ABSTRACT
In order to better understand the phenomenon of viscoelasticity in particulate porous media, several investigators have used idealized porous media, for which experimental results could be compared with theoretical predictions computed using reasonable fluid constitutive equations. The flow geometries employed include converging channels, tubes consisting of short, alternating segments of two different diameters, and tubes having sinusoidal axial variations in diameter. From these studies, a number of clear results have emerged. As with real porous media. "shear thickening" is observed for the flow of dilute polymer solutions, and distinct maxima are obtained [48-50]. Also, the extent of flow resistance depends on the geometry-for wedge-shaped channels there is no shear thickening [50], while for conical channels the apparent shear viscosity is several times the true value [48-50]; this surprising difference can be explained, at least qualitatively, by calculations done using the upper convected Maxwell
rheological equation [51]. There is also quantitative agreement between computations done with the Oldroyd B model for flow through a corrugated tube [5254] and corresponding experiments utilizing constant-viscosity elastic fluids [5455]. but in this case shear thickening is neither predicted nor observed over a shear rate range in which the experimental fluids behave like Oldroyd B liquids. Very large elastic effects, though. can be measured in this flow geometry for highly viscoelastic (but shear-thinning) liquids [55.561. and such effects can also be calculated using appropriate constitutive equations [53].
