Enhanced and suppressed multiscale dispersion of bidisperse inertial particles due to gravity

Using direct numerical simulation (DNS), we investigate how gravity modifies the multiscale dispersion of bidisperse inertial particles in isotropic turbulence. The DNS has a Taylor Reynolds number Rλ=398, and we simulate Stokes numbers (based on the Kolmogorov timescale) in the range St≤3 and consider Froude numbers Fr=0.052 and, corresponding to strong gravity and no gravity, respectively. The degree of bidispersity is quantified by the difference in the Stokes number of the particles, |ΔSt|. We first consider the mean-square separation of bidisperse particle pairs and find that, without gravity (i.e., Fr=), bidispersity leads to an enhancement of the the mean-square separation over a significant range of scales. When |ΔSt|≥O(1), the relative dispersion is further enhanced by gravity due to the large difference in the settling velocities of the two particles. However, when |ΔSt| 1, gravity suppresses the relative dispersion as the settling velocity contribution is small, and gravity suppresses the nonlocal contribution to the particle dynamics. In order to gain further insights, we consider separately the relative dispersion in the vertical (parallel to gravity) and horizontal directions. As expected, the vertical relative dispersion can be strongly enhanced by gravity due to differences in the settling velocities of the two particles. However, a key finding of our study is that gravity can also significantly enhance the horizontal relative dispersion. This nontrivial effect occurs because fast settling particles experience rapid fluctuations in the fluid velocity field along their trajectory, leading to enhanced particle accelerations and relative velocities. For sufficiently large initial particle separations, however, gravity can lead to a suppression of the horizontal relative dispersion. We also compute the probability density function (PDF) of the particle-pair dispersion. Our results for these PDFs show that even when Fr 1 and |ΔSt|≥O(1), the vertical relative dispersion of the particles can be strongly affected by turbulence. This occurs because although the settling velocity contribution to the relative motion is much larger than the "typical" velocities of the turbulence when Fr 1 and |ΔSt|≥O(1), due to intermittency, there are significant regions of the flow where the turbulent velocities are of the same order as the settling velocity. These findings imply that in many applications where Rλ 1 the effect of turbulence on the vertical relative dispersion of settling bidisperse particles may never be ignored, even if the particles are settling rapidly.

Duke Scholars

Author Andrew Bragg Civil and Environmental Engineering