Theses and Dissertations from UMD
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Item Simulation of polymeric drop dynamics: Effect of photopolymerization, impact velocity, and multi-material coalescence(2023) Sivasankar, Vishal Sankar; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past couple of decades, additive manufacturing has emerged as one of the most promising manufacturing tools and has rightfully garnered the attention of researchers across various fields ranging from biochemistry and medicine to energy and infrastructure. Especially, direct-ink-writing methods (e.g., inkjet printing, aerosol jet printing, or AJ printing, etc.) have been widely studied because of their ability to print highly complex geometries with finer resolution. In order to design a more efficient droplet-based direct ink writing system, it is essential to understand the deposition process and the post-deposition dynamics of the drop. The post-deposition drop dynamics dictate the spreading radius of the drop and hence the print resolution. Such an understanding is even more critical when there are multiple drops interacting with each other, given the fact that such interactions determine the presence/absence of surface defects in addition to determining the print resolution. Moreover, to have a holistic understanding of the post-deposition process, it is essential to further account for the droplet solidification mechanisms (for example, through effects such as in-situ curing) that might interplay with multiple drop dynamics events (such as drop spreading, drop coalescence, drop impact, etc.). In this dissertation, computation fluid dynamics (CFD) frameworks have been developed to investigate the facets dictating the post-deposition dynamics of one (or several) solidifying polymer drops, with these dynamics show-casing the different post-deposition events that are intrinsic to the droplet-based additive manufacturing processes. First, we considered a situation where the polymeric drop undergoes simultaneous spreading and photopolymerization, with the timescales of the spreading and photopolymerization events being τ? and τ? respectively. The findings from this work confirmed the significant impact of the ratio of timescales (τ? and τ?) on the thermo-fluidic-solutal dynamics of the polymeric drops. Moreover, the evolution of the curing front showed distinct behaviors as a function of the timescale ratio.Subsequently, the effect of the interaction of multiple polymeric drops during the post-deposition event, as seen in the typical printing process, was investigated. Specifically, we studied the effect of drop impact on the coalescence dynamics of two polymeric drops of identical and different sizes. The study revealed the presence of two distinct stages of coalescence. The early-stage coalescence was found to be enhanced with an increase in the impact velocity; however, the late-stage coalescence behavior remained unaffected by the impact velocity. Further, the coalescence dynamics of polymeric drops of different materials, as witnessed in multi-jet printing, was probed. This study shed light on the mechanisms that drive the mixing process at different stages of drop coalescence. Finally, we evaluated the effects of the in-situ photopolymerization on the coalescence dynamics of multiple polymeric drops deposited on a substrate. Here too the comparative values of the drop dynamics timescale and the photopolymerization became important. Our results show three-distinct regimes characterizing the bridge growth which was further validated through physics-based theoretical scaling. This study would provide key insights into the direct-ink writing process and would aid in designing parameters for polymer-based additive manufacturing and product repair.Item DETAILED PIV MEASUREMENTS ON PARTICLE-TURBULENCE INTERACTION IN OSCILLATORY SHEET FLOW: IN DILUTE REGIME(2022) LIU, CHANG; Kiger, Kenneth KK; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An experimental investigation of sediment transport mechanisms under oscillatory sheet flow condition is conducted. Focus is placed upon the dilute regime of solid-liquid transport with volume concentrations C=0.01, where significant fluid turbulence is present, strong particle-turbulence interaction occurs and inter-particle collisions can be neglected. Understanding the coupling dynamics between phases is critical for the validation and improvement of the existing numerical models. Simultaneous determination of the dynamics of each phase is often prohibitively expensive to acquire by direct numerical simulation and poses significant challenges to experimental measurements. In our experiment, a U-shaped water tunnel is used to create highly repeatable oscillatory sheet flow conditions over a mobile bed. The test section of the tunnel is 375 cm in length, with a cross-sectional area of 30X45 cm^2. The sediment is modeled using narrowly sorted spherical soda lime glass beads with a mean diameter of d=240 \mu m and a specific gravity of s=2.5. The efforts have been made in two directions: the measurement technique development and the application of the developed technique to an oscillatory sheet flow. First, a novel measurement technique, based upon Particle Image Velocimetry (PIV), was developed and validated, that aims for a simultaneous measurement of both phases. For the sediment phase measurement, the multi-camera single-plane (MCSP) method was developed to reconstruct particle's instantaneous 3D positions towards a higher concentration. This was followed by Lagrangian particle tracking (LPT) to link the reconstructed particles over successive frames, with an in-house developed algorithm based upon shake-the-box (STB, Schanz et al. Experiments in Fluids 2016). For the carrier phase measurement, stereoscopic PIV (SPIV) was implemented. In order to reduce the cross-talk errors due to the presence of sediment particles, the apertured filter method was developed to produce adequate image quality of both phases allowing for a reliable extraction of each phase independently. Second, the developed measurement technique was applied in a sinusoidal oscillatory sheet flow (period, T=5 s, and peak free stream velocity, Uo,p=1 m/s) to provide a whole field, phase-locked time-resolved, particle resolved and concurrent measurement of