Derivation of a multilayer approach to model suspended sediment transport: application to hyperpycnal and hypopycnal plumes

We propose a multi-layer approach to simulate hyperpycnal and hypopycnal plumes in flows with free surface. The model allows to compute the vertical profile of the horizontal and the vertical components of the velocity of the fluid flow. The model can describe as well the vertical profile of the sediment concentration and the velocity components of each one of the sediment species that form the turbidity current. To do so, it takes into account the settling velocity of the particles and their interaction with the fluid. This allows to better describe the phenomena than a single layer approach. It is in better agreement with the physics of the problem and gives promising results. The numerical simulation is carried out by rewriting the multi-layer approach in a compact formulation, which corresponds to a system with non-conservative products, and using path-conservative numerical scheme. Numerical results are presented in order to show the potential of the model

Defining risk factors associated with difficult peripheral venous Cannulation: A systematic review and meta-analysis

Peripheral venous catheterization is a common technique in hospitals which is not always successful, resulting in multiple punctures and degradation of the vessels. This scenario, which we have termed 'difficult peripheral venous access', is associated to delays in care, obtention of samples or diagnosis, as well as a higher use of central catheters. This study intends to identify risk factors associated to the incidence of 'difficult peripheral venous access' in adults at hospital. We designed a systematic review of published studies (protocol PROSPERO 2018 CRD42018089160). We conducted structured electronic searches using key words and specific vocabulary, as well as directed searches in several databases. After validity analysis, we selected 7 studies with observational methodology. We found great variability in the definition of 'difficult peripheral venous access' and in the variables proposed as risk factors. Statistically significant factors through studies include demographic and anthropometric variables (gender, Body Mass Index), as well as medical and health conditions (diabetes, renal insufficiency, parenteral drug abuse, cancer chemotherapy), together with variables related to the vein or vascular access (vein visibility and palpability, vessel diameter, previous history of difficulty). Some studies have also considered variables related to the professional performing the technique. Meta-analyses were carried out for gender and obesity as potential risk factors. Only obesity appeared as a statistically significant risk factor with OR of 1.48; 95% CI (1.03 to 1.93; p = 0.016). Methodological heterogeneity prevented the development of further meta-analyses. It is essential to design future studies with diverse hospital populations, in which a wide selection of potential risk factors can be studied in a unique analysis. Our work identifies the most relevant variables that should be included in those studies

An MPI-CUDA implementation of an improved Roe method for two-layer shallow water systems

The numerical solution of two-layer shallow water systems is required to simulate accurately stratified fluids, which are ubiquitous in nature: they appear in atmospheric flows, ocean currents, oil spills, . . . Moreover, the implementation of the numerical schemes to solve these models in realistic scenarios imposes huge demands of computing power. In this paper, we tackle the acceleration of these simulations in triangular meshes by exploiting the combined power of several CUDA-enabled GPUs in a GPU cluster. For that purpose, an improvement of a path conservative Roe type finite volume scheme which is specially suitable for GPU implementation is presented, and a distributed implementation of this scheme which uses CUDA and MPI to exploit the potential of a GPU cluster is developed. This implementation overlaps MPI communication with CPU-GPU memory transfers and GPU computation to increase efficiency. Several numerical experiments performed on a cluster of modern CUDA-enabled GPUs show the efficiency of the distributed solver

A second order pvm flux limiter method. Application to magnetohydrodynamics and shallow stratified flows

In this work we propose a second order flux limiter finite volume method, named PVM-2U-FL, that only uses information of the two external waves of the hyperbolic system. This method could be seen as a natural extension of the well known WAF method introduced by Prof. Toro in [21]. We prove that independently of the number of unknowns of the 1D system, it recovers the second order accuracy at regular zones, while in presence of discontinuities, the scheme degenerates to PVM-2U method, which can be seen as an improvement of the HLL method (see [4], [8]). Another interesting property of the method is that it does not need any spectral decomposition of the Jacobian or Roe matrix associated to the flux function. Therefore, it can be easily applied to systems with a large number of unknowns or in situations where no analytical expression of the eigenvalues or eigenvectors are known. In this work, we apply the proposed method to Magnetohydrodynamics and to stratified multilayer flows. Comparison with the twowaves WAF and HLL-MUSCL methods are also presented. The numerical results show that PVM-2U-FL is the most efficient and accurate among them

