Phd in AERONAUTICS AND SPACE ENGINEERING

Thesis title: Comparative analysis of volume of fluid and phase–field methods for numerical simulation of gas–liquid sloshing phenomena

This thesis provides a comparative analysis of Volume of Fluid (VOF) and Phase-Field (PF) methods for accurately modeling gas-liquid multiphase flows, specifically in scenarios of liquid sloshing within partially filled rectangular containers. An in-house Navier-Stokes multiphase solver is used to evaluate four primary interface-capturing methods: Conservative Allen-Cahn (CAC), Profile-Corrected Cahn-Hilliard (CCH), Algebraic TVD Volume of Fluid (AVOF), and Geometric PLIC Volume of Fluid (GVOF). Using benchmark tests such as the Rider-Kothe vortex flow, stationary drop, Rayleigh-Taylor instability, and phase inversion in a closed box, the study assesses the strengths and limitations of each method regarding accuracy, mass and energy conservation, and computational efficiency. Beyond standard validations, these methods are applied to simulate vertical sloshing in rectangular tanks, with a focus on characterizing the effects of vertical acceleration and excitation frequency on flow dynamics. This analysis provides insights into sloshing-induced energy dissipation, a critical factor in aerospace engineering. The flow dynamics is found to be significantly affected by the forcing parameters, and to exhibit more chaotic and three-dimensional nature in cases with strong acceleration and low forcing frequency. As a consequence, certain properties as the energy dissipation and the mixing efficiency of the system are poorly predicted from two-dimensional simulations in that range of parameters, making more expensive three-dimensional simulations necessary. Among the methods, the GVOF approach consistently demonstrated superior accuracy in predicting the interface geometry and maintaining mass conservation, making it a reliable choice for a wide range of numerical simulations. In contrast, while AVOF method provides generally accurate results and conservation properties in validation tests, its performance deteriorates in highly nonlinear scenarios with extensive mixing, such as sloshing with higher acceleration amplitudes, where the mass transfer error is double that of the other methods. The PF methods yield less accurate predictions of interface elevation in sloshing scenarios with small free-surface oscillations, hence lower fidelity in estimating energy exchanges between the fluid and structure. The CAC method proves computationally efficient and simple to implement but faces time step limitations that necessitate future improvement for high-fidelity simulations. Finally, the CCH method performs least accurately across test conditions and it also incurs the highest computational cost among the methods tested.