High resolution simulations of the head-on collision of white dwarfs

The direct impact of white dwarfs has been suggested as a plausible channel for type Ia supernovae. In spite of their (a priori) rareness, in highly populated globular clusters and in galactic centers, where the amount of white dwarfs is considerable, the rate of violent collisions between two of them might be non-negligible. Even more, there are indications that binary white dwarf systems orbited by a third stellar-mass body have an important chance to induce a clean head-on collision. Therefore, this scenario represents a source of contamination for the supernova light-curves sample that it is used as standard candles in cosmology, and it deserves further investigation. Some groups have conducted numerical simulations of this scenario, but their results show several differences. In this paper we address some of the possible sources of these differences, presenting the results of high resolution hydrodynamical simulations jointly with a detailed nuclear post-processing of the nuclear abundances, to check the viability of white dwarf collisions to produce significant amounts of 56Ni. To that purpose, we use a 2D-axial symmetric smoothed particle hydrodynamic code to obtain a resolution considerably higher than in previous studies. In this work, we also study how the initial mass and nuclear composition affect the results. The gravitational wave emission is also calculated, as this is a unique signature of this kind of events. All calculated models produce a significant amount of 56Ni, ranging from 0.1 Msun to 1.1 Msun, compatible not only with normal-Branch type Ia supernova but also with the subluminous and super-Chandrasekhar subset. Nevertheless, the distribution mass-function of white dwarfs favors collisions among 0.6-0.7 Msun objects, leading to subluminous events.Comment: 24 pages, 12 figures, accepted for publication in MNRA

Hamming weights and Betti numbers of Stanley-Reisner rings associated to matroids

To each linear code over a finite field we associate the matroid of its parity check matrix. We show to what extent one can determine the generalized Hamming weights of the code (or defined for a matroid in general) from various sets of Betti numbers of Stanley-Reisner rings of simplicial complexes associated to the matroid

Onshore Wind Farm Modelling

We present a Computational Fluid Dynamics (CFD) modeling strategy for onshore windfarms aimed at predicting and optimizing the production of farms using a CFD model that includes me-teorological data assimilation, complex terrain and wind turbine effects. The model involves the solutionof the Reynolds-Averaged Navier-Stokes (RANS) equations together with a κ-ε turbulence model spe-cially designed for the Atmospheric Boundary Layer (ABL). The model involves automatic meshing andgeneration of boundary conditions with atmospheric boundary layer shape for the entering wind flow.As the integration of the model up to the ground surface is still not viable for complex terrains, a specificlaw of the wall including roughness effects is implemented. The wake effects and the aerodynamic be-havior of the wind turbines are described using the actuator disk model, upon which a volumetric forceis included in the momentum equations. The placement of the wind turbines and a mesh refinement forthe near wakes is done by means of a Chimera method. The model is implemented in Alya, a HighPerformance Computing (HPC) multi physics parallel solver based on finite elements and developed atBarcelona Supercomputing Center.Laboratorio de Capa Límite y Fluidodinámica AmbientalGrupo Fluidodinámica Computaciona

Onshore Wind Farm Modelling

We present a Computational Fluid Dynamics (CFD) modeling strategy for onshore windfarms aimed at predicting and optimizing the production of farms using a CFD model that includes me-teorological data assimilation, complex terrain and wind turbine effects. The model involves the solutionof the Reynolds-Averaged Navier-Stokes (RANS) equations together with a κ-ε turbulence model spe-cially designed for the Atmospheric Boundary Layer (ABL). The model involves automatic meshing andgeneration of boundary conditions with atmospheric boundary layer shape for the entering wind flow.As the integration of the model up to the ground surface is still not viable for complex terrains, a specificlaw of the wall including roughness effects is implemented. The wake effects and the aerodynamic be-havior of the wind turbines are described using the actuator disk model, upon which a volumetric forceis included in the momentum equations. The placement of the wind turbines and a mesh refinement forthe near wakes is done by means of a Chimera method. The model is implemented in Alya, a HighPerformance Computing (HPC) multi physics parallel solver based on finite elements and developed atBarcelona Supercomputing Center.Laboratorio de Capa Límite y Fluidodinámica AmbientalGrupo Fluidodinámica Computaciona

