Pulsatile blood flow-split in aortic arch dissection

Abnormalities of the aortic arch, as the most proximal site of the cardiovascular system, are of great interest due to its major role in blood distribution to all downstream members. Wall dissection is one of the disorders that an aorta may suffer due to hypertension or degradation of aortic wall properties. A geometrical change of the aortic arch caused by the dissected wall, and consequently the blood flow path, makes the time-varying flow curves to be different in comparison to the healthy aortic arch. This phenomenon modifies wall shear stress (WSS) history during the cardiac cycle. In the current work, the pulsatile blood flow in a typical Stanford A (DeBakey II) dissected aorta is simulated by CFD technique, STAR-CCM+. The boundary conditions are calculated based on a combination of the impedance boundary condition and the auto-regulation concept in the cardiovascular system.

Three-dimensional flow instability in a lid-driven isosceles triangular cavity

Linear three-dimensional modal instability of steady laminar two-dimensional states developing in a lid-driven cavity of isosceles triangular cross-section is investigated theoretically and experimentally for the case in which the equal sides form a rectangular corner. An asymmetric steady two-dimensional motion is driven by the steady motion of one of the equal sides. If the side moves away from the rectangular corner, a stationary three-dimensional instability is found. If the motion is directed towards the corner, the instability is oscillatory. The respective critical Reynolds numbers are identified both theoretically and experimentally. The neutral curves pertinent to the two configurations and the properties of the respective leading eigenmodes are documented and analogies to instabilities in rectangular lid-driven cavities are discussed.

Linear instability of orthogonal compressible leading-edge boundary layer flow

Instability analysis of compressible orthogonal swept leading-edge boundary layer flow was performed in the context of BiGlobal linear theory. 1, 2 An algorithm was developed exploiting the sparsity characteristics of the matrix discretizing the PDE-based eigenvalue problem. This allowed use of the MUMPS sparse linear algebra package 3 to obtain a direct solution of the linear systems associated with the Arnoldi iteration. The developed algorithm was then applied to efficiently analyze the effect of compressibility on the stability of the swept leading-edge boundary layer and obtain neutral curves of this flow as a function of the Mach number in the range 0 ≤ Ma ≤ 1. The present numerical results fully confirmed the asymptotic theory results of Theofilis et al. 4 Up to the maximum Mach number value studied, it was found that an increase of this parameter reduces the critical Reynolds number and the range of the unstable spanwise wavenumbers.

On linear instability mechanisms in incompressible open cavity flow

A theoretical study of linear global instability of incompressible flow over a rectangular spanwise-periodic open cavity in an unconfined domain is presented. Comparisons with the limited number of results available in the literature are shown. Subsequently, the parameter space is scanned in a systematic manner, varying Reynolds number, incoming boundary-layer thickness and length-to-depth aspect ratio. This permits documenting the neutral curves and leading eigenmode characteristics of this flow. Correlations constructed using the results obtained collapse all available theoretical data on the three-dimensional instabilities.

Guidelines for the design of an environmental flow regime program at the regional scale: Application of the ELOHA framework in the Ebro River basin

