  We numerically study the impact-induced hardening in dense suspensions. We
employ the lattice Boltzmann method and perform simulations of dense
suspensions under impacts, which incorporate the contact between suspended
particles with the free surface of the suspension. Our simulation for a
free-falling impactor on a dense suspension reproduces experimental results,
where rebound takes place for frictional particles at high-speed impact and
high volume fraction shortly after the impact before subsequently sinking. We
found that the shear stress of the suspension is not affected by the impact,
which clearly distinguishes the impact-induced hardening from the discontinuous
shear thickening. Instead, we found the existence of a localized region with
distinctively high value of normal stress corresponding to the dynamically
jammed region. Our simulation indicates that the frictional interaction between
suspended particles is important for the impact-induced hardening to maintain
the dynamically jammed region. Furthermore, persistent homology analysis
successfully elucidates the topological structure of force chains.
