arXiv subjects

J. Klapp

Publications and source records attributed to J. Klapp.

At least 19 recordsLinked to original sources

arXiv2021

Endeavors in the DualSPHysics code to evaluate heat transfer in nuclear systems

This article condenses current endeavors and improvements in the expansion of applications of the DualSPHysics code to analyze heat transfer in a nuclear reactor core. This includes the essential conservation equations and certain physical considerations, particularly the thermal conductivity variable model, considering changes in the reference density to maintain the accuracy in the solution. Conventionally, to study these sorts of systems, Eulerian methods have been developed, nevertheless, this kind of method based on well-defined mesh shows major restrictions. The DualSPHysics code, based on Smoothed Particle Hydrodynamics (SPH) technique, has shown to be a real and robust alternative since it involves a free mesh approach, and the numerical method is very well parallelized in both computational and graphical process units (CPU and GPU). The results for the improvements developed in the present work show an exceptionally good approximation with other simulation approaches and also with experimental observation in the three cases studied (1) heat transfer analysis in a bidimensional system with thermal conductivity coefficient k variable, (2) natural convection heat transfer in a horizontal cylindrical ring similar to the space between the fuel rod and the cladding and (3) heat transfer in an experimental nuclear fuel rod square arrangement like in a Pressurized Water Reactor (PWR) nuclear core. Enhancements to this code (DualSPHysics) to use it in nuclear applications are fundamental in the exploitation of this technique in crucial areas of study.

physics.flu-dyn↗

arXiv2021

Lagrangian approach for the study of heat transfer in a nuclear reactor core using the SPH methodology

Numerical modeling simulations and the use of high-performance computing are fundamental for detailed safety analysis, control and operation of a nuclear reactor, allowing the study and analysis of problems related to thermal-hydraulics, neutronic, and the dynamic of fluids that are involved in these systems. In this work, we introduce the basis for the implementation of the smoothed particle hydrodynamics (SPH) approach to analyze heat transfer in a nuclear reactor core. Heat transfer by means of convection is of great importance in many engineering applications and especially in the analysis of heat transfer in nuclear reactors. As a first approach, the natural convection in the gap (space that exists between the fuel rod and the cladding) can be analyzed helping to reduce uncertainty in such calculations that usually relies on empirical correlations while using other numerical tools. The numerical method developed in this work was validated while comparing the results obtained in previous numerical simulations and experimental data reported in the literature showing that our implementation is suitable for the study of heat transfer in nuclear reactors. Numerical simulations were done with the DualSPHysics open source code that allows performing parallel calculations using different numbers of cores. The current implementation is a version written in CUDA (Compute Unified Device Architecture) that allows also the use of GPU processors (Graphics Processor Unit) to accelerate the calculations in parallel using a large number of cores contained in the GPU. This makes it possible to analyze large systems using a reasonable computer time. The obtained results verified and validated our method and allowed us to have a strong solver for future applications of heat transfer in nuclear reactors fuel inside the reactor cores.

physics.flu-dyn↗

arXiv2021

SPH simulations of turbulent flow in curved pipes with different geometries. A comparison with experiments

The swirling secondary flow in curved pipes is studied in three-space dimensions using a weakly compressible Smoothed Particle Hydrodynamics (WCSPH) formulation coupled to new non-reflecting outflow boundary conditions. A large-eddy simulation (LES) model for turbulence is benchmarked with existing experimental data. After validation of the present model against experimental results for a 90 grades pipe bend, a detailed numerical study aimed at reproducing experimental flow measurements for a wide range of Reynolds numbers has been performed for different pipe geometries, including U pipe bends, S-shaped pipes and helically coiled pipes. In all cases, the SPH calculated behavior shows reasonably good agreement with the measurements across and downstream the bend in terms of streamwise velocity profiles and cross-sectional contours. Maximum mean-root-square deviations from the experimentally obtained profiles are always less than 1.8%. This combined with the very good matching between the SPH and the experimental cross-sectional contours shows the uprising capabilities of the present scheme for handling engineering applications with streamline curvature, such as flows in bends and manifolds.

