5.2. Rapid granular stresses
Rapid granular flows provide a forum in which it is possible
to observe the interplay between the collisional stresses (Eq. (1))
and streaming stresses (Eq. (2)). (This is largely because the
flow will be in the rapid regime whenever the concentration is
small enough that streaming stresses become significant.) Fig.
12 shows the dimensionless shear stresses, scaled as in Eq. (11),
generated in a simple shear flow. Note that the simulation data
asymptotes to ∞ both as ν→0 and as ν approaches a “random
close pack” (the concentration of a randomly assembled volume
of spheres) at ν≈0.64. The latter limit occurs because in a
random close pack, the particles are in intimate contact and the
collision rate is approaching ∞; thus this asymptote occurs as a
singularity in the contact stress tensor (Eq. (1)). It is also a
byproduct of the rigid particle assumptions that lie at the heart of
rapid flow ideas; because the particles cannot deform, it would
require an infinite stress to shear a material near the random
close pack. But the singularity would disappear for real particles
with finite elastic moduli, as by deforming their shape, the
particles can be forced to shear at any concentration. In addition,
the stresses demonstrate a singular behavior as ν→0. This is a
reflection of the fact that the granular temperature becomes
infinite in that limit (as seen in Fig. 10). As the granular
temperature is closely related to the streaming stresses, obvious
from Eq. (14), the stresses demonstrate the same asymptotic
behavior as the granular temperature.