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978-3-8439-3102-1, Reihe Strömungsmechanik

Lennart Schneiders Particle-Resolved Analysis of Turbulent Multiphase Flow by a Cut-Cell Method

154 Seiten, Dissertation Rheinisch-Westfälische Technische Hochschule Aachen (2017), Softcover, A5

Turbulent flows interacting with dispersed particles are encountered in numerous technical and natural processes, such as pulverized fuel combustion, composite material production, pollutant transport in the atmosphere and hydrosphere, formation of ice crystals in clouds, blood flow, or the deposition of dust in the human airways. Although with the aid of modern supercomputers increasingly accurate predictions of complex turbulent suspensions could be realized in principle, the inadequacy or lack of particle and turbulence models lowers the significance of such simulations. To validate and to derive new modeling approaches, so-called particle-resolved simulations, in which the particle surfaces and the locally disturbed flow field in their vicinity are fully resolved, are ideally suited. In this thesis, this technique is applied to study the interaction of isotropic turbulence with particles whose diameter is on the order of the smallest eddies of the carrier flow.

A new numerical method for particle-resolved simulations is presented. The new scheme exhibits an accurate and robust discretization near moving boundaries, a strict conservation of mass, and a dynamic load balancing strategy for efficient parallel computing on dynamic meshes. Moreover, spurious force oscillations, which are inherent to this class of methods, are suppressed by the formulation of smoothed discretization operators. A new explicit Runge-Kutta time-stepping scheme is proposed which is highly efficient for fluid-structure interaction problems.

The new numerical framework is used to study the interaction of decaying isotropic turbulence with tens of thousands of spherical and non-spherical particles. A detailed analysis of the budget of turbulent kinetic energy evidences the particles to absorb energy from the large scales of the carrier flow. The particles locally increase the level of dissipation due to the intense strain rate generated near the particle surfaces. An analytical expression for the instantaneous rate of viscous dissipation induced by each particle is derived from the conservation laws and verified numerically. Based on these findings, an improved model for the kinetic energy balance of the multiphase system is proposed. Finally, the accuracy of two state-of-the-art Lagrangian particle models, one derived for spherical and one for ellipsoidal particles, is assessed via a direct comparison with the results of the particle-resolved simulations.