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Open Access Dissertations

Rising Bubble Posted: Wed, 03 Jul 2013 two-phase 3d rising Visualization of surface (triangles) and background (tetrahedrons) meshes for two-phase flows simulation of an air bubble in a water-sugar solution using an 3D Finite Element Arbitrary Lagrangian Eulerian code. As can be seen, the background mesh is more refined below the air bubble. Rising bubble with low density ratio In this section, the bubble behavior at A r = 8. 75, B o = 116 condition with density ratio λ = 2 and viscosity ratio χ = 100 will be simulated for two purposes: one is to study the convergence of the proposed the model, the other is to compare the difference of the proposed PMC scheme and the.

Date of Award

2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Ocean Engineering

Department

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Ocean Engineering

First Advisor

Stephan T. Grilli

Abstract

Simulating complex fluid flow have always been one the most challenging problem in Computational Fluid Dynamics (CFD). Most of these difficulties come from the deficiencies of Classical CFD method in computational time and boundary implementations. Recently the Lattice Boltzmann Method (LBM) has been recognized as an alternative to the classical CFD methods for its advantages such as, easy boundary implementation, suitability for parallel simulation and no need for the Poisson pressure solver.

The LBM is based on the Boltzmann equation, which governs the dynamics of molecular probability distribution functions, from a microscopic scale point of view. The primary variable of the Boltzmann equation is the particle distribution function f(x, t, ξ) which describes the probability to find a particle with microscopic velocity ξ at point χ and time t. The macroscopic fluid properties (velocities, pressure) are computed as moments of these particle distribution functions.

This dissertation investigates LBM for complex multiphase fluid simulation in three manuscripts. In the first manuscript a novel LBM scheme is developed for simulating multiphase flow simulation with high-density ratio. In previous multiphase flow simulations the maximum fluids density ratio achievable in computations was limited by the occurrence of instabilities for high density ratio values (typically larger than 10-20). Overcoming this limitation is one of the most challenging current issues in the LBM modeling of multiphase flows and the subject of the first manuscripts, as we aim at modeling complex flows at an air-water interface, whose density ratio is about 1,000.

It should be pointed out that although the numerical scheme in manuscript one is able to simulate large density ratio multiphase flow for moderate Reynolds numbers it is not stable for high Reynolds numbers. Eliminating this deficiency of the first manuscript for simulating of multiphase flows with high density ratios and high Reynolds numbers is the subject of second manuscript. In the second manuscript a novel LBM method is introduced which is able to simulate multiphase flow with arbitrary Reynolds and density ratios. The resulting algorithm is applied to several test cases, such as droplet splash, rising bubble and wave braking. The good agreement between numerical results and existing data demonstrate that the newly developed model is a useful tool for simulating complex multiphase flows.

In the third manuscript we study the effects of point-wise particles on turbulent channel flow. Investigating particle-laden turbulent flows is an important fluid mechanics problem as it occurs frequently in nature. This work has been done in collaboration with Professor Tetsu Hara and Yackar Mauzole in Graduate School of Oceanography at the University of Rhode Island to study the sea spray generation effects of turbulent layer above see surface in high wind condition. In this work the Lattice Boltzmann method is coupled to a Lagrangian particle tracking approach for simulating a system of fluid-particle flow.

In all three manuscripts the numerical schemes are developed in nVIDIA CUDA framework, which made it possible to efficiency implemented on General Purpose Graphical Processing Unit (GPGPU) to numerously increase the computational speed of the numerical code.

Recommended Citation

Banari, Amir, 'Lattice Boltzmann Simulation of Multiphase Flows; Application to Wave Breaking and Sea Spray Generation' (2014). Open Access Dissertations. Paper 282.
https://digitalcommons.uri.edu/oa_diss/282

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Adaptive Mesh Refinement: Quantitative Computation of a Rising Bubble Using COMSOL Multiphysics®

The mesh is a key component in numerical simulations as it represents the spatial discretization of the model geometry. To accurately measure the variation of the unknowns, a relevant mesh should have a high density of degrees of freedom in regions where the norm of the gradient of the quantity of interest is significant. Fine meshes, on the other hand tend to induce long computational times, especially when complex 3D physics (fluid mechanics, electromagnetism …) are involved. Another sensitive point regarding meshes is the fact that regions with high gradients are likely to move during time dependent studies hence it can be difficult to define a fixed mesh remaining relevant with time.The Adaptive Mesh Refinement (AMR) method implemented in COMSOL Multiphysics® can help to mitigate computational time while maintaining precision. Instead of using a fixed mesh throughout the simulation, the initial mesh is adapted to the solution while the simulation is computed. This adaptation is done by estimating an error indicator on the initial coarse mesh to identify high gradients areas, where the mesh will be refined.In this paper, rising bubbles models from literature, in both 2D and 3D are simulated. The AMR method is particularly relevant for the modelling of a two-phase flow since a fine mesh is needed around the interface, which evolves with time. In both 2D and 3D cases, the computational time is significantly reduced, by a factor of 5 in the 2D case while the precision is kept constant. The gain in both computational time and accuracy is quantified through a comparison between the results from the literature and those using the AMR technique.The efficiency of the AMR method is demonstrated here on the rise of a bubble to keep the physics involved simple enough. However, as the technique is particularly well suited for modelling problems involving high gradients in the field of interest, moving meshes and deformed geometry, it can be used on complex industrial cases such as numerical simulations of welding or additive manufacturing.

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