  Using Direct Numerical Simulations (DNS), we examine the effects of Taylor
Reynolds number, $R_\lambda$, and Froude number, $Fr$, on the motion of
settling, bidisperse inertial particles in isotropic turbulence. Particle
accelerations play a key role in the relative motion of bidisperse particles,
and we find that reducing $Fr$ leads to an enhancement of the accelerations,
but a suppression of their intermittency. For Stokes numbers $St>1$, the effect
of $R_\lambda$ on the accelerations is enhanced by gravity, since settling
causes the particle accelerations to be affected by a larger range of flow
scales. The results for the Probability Density Function (PDF) of the particle
relative velocities show that for bidisperse particles, decreasing $Fr$ leads
to an enhancement of their relative velocities in both the vertical (parallel
to gravity) and horizontal directions. Importantly, our results show that even
when the particles are settling very fast, turbulence continues to play a key
role in their vertical relative velocities, and increasingly so as $R_\lambda$
is increased. This occurs because although the settling velocity may be much
larger than typical velocities of the turbulence, due to intermittency, there
are significant regions of the flow where the turbulence contribution to the
particle motion is of the same order as that from gravitational settling...