La gestión de los recursos hídricos se convierte en un reto del presente y del futuro frente a un panorama de continuo incremento de la demanda de agua debido al crecimiento de la población, el crecimiento del desarrollo económico y los posibles efectos del calentamiento global. La política hidráulica desde los años 60 en España se ha centrado en la construcción de infraestructuras que han producido graves alteraciones en el régimen natural de los ríos. Estas alteraciones han provocado y acrecentado los impactos sobre los ecosistemas fluviales y ribereños. Desde los años 90, sin embargo, ha aumentado el interés de la sociedad para conservar estos ecosistemas. El concepto de caudales ambientales consiste en un régimen de caudales que simula las características principales del régimen natural. Los caudales ambientales están diseñados para conservar la estructura y funcionalidad de los ecosistemas asociados al régimen fluvial, bajo la hipótesis de que los elementos que conforman estos ecosistemas están profundamente adaptados al régimen natural de caudales, y que cualquier alteración del régimen natural puede provocar graves daños a todo el sistema. El método ELOHA (Ecological Limits of Hydrological Alteration) tiene como finalidad identificar las componentes del régimen natural de caudales que son clave para mantener el equilibrio de los ecosistemas asociados, y estimar los límites máximos de alteración de estas componentes para garantizar su buen estado. Esta tesis presenta la aplicación del método ELOHA en la cuenca del Ebro. La cuenca del Ebro está profundamente regulada e intervenida por el hombre, y sólo las cabeceras de los principales afluentes del Ebro gozan todavía de un régimen total o cuasi natural. La tesis se estructura en seis capítulos que desarrollan las diferentes partes del método. El primer capítulo explica cómo se originó el concepto “caudales ambientales” y en qué consiste el método ELOHA. El segundo capítulo describe el área de estudio. El tercer capítulo realiza una clasificación de los regímenes naturales de la cuenca (RNC) del Ebro, basada en series de datos de caudal mínimamente alterado y usando exclusivamente parámetros hidrológicos. Se identificaron seis tipos diferentes de régimen natural: pluvial mediterráneo, nivo-pluvial, pluvial mediterréaneo con una fuerte componente del caudal base, pluvial oceánico, pluvio-nival oceánico y Mediterráneo. En el cuarto capítulo se realiza una regionalización a toda la cuenca del Ebro de los seis RNC encontrados en la cueca. Mediante parámetros climáticos y fisiográficos se extrapola la información del tipo de RNC a puntos donde no existen datos de caudal inalterado. El patrón geográfico de los tipos de régimen fluvial obtenido con la regionalización resultó ser coincidente con el patrón obtenido a través de la clasificación hidrológica. El quinto capítulo presenta la validación biológica de los procesos de clasificación anteriores: clasificación hidrológica y regionalización. La validación biológica de los tipos de regímenes fluviales es imprescindible, puesto que los diferentes tipos de régimen fluvial van a servir de unidades de gestión para favorecer el mantenimiento de los ecosistemas fluviales. Se encontraron diferencias significativas entre comunidades biológicas en cinco de los seis tipos de RNC encontrados en la cuenca. Finalmente, en el sexto capítulo se estudian las relaciones hidro-ecológicas existentes en tres de los seis tipos de régimen fluvial encontrados en la cuenca del Ebro. Mediante la construcción de curvas hidro-ecológicas a lo largo de un gradiente de alteración hidrológica, se pueden sugerir los límites de alteración hidrológica (ELOHAs) para garantizar el buen estado ecológico en cada uno de los tipos fluviales estudiados. Se establecieron ELOHAs en tres de los seis tipos de RNC de la cuenca del Ebro Esta tesis, además, pone en evidencia la falta de datos biológicos asociados a registros de caudal. Para llevar a cabo la implantación de un régimen de caudales ambientales en la cuenca, la ubicación de los puntos de muestreo biológico cercanos a estaciones de aforo es imprescindible para poder extraer relaciones causa-efecto de la gestión hidrológica sobre los ecosistemas dependientes. ABSTRACT In view of a growing freshwater demand because of population raising, improvement of economies and the potential effects of climate change, water resources management has become a challenge for present and future societies. Water policies in Spain have been focused from the 60’s on constructing hydraulic infrastructures, in order to dampen flow variability and granting water availability along the year. Consequently, natural flow regimes have been deeply altered and so the depending habitats and its ecosystems. However, an increasing acknowledgment of societies for preserving healthy freshwater ecosystems started in the 90’s and agreed that to maintain healthy freshwater ecosystems, it was necessary to set environmental flow regimes based on the natural flow variability. The Natural Flow Regime paradigm (Richter et al. 1996, Poff et al. 1997) bases on the hypothesis that freshwater ecosystems are made up by elements adapted to natural flow conditions, and any change on these conditions can provoke deep impacts on the whole system. Environmental flow regime concept consists in designing a flow regime that emulates natural flow characteristics, so that ecosystem structure, functions and services are maintained. ELOHA framework (Ecological Limits of Hydrological Alteration) aims to identify key features of the natural flow regime (NFR) that are needed to maintain and preserve healthy freshwater and riparian ecosystems. Moreover, ELOHA framework aims to quantify thresholds of alteration of these flow features according to ecological impacts. This thesis describes the application of the ELOHA framework in the Ebro River Basin. The Ebro River basin is the second largest basin in Spain and it is highly regulated for human demands. Only the Ebro headwaters tributaries still have completely unimpaired flow regime. The thesis has six chapters and the process is described step by step. The first chapter makes an introduction to the origin of the environmental flow concept and the necessity to come up. The second chapter shows a description of the study area. The third chapter develops a classification of NFRs in the basin based on natural flow data and using exclusively hydrological parameters. Six NFRs were found in the basin: continental Mediterranean-pluvial, nivo-pluvial, continental Mediterranean pluvial (with groundwater-dominated flow pattern), pluvio-oceanic, pluvio-nival-oceanic and Mediterranean. The fourth chapter develops a regionalization of the six NFR types across the basin by using climatic and physiographic variables. The geographical pattern obtained from the regionalization process was consistent with the pattern obtained with the hydrologic classification. The fifth chapter performs a biological validation of both classifications, obtained from the hydrologic classification and the posterior extrapolation. When the aim of flow classification is managing water resources according to ecosystem requirements, a validation based on biological data is compulsory. We found significant differences in reference macroinvertebrate communities between five over the six NFR types identified in the Ebro River basin. Finally, in the sixth chapter we explored the existence of significant and explicative flow alteration-ecological response relationships (FA-E curves) within NFR types in the Ebro River basin. The aim of these curves is to find out thresholds of hydrological alteration (ELOHAs), in order to preserve healthy freshwater ecosystem. We set ELOHA values in three NFR types identified in the Ebro River basin. During the development of this thesis, an inadequate biological monitoring in the Ebro River basin was identified. The design and establishment of appropriate monitoring arrangements is a critical final step in the assessment and implementation of environmental flows. Cause-effect relationships between hydrology and macroinvertebrate community condition are the principal data that sustain FA-E curves. Therefore, both data sites must be closely located, so that the effects of external factors are minimized. The scarce hydro-biological pairs of data available in the basin prevented us to apply the ELOHA method at all NFR types.

