The influence of Reynolds and Froude number on the motion of bidisperse inertialparticles in turbulence

Using Direct Numerical Simulations (DNS), we consider the effects of Taylor Reynolds number ($R_\lambda$), Froude number ($Fr$), and Stokes number ($St$) on the motion of bidisperse particles in turbulence. Particle accelerations play a key role in the relative motion of bidisperse particles, and we find that reducing $Fr$ enhances the accelerations, but suppresses their intermittency. Probability Density Functions (PDF) of the relative velocities show that even when the particles are settling rapidly, turbulence still plays a key role in their motion parallel to the direction of gravity, and all the more 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 turbulent velocities are of the same order as the settling velocity. Increasing $R_\lambda$ enhances the non-Gaussianity of the relative velocity PDFs, while reducing $Fr$ has the opposite effect, and for $Fr\ll 1$, the PDFs become close to Gaussian (except for weak bidispersity). Finally, we observe that low-order statistics related to collision rates, while strongly affected by $Fr$ and $St$, are only very weakly affected by $R_\lambda$ when $St\leq O(1)$.