Now showing 1 - 4 of 4
  • Publication
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    A deterministic breakup model for Euler–Lagrange simulations of turbulent microbubble-laden flows
    (Pergamon Press, 2020-02) ;
    © 2019 The Authors The present study is concerned with breakup models for microbubbles in turbulent flows. Analyzing the different physical mechanisms responsible for breakup based on a literature review, breakage due to turbulent fluctuations in the inertial subrange is identified as the most important one. Widely used breakup models for this mechanism are discussed concerning their advantages and drawbacks with special emphasis on thoughts how these models developed in the Euler-Euler context can be transferred into the Euler-Lagrange approach favored in this study. The most promising model is chosen as a basis and then implemented in an efficient bubble tracking scheme relying on the large-eddy simulation technique. The size of the daughter bubbles is deterministically estimated based on the breakup mechanism. Furthermore, a physically motivated model for the axis along which bubbles separate and for the separation velocity of the daughter bubbles is developed. Lastly, an estimate of the time lag between two successive breakup processes is provided. The simulation methodology is validated against an experimental study by Martínez-Bazán et al. (1999) investigating bubble breakup within a turbulent jet flow. The predicted results are found to be in reasonable agreement with the measurements. Furthermore, the effect of coalescence and other properties of the bubbles on the breakup behavior is investigated.
  • Publication
    Metadata only
    Influence of a cost–efficient Langevin subgrid-scale model on the dispersed phase of large–eddy simulations of turbulent bubble–laden and particle–laden flows
    (Pergamon Press, 2017) ;
    © 2016 Elsevier Ltd The paper is concerned with the development of a cost-efficient Langevin subgrid-scale model and the analysis of its influence on the dispersed phase of turbulent bubble–laden and particle–laden flows. For this purpose, the Langevin subgrid-scale model of [Pozorski, J., Apte, S. V., 2009. Filtered particle tracking in isotropic turbulence and stochastic modeling of subgrid-scale dispersion. Int. J. Multiphase Flow 35, 118–128.] is chosen as the starting point since it takes the temporal correlations of the subgrid-scale velocity fluctuations, the crossing-trajectory and the continuity effect into account. Based on the idea of [Minier, J.-P., Peirano, E., Chibbaro, S., 2004. PDF model based on Langevin equation for polydispersed two-phase flows applied to a bluff-body gas-solid flow. Phys. Fluids 16, 2419–2431.] [Minier, J.-P., Chibbaro, S., Pope, S. B., 2014. Guidelines for the formulation of Lagrangian stochastic models for particle simulations of single-phase and dispersed two-phase turbulent flows. Phys. Fluids 26, 113303.] to formulate the drift and diffusion terms in matrix form, the model is extended for an arbitrary direction of the particle motion. Considering turbulent downward channel flows of different setups covering a large range of parameters, the influence of the subgrid-scale model is analyzed. After a detailed validation of the bubble–laden flow the Langevin model and a simple trivial model are applied to investigate the effect of the subgrid-scales. It is found that the Langevin subgrid-scale model only marginally changes the velocity statistics or the volume fraction of the bubbles, which can be attributed to the small magnitude of the subgrid-scale velocities obtained by the Langevin model. The model is able to estimate the correct level of the turbulent kinetic energy of the subgrid-scales. Similar results are found for the second setup consisting of solid particles of Stokes number St+=1.67. In this case the influence of the Langevin subgrid-scale model on the velocity statistics of the particles is found to be more pronounced. Furthermore, it is observed that the model leads to a strongly increased volume fraction of the particles at the walls and thus to a significant increase of particle-wall collisions. To further investigate this behavior and to analyze the impact of the particle inertia, additional simulations containing smaller particles (St+=1 and 0.1) are carried out. The results show that the influence of the Langevin subgrid-scale model on the velocity fluctuations and the volume fraction increases with decreasing Stokes number. Thus, for these cases the extended but nevertheless still cost-efficient Langevin model is a reasonable approach.
  • Publication
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    Euler-Lagrange Simulations of Turbulent Two-phase Flows with an Advanced Deterministic Bubble Coalescence Model
    (Wiley-VCH, 2017) ;
    The present paper is concerned with an advanced version of the deterministic coalescence model [1] and its application to turbulent bubble-laden flows. The major drawback of this film drainage model - its incapability to handle large numbers of coalescence processes - is avoided by an a-priori determination of a parametric relation for the transition time between the main drainage mechanisms. The comparison with experimental results yields a good overall agreement, while the application to a downward pipe flow demonstrates the ability of the model to efficiently handle large numbers of coalescence processes.