A FIC-based stabilized finite element formulation for turbulent flows

We present a new stabilized finite element (FEM) formulation for incompressible flows based on the Finite Increment Calculus (FIC) framework. In comparison to existing FIC approaches for fluids, this formulation involves a new term in the momentum equation, which introduces non-isotropic dissipation in the direction of velocity gradients. We also follow a new approach to the derivation of the stabilized mass equation, inspired by recent developments for quasi-incompressible flows. The presented FIC-FEM formulation is used to simulate turbulent flows, using the dissipation introduced by the method to account for turbulent dissipation in the style of implicit large eddy simulation.Peer ReviewedPostprint (author's final draft

p-probabilistic k-anonymous microaggregation for the anonymization of surveys with uncertain participation

We develop a probabilistic variant of k-anonymous microaggregation which we term p-probabilistic resorting to a statistical model of respondent participation in order to aggregate quasi-identifiers in such a manner that k-anonymity is concordantly enforced with a parametric probabilistic guarantee. Succinctly owing the possibility that some respondents may not finally participate, sufficiently larger cells are created striving to satisfy k-anonymity with probability at least p. The microaggregation function is designed before the respondents submit their confidential data. More precisely, a specification of the function is sent to them which they may verify and apply to their quasi-identifying demographic variables prior to submitting the microaggregated data along with the confidential attributes to an authorized repository. We propose a number of metrics to assess the performance of our probabilistic approach in terms of anonymity and distortion which we proceed to investigate theoretically in depth and empirically with synthetic and standardized data. We stress that in addition to constituting a functional extension of traditional microaggregation, thereby broadening its applicability to the anonymization of statistical databases in a wide variety of contexts, the relaxation of trust assumptions is arguably expected to have a considerable impact on user acceptance and ultimately on data utility through mere availability.Peer ReviewedPostprint (author's final draft

Size Effect and Other Effects on Mode I Fracture Toughness Using Two Testing Methods

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG[Abstract] Mode I fracture toughness (KIC) is an intrinsic material property that quantifies its resistance to tensile fracture propagation. The International Society for Rock Mechanics has endorsed four methods to determine the KIC of rock, namely, the short rod, chevron bend, cracked chevron notched Brazilian disc, and semi-circular bend (SCB) methods. In this study, we compare the results of the SCB technique with those of the recently proposed pseudo-compact tension (pCT) test, which has proven to be convenient for the assessment of KIC in both fragile and ductile rocks. We select the SCB as a benchmark method due to its popularity, simplicity, and straightforward testing configuration. We discuss the results of 146 tests performed with different lithologies (Arcera, Pinacas and Corvio sandstones, and Blanco Mera granite), different sample sizes (100, 50 and 38 mm diameter), and a range of notch lengths. We also assess test repeatability and intercomparability of the results obtained using the two techniques. Compared with the SCB test, the pCT test allows for improved control of the specimen behaviour after the peak load, which provides a greater wealth of fracture mechanics information. pCT specimens yield results with higher repeatability than SCB samples. Although we observe that KIC tends to decrease with an increase in the notch length ratio, this effect appears to be non-significant based on statistical assessments. Accordingly, the corresponding mean KIC values are comparable for medium- and large specimens. The influence of specimen size is more pronounced in the SCB tests, while the pCT tests show less dependence for harder lithologies. Therefore, to set up minimum specimen requirements for fracture toughness testing, in addition to geometrical constraints, some key lithology-dependent properties (strength, mineralogy, grain size, etc.) should also be considered. Further methodological considerations related to test execution are also discussed.This work was funded by Repsol S.A. and supported by the Xunta de Galicia, the European Union (European Social Fund - ESF) and the MINECO/AEI/FEDER EU project BIA2017-87066-R. Funding for open access charge: Universidade da Coruña/CISUG

Desarrollo e implantación, a nivel comercial, de un nuevo modelo operativo en una compañía de seguros para la distribución de productos dirigidos a las empresas

Una compañía aseguradora multinacional, generalista y con un ámbito de distribución nacional (para este proyecto la llamaremos Alfa), tiene como estrategia definida y aplicada desde el 2002 el focalizar sus esfuerzos, tanto técnicos como comerciales, hacia la distribución de productos dirigidos a particulares sin dedicar ni enfoque ni estructura a los productos para las empresas. Los motivos que propiciaron esta decisión fueron varios pero el más importante fue sin duda el que, entre 2001 y 2002, la compañía Alfa registró grandes pérdidas en los productos de empresas hecho que, junto con el endurecimiento general del mercado asegurador que sobrevino después del 11-S, hizo replantear la estrategia y focalizar esfuerzos en el mercado de particulares, saneando la cartera de empresas y reduciendo drásticamente la nueva contratación de pólizas de esos ramos. Por aquel entonces, antes del 2002, la compañía Alfa disponía de una potente estructura técnica y comercial para atender las necesidades de las empresas, pero esta decisión motivó que los efectivos especializados en estos ramos se redistribuyeran en los ramos de particulares, lo cual llevo, a medio plazo, a una pérdida de “know-how” generalizado así como salidas hacia otras entidades aseguradoras de las personas mejor preparadas. Ahora, esta compañía quiere volver a entrar en la comercialización de esta gama de productos; la situación actual del mercado se puede clasificar de “blanda” en el sentido de que existe una guerra generalizada de precios y voracidad extrema por conseguir clientes, sin que las primas y, en general, las condiciones de cobertura sea el mayor impedimento para la nueva contratación de las pólizas de seguro. El proyecto incide en el desarrollo del nuevo modelo operativo para la comercialización de los productos dirigidos a las empresas así como su posterior implantación, teniendo en cuenta tanto el entorno económico actual como la situación en la que se encuentra la compañía Alfa. No forma parte de este proyecto la definición del modelo operativo técnico (posicionamiento de tarifas así como redefinición de garantías, franquicias y límites de los productos afectados). La aplicación de los resultados y conclusiones de este proyecto permitirán a la compañía aseguradora Alfa ofrecer a sus clientes, a través de los mediadores con los que trabaja, una respuesta global a sus necesidades incluyendo las referentes a los productos de empresas

