Bubble Induced Mixing with Smoothed Particle Hydrodynamics

W ithin this thesis, the Computational Fluid Dynamics method Smoothed Particle Hydrodynamics (SPH) is applied to simulate bubble induced mixing in wastewater treatment plants for the first time. To do so, a new SPH multiphase algorithm is presented that is especially designed to be very cheap in terms of computational costs and solves the problem that air bubbles are only stable for a short time in SPH and require a high resolution. The latter algorithm involves a novel coupling mechanism, a buoyancy model and an optional controller mechanism. A method to handle multiphase-inflow into a hydrodynamic tank is realized, which is a novelty as this topic is not described until now for the SPH methodology. In order to allow for mass conservation in long-term simulations, a simple heuristic for degassing air bubbles at the surface is proposed. To further improve the realism and computational efficiency of the used wastewater treament plant model, particle refinement is studied and successfully applied to the inflow particles. Further enhancements include the implementation of the well known density correction called Moving Least Squares that allows for a better behaviour near the surface and significantly delays a weak instability occurring in long-time simulations of hydrodynamic tanks by more than an order of magnitude. It is shown that a different initialization process for tank-simulations improves not only the physical behaviour as well but reduces the initialization overhead as well to a fraction of its previous workload. All the previous methods are combined together and as a result the expected evolution of two global vortices inside a tank is found. Therefore, this work successfully solves one of the problems needed to overcome for computational modelling of wastewater treatment plants with SPH in the future.