Direct numerical simulation of multi-phase flow in complex media

Tesi en modalitat de compendi de publicacionsIn numerous applications, two-phase liquid-gas transport at sub-millimeter length scales plays a substantial role in the determination of the behavior of the system at hand. As its main application, the present work focuses on the polymer electrolyte membrane (PEM) fuel cells. Desirable performance and operational life-time of this class of high-throughput energy conversion devices requires an effective water management, which per se relies on proper prediction of the water-air transport mechanisms. Such two-phase flow involves interfacial forces and phenomena, like hysteresis, that are associated with the physicochemical properties the liquid, gas, and if present, the solid substrate. In this context, numerical modeling is a viable means to obtain valuable predictive understanding of the transport mechanisms, specially for cases that experimental analyses are complicated and/or prohibitively expensive. In this work, an efficient finite element/level-set framework is developed for three-dimensional simulation of two-phase flow. In order to achieve a robust solver for practical applications, the physical complexities are consistently included and the involved numerical issues are properly tackled; the pressure discontinuity at the liquid-gas interface is consistently captured by utilizing an enriched finite element space. The method is stabilized within the framework of variational multiscale stabilization technique. A novel treatment is further proposed for the small-cut instability problem. It is shown that the proposed model can provide accurate results minimizing the spurious currents. A robust technique is also developed in order to filter out the possible noises in the level-set field. It is shown that it is a key to prevent irregularities caused by the persistent remnant of the spurious currents. It is shown how the well-established contact-line models can be incorporated into the variational formulation. The importance of the inclusion of the sub-elemental hydrodynamics is also elaborated. The results presented in the present work rely on the combination of the linearized molecular kinetic and the hydrodynamic theories. Recalling the realistic behavior of liquids in contact with solid substrates, the contact--angle hysteresis phenomenon is taken into account by imposing a consistent pinning/unpinning mechanism developed within the framework of the level-set method. Aside from the main developments, a novel technique is also proposed to significantly improve the accuracy and minimize the the loss in the geometrical features of the interface during the level-set convection based on the back and forth error compensation correction (BFECC) algorithm. Within the context of this thesis, the numerical model is validated for various cases of gas bubble in a liquid and liquid droplets in a gas. For the latter scenario, besides free droplets, the accuracy of the proposed numerical method is assessed for capturing the dynamics droplets spreading on solid substrates. The performance of the model is then analyzed for the capturing the configuration of a water droplet on an inclined substrate in the presence the contact--angle hysteresis. The proposed method is finally employed to simulate the dynamics of a water droplet confined in a gas channel and exposed to air-flow.Existen numerosas aplicaciones industriales en las que transporte bifásico (líquido-gas) a escalas submilimétricas resulta crucial para la determinación del comportamiento del sistema en cuestión. Entre todas ellas, el presente trabajo se centra en las pilas de combustible con membrana de electrolito polimérico (PEMFC). El rendimiento deseable y la vida útil operativa de esta clase de dispositivos de conversión de energía de alto rendimiento requieren una gestión eficaz del agua (conocida como “water management”), que per se depende de la predicción adecuada de los mecanismos de transporte de agua y aire. Así pues, el análisis del flujo microfluídico de dos fases obliga considerar fuerzas y fenómenos interfaciales, tales como la histéresis, que están asociados con las propiedades fisicoquímicas del líquido, el gas y, si está presente, el sustrato sólido. En este contexto, la modelización numérica es una alternativa viable para obtener una predicción precisa de los mecanismos de transporte, especialmente en aquellos casos en los que los análisis experimentales son prohibitivos, ya sea por su complejidad o coste económico. En este trabajo, se desarrolla un marco eficiente, basado en la combinación del método de elementos finitos y el método de “level-set”, para la simulación tridimensional de flujos bifásicos. Con el fin de lograr una herramienta numérica robusta para aplicaciones prácticas, las complejidades físicas se incluyen consistentemente y los problemas numéricos involucrados se abordan adecuadamente. Concretamente, la discontinuidad de la presión en la interfaz líquido-gas se captura consistentemente utilizando un espacio de elementos finitos enriquecido. La estabilización del método se consigue mediante la introducción de la técnica de multiescalas variacionales. Asimismo, se propone también un tratamiento novedoso para el problema de la inestabilidad de tipo “small-cut”. Se muestra que el modelo propuesto puede proporcionar resultados precisos minimizando las corrientes espurias en la interfaz liquido-gas. Complementariamente, se presenta una nueva metodología para filtrar el ruido en el campo de “level-set”. Esta metodología resulta ser crucial para prevenir las irregularidades provocadas por el remanente persistente de las corrientes espurias. El comportamiento de la línea de contacto es considerado a través de la inclusión los modelos correspondientes en la formulación variacional. A este respecto, el presente trabajo aborda la importancia de la inclusión de la hidrodinámica subelemental. Los resultados presentados se basan en la combinación de la cinética molecular linealizada y las teorías hidrodinámicas. Para representación del comportamiento realista de los líquidos en contacto con sustratos sólidos, el fenómeno de histéresis del ángulo de contacto se tiene en cuenta imponiendo un mecanismo de anclado / desanclado consistente desarrollado en el marco del método de level-set. Aparte de los desarrollos principales, también se propone una técnica novedosa para la convección de la función ”level-set”. Ésta permite mejorar significativamente la precisión, minimizando a su vez la pérdida en las características geométricas de la interfaz asociadas al transporte. Esta nueva metodología está basada en el algoritmo de corrección de compensación de errores (BFECC). La herramienta numérica desarrollada en esta tesis es validada para varios casos que involucran burbujas de gas en un líquido y pequeñas gotas de líquido en un gas. Para el último escenario, además de las gotas libres, se evalúa la precisión de la herramienta propuesta para capturar la dinámica de las gotas sobre sustratos sólidos. A continuación, se analiza el rendimiento del modelo para capturar la configuración de una gota de agua sobre un sustrato inclinado en presencia de la histéresis del ángulo de contacto. El método propuesto finalmente se aplicaPostprint (published version

