Browsing by Subject "Multiphase flows"

Now showing 1 - 3 of 3
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    Methods for the Modeling and Simulation of Sprays and Other Interfacial Flows
    (2019-09) Wenzel, Everett
    Interfacial multiphase flows involve the motion of at least two fluids separated by surface tension. Atomizing interfacial flows, colloquially known as sprays, are among the most important fluid dynamic systems because of their ubiquity; power generation, delivery of aerosolized medicines, and productive produce farming all depend fundamentally on the detailed control of sprays. Atomization remains poorly understood because of a historical and persisting inability to accurately and affordably measure the dynamics inside and near the spray orifice outlet -- it is therefore desirable to be able to numerically simulate sprays with high fidelity. This dissertation presents computational methods that aim to improve current shortcomings in the modeling and simulation of sprays. Accurately characterizing the interfacial curvature of poorly-resolved liquid structures is addressed by deriving a series of finite particle methods for computing curvature. The methods are verified in analytical curvature tests, and validated against the oscillation frequency of ethanol droplets in air. The finite particle method, leveraging dynamic length scale modification, is demonstrated to out-perform the widely-used height function approach. Tracking the location of interfaces is also addressed, for which a coupled Eulerian-Lagrangian point mass particle scheme is introduced that preserves a well-distributed particle field, can be applied to an arbitrary number of fluids, and does not limit the simulation time step. The Eulerian-Lagrangian method is demonstrated to out-perform contemporary geometric volume of fluid methods at resolutions relevant to spray simulation in a variety of analytical phase tracking tests, and is dynamically evaluated by simulating extending three-phase elliptical regions, droplet dynamics, and Rayleigh-Taylor instabilities. The Eulerian-Lagrangian method is then extended to an approach for consistently and conservatively solving multiphase convection-diffusion problems -- this extension is verified via two analytical heat transfer problems, and robustness is demonstrated by simulating heated air blast atomization. Each of these tests conserves thermal energy and preserves boundedness of the temperature field. This dissertation concludes by outlining paths for consistently and conservatively solving the multiphase Navier-Stokes equations and the multiphase large eddy simulation equations in the coupled Eulerian-Lagrangian point mass particle framework.
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    Numerical simulation and modeling of two-phase flows using the volume-of-fluid method
    (2024-07) Fakhreddine, Ali
    Two-phase flows are of great interest in natural and industrial applications such as cavitation, spray atomization, and bubbly flows. As a two-phase system grows in complexity, analytical approximations of fluid behavior and experimental observations become more challenging. To make the detailed study of multiphase flows possible, numerical methods have been developed over the past seventy years to provide physical insight into areas where experiments and models are not feasible. The volume-of-fluid (VOF) method is a numerical interface-capturing method used to track a material interface in an Eulerian frame of reference. It is also one of the most widely used methods in the multiphase community due to its unique features such as mass conservation and interface sharpness. In this work, we extend the traditional use of the VOF method to a Lagrangian framework that complements the Eulerian description. The color function C is used to create a Lagrangian mapping of the dispersed phase in the carrier fluid where each dispersed body is assigned a unique identifier. The method of phase tagging adopted from Herrmann [1] is implemented in the in-house two-phase flow solver NS-VOF and is simultaneously extended to incorporate an id maintenance algorithm. This additional capability allows the temporal tracking of a moving body when historical information about the body is of interest. Following the extensive verification of the parallel tagging algorithm, the Lagrangian information retrieved from the Eulerian field is used to post-process the direct numerical simulation (DNS) results of a dilute buoyancy-driven bubbly flow ( ɑ=2.71%) at Bo=2.5 and Mo=5.9 x 10⁻⁴ for different density ratios (η⍴=10, 50, and 100) and initial bubble distributions. The statistical results obtained in the nearly spherical regime show that the evolution of the bubbly flow is more sensitive to initial bubble distribution than it is sensitive to an increase in η⍴. For any flow configuration