(249d) Instability Formation in Granular Taylor-Couette Flows

Flow instabilities, such as clustering inhomogeneities, can dramatically influence microscopic and macroscopic properties of granular materials. Taylor-Couette flows represent a simple model geometry encompassing both shear and physical boundary interactions ? essential ingredients of practical flows. Taylor?Couette fluid flow research in past years has yielded extremely rich information on dynamical behavior. Equivalent steps for granular flows are far from complete.

Instabilities in Taylor-Couette flows of dilute, monodisperse, inelastic granular materials were studied with the use of particle dynamic simulations. The development of density inhomogeneities was observed in pseudo two-dimensional and three-dimensional systems. Fully-developed states were quantified using Fourier methods. For the pseudo two-dimensional case, two main instability modes are observed. The first instability constitutes a density gradient in the radial direction which develops as result of a granular temperature gradient along the inner-outter cylinder gap. As the amount of energy dissipation in the system is increased, a second instability forms in the azimuthal direction analogous to the previously reported particle plug development in one-dimensional flows. When similar flows are studied in a three dimensional system, an additional instability mode develops along the axis. The density inhomogeneities strongly impact macroscopic behavior, as stresses and granular temperatures deviate by up to an order of magnitude from mean values. The influence of solids fraction, material elasticity, surface friction, and gap width on instability formation is also shown.