physics.flu-dyn↗

arXiv2020

SPH modeling of natural convection around a heated horizontal cylinder: A comparison with experiments

An experimental and numerical smoothed particle hydrodynamics (SPH) analysis was performed for the convective flow arising from a horizontal, thin cylindrical heat source enclosed in a glycerin-filled, slender enclosure at low Rayleigh numbers ($1.18\leq {\rm Ra}\leq 242$). Both the experiments and the SPH calculations were performed for positive ($0.1\leqΔT\leq 10$ K) and negative ($-10\leqΔT\leq -0.1$ K) temperature differences between the source and the surrounding fluid. In all cases a pair of steady, counter-rotating vortices is formed, accompanied by a plume of vertically ascending flow just above the source for $ΔT>0$ and a vertically descending flow just below the source for $ΔT<0$. The maximum flow velocities always occur within the ascending/descending plumes. The SPH predictions are found to match the experimental observations acceptably well with root-mean-square errors in the velocity profiles of the order of $\sim 10^{-5}$ m s$^{-1}$. The fact that the SPH method is able to reveal the detailed features of the flow phenomenon demonstrates the correctness of the approach.

physics.flu-dyn↗

arXiv2020

Creating and using large grids of precalculated model atmospheres for a rapid analysis of stellar spectra

$Aims.$ We present a database of 43,340 atmospheric models ($\sim$80,000 models at the conclusion of the project) for stars with stellar masses between 9 and 120 $M_{\odot}$, covering the region of the OB main-sequence and Wolf-Rayet (W-R) stars in the Hertzsprung--Russell (H--R) diagram. $Methods.$ The models were calculated using the ABACUS I supercomputer and the stellar atmosphere code CMFGEN. $Results.$ The parameter space has six dimensions: the effective temperature $T_{eff}$, the luminosity $L$, the metallicity $Z$, and three stellar wind parameters: the exponent $β$, the terminal velocity $V_{\infty}$, and the volume filling factor $F_{cl}$. For each model, we also calculate synthetic spectra in the UV (900-2000 A), optical (3500-7000 A), and near-IR (10000-40000 A) regions. To facilitate comparison with observations, the synthetic spectra can be rotationally broadened using ROTIN3, by covering vsin(i) velocities between 10 and 350 km s$^{-1}$ with steps of 10 km s$^{-1}$. $Conclusions.$ We also present the results of the reanalysis of $ε$ Ori using our grid to demonstrate the benefits of databases of precalculated models. Our analysis succeeded in reproducing the best-fit parameter ranges of the original study, although our results favor the higher end of the mass-loss range and a lower level of clumping. Our results indirectly suggest that the resonance lines in the UV range are strongly affected by the velocity-space porosity, as has been suggested by recent theoretical calculations and numerical simulations.

astro-ph.SR↗

arXiv2019

Radiative non-isothermal Bondi accretion onto a massive black hole

In this paper, we present the classical Bondi accretion theory for the case of non-isothermal accretion processes onto a supermassive black hole (SMBH), including the effects of X-ray heating and the radiation force due to electron scattering and spectral lines. The radiation field is calculated by considering an optically thick, geometrically thin, standard accretion disk as the emitter of UV photons and a spherical central object as a source of X-ray emission. In the present analysis, the UV emission from the accretion disk is assumed to have an angular dependence, while the X-ray/central object radiation is assumed to be isotropic. This allows us to build streamlines in any angular direction we need to. The influence of both types of radiation is evaluated for different flux fractions of the X-ray and UV emissions with and without the effects of spectral line driving. We find that the radiation emitted near the SMBH interacts with the infalling matter and modifies the accretion dynamics. In the presence of line driving, a transition resembles from pure type 1 & 2 to type 5 solutions (see Fig2.1 of Frank etal. 2002), which takes place regardless of whether or not the UV emission dominates over the X-ray emission. We compute the radiative factors at which this transition occurs, and discard type 5 solution from all our models. Estimated values of the accretion radius and accretion rate in terms of the classical Bondi values are also given. The results are useful for the construction of proper initial conditions for time-dependent hydrodynamical simulations of accretion flows onto SMBH at the centre of galaxies.