Onshore Wind Farm Modelling

We present a Computational Fluid Dynamics (CFD) modeling strategy for onshore windfarms aimed at predicting and optimizing the production of farms using a CFD model that includes me-teorological data assimilation, complex terrain and wind turbine effects. The model involves the solutionof the Reynolds-Averaged Navier-Stokes (RANS) equations together with a κ-ε turbulence model spe-cially designed for the Atmospheric Boundary Layer (ABL). The model involves automatic meshing andgeneration of boundary conditions with atmospheric boundary layer shape for the entering wind flow.As the integration of the model up to the ground surface is still not viable for complex terrains, a specificlaw of the wall including roughness effects is implemented. The wake effects and the aerodynamic be-havior of the wind turbines are described using the actuator disk model, upon which a volumetric forceis included in the momentum equations. The placement of the wind turbines and a mesh refinement forthe near wakes is done by means of a Chimera method. The model is implemented in Alya, a HighPerformance Computing (HPC) multi physics parallel solver based on finite elements and developed atBarcelona Supercomputing Center.Laboratorio de Capa Límite y Fluidodinámica AmbientalGrupo Fluidodinámica Computaciona

A hybrid actuator disc - full rotor CFD methodology for modelling the effects of wind turbine wake interactions on performance

The performance of individual wind turbines is crucial for maximum energy yield, however, their performance is often reduced when turbines are placed together in an array. The wake produced by the rotors interacts with downstream turbines, resulting in a reduction in power output. In this paper, we demonstrate a new and faster modelling technique which combines actuator disc theory, modelled using wind tunnel validated Computational Fluid Dynamics (CFD), and integrated into full rotor CFD simulations. This novel hybrid of techniques results in the ability to analyse performance when simulating various array layouts more rapidly and accurately than using either method on its own. It is shown that there is a significant power reduction from a downstream turbine that is subjected to the wake of an upstream turbine, and that this is due to both a reduction in power in the wind and also due to changes in the aerodynamics. Analysis of static pressure along the blade showed that as a result of wake interactions, a large reduction in the suction peak along the leading edge reduced the lift generated by the rotor and so reduced the torque production and the ability for the blade to extract energy from the wind

Improving smoothed particle hydrodynamics with an integral approach to calculating gradients

In this paper we develop and test a fully conservative SPH scheme based on a tensor formulation that can be applied to simulate astrophysical systems. In the proposed scheme, derivatives are calculated from an integral expression that leads to a tensor (instead of a vectorial) estimation of gradients and reduces to the standard formulation in the continuum limit. The new formulation improves the interpolation of physical magnitudes, leading to a set of conservative equations that resembles those of standard SPH. The resulting scheme is verified using a variety of well-known tests, all of them simulated in two dimensions. We also discuss an application of the proposed tensor method to astrophysics by simulating the stability of a Sun-like polytrope calculated in three dimensions.Comment: 15 pages, 12 figures, accepted for publication in Astronomy and Astrophysics; minor changes due to language editin

Evolution of the seroprevalence of pestivirus and respiratory viral infections in Spanish feedlot lambs

The presence of respiratory viruses and pestiviruses in sheep has been widely demon-strated, and their ability to cause injury and predispose to respiratory processes have been proven experimentally. A longitudinal observational study was performed to determine the seroprevalence of bovine parainfluenza virus type 3 (BPIV-3), bovine respiratory syncytial virus (BRSV), bovine herpesvirus type 1 (BHV-1) and pestiviruses in 120 lambs at the beginning and the end of the fattening period. During this time, the animals were clinically monitored, their growth was recorded, and post-mortem examinations were performed in order to identify the presence of pneumonic lesions in the animals. Seroconversion to all viruses tested except BHV-1 was detected at the end of the period. Initially, BPIV-3 antibodies were the most frequently found, while the most common seroconversion through the analysed period occurred to BRSV. Only 10.8% of the lambs showed no detectable levels of antibodies against any of the tested viruses at the end of the survey. In addition, no statistical differences were found in the presentation of respiratory clinical signs, pneumonic lesions nor in the production performance between lambs that seroconverted and those which did not, except in the case of pestiviruses. The seroconversion to pestiviruses was associated with a reduction in the final weight of the lambs

Parameterization of the atmospheric boundary layer for offshore wind resource assessment with a limited- length-scale k-ε model

Abstract The structure of the atmospheric boundary layer (ABL) is modelled with the limitedlength-scale k-ε model of Apsley and Castro. Contrary to the standard k-ε model, the limited-length-scale k-ε model imposes a maximum mixing length which is derived from the boundary layer height, for neutral and unstable atmospheric situations, or by Monin-Obukhov length when the atmosphere is stably stratified. The model is first verified reproducing the famous Leipzig wind profile. Then the performance of the model is tested with measurements from FINO-1 platform using sonic anemometers to derive the appropriate maximum mixing length