Late-type members of young stellar kinematic groups - I. Single stars

This is the first paper of a series aimed at studying the properties of late-type members of young stellar kinematic groups. We concentrate our study on classical young moving groups such as the Local Association (Pleiades moving group, 20-150 Myr), IC 2391 supercluster (35 Myr), Ursa Major group (Sirius supercluster, 300 Myr), and Hyades supercluster (600 Myr), as well as on recently identified groups such as the Castor moving group (200 Myr). In this paper we compile a preliminary list of single late-type possible members of some of these young stellar kinematic groups. Stars are selected from previously established members of stellar kinematic groups based on photometric and kinematic properties as well as from candidates based on other criteria such as their level of chromospheric activity, rotation rate and lithium abundance. Precise measurements of proper motions and parallaxes taken from the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, and published radial velocity measurements are used to calculate the Galactic space motions (U, V W) and to apply Eggen's kinematic criteria in order to determine the membership of the selected stars to the different groups. Additional criteria using age-dating methods for late-type stars will be applied in forthcoming papers of this series. A further study of the list of stars compiled here could lead to a better understanding of the chromospheric activity and their age evolution, as well as of the star formation history in the solar neighbourhood. In addition, these stars are also potential search targets for direct imaging detection of substellar companions

Ultraviolet spectroscopy of the hotspot in the classical T Tauri star DI Cep: Observational indications of magnetically channelled accretion

T Tauri stars (TTS) are low-mass pre-main-sequence stars that are accreting mass from the surrounding disc. The hotspots detected in some of them are probably heated by the release of gravitational energy in the accretion of the disc material on to the star. In this work we study the UV spectrum of the hotspot detected in DI Cep to constrain the physical mechanisms heating the spot and to study the possible role of the magnetic field in channelling the accretion flow. DI Cep is a classical TTS, classified as G8 IV, with a hotspot (T similar to 8500 K) covering 1-3 per cent of the visible hemisphere. We have carried out a monitoring campaign with the Short Wavelength spectrograph (1200-2000 Angstrom) and the optical FES Camera of the International Ultraviolet Explorer (IUE) from 1992 July 12 to 26. The UV spectrum of DI Cep shows excess emission in the continuum from 1700 Angstrom towards longer wavelengths with respect to a G8 IV star. The far-UV spectrum is dominated by strong emission lines of OI, CIV, Si IV, Si II and Si III], with typical surface fluxes of similar to 10(6) erg cm(-2) s(-1). The UV fluxes (lines and continuum) vary in phase and reach the maximum when the optical flux (FES) does. The light curves are similar in all the lines: the emission from the hotspot is detected above a baseline flwe probably produced by the stellar atmosphere. There is a broad range of temperatures in the hotspot (from 10(4) to 10.5 K) that is similar to that observed in the plages of magnetically active cool stars (e.g. II Peg). However, in DI Cep the light cuwes of the UV lines and continuum are correlated with the optical continuum (T/-bnnd) light curve. DI Cep as a whole deviates only slightly from active stars in the CIV-Si II and CIV-CII flux-flux relations (there is a factor of 2 excess of Si II with respect to CIV when compared with the regression line fitted to active stars). This suggests that the chromosphere and transition region of DI Cep are heated by a mechanism similar to that of the active main-sequence stars. However, the spot is significantly shifted from these relations in the flux-flux diagrams, displaying an excess of Si II (or a defect of CIV) with respect to the surface fluxes emitted by magnetically active stars. The spot alone radiates as much energy as the rest of the atmosphere, and the spot surface fluxes are similar to 10(8) erg cm(-2) s(-1) (typically 2 orders of magnitude larger than those corresponding to the atmosphere). Our observations support the theories in which the accreting material is magnetically channelled on to the stellar surface. Variations in the temperature of the spot between observations taken 1 year apart suggest that the infalling material is more likely channelled by a transient loop structure attached to the star than by a strong stellar dipolar field. The total energy radiated in the far UV lines plus the UV continuum excess is greater than or equal to 0.07 L.. This can be accounted for by the accretion of greater than or equal to 6x10(-9) M. yr(-1) from the corotation radius (8.3 R*)