2.2 iSALE simulation set-up
Davison et al (2011) show that simulations at this scale can be
approximated well with resolutions of 12-20 cells per projectile radius
(CPPR), with little increase in accuracy beyond 20 CPPR for crater
depth. As such, our simulations employ an Eulerian grid with square
1.5785x10-4 m cells, corresponding to resolutions of
20 CPPR. The high-resolution zone (HRZ) of this mesh consists of 220 x
360 cells. Beyond the HRZ, cell-size increases by a constant fraction of
1.05. As these are 2D simulations, we use cylindrical symmetry to
approximate vertical impacts along the axis of symmetry (the y-axis).
The right and left sides of the mesh have free-slip boundary conditions.
The bottom and top have no-slip and outflow boundary conditions,
respectively. Projectiles were simulated as 3.175 mm (aluminum and
quartz) and 6.35 mm (quartz only) spheres with impact velocities that
correspond to those reported from the experiments by Marchi et al.
(2020).
For small-scale simulations such as these, the time when the transient
crater is formed (when there is the maximum volume) is generally the
same time as when the crater has finished forming (change in depth and
diameter is minimized). Further, observations from the in situ
experiments from Marchi et al (2020) show that the modification stage
for impacts into these metallic materials is virtually non-existent,
resulting in crater rims that appear frozen in place. For the materials
in this work, the timeframe in which the transient crater is formed (and
therefore, where we are approximating the end of the crater formation)
seems to occur after 30-80 microseconds. For continuity, we take the
longer end-member of 80 microseconds as the final crater stage to
compare the results of all simulated craters with the experimental
results. We ran simulations through 150 microseconds.
We simulated impacts into four different target materials with two
different impactor materials based on laboratory measurements of the
targets used in Marchi et al (2020). Table 3 lists the full details of
the properties for each material. The quartz projectiles were
approximated by using the ANEOS equation of state for granite (Ivanov,
2000 and Pierazzo et al., 1997) and a thermal softening model after
Ohnaka et al. (1995). Strength in the quartz projectiles is approximated
by the “ROCK” strength model (Collins, 2004)—a pressure-dependent
strength model which reduces shear strength with the accumulation of
damage. The aluminum projectiles were approximated using the Tillotson
equation of state for aluminum (Tillotson, 1962) and the JC strength and
thermal softening (Johnson and Cook, 1983) models.
The experiments performed by Marchi et al. (2020) simulated in this work
were on three distinct manufactured types, henceforth referred to as
I-90 (a), I-90 (b), and I-94 (b) where each refers to Ingot (I), the
compositional make-up (90 or 94 % Fe, and 10 or 6 % Ni, respectively),
and which batch of experiments (a - hypervelocity experiments which took
place in 2016 versus b- hypervelocity experiments that took place in
2019). We also simulated the five vertical impacts into the Gibeon
meteorite targets. All target materials were simulated using the ANEOS
equation of state for iron (Ivanov, 2000) and the JC strength and
thermal softening (Johnson and Cook, 1983) models. For each distinct
target material, we determined the necessary JC strength model
parameters (see Table 2) from mechanical characterization tests
performed in the laboratory. Finally, we define the surface temperature
of the target in the simulation to match that of the temperature of the
target from the in situ impact experiments (as temperature was noted to
have a distinct effect on the material strength, discussed further in
the next section).