OpenUEBA – A systematic approach to learn behavioural patterns

For years, Security Operations Centers (SOC) have resorted to SIEM and IDS tools as the core defence shield, offering reactive detection capabilities against latent threats. Despite the effectiveness of the tools described above, cybercriminal groups have professionalized themselves by launching very sophisticated campaigns that unfortunately, go unnoticed by current detection tools. In order to revolutionize the current range of security tools, we present our vision and advances in openUEBA; An open-source framework focused on the study of the behaviour of users and entities on the network; Where through state-of-the-art Artificial Intelligence techniques are learn behavioural patterns of those users who later fall into cyber attacks. With the learnt knowledge, the tool calculates the user exposure; in other words, it predicts which users will be victims of latent threats, allowing the analyst to make preventive decisions.Peer ReviewedPostprint (published version

Photoelastic stress analysis of mode I fracture toughness tests using PMMA samples

[Abstract:] Rocks are usually inhomogeneous and anisotropic materials. The presence of foliation planes, grain boundaries or even microcracks may alter the stress distribution. In order to identify whether unusual behaviours in rocks are due to these imperfections or result from other factors (e.g. experimental configuration), the analyses of homogenous and isotropic materials is an useful approach. We have performed a series of mode I fracture toughness (KIC) tests using polymethyl methacrylate (PMMA) samples, which has the advantage of allowing photoelastic stress analysis based on its birefringent nature. Three different testing configurations were considered in the study: S\ emi-circular bend (SCB) test, the pseudo-compact tension (pCT) test, and a new alternative configuration based on the previous two that we have called pseudo-SCB (pSCB) test. To perform the photoelastic analysis, all the experiments were complemented with a specially-designed experimental setup consisting in two orthogonally arranged circular polarizers placed on both sides of the tested specimens. Using a source of white (polychromatic) light on one end it is possible to record the stress distribution using a digital camera aligned with the samples on the other end. As the load increases, a distinct evolving pattern of colour fringes can be visualized in the samples illustrating the spatially distributed stress levels. Based on this analysis we observe in some of the tests performed non-symmetrical stress fields. Although this behaviour could be related with the testing configuration, results suggest that other features, such as the shape of the notch tip, imperfections in sample preparation, or the misalignment of the samples in the testing device may also have an influence in stress distribution.Ministerio de Asuntos Económicos y Transformación Digital, MINECO; BIA2017- 87066-

Applications of turbulence modeling in civil engineering

This work explores the use of stabilized finite element formulations for the incompressible Navier-Stokes equations to simulate turbulent flow problems. Turbulence is a challenging problem due to its complex and dynamic nature and its simulation if further complicated by the fact that it involves fluid motions at vastly different length and time scales, requiring fine meshes and long simulation times. A solution to this issue is turbulence modeling, in which only the large scale part of the solution is retained and the effect of smaller turbulent motions is represented by a model, which is generally dissipative in nature. In the context of finite element simulations for fluids, a second problem is the apparition of numerical instabilities. These can be avoided by the use of stabilized formulations, in which the problem is modified to ensure that it has a stable solution. Since stabilization methods typically introduce numerical dissipation, the relation between numerical and physical dissipation plays a crucial role in the accuracy of turbulent flow simulations. We investigate this issue by studying the behavior of stabilized finite element formulations based on the Variational Multiscale framework and on Finite Calculus, analyzing the results they provide for well-known turbulent problems, with the final goal of obtaining a method that both ensures numerical stability and introduces physically correct turbulent dissipation. Given that, even with the use of turbulence models, turbulent flow problems require significant computational resources, we also focused on programming and parallel implementation aspects of finite element codes, and in particular in ensuring that our solver can perform efficiently on distributed memory architectures and high-performance computing clusters. Finally, we have developed an adaptive mesh refinement technique to improve the quality of unstructured tetrahedral meshes, again with the goal of enabling the simulation of large turbulent flow problems. This technique combines an error estimator based on Variational Multiscale principles with a simple refinement procedure designed to work in a distributed memory context and we have applied it to the simulation of both turbulent and non-Newtonian flow problems.Postprint (published version

A cooling system for a hybrid PV/thermal linear concentrator

This paper presents the thermal evaluation of an evacuated PVT collector designed to operate under concentrated radiation (15 suns). Finite volume 3D numerical computations have been carried out in order to study the thermal characteristics of different rectangular cross section aluminium pipes and to test the performance of the PVT collector with several laminar flow rates. Experiments with the same laminar flows show the same behavior than in the numerical results