  Motivated by its importance for modeling dust particle growth in
protoplanetary disks, we study turbulence-induced collision statistics of
inertial particles as a function of the particle friction time, tau_p. We show
that turbulent clustering significantly enhances the collision rate for
particles of similar sizes with tau_p corresponding to the inertial range of
the flow. If the friction time, tau_p,h, of the larger particle is in the
inertial range, the collision kernel per unit cross section increases with
increasing friction time, tau_p,l, of the smaller particle, and reaches the
maximum at tau_p,l = tau_p,h, where the clustering effect peaks. This feature
is not captured by the commonly-used kernel formula, which neglects the effect
of clustering. We argue that turbulent clustering helps alleviate the bouncing
barrier problem for planetesimal formation. We also investigate the collision
velocity statistics using a collision-rate weighting factor to account for
higher collision frequency for particle pairs with larger relative velocity.
For tau_p,h in the inertial range, the rms relative velocity with
collision-rate weighting is found to be invariant with tau_p,l and scales with
tau_p,h roughly as ~ tau_p,h^(1/2). The weighting factor favors collisions with
larger relative velocity, and including it leads to more destructive and less
sticking collisions. We compare two collision kernel formulations based on
spherical and cylindrical geometries. The two formulations give consistent
results for the collision rate and the collision-rate weighted statistics,
except that the spherical formulation predicts more head-on collisions than the
cylindrical formulation.