in the simulated regime, the bubble swarm does not reach a homogeneous state. As the density ratio is increased, the centroid of the bubble swarm was found to deflect away from the center and closer to the wall. This effect is less deterministic upon varying the initial bubble distribution. Before simulating the bubbly flow problem with Lagrangian tagging, NS-VOF was validated for buoyancy-driven flow using the benchmark case of a single buoyant bubble rising at Mo = [5.51, 41.1, 266, 848] and Bo = 116. The rise velocity U𝑇, drag coefficient CD, and interface shape were compared to existing literature [2-4] where good agreement was found. We also derive a directionally-split geometric VOF approach to study curvature flow problems. This approach comes in the context of expanding the range of applications of VOF to encompass a wider array of problems that are of particular relevance to aerospace engineering such as flame propagation in propulsion systems and crystal growth on wings. The VOF approach is derived from variational principles. Additionally, it uses the idea that the role of curvature in a speed function 𝑉 is analogous to the role of viscosity in the corresponding hyperbolic conservation law to propagate complex topologies where singularities may exist. Both constrained and free curvature flow problems are simulated, and the results are compared to solutions obtained from two level set formulations, the traditional LSM and distance regularized level set evolution (DRLSE). The VOF approach performed better than LSM with reinitialization, especially in high-curvature problems, and compared very well with DRLSE. The numerical approximation of the Dirac delta to compute 𝜅 is shown to have a direct effect on the accuracy of the final equilibrium solution and an alternative definition is proposed such that 𝛿(C)=4C(1-C).
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    Onset of dynamic wetting failure: the mechanics of high-speed fluid displacement
    (2013-07) Vandre, Eric Allen
    Dynamic wetting is crucial to processes where a liquid displaces another fluid along a solid surface, such as the deposition of a coating liquid onto a moving substrate. Numerous studies report the failure of dynamic wetting when process speed exceeds some critical value. Typically, wetting failure is a precursor to air entrainment, which produces catastrophic defects in coatings. However, the hydrodynamic factors that influence the transition to wetting failure remain poorly understood from empirical and theoretical perspectives. This work investigates the fundamentals of wetting failure in a variety of systems that are relevant to industrial coating flows. A hydrodynamic model is developed for planar and axisymmetric geometries where an advancing fluid displaces a receding fluid along a smooth, moving substrate. Numerical solutions predict the onset of wetting failure at a critical substrate speed, which coincides with a turning point in the steady-state solution path for a given set of system parameters. Flow-field analysis reveals a physical mechanism where wetting failure results when capillary forces can no longer support the pressure gradients necessary to steadily displace the receding fluid.Novel experimental systems are used to measure the substrate speeds and meniscus shapes associated with the onset of air entrainment during wetting failure. Using high-speed visualization techniques, air entrainment is identified by the elongation of triangular air films with system-dependent size. Air films become unstable to thickness perturbations and ultimately rupture, leading to the entrainment of air bubbles. Meniscus confinement in a narrow gap between the substrate and a stationary plate is shown to delay air entrainment to higher speeds for a variety of water/glycerol solutions. In addition, liquid pressurization (relative to ambient air) further postpones air entrainment when the meniscus is located near a sharp corner along the plate. Recorded critical speeds compare well to predictions from the model, supporting the hydrodynamic mechanism for the onset of wetting failure. Lastly, the common practice of curtain coating is investigated using the hydrodynamic model. Due to the complexity of this system, a new hybrid method is developed to reduce computational cost associated with the numerical analysis. Results show that the onset of wetting failure varies strongly with the operating conditions of this system. In addition, stresses from the air flow dramatically affect the steady wetting behavior of curtain coating. Ultimately, these findings emphasize the important role of two-fluid displacement mechanics during high-speed wetting. Although this work was motivated by coating flows, it is also relevant to a number of other applications such as microfluidic devices, oil-recovery systems, and splashing droplets.