2019 EJRNL PP MOHAMMADREZA MOMENIFAR 1.pdf?
Terbatas Ratnasari
» ITB
Terbatas Ratnasari
» ITB
Using direct numerical simulations, we examine the effects of the Taylor Reynolds
number, R? ? u
?/? (where u, ?, and ? denote the fluid root-mean-squared velocity, the
Taylor microscale and the fluid kinematic viscosity, respectively), and Froude number,
Fr ? a?/g (where a? is the Kolmogorov acceleration, and g is the acceleration due to
gravity), on the motion of small, spherical, settling, bidisperse inertial particles (characterized
by their Stokes number St ? ?p/??, which is the ratio of the particle response
time to the Kolmogorov timescale) 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? 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 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? 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. Increasing R? enhances the non-Gaussianity of the relative velocity PDFs, while
reducing Fr has the opposite effect, and for fast settling particles, the PDFs become
approximately Gaussian. Finally, we observe that low-order statistics such as the radial
distribution function and the particle collision kernel are strongly affected by Fr and St,
and especially by the degree of bidispersity of the particles. However, we also find that
these low-order statistics are very weakly affected by R? when St O(1), irrespective of
the degree of bidispersity. Therefore, although the mechanisms controlling the collision
rates of monodisperse and bidisperse particles are different, they share the property of a
weak sensitivity to R? when St O(1).