Thesis SIMULACIÓN NUMÉRICA BIFÁSICA DE UN CANAL ABIERTO ATRAVESADO POR UN OBSTÁCULO CILÍNDRICO HORIZONTAL: APLICACIÓN AL FENÓMENO DE SOCAVACIÓN
Date
2016-03
Authors
GUZMÁN CASTRO, PABLO JAVIER
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Abstract
El presente trabajo aborda el estudio de las estructuras de flujo al interior de canales abiertos, y la socavación en lechos de ríos, ya que dicho fenómeno produce impacto ambiental al generar una modificación de los ecosistemas milenarios que se dan en estas bioregiones. La idea principal de este trabajo es caracterizar y cuantificar la influencia que ejerce al interior del flujo la introducción de un obstáculo cilíndrico horizontal, es decir, un ducto cerrado y de sección circular que atraviesa el cauce de orilla a orilla. Dada la información que existe de estudios anteriores, se presume que la influencia del obstáculo en la estructura del flujo es tal, que genera una zona de socavación en las áreas del lecho ubicadas inmediatamente bajo el ducto, y luego aguas abajo de este (lo que también provoca un segundo fenómeno que va de la mano con la socavación, de arrastre y transporte de sedimentos que no será incluido en este estudio). De inmediato cabe indicar que el efecto de socavación y de arrastre de material biológico está presente de manera natural en la vida de los cauces, por la presencia de obstáculos naturales como grandes rocas. Sin embargo, este trabajo se enfoca en aquellos obstáculos artificiales impuestos por el hombre como ductos de transporte de materias primas industriales, riles o agua potable, y que son susceptibles de analizar en casos. En concreto, lo que se hizo fue resolver un conjunto de ecuaciones aplicadas al flujo del rio en los casos que se simularon, contemplando la presencia de la atmósfera. Aquí es necesario un espacio para enfatizar la importancia del hecho que la simulación sea bifásica, es decir, que considere dentro del dominio espacial simulado un compartimento para cada uno de los fluidos presentes, agua y aire, ya que parte de las hipótesis que se generaron al leer las últimas publicaciones del tema es que la influencia que ejerce la interfase sobre la estructura interna del flujo de agua se hace más importante en la medida que se acorta la distancia entre el ducto y la interfase. Para abordar este aspecto se utilizó el método VOF (Volume Of Fluid), que puede consultarse al interior del trabajo. Las ecuaciones mencionadas fueron modificadas por el efecto de la turbulencia a partir de cierto número de Reynolds. Este efecto genera la proliferación de estructuras coherentes de flujo que tienen efectos en el comportamiento del flujo a través del tiempo, en ese sentido la velocidad se subdivide, en el dominio espacial, en una componente promedio y otra fluctuante, y se introduce el método LES (Large Eddy Simulation) que realiza lo anterior con la introducción de un filtro espacial para los campos de resultados instantáneos, calculando los campos promedio y modelando los campos de fluctuaciones. Se generó un primer grupo de simulaciones con un número de Froude Fr=0.44 y otro grupo con Fr=0.88. Entonces, se muestran los resultados obtenidos que sean de mayor interés para el análisis, previa validación de los mismos (estudio sobre el número de Strouhal y sobre la Ley Logarítmica en perfiles de velocidad turbulentos). Se presentan resultados cualitativos (capturas a cortes longitudinales, z=4D, de la interfase aire-agua y de los campos de velocidad promedio, presión promedio, de fases y de vorticidad) y luego se realiza un análisis cuantitativo tanto sobre el desprendimiento de la capa límite en la superficie del cilindro como sobre el esfuerzo de corte, a nivel del lecho del río, y de su variación a lo largo de este, en presencia de las condiciones de flujo impuestas y del incremento de h entre cada caso. El análisis de los resultados reveló un comportamiento cualitativo característico, mientras que el análisis cuantitativo reveló relaciones importantes entre los esfuerzos de corte (asociados a la socavación), la altura relativa h y el número de Froude. Se observa que los resultados concuerdan con lo esperado y dan nuevas luces acerca de la acción de los obstáculos impuestos a flujos de ríos o cauces, y con esto su impacto ambiental, sentando bases para una mayor comprensión de la interacción del hombre con el entorno y también para generar en un futuro normativa adecuada en un terreno en que no se ha trabajado suficiente en términos prácticos.
