Geometry-Driven Detection, Tracking and Visual Analysis of Viscous and Gravitational Fingers

Viscous and gravitational flow instabilities cause a displacement front to break up into finger-like fluids. The detection and evolutionary analysis of these fingering instabilities are critical in multiple scientific disciplines such as fluid mechanics and hydrogeology. However, previous detection methods of the viscous and gravitational fingers are based on density thresholding, which provides limited geometric information of the fingers. The geometric structures of fingers and their evolution are important yet little studied in the literature. In this work, we explore the geometric detection and evolution of the fingers in detail to elucidate the dynamics of the instability. We propose a ridge voxel detection method to guide the extraction of finger cores from three-dimensional (3D) scalar fields. After skeletonizing finger cores into skeletons, we design a spanning tree based approach to capture how fingers branch spatially from the finger skeletons. Finally, we devise a novel geometric-glyph augmented tracking graph to study how the fingers and their branches grow, merge, and split over time. Feedback from earth scientists demonstrates the usefulness of our approach to performing spatio-temporal geometric analyses of fingers.Comment: Published at IEEE Transactions on Visualization and Computer Graphic

Visual Analysis of Two-Phase Flow Displacement Processes in Porous Media

We present the visual analysis of our novel parameter study of porous media experiments, focusing on gaining a better understanding of drainage processes on the micro-scale. We analyze the temporal evolution of extracted characteristic values, and discuss how to directly compare experiments that exhibit processes at different temporal scales due to varying boundary and physical conditions. To enable spatio-temporal analysis, we introduce a new abstract visual representation showing which paths through the porous media were occupied to what extent, e.g., allowing for classification into viscous and capillary regimes. This joint work of porous media experts and visualization researchers yields new insights regarding immiscible two-phase flow on the micro-scale toward the overarching goal of characterizing flow based on boundary conditions and physical fluid properties

Visual Analysis of Two-Phase Flow Displacement Processes in Porous Media

We present the visual analysis of our novel parameter study of porous media experiments, focusing on gaining a better understanding of drainage processes on the micro-scale. We analyze the temporal evolution of extracted characteristic values, and discuss how to directly compare experiments that exhibit processes at different temporal scales due to varying boundary and physical conditions. To enable spatio-temporal analysis, we introduce a new abstract visual representation showing which paths through the porous media were occupied to what extent, e.g., allowing for classification into viscous and capillary regimes. This joint work of porous media experts and visualization researchers yields new insights regarding immiscible two-phase flow on the micro-scale toward the overarching goal of characterizing flow based on boundary conditions and physical fluid properties

Visual Analysis of Two-Phase Flow Displacement Processes in Porous Media

We present the visual analysis of our novel parameter study of porous media experiments, focusing on gaining a better understanding of drainage processes on the micro-scale. We analyze the temporal evolution of extracted characteristic values, and discuss how to directly compare experiments that exhibit processes at different temporal scales due to varying boundary and physical conditions. To enable spatio-temporal analysis, we introduce a new abstract visual representation showing which paths through the porous media were occupied to what extent, e.g., allowing for classification into viscous and capillary regimes. This joint work of porous media experts and visualization researchers yields new insights regarding immiscible two-phase flow on the micro-scale toward the overarching goal of characterizing flow based on boundary conditions and physical fluid properties

