Effect of stent position on flow characteristics in a cerebral aneurysm

The position of an intracranial stent in relation to the ostium of a cerebral aneurysm can significantly affect the blood flow characteristics through the ostium and inside the aneurysm. An idealised cerebral artery and aneurysm were simulated with a pulsatile flow. Simulation results show that the effect on mass inflow between two stent positions is about 20% whereas the difference in the porosity effect of the pattern at these two positions is around 3%. The remainder may be attributed to differences in flow velocity profile across the stent into the aneurysm. The implications for clinical practice are an important consideration as the surgeon may place the stent in any position between the two investigated and hence this will lead to markedly different stent performance. Therefore, computational tools that take into account the variability of stent placement will be valuable for assisting surgical planning

A Lagrangian finite element method for the simulation of 3D compressible flows

The numerical solution of compressible fluid flows is of paramount importance in many industrial and engineering applications. Compared to the classical fluid dynamics, the introduction of the fluid compressibility changes the formulation of the problem and consequently its computational treatment. Among the possible numerical solutions of compressible flow problems, the finite element method has always been privileged. However, the standard Eulerian approaches with fixed domain are not particularly suited to represent the strong shock waves and the significant movement of the external boundaries. On the contrary, in problems characterized by evolving surfaces, Lagrangian approaches can be very effective. The governing equations of compressible flow problems are mass, momentum and energy conservation. These equations are discretized in the spirit of the Lagrangian Particle Finite Element Method (PFEM). The strong distortions of the mesh, typical of the Lagrangian approaches, are managed with a continuous remeshing of the computational domain. The nodal unknowns are velocities, density and internal energy. To fully exploit the potential of continuous remeshing, only nodal variables are stored and consequently only linear interpolation are used. In addition, an artificial viscosity has been introduced to stabilize the formation and propagation of shock waves. Finally, explicit time integration of the governing equations enables a highly efficient solution of the discretized problem. The proposed approach has been validated against typical benchmarks of gas dynamics in the presence of strong shock waves. A very good agreement has been shown in all the tests proving the excellent accuracy and versatility of the proposed method

Aortendilatation und Funktion des linken Ventrikels bei bikuspider Aortenklappe

Einleitung: Die bikuspide Aortenklappe (BAK) ist mit einer Prävalenz von 0.5-2% der häufigste angeborene Herzfehler. Die Dilatation der Aorta ascendens ist eine häufige Begleiterkrankung der BAK, die im schlimmsten Fall zum plötzlichen Herztod bei Athleten führen kann. Gerade im Leistungssport sind betreuende Fachpersonen und die Athleten selbst noch zu wenig über die Pathophysiologie und die individuelle Betreuung der BAK Patienten aufgeklärt. Ziel: Das Ziel dieses Reviews ist es, zu untersuchen, welchen Einfluss die BAK auf die Funktion des linken Ventrikels (LV) und die Aortendilatation hat. Methodik: Die Erarbeitung des Reviews erfolgte durch eine systematische Literaturrecherche mittels den medizinischen Datenbanken Pubmed, Medline und CINAHL, Referenzlisten sowie Anfragen an Fachpersonen. Anhand von definierten Ein- und Ausschlusskriterien wurde selektioniert. Resultate: Es wurden fünf Studien mit passendem Studiendesign gefunden. Bezüglich der Aortendilatation konnte bei BAK Patienten eine signifikante Zunahme gegenüber Probanden mit trikuspider Aortenklappe beobachtet werden. Hinsichtlich der Parameter des LV gab es keine einheitlichen Ergebnisse. Schlussfolgerung: Es konnte bestätigt werden, dass die BAK zu einer Aortendilatation führt. Über eine veränderte Funktion des LV hingegen, kann keine klare Aussage gemacht werden. Es besteht noch Forschungsbedarf

