Also, the gas kinetic theory on which rapid-flow models is
based, assumes that the molecular collisions are elastic in the
sense that they do not dissipate energy. In converting the theory
to granular materials, the inelasticity of granular impacts is
largely accounted for by the granular energy Eq. (18). But one
must compute integrals over a velocity distribution functions in
order to compute the constitutive relationships, (19–22) and the
methods for computing the distribution function restrict the
system to “nearly elastic particles”, roughly ε=0.9 and above.
This severely limits the materials that may be modeled with
these methods.There is also a more subtle problem. Notice that
all of the constitutive laws in (19–22) obtain their rates through
the granular temperature T. This implicitly assumes that the
magnitude of the thermal velocities (T1/2) is much larger than
the relative velocities induced by the shear (dγ). In terms of Fig.
10 this means that S≪1, which is only observed at extremely
small solid concentrations. However, Fig. 10 shows that over
most of the range of solid concentrations, S≈1, indicating that
dγ∼T1/2. Thus, the mean shear and the temperature are equally
important in driving the relative motion between particles, the
collision rate, and thus the transport rates in a rapid granular
flow. As the kinetic theories depend only on the temperature to
govern transport, they most likely to either underpredict the
transport rate or overpredict the granular temperature