TY - JOUR

T1 - Orientation statistics and settling velocity of ellipsoids in decaying turbulence

AU - Siewert, C.

AU - Kunnen, R.P.J.

AU - Meinke, M.

AU - Schröder, W.

PY - 2014

Y1 - 2014

N2 - Motivated by applications in engineering as well as in other disciplines where the motion of
particles in a turbulent flow field is important, the orientation and settling velocity of ellipsoidal
particles in a spatially decaying isotropic turbulent flow are numerically investigated. With respect to
cloud microphysics ellipsoidal particles of various shapes are interpreted as archetypes of regular ice
crystals, i.e., plates and columns approximated by oblate and prolate ellipsoids. The motion of 19
million small and heavy ellipsoidal particles is tracked by a Lagrangian point-particle model based on
Stokes flow conditions. Five types of ellipsoids of revolution such as prolates, spheres, and oblates are
considered. The orientation and settling velocity statistics are gathered at six turbulence intensities
characterized by the turbulent kinetic energy dissipation rate ranging from 30 to 250 cm2s-3. It is
shown that the preferential orientation of ellipsoids is disturbed by the turbulent fluctuations of the
fluid forces and moments. As the turbulence intensity increases the orientation probability distribution
becomes more and more uniform. That is, the settling velocity of the ellipsoids is influenced by the
turbulence level since the drag force is dependent on the orientation. The effect is more pronounced,
the longer the prolate or the flatter the oblate is. The theoretical settling velocity based on the
orientation probability of the non-spherical particles is smaller than that found in the simulation. The
results show the existence of the preferential sweeping phenomenon also for non-spherical particles.
These two effects of turbulence on the motion of ellipsoids change the settling velocity and as such the
swept volume, that is expected to result in modified collision probabilities of ellipsoid-shaped particles.

AB - Motivated by applications in engineering as well as in other disciplines where the motion of
particles in a turbulent flow field is important, the orientation and settling velocity of ellipsoidal
particles in a spatially decaying isotropic turbulent flow are numerically investigated. With respect to
cloud microphysics ellipsoidal particles of various shapes are interpreted as archetypes of regular ice
crystals, i.e., plates and columns approximated by oblate and prolate ellipsoids. The motion of 19
million small and heavy ellipsoidal particles is tracked by a Lagrangian point-particle model based on
Stokes flow conditions. Five types of ellipsoids of revolution such as prolates, spheres, and oblates are
considered. The orientation and settling velocity statistics are gathered at six turbulence intensities
characterized by the turbulent kinetic energy dissipation rate ranging from 30 to 250 cm2s-3. It is
shown that the preferential orientation of ellipsoids is disturbed by the turbulent fluctuations of the
fluid forces and moments. As the turbulence intensity increases the orientation probability distribution
becomes more and more uniform. That is, the settling velocity of the ellipsoids is influenced by the
turbulence level since the drag force is dependent on the orientation. The effect is more pronounced,
the longer the prolate or the flatter the oblate is. The theoretical settling velocity based on the
orientation probability of the non-spherical particles is smaller than that found in the simulation. The
results show the existence of the preferential sweeping phenomenon also for non-spherical particles.
These two effects of turbulence on the motion of ellipsoids change the settling velocity and as such the
swept volume, that is expected to result in modified collision probabilities of ellipsoid-shaped particles.

U2 - 10.1016/j.atmosres.2013.08.011

DO - 10.1016/j.atmosres.2013.08.011

M3 - Article

SN - 0169-8095

VL - 142

SP - 45

EP - 56

JO - Atmospheric Research

JF - Atmospheric Research

ER -