Green's functions for the evaluation of anchor losses in mems

The issue of dissipation has a peculiar importance in micro-electro-mechanical-structures (MEMS). Among the sources of damping that affect their performance, the most relevant are [1]: thermoelastic coupling, air damping, intrinsic material losses, electrical loading due to electrode routing, anchor losses. Moreover, recent experimental results indicate the presence of additional temperature dependent dissipation mechanisms which are not yet fully understood (see e.g. [2, 12]). In a resonating structure the quality factor Q is defined as: Q = 2πW/ΔW (1) where ΔW and W are the energy lost per cycle and the maximum value of energy stored in the resonator, respectively. According to eq. (1), the magnitude of Q ultimately depends on the level of energy loss (or damping) in a resonator. The focus of the present contribution is set on anchor losses and the impact they have in the presence of axial loads. Anchor losses are due to the scattering of elastic waves from the resonator into the substrate. Since the latter is typically much larger than the resonator itself, it is assumed that all the elastic energy entering the substrate through the anchors is eventually dissipated. The semi-analytical evaluation of anchor losses has been addressed in several papers with different levels of accuracy [3, 6]. These contributions consider a resonator resting on elastic half-spaces and assume a weak coupling, in the sense that the mechanical mode, as well as the mechanical actions transmitted to the substrate, are those of a rigidly clamped resonator. The displacements and rotations induced in the half-space are provided by suitable Green's functions. Photiadis, Judge et al. [7] studied analytically the case of a 3D cantilever beam attached either to a semi-infinite space or to a semi-infinite plate of finite thickness. Their results are based on the semi-exact Green's functions established in [4]. More recently Wilson-Rae et al. [9, 10] generalized all these approaches using the involved framework of radiation tunnelling in photonics. Unfortunately, these contributions provide estimates of quality factors that differ quantitatively. In this paper we revisit the procedure of [7], which rests on simple mechanical principles, but starting from the exact Green's functions for the half space studied by Pak [14]. Through a careful analysis utilizing the theory of residues and inspired by the work of Achenbach [15], we show that the results obtained coincide exactly with those of [9], but for the case of torsion

On the application of Fast Multipole Methods to Helmholtz problems with complex wavenumber

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From Employee to Homo Faber? Considerations About Union Renewal and Informal Workers in Brasil and India

For the purpose of trade union renewal, it is suggested that trade unions need to convert themselves from being institutions centred on employer-employee relations to open source ones engaged with broader social justice issues. In this article, we offer two elements to the debate on trade union revival: first, we focus on two rapidly emerging economies with a corporatist and state-centered union structure (i.e., Brazil and India); second, in the context of these two countries, we challenge the idea that informal workers are a burden for trade union organizations. We consider the possible contributions that informal workers could make towards the renewal of trade unions in these two countries. We argue that trade unions could take advantage of these contributions if they overcome the employee horizon, which originated in Western countries and excludes millions of workers from its purview in Brazil and India. We propose the concept of “homo faber” as a new horizon for trade union organization, which is inclusive of both formal as well as informal workers

Rationalizing the Irrational: Making Sense of (In)consistency among Union Members and Non-Members

Focusing on 13 OECD countries over 25 years, we examine the factors that explain why a sizable fraction of wage-earners exhibit an inconsistency between their union membership status and their confidence in unions by being either confident non-members or non-confident members. While structural factors associated with joining constraints generate inconsistency in specific labour market categories, wage-earners who have extreme ideological orientations and are highly interested in politics are much less likely to exhibit inconsistency across time and countries. For individuals who have intermediate ideological orientations and are not very interested in politics, differences in terms of non-member and member inconsistency between countries are explainable through contextual variables such as economic conditions, the level of employment protection, and historical legacies. Implications for union membership research and union strategies are discussed.Diverging membership and confidence trends: Introduction At the sources of inconsistency: Theoretical background Empirical strategy: Data and methodology Descriptive and explanatory angles: Results Inconsistently inconsistent empirical patterns: Discussion Moody members versus hopeful outsiders? Conclusions Acknowledgements Notes Reference

Double Diffusion Encoding Prevents Degeneracy in Parameter Estimation of Biophysical Models in Diffusion MRI

Purpose: Biophysical tissue models are increasingly used in the interpretation of diffusion MRI (dMRI) data, with the potential to provide specific biomarkers of brain microstructural changes. However, the general Standard Model has recently shown that model parameter estimation from dMRI data is ill-posed unless very strong magnetic gradients are used. We analyse this issue for the Neurite Orientation Dispersion and Density Imaging with Diffusivity Assessment (NODDIDA) model and demonstrate that its extension from Single Diffusion Encoding (SDE) to Double Diffusion Encoding (DDE) solves the ill-posedness and increases the accuracy of the parameter estimation. Methods: We analyse theoretically the cumulant expansion up to fourth order in b of SDE and DDE signals. Additionally, we perform in silico experiments to compare SDE and DDE capabilities under similar noise conditions. Results: We prove analytically that DDE provides invariant information non-accessible from SDE, which makes the NODDIDA parameter estimation injective. The in silico experiments show that DDE reduces the bias and mean square error of the estimation along the whole feasible region of 5D model parameter space. Conclusions: DDE adds additional information for estimating the model parameters, unexplored by SDE, which is enough to solve the degeneracy in the NODDIDA model parameter estimation.Comment: 22 pages, 7 figure