Electrostatic control and enhancement of film boiling heat transfer

Boiling heat transfer is severely degraded at high surface temperatures due to the formation of a vapor layer at the surface, commonly known as the Leidenfrost effect. Heat transfer is limited to a critical heat flux (CHF); higher heat fluxes lead to surface dryout and temperature excursions. An externally applied electric field in the vapor layer can significantly enhance boiling heat transfer for electrically conducting or polar liquids. In such liquids, the electric field is concentrated in the vapor layer, and promotes liquid-surface contact, which can significantly enhance boiling heat transfer. This dissertation is a fundamental study of the influence of concentrated interfacial electric fields on film boiling heat transfer for liquids with finite electrical conductivity (like water and organic solvents). This dissertation describes experimental, analytical and numerical studies on various aspects of the physics underlying electrostatic suppression of film boiling. This dissertation also quantifies the heat transfer benefits associated with electrostatic suppression of film boiling. This dissertation is divided into five main studies, which analyze different aspects of electrostatic suppression of the Leidenfrost state. The first part of this dissertation (Chapter 2) describes droplet-based experimental investigations on electrostatic suppression of the Leidenfrost state. It is demonstrated that the Leidenfrost state can be suppressed and surface dryout can be prevented using externally applied electric fields (AC or DC). Elimination of the Leidenfrost state increases heat dissipation capacity by more than one order of magnitude. In preliminary experiments, heat removal capacities exceeding 500 W/cm² are measured for water, which is five times the CHF of water on common engineering surfaces. A multiphysics analytical model is developed to predict the vapor layer thickness in the Leidenfrost state. The second part of this dissertation (Chapter 3) analyzes the fundamental mechanisms underlying electrostatic suppression of Leidenfrost state. It is shown that the interplay of destabilizing and stabilizing forces determines the minimum (threshold) voltage required to suppress the Leidenfrost state. Detailed linear instability analysis is conducted to investigate the growth of electrostatically-induced perturbations on the liquid-vapor interface in the Leidenfrost state, and predict the threshold voltage required for suppression. The third part of this dissertation (Chapter 4) focuses on suppression of the Leidenfrost state on soft, deformable surfaces, like liquids. It is seen that the nature of electrostatic suppression on a deformable liquid substrate is drastically different from that on a solid substrate. This is due to the existence of an electric field inside the substrate and the deformability of the substrate. A multiphysics analytical model is developed to predict the vapor layer thickness on deformable liquids. The fourth part of this dissertation (Chapter 5) includes experimental studies on suppression of film boiling during high temperature quenching of metals. It is shown that an electric field can fundamentally change the boiling patterns, wherein the stable vapor layer (film boiling) is replaced by intermittent wetting of the surface. This fundamental switch in the heat transfer mode significantly accelerates cooling during quenching. An order of magnitude increase in the cooling rate is observed, with the heat transfer seen approaching saturation at higher voltages. An analytical model is developed to extract voltage dependent heat transfer rates from the measured cooling curve. The fifth part of this dissertation (Chapter 6) develops the concept of using acoustic signature tracking to study electrostatic suppression of film boiling. It is shown that acoustic signature tracking can be the basis for objective measurements of the threshold voltage and frequency required for suppression. Acoustic signature tracking can also detect various boiling patterns associated with electrostatically-assisted quenching. With appropriate calibration, this technique can be used to estimate surface temperatures, heat flux and onset of dryout associated with electrically enhanced boiling. In summary, this dissertation has led to seminal contributions in the field of boiling heat transfer, and essentially opened up a new area of study in the field. This work has shown that electric fields can make the CHF limit irrelevant, and reshape the boiling curve. The present work lays the foundations for electrically tunable boiling heat transfer with conducting liquids. The impact of the proposed work is evident in the area of quenching, where electrically tunable cooling offers a new tool to control the microstructure and mechanical properties of metals.Mechanical Engineerin

The Influence of Particle Concentration on the Formation of Settling-Driven Gravitational Instabilities at the Base of Volcanic Clouds

Settling-driven gravitational instabilities observed at the base of volcanic ash clouds have the potential to play a substantial role in volcanic ash sedimentation. They originate from a narrow, gravitationally unstable region called a Particle Boundary Layer (PBL) that forms at the lower cloud-atmosphere interface and generates downward-moving ash fingers that enhance the ash sedimentation rate. We use scaled laboratory experiments in combination with particle imaging and Planar Laser Induced Fluorescence (PLIF) techniques to investigate the effect of particle concentration on PBL and finger formation. Results show that, as particles settle across an initial density interface and are incorporated within the dense underlying fluid, the PBL grows below the interface as a narrow region of small excess density. This detaches upon reaching a critical thickness, that scales with (ν2/g′)1/3, where ν is the kinematic viscosity and g′ is the reduced gravity of the PBL, leading to the formation of fingers. During this process, the fluid above and below the interface remains poorly mixed, with only small quantities of the upper fluid phase being injected through fingers. In addition, our measurements confirm previous findings over a wider set of initial conditions that show that both the number of fingers and their velocity increase with particle concentration. We also quantify how the vertical particle mass flux below the particle suspension evolves with time and with the particle concentration. Finally, we identify a dimensionless number that depends on the measurable cloud mass-loading and thickness, which can be used to assess the potential for settling-driven gravitational instabilities to form. Our results suggest that fingers from volcanic clouds characterised by high ash concentrations not only are more likely to develop, but they are also expected to form more quickly and propagate at higher velocities than fingers associated with ash-poor clouds.</jats:p