Ph.D Thesis

Ph.D StudentWirzberger Hod
SubjectDynamics and Coagulation of Non Spherical Particles
Settling in Shear Flows
DepartmentDepartment of Mechanical Engineering
Supervisor PROFESSOR EMERITUS Michael Shapiro


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.