Abstract

# The problem of coagulation of non-spherical particles in
general shear and turbulent shear flows is very important in many fields
including atmospheric and aerosol sciences and in particular in the context of
cloud physics. Evaluation of the coagulation rates of non-spherical ice
crystals with water droplets is an important factor affecting cloud spectrum
evolution, attracting attention of cloud physicists. No general theory of
coagulation of nonspherical orientable particles has been developed. In this
work we develop a model for the coagulation rates (collision volumes) of
nonspherical particles moving in general shear flows. We calculated the
collision volume coagulation rate of nonspherical particles of different sizes
in general shear flows and compared with known data for spherical particles. A
statistical investigation is performed of the collision volumes of nonspherical
ice particles moving in shear flows representative of atmospheric turbulent
flows prevailing in cumulus clouds. We calculated the mutual collisions
(swept) volumes for pairs of spheroidal ice crystals in the absence of
hydrodynamic interactions. For particles of significantly different sizes the
collision volume depends on the shape and dynamic properties of the larger
spheroid and thus constitutes its inherent property. The coagulation rate of
spheroids is significantly different than spheres due to dependence of
spheroid’s relative velocity on shape and orientation. Our statistical study of
*inherent collision volume* showed that in atmospheric conditions the
gravitational mechanism is dominant for small spheroids, whereas for large
inertial spheroids the shear flow field induces orientation distribution, which
leads to an increase of the time-average coagulation rate and its variance, as
compared to spheres. For spheroids of similar sizes the *mutual coagulation
rate* is correlated with characteristic shear of the flow field.