astro-ph.HE↗

arXiv2018

Numerical simulation of a temporary repository of radioactive material

The use of computer simulations techniques is an advantageous tool in order to evaluate and select the most appropriated site for radionuclides confinement. Modelling different scenarios allow to take decisions about which is the most safety place for the final repository. In this work, a bidimensional numerical simulation model for the analysis of dispersion of contaminants trough a saturated porous media using finite element method (FEM), was applied to study the transport of radioisotopes in a temporary nuclear repository localized in the Vadose Zone at Peña Blanca, México. The 2D model used consider the Darcy's law for calculating the velocity field, which is the input data for in a second computation to solve the mass transport equation. Taking into account radionuclides decay the transport of long lived U-series daughters such as ${}^{238}\!\text{U}$, ${}^{234}\!\text{U}$, and ${}^{230}\!\text{Th}$ is evaluated. The model was validated using experimental data reported in the literature obtaining good agreement between the numerical results and the available experimental data. The simulations show preferential routes that the contaminant plume follows over time. The radionuclide flow is highly irregular and it is influenced by failures in the area and its interactions in the fluid-solid matrix. The resulting radionuclide concentration distribution is as expected. The most important result of this work is the development of a validated model to describe the migration of radionuclides in saturated porous media with some fractures.

physics.comp-ph↗

arXiv2018

Impetus: Consistent SPH calculations of 3D spherical Bondi accretion onto a black hole

We present three-dimensional calculations of spherically symmetric Bondi accretion onto a stationary supermassive black hole (SMBH) of mass $10^{8}$ $M_{\odot}$ within a radial range of $0.02-10$ pc, using a modified version of the smoothed particle hydrodynamics (SPH) \gad \sp code, which ensures approximate first-order consistency (i.e., second-order accuracy) for the particle approximation. First-order consistency is restored by allowing the number of neighbours, $n_{\rm neigh}$, and the smoothing length, $h$, to vary with the total number of particles, $N$, such that the asymptotic limits $n_{\rm neigh}\to\infty$ and $h\to 0$ hold as $N\to\infty$. The ability of the method to reproduce the isothermal ($γ=1$) and adiabatic ($γ=5/3$) Bondi accretion is investigated with increased spatial resolution. In particular, for the isothermal models the numerical radial profiles closely match the Bondi solution, except near the accretor, where the density and radial velocity are slightly underestimated. However, as $n_{\rm neigh}$ is increased and $h$ is decreased, the calculations approach first-order consistency and the deviations from the Bondi solution decrease. The density and radial velocity profiles for the adiabatic models are qualitatively similar to those for the isothermal Bondi accretion. Steady-state Bondi accretion is reproduced by the highly resolved consistent models with a percent relative error of $\lesssim 1$\% for $γ=1$ and $\sim 9$\% for $γ=5/3$, with the adiabatic accretion taking longer than the isothermal case to reach steady flow. The performance of the method is assessed by comparing the results with those obtained using the standard Gadget and the Gizmo codes.