Pulsatile blood flow interaction with arterial walls of aorta : autoregulation and impedance pressure boundary condition and its biomedical applications

Para las decisiones urgentes sobre intervenciones quirúrgicas en el sistema cardiovascular se necesitan simulaciones computacionales con resultados fiables y que consuman un tiempo de cálculo razonable. Durante años los investigadores han trabajado en diversos métodos numéricos de cálculo que resulten atractivos para los cirujanos. Estos métodos, precisos pero costosos desde el punto de vista del coste computacional, crean un desajuste entre la oferta de los ingenieros que realizan las simulaciones y los médicos que operan en el quirófano. Por otra parte, los métodos de cálculo más simplificados reducen el tiempo de cálculo pero pueden proporcionar resultados no realistas. El objetivo de esta tesis es combinar los conceptos de autorregulación e impedancia del sistema circulatorio, la interacción flujo sanguíneo-pared arterial y modelos geométricos idealizados tridimensionales de las arterias pero sin pérdida de realismo, con objeto de proponer una metodología de simulación que proporcione resultados correctos y completos, con tiempos de cálculo moderados. En las simulaciones numéricas, las condiciones de contorno basadas en historias de presión presentan inconvenientes por ser difícil conocerlas con detalle, y porque los resultados son muy sensibles ante pequeñas variaciones de dichas historias. La metodología propuesta se basa en los conceptos de autorregulación, para imponer la demanda de flujo aguas abajo del modelo en el ciclo cardiaco, y la impedancia, para representar el efecto que ejerce el flujo en el resto del sistema circulatorio sobre las arterias modeladas. De este modo las historias de presión en el contorno son resultados del cálculo, que se obtienen de manera iterativa. El método propuesto se aplica en una geometría idealizada del arco aórtico sin patologías y en otra geometría correspondiente a una disección Stanford de tipo A, considerando la interacción del flujo pulsátil con las paredes arteriales. El efecto de los tejidos circundantes también se incorpora en los modelos. También se hacen aplicaciones considerando la interacción en una geometría especifica de un paciente anciano que proviene de una tomografía computarizada. Finalmente se analiza una disección Stanford tipo B con tres modelos que incluyen la fenestración del saco. Clinicians demand fast and reliable numerical results of cardiovascular biomechanic simulations for their urgent pre-surgery decissions. Researchers during many years have work on different numerical methods in order to attract the clinicians' confidence to their colorful contours. Though precise but expensive and time-consuming methodologies create a gap between numerical biomechanics and hospital personnel. On the other hand, simulation simplifications with the aim of reduction in computational time may cause in production of unrealistic outcomes. The main objective of the current investigation is to combine ideas such as autoregulation, impedance, fluid-solid interaction and idealized geometries in order to propose a computationally cheap methodology without excessive or unrealistic simplifications. The pressure boundary conditions are critical and polemic in numerical simulations of cardiovascular system, in which a specific arterial site is of interest and the rest of the netwrok is neglected but represented by a boundary condition. The proposed methodology is a pressure boundary condition which takes advantage of numerical simplicity of application of an imposed pressure boundary condition on outlets, while it includes more sophisticated concepts such as autoregulation and impedance to gain more realistic results. Incorporation of autoregulation and impedance converts the pressure boundary conditions to an active and dynamic boundary conditions, receiving feedback from the results during the numerical calculations and comparing them with the physiological requirements. On the other hand, the impedance boundary condition defines the shapes of the pressure history curves applied at outlets. The applications of the proposed method are seen on idealized geometry of the healthy arotic arch as well as idealized Stanford type A dissection, considering the interaction of the arterial walls with the pulsatile blood flow. The effect of surrounding tissues is incorporated and studied in the models. The simulations continue with FSI analysis of a patient-specific CT scanned geometry of an old individual. Finally, inspiring of the statistic results of mortality rates in Stanford type B dissection, three models of fenestrated dissection sac is studied and discussed. Applying the developed boundary condition, an alternative hypothesis is proposed by the author with respect to the decrease in mortality rates in patients with fenestrations.