A relationship between pressure and flow rate is presented for describing the flow regime for laboratory permeability tests. The equation is unique to the test specimen, the fluid and the apparatus used, and is termed the Characteristic Flow Equation or “CFE”. A CFE can be established for any laboratory conductivity test including rigid-wall permeability, permittivity and transmissivity tests.The Characteristic Flow Equation is a quadratic formula consisting of two terms; one where head loss is directly proportional to the flow velocity and one where head loss is proportional to the velocity squared. These terms account for two sources of head loss that accumulate as the fluid passes into, through, and out of, the material; one that is due to viscous resistance referred to as the “friction term”, and one that is due to losses of kinetic energy referred to as the “inertia term”.The definitions of the variables that comprise the two coefficients in a CFE are a function of the hydraulic details of the test method and the associated measurement parameters.This paper presents a review of research on non-linear flow in porous media, the application of the Characteristic Flow Equation (CFE) for determining the hydraulic permittivity and transmissivity of geosynthetic materials using air as the test fluid instead of water, and an algorithm for determining the Percent Open Area of woven geotextiles.An example showing how the CFE theory can be used to evaluate the hydraulic behavior of a laboratory conductivity test, as well as other applications of the CFE and the potential benefits of testing with air instead of water are discussed in closing.