Complexity Analysis Of Next-Generation VVC Encoding and Decoding

While the next generation video compression standard, Versatile Video Coding (VVC), provides a superior compression efficiency, its computational complexity dramatically increases. This paper thoroughly analyzes this complexity for both encoder and decoder of VVC Test Model 6, by quantifying the complexity break-down for each coding tool and measuring the complexity and memory requirements for VVC encoding/decoding. These extensive analyses are performed for six video sequences of 720p, 1080p, and 2160p, under Low-Delay (LD), Random-Access (RA), and All-Intra (AI) conditions (a total of 320 encoding/decoding). Results indicate that the VVC encoder and decoder are 5x and 1.5x more complex compared to HEVC in LD, and 31x and 1.8x in AI, respectively. Detailed analysis of coding tools reveals that in LD on average, motion estimation tools with 53%, transformation and quantization with 22%, and entropy coding with 7% dominate the encoding complexity. In decoding, loop filters with 30%, motion compensation with 20%, and entropy decoding with 16%, are the most complex modules. Moreover, the required memory bandwidth for VVC encoding/decoding are measured through memory profiling, which are 30x and 3x of HEVC. The reported results and insights are a guide for future research and implementations of energy-efficient VVC encoder/decoder.Comment: IEEE ICIP 202

Economic Austerity along with Resistive Economy as Two Actuating Forces in Iran's Economy

Resistive economy is an attempt to control and counteract effects of international limitations against a zone or any country. In fact, it is necessary to resist counteracting threats and turning them to opportunity and obtaining national strengths. In an ideal situation, resistive economy can remove the crisis resulted from international limitations. Certainly, public belief and cooperation and rational and wise management are prerequisites for such goal. Resistive economy is a decrease of dependencies and emphasis on benefits of domestic production, efficiency of human resources and trying for self-sufficiency. Of course, a real resistive economy is a dynamic one not a stable and closed economy, as resistance is defined to remove obstacles of development and to try for improving level of economy. By studying Iran's economy during recent years, it is found out that international limitations are not the only negative factor influencing the Iranian economy. Rather, other factors such as failure in proper execution of subsidies purposefulness law as a public service of government has caused shortage in budget and also improper consumption of national and natural resources. To counteract the negative effect of such factors, a kind of economic austerity at the same time with the Resistive economy should be executed by the policymakers of economy. The fact that austerity is imposed on the economy and its critical situation does not seem to be tolerable but in long term it can help restore the economy. Consequently, the Iranian economy should turn to austerity in long term

The Impact of Tourism Industry on Increasing Employment and Income in Daylam Port

The aim of this study was to analyze the impact of tourism industry on increasing employment and income in Daylam Port. Field data were used in this study and the hypotheses, based on statistical analysis, were studied and tested by SPSS software. The results indicated that the activities and employment in this industry along with the not very significant role of industry, services and agriculture had a significant economic impact on the port city and led to increase in income and employment, and cost in the city

A New Approach to the Optimal Placement of the Viscous Damper Based on the Static Force Distribution Pattern

Viscous dampers (VDs) are currently used an effective earthquake risk reduction measure. Due to the high cost of this type of dampers, an optimal damper layout across the stories will specifically improve the seismic response and reduce building costs. This paper introduces a new simple three-stage method to determine the optimal placement of VDs on different stories of reinforced concrete structures. In the first stage, the damping demand of each story was determined using the distribution pattern of earthquake forces by the equivalent static method and the story velocity obtained through time-history analysis. In the second stage, the number of dampers required for the structure was calculated, and the location and damping percentage of dampers were precisely determined through an iterative process in the third stage. An indicator representing all basic structural responses was used to evaluate the multiple choices for the damper layout. This process was evaluated for 4, 8 story concrete frames under a near-field earthquake. The results indicated the efficiency of the proposed method in determining the location and damping of VDs on different stories of the structure.

