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

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

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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

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

Precision Imaging: more descriptive, predictive and integrative imaging

Medical image analysis has grown into a matured field challenged by progress made across all medical imaging technologies and more recent breakthroughs in biological imaging. The cross-fertilisation between medical image analysis, biomedical imaging physics and technology, and domain knowledge from medicine and biology has spurred a truly interdisciplinary effort that stretched outside the original boundaries of the disciplines that gave birth to this field and created stimulating and enriching synergies. Consideration on how the field has evolved and the experience of the work carried out over the last 15 years in our centre, has led us to envision a future emphasis of medical imaging in Precision Imaging. Precision Imaging is not a new discipline but rather a distinct emphasis in medical imaging borne at the cross-roads between, and unifying the efforts behind mechanistic and phenomenological modelbased imaging. It captures three main directions in the effort to deal with the information deluge in imaging sciences, and thus achieve wisdom from data, information, and knowledge. Precision Imaging is finally characterised by being descriptive, predictive and integrative about the imaged object. This paper provides a brief and personal perspective on how the field has evolved, summarises and formalises our vision of Precision Imaging for Precision Medicine, and highlights some connections with past research and current trends in the field

In the Interest of Everyone? Support for Social Movement Unionism among Union Officials in Quebec (Canada)

Using a mixed-method research design involving interviews and a survey, we examined how union officials in Quebec perceive social movement unionism (SMU). We show that union officials view SMU as a multifaceted phenomenon with ideal and pragmatic dimensions. They are torn between strong support for the ideals of SMU and a practical reluctance to use members’ dues to provide services to non-members. Experience with civil society organisations mitigates this tension, encouraging union officials to defend the interests of everyone not only as an ideal, but also as a strategy that allows unions to protect members and unrepresented workers.1. WITHIN AND BEYOND UNIONS: INTRODUCTION 2. ROOTS AND RAMIFICATIONS OF SMU: THEORETICAL FRAMEWORK 3. A MIXED-METHOD APPROACH APPLIED TO THE CASE OF QUEBEC TUs 4. SMU AS A MULTIDIMENSIONAL CONSTRUCT: AN EXPLORATIVE QUANTITATIVE ANALYSIS 5. IDEALLY INCLINED, PRAGMATICALLY RELUCTANT: DISCUSSION 6. IDEAL VERSUS PRAGMATIC TENSION IN SMU: DELVING DEEPER AND WIDER ACKNOWLEDGEMENTS Supporting Information REFERENCE

Experimental Analysis of Concrete Strength at High Temperatures and after Cooling

In recent years, the cement industry has been criticized for emitting large amounts of carbon dioxide; hence it is developing environment-friendly cement, e.g., blended, supersulfated slag cement (SSC). This paper presents an experimental analysis of the compressive strength development of concrete made from blended cement in comparison to ordinary cement at high temperature. Three different types of cement were used during these tests, an ordinary portland cement (CEM I), a portland limestone cement (CEM II-A-LL) and a new, supersulfated slag cement (SSC). The compressive strength development for a full thermal cycle, including cooling down phase, was investigated on concrete cylinders. It is shown that the SSC concrete specimens perform similar to ordinary cement specimens.

A Parametrical Finite Element Formulation of the Bloch-Torrey Equation for NMR Applications

We present a finite element formulation of the full Bloch-Torrey equation for nuclear magnetic resonance (NMR) applications. We obtained parametrical expressions that allow us to compute the involved matrices in a simple and fast way for any spatial convergence order. The framework here proposed is valid for many problems related to MR, as diffusion and perfusion MRI

Application of optimally-shaped phononic crystals to reduce anchor losses of MEMS resonators

This work is focused on the application of Phononic Crystals to reduce anchor losses of MEMS contour mode resonators. Anchor losses dominates the losses in these type of released resonators at low frequency and at low temperature. The use of phononic crystals, intended as finite-periodic distribution of holes in the anchor, is fully compatible with fabrication processes and moreover it is easy to implement. The numerical results obtained in this work show how the use of these crystals can significantly reduce the anchor losses: without the use of the crystal the Q-factor related to only anchor losses is 344, with the use of the crystal it can reach up to 105900