The present work deals with the study of flow structures into open channels, and the scour in riverbeds, since this phenomenon produces environmental impact by generating a modification of the ancient ecosystems that occur in these bioregions. The main idea of this work is to characterize and quantify the influence into the flow that the introduction of a horizontal cylindrical obstacle does, i.e. a closed duct with a circular section that crosses the channel from bank to bank. Given the information that exists in previous studies, it is presumed that the influence of the obstacle in the flow structure is such that it generates one undercut region in the bed areas located immediately beneath the duct, and downstream of this (which also causes a second phenomenon that goes hand in hand with the undermining, drag and sediment transport will not be included in this study).Immediately it should be noted that the effect of undermining and drag biological material is present naturally in the life of the channels, with the presence of natural obstacles such as large rocks. However, this work focuses on those artificial obstacles imposed by man as pipeline transport of industrial raw materials, sterile or potable water, and are likely to analyze cases. Specifically, what was done was to solve a set of equations applied to the flow of the river in the simulated cases, considering the presence of the atmosphere. Here, a space is necessary to emphasize the importance of the fact that the simulation is biphasic, i.e., to consider within the spatial domain simulated a compartment for each of the fluids, water and air as part of the hypotheses generated to read the latest posts of the matter is that the influence of the interface on the internal structure of the water flow. It becomes more important as the distance between the duct and the interface is shortened. To address this aspect used the VOF (Volume Of Fluid) method, which is available within the work. The equations above were modified by the effect of turbulence after a certain number of Reynolds. This effect generates the proliferation of coherent flow structures which impact on the flow behavior over time, in that sense the speed is divided in the spatial domain, in an average component and a fluctuating, and LES method is introduced (Large Eddy Simulation) performing the above with the introduction of a spatial filter for the fields of instant results, calculating the average fields and modeling fluctuations fields.A first group of simulations were generated with a Froude number Fr = 0.44 and another group with Fr = 0.88. Then, the results that are most relevant to the analysis, after validation of these (study on the Strouhal number and the logarithmic law in turbulent velocity profiles) are shown. Qualitative results are presented (catches longitudinal cuts, z = 4D, the air-water interface and the fields of average speed, average pressure, phase and vorticity) and then a quantitative analysis is performed both on the evolution of the boundary layer on the surface of the cylinder and on the shear stress at the level of the riverbed, and its variation over this, in the presence of flow conditions imposed and the increase of h between each case.The analysis of the results revealed a characteristic qualitative behavior, whereas quantitative analysis revealed significant relationship between shear stress (associated with undermining), the relative height h and the Froude number. It is observed that the results are consistent with expectations and give new lights about the action of the obstacles imposed in rivers streams, and thereby their environmental impact, laying groundwork for a greater understanding of human interaction with the environment and also in the future to generate adequate normative on land that has not been done enough in practical terms.
The present work deals with the study of flow structures into open channels, and the scour in riverbeds, since this phenomenon produces environmental impact by generating a modification of the ancient ecosystems that occur in these bioregions. The main idea of this work is to characterize and quantify the influence into the flow that the introduction of a horizontal cylindrical obstacle does, i.e. a closed duct with a circular section that crosses the channel from bank to bank. Given the information that exists in previous studies, it is presumed that the influence of the obstacle in the flow structure is such that it generates one undercut region in the bed areas located immediately beneath the duct, and downstream of this (which also causes a second phenomenon that goes hand in hand with the undermining, drag and sediment transport will not be included in this study).Immediately it should be noted that the effect of undermining and drag biological material is present naturally in the life of the channels, with the presence of natural obstacles such as large rocks. However, this work focuses on those artificial obstacles imposed by man as pipeline transport of industrial raw materials, sterile or potable water, and are likely to analyze cases. Specifically, what was done was to solve a set of equations applied to the flow of the river in the simulated cases, considering the presence of the atmosphere. Here, a space is necessary to emphasize the importance of the fact that the simulation is biphasic, i.e., to consider within the spatial domain simulated a compartment for each of the fluids, water and air as part of the hypotheses generated to read the latest posts of the matter is that the influence of the interface on the internal structure of the water flow. It becomes more important as the distance between the duct and the interface is shortened. To address this aspect used the VOF (Volume Of Fluid) method, which is available within the work. The equations above were modified by the effect of turbulence after a certain number of Reynolds. This effect generates the proliferation of coherent flow structures which impact on the flow behavior over time, in that sense the speed is divided in the spatial domain, in an average component and a fluctuating, and LES method is introduced (Large Eddy Simulation) performing the above with the introduction of a spatial filter for the fields of instant results, calculating the average fields and modeling fluctuations fields.A first group of simulations were generated with a Froude number Fr = 0.44 and another group with Fr = 0.88. Then, the results that are most relevant to the analysis, after validation of these (study on the Strouhal number and the logarithmic law in turbulent velocity profiles) are shown. Qualitative results are presented (catches longitudinal cuts, z = 4D, the air-water interface and the fields of average speed, average pressure, phase and vorticity) and then a quantitative analysis is performed both on the evolution of the boundary layer on the surface of the cylinder and on the shear stress at the level of the riverbed, and its variation over this, in the presence of flow conditions imposed and the increase of h between each case.The analysis of the results revealed a characteristic qualitative behavior, whereas quantitative analysis revealed significant relationship between shear stress (associated with undermining), the relative height h and the Froude number. It is observed that the results are consistent with expectations and give new lights about the action of the obstacles imposed in rivers streams, and thereby their environmental impact, laying groundwork for a greater understanding of human interaction with the environment and also in the future to generate adequate normative on land that has not been done enough in practical terms.
Description
Catalogado desde la version PDF de la tesis.
Keywords
LES , OPENFOAM , SIMULACION NUMERICA
