Turbulent particle suspensions

Particle suspensions refer to tiny solid particles embedded in a carrier fluid which can be either a gas or a liquid. The mixture is thus a two-phase flow and most often the fluid motion is turbulent and chaotic rather than smooth and regular. The vast majority of studies of such two-phase flow mixtures have assumed that the particles are spheres. In practice, however, the particle shape is different from spherical, for instance micro-organisms like phytoplankton in the ocean and volcanic ash particles in the atmosphere. In the first part of the project we studied the translational and rotational motion of prolate spheroids which represent a special case of tri-axial ellipsoids. We simulated turbulent flow in a plane channel and examined how the elongated rod-like particles oriented themselves in the turbulent flow field and how the particle orientation varied from the near-wall region to the center of the channel. In a simplified setting, a chaotic behaviour may occur and we have explored in which parameter ranges (particle inertia and particle elongation) chaotic rotation arises. The results from our extensive computer simulations have also been compared with data from experimental measurements at KTH in Stockholm where a wood fiber suspension was allowed to flow along a slightly inclined plate, a so-called water table. The orientation of the wood fibers has particular relevance for the product quality in paper making. Together with a group at University of Udine in Italy, we have made a comprehensive investigation of how rod-like particles translate at a different rate than the fluid. The so-called slip-velocity has been examined over a fairly wide parameter space (shape, inertia). Interactions between spherical particles and a turbulent fluid flow have been investigated in great detail with the view to understand the energy conversion mechanisms between the fluid and particle phases. In this way we introduced the novel concept particle dissipation which represents a loss of mechanical energy from the fluid-particle suspension. A continuum model of particulate flow based on micro-polar fluid theory has been examined and we have suggested a bridge to close the gap between conventional continuum mechanical modelling and the discrete Lagrangian point-particle approach which we are using in the computer simulations. Since almost all our studies were carried out in a plane channel flow, we investigated jointly with University of California at Berkeley how the high anistropy of the fluid rotation in the near-wall regions almost vanished in the center of the channel. This is believed to have major influence on the anisotropy of particle rotation in the different parts of the channel. We next extended our investigations to include oblate spheroids in addition to prolate spheroids. This enabled us to explore the rather different orientational and rotational behaviour of disk-like compared with rod-like particles. This pioneering study was followed up by an investigation of non-inertial spheroids in channel flow turbulence. Such particles passively translate along with the flow. But due to their non-spherical shape they rotate differently from the fluid rotation. Finally, the behavior of non-axisymmetric ellipsoids, i.e. with three different axes, was studied. We observed that they sometimes rotated as prolate spheroids and sometimes as oblate spheroids and their mode of rotation depended on the ratios between the three different particle axes. Nearly 50 researchers participated in EUROMECH Colloquium no 566 on "Anisotropic Particles in Turbulence" organized by the project leader 9th -11th June 2015 at NTNU in Trondheim. Mr Niranjan R. Challabotla who has been funded by the project will defend his doctoral thesis Nonspherical particle suspension in wall turbulence on 11th November 2016.

Suspensions of tiny solid particles in a carrier fluid are most often treated as a mixture of solid spheres and a gas or liquid fluid phase. In practice, however, the particle shape is different from spherical, for instance micro-organisms like phytoplank ton in the ocean and volcanic ash particles in the atmosphere. We aim to study the motion of more complex particles for which a tri-axial ellipsoid will serve as the prototype. Both the translational and rotational motion of such particles will be studied in a variety of different computer experiments. Some of the simulations will be supplemented by laboratory measurements and flow visualisations in a water-table flow at KTH in Stockholm. Different combinations of the ratios between the three semi-axes of the ellipsoids will studied and results also compared with the limiting case of prolate spheroids. The dynamics of ellipsoidal particles in massively separated flows will also be investigated. The project aims to add further insight into the dynamics o f tri-axial ellipsoidal particles. A continuum model of particulate flow based on micro-polar fluid theory will be examined in great detail and compared with data from the computer simulations. We believe that we will be able to bridge the gap between the continuum mechanical modelling and the discrete Lagrangian point-particle approach which is to be used in the computational experiments.