Note that k / (ρd3γ2)=(τ /ρd2γ2) / (τd/ k) and is thus the ratio of
Bagnold's inertial to the elastic stress scalings. A similar
parameter was first proposed by Babic et al. [69]. Campbell [28]
gives several interpretations for this parameter, but the most
useful is that k⁎ represents (d/ δi)2 where δi is degree of
deformation expected from the impact by a particle moving at
the shear velocity, dγ, making k⁎ a measure of inertially induced
deformation, much as (τd/k) is a measure of elastic deformation.
As such the value of k⁎ reflects the relative effects of elastic to
inertial forces, i.e. in principle at large k⁎ elastic forces dominate
and at small k⁎, inertial forces dominate.
Fig. 13 shows the dimensionless normal stress, scaled
elastically as τyyd/k plotted against the stiffness parameter. The
plot is marked to show the division into the Elastic–Quasistatic
and Elastic–Inertial regimes, which are differentiated solely by
the fact that the stresses are independent (Elastic–Quasistatic)
or dependent (Elastic–Inertial) on the shear rate γ, and thus on
the parameter k / (ρd3γ2). As expected, the flow transitions from
Elastic–Quasistatic to Elastic–Inertial as the shear rate
increases, (k/(ρd3γ2) decreases). Each point was taken for
three particle diameters and as many as three stiffnesses and
thus each point represents up to 9 overlapping points illustrating
the robustness of the scaling. These data are for a single solid
fraction ν=0.6, which lies below a random close-pack