Ezequiel Medici

Modeling shock waves generated by explosive volcanic eruptions

Atmospheric shock waves induced by explosive volcanic eruptions can provide valuable information about eruption characteristics. Shock waves are manifested as pressure-density gradients that can be remotely observed with relatively little noise. Field measurements of expanding shock waves can be directly recorded by pressure transducers or imaged under the proper illumination and atmospheric conditions. In this paper, an open-ended shock tube was used to generate weak shock waves in the laboratory that are representative of explosive volcanic eruptions. They indicate that strong shock wave theory can be used for modeling moderate volcanic eruptions. Based on that finding, we use strong shock theory to estimate the sudden explosive energy released from several explosive eruptions. Our energy calculations are well correlated with total energy estimates derived from plume height or erupted mass.

Scaling percolation in thin porous layers

Percolation in porous media is a complex process that depends on the flow rate, material, and fluids properties as well as the boundary conditions. Traditional methods of characterizing percolation rely upon visual observation of a flow pattern or a pressure-saturation relation valid only in the limit of no flow. In this paper, the dynamics of fluid percolation in thin porous media is approached through a new scaling. This new scaling in conjunction with the capillary number and the viscosity ratio has resulted in a linear non-dimensional correlation of the percolation pressure and wetted area in time unique to each porous media. The effect of different percolation flow patterns on the dynamic pressure-saturation relation can be condensed into a linear correlation using this scaling. The general trend and implications of the scaling have been analyzed using an analytical model of a fluid percolating between two parallel plates and by experimental testing on thin porous media. Cathode porous transport laye...

A new experiment for investigating evaporation and condensation of cryogenic propellants

Passive and active technologies have been used to control propellant boil-off, but the current state of understanding of cryogenic evaporation and condensation in microgravity is insufficient for designing large cryogenic depots critical to the long-term space exploration missions. One of the key factors limiting the ability to design such systems is the uncertainty in the accommodation coefficients (evaporation and condensation), which are inputs for kinetic modeling of phase change. A novel, combined experimental and computational approach is being used to determine the accommodation coefficients for liquid hydrogen and liquid methane. The experimental effort utilizes the Neutron Imaging Facility located at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland to image evaporation and condensation of hydrogenated propellants inside of metallic containers. The computational effort includes numerical solution of a model for phase change in the contact line and thin film regions as well as an CFD effort for determining the appropriate thermal boundary conditions for the numerical solution of the evaporating and condensing liquid. Using all three methods, there is the possibility of extracting the accommodation coefficients from the experimental observations. The experiments are the first known observation of a liquid hydrogen menisci condensing and evaporating inside aluminum and stainless steel cylinders. The experimental technique, complimentary computational thermal model and meniscus shape determination are reported. The computational thermal model has been shown to accurately track the transient thermal response of the test cells. The meniscus shape determination suggests the presence of a finite contact angle, albeit very small, between liquid hydrogen and aluminum oxide.

Modeling and Diagnostics of Fuel Cell Porous Media for Improving Water Transport

When a fuel cell is operating at high current density, water accumulation is a significant cause of performance and component degradation. Investigating the water transport inside the fuel cell is a challenging task due to opacity of the components, the randomness of the porous materials, and the difficulty in gain access to the interior for measurement due to the small dimensions of components. Numerical simulation can provide a good insight of the evolution of the water transport under different working condition. However, the validation of those simulations is remains an issue due the same experimental obstacles associated with in-situ measurements. The discussion herein will focus on pore-network modeling of the water transport on the PTL and the insights gained from simulations as well as in the validation technique. The implications of a recently published criterion to characterize PTL, based on percolation theory, and validate numerical simulation are discussed.

An automatic method for detecting sliding railway wheels and hot bearings using thermal imagery

One of the most important safety-related tasks in the rail industry is an early detection of defective rolling stock components. Railway wheels and wheel bearings are the two components prone to damages due to their interactions with brakes and railway track, which makes them a high priority when the rail industry investigates improvements in the current detection processes. One of the specific wheel defects is a flat wheel, which is often caused by a sliding wheel during a heavy braking application. The main contribution of this paper is the development of a computer vision method for automatically detecting the sliding wheels from images taken by wayside thermal cameras. As a byproduct, the process will also include a method for detecting hot bearings from the same images. We first discuss our automatic detection and segmentation method, which identifies the wheel and bearing portion of the image. Then, we develop a method, using histogram of oriented gradients to extract the features of these regions. These feature descriptors are later employed by support vector machine to build a fast classifier with a good detection rate, which can detect abnormalities in the wheel. At the end, we train our algorithm using simulated images of sliding wheels and test it on several thermal images collected in a revenue service by the Union Pacific Railroad in North America.

A Quantitative Technique to Compare Experimental Observations and Numerical Simulations of Percolation in Thin Porous Materials

Quantitative validation of numerical simulations of percolation in porous media has always been challenging due to the stochastic nature of the material structure and morphology regardless of the numerical technique being used. In this article, we present a technique that allows for a quantitative comparison between numerical and experimental percolation. The experimental observations are of injection pressure and liquid intrusion within thin porous materials in a Hele-Shaw style test. The thin porous materials tested had a surface treatment such the contact angle was larger than

Sensitivity of Thermal Transport and Phase-Change in Thin Porous Layers to the Distribution of the Solid Matrix

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2D Parametric Study of Viscous Fingering

90∘ resulting drainage percolation. Flow rates were adjusted to encompass the range of capillary numbers for drainage flow patterns between the stable displacement and capillary fingering. In parallel to the experimental observations, a series of numerical simulations using a two-dimensional pore network model were performed mimicking the Hele-Shaw experiments. The material properties of the pore network realizations, pore size distribution, contact angle, and representative volume for the pore space were based on measurements of the porous materials tested. Boundary and initial conditions were matched between numerical simulations and experiments. To compare and validate the numerical simulation against the experiments, a new scaling of the dissipated energy during percolation in thin porous media was used. This scaling has been recently used to identify the transition between capillary fingering and stable displacement for drainage in different thin porous materials and provides a unique method for characterizing percolation in thin porous materials. Though the experiments and simulations presented in this article are for drainage, the technique described is equally applicable to imbibition. Excellent agreement is obtained between experiment and simulation with clear delineation between different types of thin porous materials.