Santiago Urquiza

Numerical modeling of welded joints by the "Friction Stir Welding" process

The present work is aimed to simulate the Friction Stir Welding process as a three-dimensional thermally coupled viscoplastic flow. A Finite Element technique is employed, within the context of a general purpose FEM framework, to provide the temperature distributions and the patterns of plastic flow for the material involved in the welded joints. The computational tool presented here may be of great relevance for technologist seeking to set the process control variables, as they are intended to obtain suitable material properties that yield the adequate on service response of the structural components.

HeMoLab - Hemodynamics Modelling Laboratory

In this work we present HeMoLab (Hemodynamics Modeling Laboratory), a computational environment for modeling the Human Cardiovascular System. Its integrates novel computational tools, running from medical image processing to numerical simulation and visualization. As a simulation tool, it allows to accommodate complex physiological and/or pathophysiological (virtual) scenarios aimed to retrieve detailed information from the numerical computations. Such application makes possible to speed up research in the study and analysis of the cardiovascular system and, to provide a virtual laboratory for medical training and education, and specialized Human Resources development. In order to demonstrate the modeling and simulation capabilities of HeMoLab some cases of use are presented.

Modelado numérico del proceso de pultrusión en materiales compuestos

Received: February 14, 2003; Revised: June 26, 2003A mathematical pultrusion model of a thermosetting matrix composite is presented. The en-ergy balance was solved by Finite Elements techniques using a program developed for this pur-pose. This model is applied to describe the temperature and resin conversion profiles along thedie. This study was developed considering different pulling velocity, die temperature, differentcomposite and different sections die. The model outcomes are verified through comparison withthe experimental results reported in the literature.

On the effect of preload and pre-stretch on hemodynamic simulations: an integrative approach

In this work, we address the simulation of three-dimensional arterial blood flow and its effect on the stress state of arterial walls. The novel contribution is the unprecedented combination of several modeling techniques to account for (1) the fact that known configurations for the arterial wall are in a preloaded state, (2) the compliance of the vessel segments, (3) proper boundary data over the non-physical interfaces resulting from the isolation of an arterial district from the rest of the arterial tree, (4) the presence of surrounding tissues in which the vessel is embedded and (5) residual stress state due to pre-stretch. Firstly, we formulate both the forward mechanical problem when the reference (zero-load) configuration is assumed to be known and, the preload problem arising when the known domain is a configuration at equilibrium with a certain load state (typically due to internal pressure and tethering forces). Then, two additional complexities are faced: the fluid–structure interaction problem that follows when the compliant vessels are coupled with the blood flow, and the introduction of non-physical boundaries coming from the artificial isolation of the arterial district from the original vessel. This, in turn, posses the problem of coupling dimensionally heterogeneous models to incorporate the effect of upstream and downstream systemic impedances. Additionally, a viscoelastic support on the external surface of the vessel is also incorporated. Two examples are presented to quantify in a physiologically consistent scenario the differences in simulation results when either considering or not the preload state of arterial walls. These computational simulations shed light on the validity of simplifying hypotheses in most hemodynamic models.

Sensitivity of Blood Flow Patterns to the Constitutive Law of the Fluid

It is well known that hemodynamic factors are strongly influenced by the arterial geometry. Combining computational fluid dynamics with three-dimensional medical data makes possible the study of blood flow in real geometries. By this means we are able to analyze the sensitivity of hemodynamic factors due to shape changes in vascular districts. It is also well known that the fluid’s constitutive law very much influences the flow structure in singularities, such as the carotid sinus. In this work we quantify the sensitivity of blood flow for a model of a carotid bifurcation due to different constitutive behavior. In this case, the flow pattern for the Casson constitutive equation for the fluid is compared with its Newtonian counterpart. To this end a multidimensional 3D-1D FEM model of the whole arterial tree is implemented. It comprises a 3D compliant model of the carotid bifurcation coupled with a 1D model for the remaining part of the arterial tree. With this approach, difficulties arising from the treatment of boundary conditions for the 3D model are naturally handled. In particular, two carotid bifurcation geometries were analyzed. One of them corresponds to a standard model taken from literature. The second geometry was acquired from a patient-specific angiography using image segmentation and reconstruction techniques. Detailed hemodynamic factors and flow patterns in the carotid bifurcation are provided. Finally, this data is analyzed in order to determine how these results are affected in the different cases under study.

Sensitivity analysis and parameter estimation of heat transfer and material flow models in friction stir welding

Although numerical models of heat transfer and material flow have contributed to understand the underlying mechanisms of friction stir welding (FSW), there are certain input model parameters that can not be easily determined. Thus, the model predictions do not always agree with experimental results. In this work, sensitivity analysis and parameter estimation were applied to test heat transfer and material flow models. A forward-difference approximation was used to compute the sensitivity of the solution with respect to the unknown model parameters. The Levenberg-Marquardt (LM) method was applied to solve the nonlinear parameter estimation problem. The numerical models were developed by the finite element method (FEM). The way in which the unknown model parameters independently affect the results and the importance of the location of reference points that take part in the objective function were determined.

Coupled models technology in multi-scale computational haemodynamics

The goal of this work is to gather recent advances in computational modelling aimed at tackling problems in the field of computational hemodynamics to provide insight about the flow regimes prevailing in certain vascular districts. These models are devised to retrieve the relevant physical phenomena from the different geometrical scales encountered within the cardiovascular system. Hence, we resort to the use of dimensionally-heterogeneous models, whose derivation follows a unified variational approach. Also, some issues that emerge with the use of such technology are exposed and elucidated, together with two selected examples of application. Finally, some guidelines for future research are highlighted.

Inverse solution of hemodialysis solute kinetics

A numerical solution of the inverse problem for parameter estimation of solute kinetics during hemodialysis based on a double pool model is presented. Weighted Least Squares Minimisation is employed to optimise the fit between data and estimated concentration of extracellular compartment, resulting in a non-linear set of equations where second order sensitivity parameters are involved in the solution process. The resulting systems of ordinary differential equations ( ODE ) are discretised by Finite Differences. Assuming the total distribution volume, estimation for volume ratio between intra-extracellular compartments, mass transfer coefficient of cellular membrane, dialyzer clearance and generation rate are provided. The model predicts accurately solute concentration in numerical experiment for urea and creatinine kinetics during hemodialysis simulation and performs efficiently, in terms of convergence, even if few sample points are given as input data.