astro-ph.GA↗

arXiv2017

Consistent SPH Simulations of Protostellar Collapse and Fragmentation

We study the consistency and convergence of smoothed particle hydrodynamics (SPH), as a function of the interpolation parameters, namely the number of particles $N$, the number of neighbors $n$, and the smoothing length $h$, using simulations of the collapse and fragmentation of protostellar rotating cores. The calculations are made using a modified version of the GADGET-2 code that employs an improved scheme for the artificial viscosity and power-law dependences of $n$ and $h$ on $N$, as was recently proposed by Zhu et al., which comply with the combined limit $N\to\infty$, $h\to 0$, and $n\to\infty$ with $n/N\to 0$ for full SPH consistency, as the domain resolution is increased. We apply this realization to the "standard isothermal test case" in the variant calculated by Burkert & Bodenheimer and the Gaussian cloud model of Boss to investigate the response of the method to adaptive smoothing lengths in the presence of large density and pressure gradients. The degree of consistency is measured by tracking how well the estimates of the consistency integral relations reproduce their continuous counterparts. In particular, $C^{0}$ and $C^{1}$ particle consistency is demonstrated, meaning that the calculations are close to second-order accuracy. As long as $n$ is increased with $N$, mass resolution also improves as the minimum resolvable mass $M_{\rm min}\sim n^{-1}$. This aspect allows proper calculation of small-scale structures in the flow associated with the formation and instability of protostellar disks around the growing fragments, which are seen to develop a spiral structure and fragment into close binary/multiple systems as supported by recent observations.

astro-ph.IM↗

arXiv2016

Scaling features of the tribology of polymer brushes of increasing grafting density around the mushroom to brush transition

Non equilibrium coarse grained, dissipative particle dynamics simulations of complex fluids, made up of polymer brushes tethered to planar surfaces immersed in a solvent yield non monotonic behavior of the friction coefficient as a function of the polymer grating density on the substrates, Γ, while the viscosity shows a monotonically increasing dependence on Γ. This effect is shown to be independent of the degree of polymerization, N, and the size of the system. It arises from the composition and the structure of the first particle layer adjacent to each surface that results from the confinement of the fluid. Whenever such layers are made up of as close a proportion of polymer beads to solvent particles as there are in the fluid, the friction coefficient shows a minimum, while for disparate proportions the friction coefficient grows. At the mushroom to brush transition (MBT) the viscosity scales with an exponent that depends on the characteristic exponent of the MBT (6/5) and the solvent quality exponent (ν= 0.5, for theta solvent), but it is independent of the polymerization degree (N). On the other hand, the friction coefficient at the MBT scales as μ~N^(6/5), while the grafting density at the MBT scales as Γ~ N^(-6/5) when friction is minimal, in agreement with previous scaling theories. We argue these aspects are the result of cooperative phenomena that have important implications for the understanding of biological brushes and the design of microfluidics devices, among other applications of current academic and industrial interest.

cond-mat.soft↗

arXiv2015

Formation of accretion centers in simulations of colliding uniform density H$_2$ cores

We test here the first stage of a route of modifications to be applied to the public GADGET2 code for dynamically identifying accretion centers during the collision process of two adjacent and identical gas cores. Each colliding core has a uniform density profile and rigid body rotation; its mass and size have been chosen to represent the observed core $L1544$; for the thermal and rotational energy ratios with respect to the potential energy, we assume the values $α=0.3$ and $β=0.1$, respectively. These values favor the gravitational collapse of the core. We here study cases of both -head-on and off-center collisions, in which the pre-collision velocity increases the initial sound speed of the barotropic gas by up to several times. In a simulation the accretion centers are formed by the highest density particles, so we here report their location and properties in order to realize the collision effects on the collapsing and colliding cores. In one of the models we observe a roughly spherical distribution of accretion centers located at the front wave of the collision. In a forthcoming publication we will apply the full modified GADGET code to study the collision of turbulent cores.

astro-ph.GA↗

arXiv2014

An interaction scenario of the galaxy pair NGC 3893/96 (KPG 302). A single passage?