Direct Numerical Simulation of Magnetic Particles Suspended in a Newtonian Fluid Exhibiting Finite Inertia Under SAOS

The corresponding author wishes to express his sincerest thanks to the Iran National Science Foundation (INSF) for supporting this work under Contract Number 92021291.Peer reviewedPostprin

An enhanced non-oscillatory BFECC algorithm for finite element solution of advective transport problems

In this paper, the so-called “back and forth error compensation correction (BFECC)” methodology is utilized to improve the solvers developed for the advection equation. Strict obedience to the so-called “discrete maximum principle” is enforced by incorporating a gradient–based limiter into the BFECC algorithm. The accuracy of the BFECC algorithm in capturing the steep–fronts in hyperbolic scalar–transport problems is improved by introducing a controlled anti–di¿usivity. This is achieved at the cost of performing an additional backward sub–solution–step and modifying the formulation of the error compensation accordingly. The performance of the proposed methodology is assessed by solving a series of benchmarks utilizing di¿erent combinations of the BFECC algorithms and the underlying numerical schemes. Results are presented for both the structured and unstructured meshes.This work was performed within the framework of AMADEUS project (”Advanced Multi-scAle moDEling of coupled mass transport for improving water management in fUel cellS”, reference number PGC2018-101655-B-I00) supported by the Ministerio de Ciencia, Innovacion e Universidades of Spain. The authors also acknowledge financial support of the mentioned Ministry via the “Severo Ochoa Programme” for Centres of Excellence in R&D (referece: CEX2018-000797-S) given to the International Centre for Numerical Methods in Engineering (CIMNE).Peer ReviewedPostprint (published version

Toward droplet dynamics simulation in polymer electrolyte membrane fuel cells: three-dimensional numerical modeling of confined water droplets with dynamic contact angle and hysteresis

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Mohammad R. Hashemi, Pavel B. Ryzhakov, and Riccardo Rossi, "Toward droplet dynamics simulation in polymer electrolyte membrane fuel cells: Three-dimensional numerical modeling of confined water droplets with dynamic contact angle and hysteresis", Physics of Fluids 33, 122109 (2021) and may be found at https://doi.org/10.1063/5.0073331This work focuses on three-dimensional simulation of the dynamics of droplets with contact–angle hysteresis. In order to consistently model the dynamics of the contact– line, a combination of the linear molecular kinetic theory and the hydrodynamic theory is implemented in the present numerical method. Without presetting the contact–line and/or the contact–angle, such simulations are generally prone to irregularities at the contact–line, which are mainly due to the imposition of the pinning and unpinning mechanisms associated with the hysteresis phenomenon. An effective treatment for this issue is proposed based on a simple procedure for calculating the nodal contact–angle within the framework of enriched finite element/level set method. The resulting method also benefits from a manipulated momentum conservation equation that incorporates the effect of the liquid mass conservation correction, which is essentially important for simulations with a rather long (physical) run–time. In this paper, the proposed numerical model is validated against the previously published experimental data addressing the configuration of a water droplet on a tilted rough hydrophobic surface. In this test, the effect of the contact–line pinning as the underlying mechanism for droplet hysteresis phenomenon is also studied. The model is further employed to simulate a liquid droplet confined in a channel in the presence of air flow.This work was performed within the framework of AMADEUS project (“Advanced Multi-scAle moDEling of coupled mass transport for improving water management in fUel cellS,” Reference Number PGC2018–101655-B-I00) supported by the Ministerio de Ciencia, Innovacion e Universidades of Spain. The authors acknowledge the financial support of the mentioned Ministry via the “Severo Ochoa Programme” for Centres of Excellence in R&D (Reference No. CEX2018–000797-S) given to the International Centre for Numerical Methods in Engineering (CIMNE). The authors also acknowledge PRACE for awarding us access to MareNostrum hosted by Barcelona Supercomputing Center, Spain (Project Reference No. 2010PA5560).Peer ReviewedPostprint (author's final draft

The Analysis of the Relationship between the Theoretical Knowledge of Translators and Their Practical Translation Skills: An Evaluation of Graduate Translation Courses

The present study, in the first place, attempted to examine the relationship between the theoretical knowledge of translation and practical translation skills in Iranian translators. For this purpose, 35 M.A students both males and females studying English translation were selected in Mashhad Ferdowsi University in Iran. In order to collect data two tests were applied: one made up of questions selected from the Ph.D and M.A entrance exams together with a test of actual translation from Persian to English and the reverse. Data analysis and statistical calculations through T-TEST, one way ANOVA and Pearson Correlation revealed that participants’ theoretical knowledge does not displays a significant correlation with their scores in the English to Persian practical exams, while there is a positive relationship between participants’ scores in the theoretical exams and their Persian to English translation. Also it was found that participants’ experimental experiences in translation have a significant effect on their English to Persian translation