This year research on the rheological behavior of concentrated suspen- sions has focused on the connection between viscous suspensions and granu- lar media – from wet to dry. The goal has been to understand the similarities and differences between these two classes of particulate materials. How the microstructure changes as one moves from viscous to rapid granular flow and how this microstructural evolution manifests itself in macroscopic proper- ties such as stress. The effort has been numerical, as the ASD technique has been extended to incorporate particle inertia, which is necessary to model granular systems. Please note that not all of the work described here has been supported by IFPRI; for example, the development work for ASD has been primarily supported by NASA.
The major conclusions of this study are that there basically exist two distinct states as the effect of particle inertia is varied. At low particle inertia (or low Stokes number) – wet suspensions – viscous forces dominate, stresses scale linearly with the shear rate and the fluctuational motion of the particles as characterized by the suspension or granular temperature is small. This is the so-called ‘quenched’ state first predicted by Koch and co-workers (Koch 1990, Tsao & Koch 1995). At high particle inertia (high Stokes number) the fluctuational motion is large, corresponding to a large suspension temperature – the so-called ‘ignited’ state (Sangani et al 1996). At high Stokes number – dry suspensions – stresses scale inertially and are proportional to the shear rate squared. The transition from the quenched to the ignited state occurs at a Stokes number of roughly 10 over the wide range of volume fractions studied here, 0.01 ≤ φ ≤ 0.45.