Wheel–rail contact: experimental study of the creep forces–creepage relationships

The wheel–rail contact problem plays an important role in the simulation methods used to solve railway dynamics problems. As a consequence, many different mathematical models have been developed to calculate wheel–rail contact forces. However, most of them tackle this problem purely from a theoretical point of view and need to be experimentally validated. Such validation could also reveal the influence of certain parameters not taken into account in the mathematical developments. This paper presents the steps followed in building a scaled test-bench to experimentally characterise the wheel–rail contact problem. The results of the longitudinal contact force as a function of the longitudinal creepage are obtained and the divergences with respect to Kalker's simplified theory are analysed. The influence of lateral creepage, angular velocity and certain contaminants such as cutting fluid or high positive friction modifier is also discussed

Ranking and clustering of nodes in networks with smart teleportation

Random teleportation is a necessary evil for ranking and clustering directed networks based on random walks. Teleportation enables ergodic solutions, but the solutions must necessarily depend on the exact implementation and parametrization of the teleportation. For example, in the commonly used PageRank algorithm, the teleportation rate must trade off a heavily biased solution with a uniform solution. Here we show that teleportation to links rather than nodes enables a much smoother trade-off and effectively more robust results. We also show that, by not recording the teleportation steps of the random walker, we can further reduce the effect of teleportation with dramatic effects on clustering.Comment: 10 pages, 7 figure

Numerical mode matching in dissipative silencers with temperature gradients and mean flow

This work presents a mathematical approach based on the mode matching method to compute the transmission loss of perforated dissipative silencers with temperature gradients and mean flow. Three-dimensional wave propagation is considered in silencer geometries with arbitrary, but axially uniform, cross section. To reduce the computational requirements of a full multidimensional finite element calculation, a method is developed combining axial and transversal solutions of the wave equation. First, the finite element method is employed in a twodimensional problem to extract the eigenvalues and associated eigenvectors for the silencer cross section. Mean flow as well as radial temperature gradients and the corresponding thermal-induced material heterogeneities are included in the model. Assuming a low acoustic influence of axial gradients (compared to radial variations), an axially uniform temperature field is taken into account, its value being the inlet/outlet average. A weighted residual approach is then used to match the acoustic fields (pressure and axial acoustic velocity) at the geometric discontinuities between the silencer chamber and the inlet and outlet pipes. Transmission loss predictions are compared favourably with a general three-dimensional finite element approach, offering a reduction in the computational effort

Brain germinoma presenting as a first psychotic episode in an adolescent male

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Composite Electrodes Based on Carbon Materials Decorated with Hg Nanoparticles for the Simultaneous Detection of Cd(II), Pb(II) and Cu(II)

Monitoring water quality has become a goal to prevent issues related to human health and environmental conditions. In this sense, the concentration of metal ions in water sources is screened, as these are considered persistent contaminants. In this work, we describe the implementation of customized graphite electrodes decorated with two types of Hg nanoparticles (Hg-NPs), optimized toward the electrochemical detection of Cd, Pb and Cu. Here, we combine Hg, a well-known property to form alloys with other metals, with the nanoscale features of Hg-NPs, resulting in improved electrochemical sensors towards these analytes with a substantial reduction in the used Hg amount. Hg-NPs were synthesized using poly(diallyldimethylammonium) chloride (PDDA) in a combined role as a reducing and stabilizing agent, and then appropriately characterized by means of Transmission Electron Microscopy (TEM) and Zeta Potential. The surface of composite electrodes with optimized graphite content was modified by the drop-casting of the prepared Hg-NPs. The obtained nanocomposite electrodes were morphologically characterized by Scanning Electron Microscopy (SEM), and electrochemically by Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). The results show that the Hg-NP-modified electrodes present better responses towards Cd(II), Pb(II) and Cu(II) detection in comparison with the bare graphite electrode. Analytical performance of sensors was evaluated by square-wave anodic stripping voltammetry (SWASV), obtaining a linear range of 0.005-0.5 mg·L−1 for Cd2+, of 0.028-0.37 mg·L−1 for Pb2+ and of 0.057-1.1 mg·L−1 for Cu2+. Real samples were analyzed using SWASV, showing good agreement with the recovery values of inductively coupled plasma-mass spectrometry (ICP-MS) measurement

Improved railway wheelset-track interaction model in the high-frequency domain

[EN] As it is well known, there are various phenomena related to railway train-track interaction, some of them caused by the high frequency dynamics of the system, such as rolling noise when the vehicle runs over the track, as well as squeal noise and short-pitch rail corrugation for curved tracks. Due to these phenomena and some others unsolved so far, a large effort has been made over the last 40 years in order to define suitable models to study the train-track interaction. The introduction of flexibility in wheelset and rail models was required to have a more realistic representation of the wheel-rail interaction effects at high frequencies. In recently published train-track interaction models, the rails are modelled by means of Timoshenko beam elements, valid up to 1.5 kHz for lateral rail vibration and up to 2 kHz for vertical vibration. This confines the frequency range of validity for the complete train-track model to 1.5 kHz. With the purpose of extending the range of validity above 1.5 kHz, a 3D track model based on the Moving Element Method (MEM) is developed in this paper to replace the Timoshenko beam considered in earlier studies, adopting cyclic boundary conditions and Eulerian coordinates. The MEM approach considers a mobile Finite Element (FE) mesh which moves with the vehicle, so the mass of the rail flows with the vehicle speed but in the opposite direction through the mesh. Therefore, the MEM permits to fix the contact area in the middle of a finitely long track and to refine the mesh only around the contact area, where the forces and displacements will be more significant. Additionally, a modal approach is adopted in order to reduce the number of degrees of freedom of the rail model. Both strategies lower substantially the computational cost. Simulation results are presented and discussed for different excitation sources including random rail roughness and singularities such as wheel flats. All the simulation cases are carried out for a Timoshenko beam and a 3D MEM track model in order to point out the differences in the contact forces above the range of validity of the Timoshenko beam.The authors gratefully acknowledge the financial support of Ministerio de Economía y Competitividad and the European Regional Development Fund (project TRA2013-45596-C2-1-R), as well as Generalitat Valenciana (project Prometeo/2012/023) and Ministerio de Educación, Cultura y Deporte (project SP20140659) as part of Programa Campus de Excelencia Internacional.Martínez Casas, J.; Giner Navarro, J.; Baeza González, LM.; Denia Guzmán, FD. (2017). Improved railway wheelset-track interaction model in the high-frequency domain. Journal of Computational and Applied Mathematics. 309(1):642-653. https://doi.org/10.1016/j.cam.2016.04.034S642653309