both the fluid and the sediment phase in the dilute regime (C<0.01). Such detailed measurements in sheet flow have never been reported before to the author's best knowledge. The analysis of the acquired data focused upon three phase angles when the external flow is reversing the direction. It has been found that during flow reversal, 1) distinct particle suspension mechanisms are identified in the upper dilute regime (y>11 mm) and the lower dilute regime (211 mm, while the over-sampling diminishes towards the bed; and 3) the particle velocity and their ambient fluid velocity show a strong correlation for y>11 mm and the correlation becomes weaker as approaching the bed.Item Fast Solvers and Preconditioners for Multiphase Flow in Porous Media(2018) Bui, Quan Minh; Elman, Howard; Applied Mathematics and Scientific Computation; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multiphase flow is a critical process in a wide range of applications, including carbon sequestration, contaminant remediation, and groundwater management. Typically, this pro- cess is modeled by a nonlinear system of partial differential equations derived by considering the mass conservation of each phase (e.g., oil, water), along with constitutive laws for the relationship of phase velocity to phase pressure. The problem becomes much more complex if the phases are allowed to contain multiple chemical species (also called components), as miscibility and phase transition effects need to be taken into account. The main problem with phase transition stems from the inconsistency of the primary variables such as phase pressure and phase saturation, i.e. they become ill-defined when a phase appears or dis- appears. Recently, a new approach for handling phase transition has been developed by formulating the system as a nonlinear complementarity problem (NCP). Unlike the widely used primary variable switching method (PVS), which requires a drastic reduction of the time step size when a phase appears or disappears, this approach is more robust and allows for larger time steps. One way to solve an NCP system is to reformulate the inequality constraints for the primary variables as a non-smooth equation using a complementary function (C-function). Because of the non-smoothness of the constraint equations, a semi-smooth Newton method needs to be developed. Another feature of the NCP approach is that the set of primary variables in this approach is fixed even when there is phase transition. Not only does this improve the robustness of the nonlinear solver, it opens up the possibility to use multigrid methods to solve the resulting linear system. The disadvantage of the complementarity approach, however, is that when a phase disappears, the linear system has the structure of a saddle point problem and becomes indefinite, and current algebraic multigrid (AMG) algorithms cannot be applied directly. In this work, we aim to address computational issues related to modeling multiphase flow in porous media. First, we develop and study efficient solution algorithms for solving the algebraic systems of equations derived from a fully coupled and time-implicit treatment of models of incompressible two-phase flow. We explore the performance of several precon- ditioners based on algebraic multigrid (AMG) for solving the linearized problem, including “black-box” AMG applied directly to the system, a new version of constrained pressure residual multigrid (CPR-AMG) preconditioning, and a new preconditioner derived using an approximate Schur complement arising from the block factorization of the Jacobian. We show that the new methods are the most robust with respect to problem character as de- termined by varying effects of capillary pressures, and we show that the block factorization preconditioner is both efficient and scales optimally with problem size. We then generalize the block factorization method and incorporate it into a multigrid framework which is based on the multigrid reduction technique to deal with linear systems resulting from the NCP approach for modeling compositional multiphase flow with phase transitions. We demon- strate the effectiveness and scalability of the method through numerical results for a case of two-phase, two-component flow with phase appearance/disappearance. Finally, we propose a new semi-smooth Newton method which employs a smooth version of the Fischer-Burmeister function as the C-function and evaluate its performance against the semi-smooth Newton method for two C-functions: the minimum and the Fischer-Burmeister functions. We show that the new method is robust and efficient for standard benchmark problems as well as for realistic examples with highly heterogeneous media such as the SPE10 benchmark.Item INTERFACE ADVECTION AND JUMP CONDITION CAPTURING METHODS FOR MULTIPHASE INCOMPRESSIBLE FLOW(2015) Qin, Zhipeng; Riaz, Amir; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this work, new numerical methods are proposed to efficiently resolve interfaces occurring in multiphase incompressible flows. Multiphase flow problems consist of a large class of physical phenomenon from bubbles to bow waves in ships. Over the recent decades, numerical methods are becoming an important tool in addition to pure analytical and experimental methods. However, there is still large room for improvement in existing numerical methods. Contributions are made in the field of interface advection and the jump conditions for pressure. In the case of advection, a method is developed specifically for implicit interfaces that evolve with the Eulerian advection of a scalar field. The new method is validated by comparison with the interfaces that evolve with Lagrangian advection using a connected set of marker particles. To accurately capture the jump conditions, a second order accurate method is proposed for solving the variable coefficient Poisson's equation in the discretized Navier-Stokes formulation. This new method assumes both phases exist in the interface cell and that their collective effect can be expressed by a volume fraction weighted average value. The new capabilities have been integrated to build a dynamic Navier-Stokes equation solver. The new advection scheme scheme is also associated to track the interface. The new solver is tested by applications in several two phase flow problems.