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Development of a multiscale two-fluid approach with application to droplet dynamics

Nykteri, G. (2021). Development of a multiscale two-fluid approach with application to droplet dynamics. (Unpublished Doctoral thesis, City, University of London)


The present thesis introduces a novel numerical methodology for multiscale flows with application to complex droplet fragmentation cases. The proposed methodology concerns a compressible two-fluid model developed in OpenFOAM® and provides the flexibility of dealing with the multiscale character of flow fields: the interface scales greater than the grid size are resolved using the sharp interface (VOF) methodology, while the smaller ones, representing the diffused phase, are resolved by solving an additional transport equation of the generated surface area density (Σ) of the dispersed droplet cloud. The solver switches automatically between the sharp and the diffuse interface within the Eulerian-Eulerian framework in segregated and dispersed flow regions, respectively, by employing a dynamic interface sharpening based on a flow topology detection algorithm.
Validation cases against a two-fluid shock tube and a rising bubble depict the accuracy of the numerical methodology to deal with highly compressible flows and fast changing interfaces. Initially, the functionality of the multiscale framework is demonstrated for high-speed droplet impact cases with Weber numbers above 105 and compared with new experimental data. At the investigated impact conditions, compressibility effects dominate the early stages of droplet splashing with shock waves to form and propagate inside the droplet and local Mach numbers up to 2.5 to be observed for the expelled surrounding gas outside the droplet. At the later stages of splashing, the dispersion of the dense cloud of fragments dominates and an insight into the fragments dynamics and the evolving sizes is presented. Subsequently, the droplet aerobreakup imposed by three different intensity shock waves, with Mach numbers of 1.21, 1.46 and 2.64, is investigated. The major features and physical mechanisms of breakup, including the incident shock wave dynamics and the vortices development, are accurately captured. Additionally, the dense mist development and the evolution of the underlying secondary droplets is examined under different post-shock conditions, based on the sub-grid scale modelling. Finally, the laser-induced fragmentation of a liquid droplet for different laser pulses that correspond to resulting droplet propulsion velocity values between 1.76m/s and 5.09m/s is investigated. Both the early- and later-time droplet dynamics are accurately captured and the influence of the laser energy on the droplet deformation and subsequent fragmentation is highlighted. The evolution of the produced fragments due to the rim breakup is quantified with respect to the different expansion rates and aerodynamic conditions.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
Departments: Doctoral Theses
School of Science & Technology
School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics
Text - Accepted Version
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