Using the data obtained previously from Fabry-Perot interferometry, we study the orbital characteristics of the interacting pair of galaxies KPG 302 with the aim to estimate a possible interaction history, the conditions necessary for the spiral arms formation and initial satellite mass. We found by performing N-body/SPH simulations of the interaction that a single passage can produce a grand design spiral pattern in less than 1 Gyr. Althought we reproduce most of the features with the single passage, the required satellite to host mass ratio should be 1:5, which is not confirmed with the dynamical mass estimate made from the measured rotation curve. We conclude that a more realistic interaction scenario would require several passages in order to explain the mass ratio discrepancy.

astro-ph.GA↗

arXiv2013

Nanotribology of biopolymer brushes in aqueous solution using dissipative particle dynamics simulations: an application to PEG covered liposomes in theta solvent

We undertake the investigation of sheared polymer chains grafted on flat surfaces to model liposomes covered with polyethylene glycol brushes as a case study for the mechanisms of efficient drug delivery in biologically relevant situations, for example, as carriers for topical treatments of illnesses in the human vasculature. For these applications, specific rheological properties are required, such as low viscosity at high shear rate to improve the transport of the liposomes. Therefore non - equilibrium, DPD simulations of polymer brushes of various length and shear rates are performed to obtain the average viscosity and friction coefficient of the system as functions of the shear rate and polymerization degree under theta solvent conditions, and find that the brushes experience shear thinning at large shear rates.The viscosity and the friction coefficient are shown to obey scaling laws at high shear rate in theta solvent, irrespective of the brushes degree of polymerization. These results confirm recent scaling predictions and reproduce very well trends in measurements of the viscosity at high shear of red blood cells in a liposome containing medium.

physics.bio-ph↗

arXiv2013

Silicon nanostructures toxicity. An ab inito approach

Silicon nanoparticles are widely used in the medical area and until now they have not manifested toxicological effects in humans beings. In order to understand the physical properties that determine their low-toxicity, we perform ab initio computational simulations of silicon nanoclusters, pure, p-doped and hollow structured. The topological and electronic properties obtained pointed out to the number of dangling bonds and electronic density as fundamental parameters to locate active sites and directly related to toxicity, besides the size and surface chemistry of the silicon nanoparticle.

cond-mat.mtrl-sci↗

arXiv2011

Collapse and fragmentation of Gaussian barotropic protostellar clouds

We examine the problem of the collapse and fragmentation of molecular clouds with a Gaussian density distribution with high resolution, double precision numerical simulations using the GADGET-2 code. To describe the thermodynamic properties of the cloud during the collapse -to mimic the rise of temperature predicted by radiative transfer- we use a barotropic equation of state that introduces a critical density to separate the isothermal and adiabatic regimes. We discuss the effects of this critical density in the formation of multiple systems. We confirm the tendency found for Plummer and Gaussian models that if the collapse changes from isothermal to adiabatic at earlier times that occurs for the models with a lower critical density, the collapse is slowed down, and this enhances the fragments' change to survive. However, this effect happens up to a threshold density below which single systems tend to form. On the other hand, by setting a bigger initial perturbation amplitude, the collapse is faster and in some cases a final single object is formed.

astro-ph.SR↗

arXiv2010

First Stars. I. Evolution without mass loss

The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M > 100 Mo. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with heavy elements, and other cosmological consequences. In this work, we study the structure of Zero Age Main Sequence stars for a wide mass and metallicity range and the evolution of 100, 150, 200, 250 and 300 Mo galactic and pregalactic Pop III very massive stars without mass loss, with metallicity Z=10E-6 and 10E-9, respectively. Using a stellar evolution code, a system of 10 equations together with boundary conditions are solved simultaneously. For the change of chemical composition, which determines the evolution of a star, a diffusion treatment for convection and semiconvection is used. A set of 30 nuclear reactions are solved simultaneously with the stellar structure and evolution equations. Several results on the main sequence, and during the hydrogen and helium burning phases, are described. Low metallicity massive stars are hotter and more compact and luminous than their metal enriched counterparts. Due to their high temperatures, pregalactic stars activate sooner the triple alpha reaction self-producing their own heavy elements. Both galactic and pregalactic stars are radiation pressure dominated and evolve below the Eddington luminosity limit with short lifetimes. The physical characteristics of the first stars have an important influence in predictions of the ionizing photon yields from the first luminous objects; also they develop large convective cores with important helium core masses which are important for explosion calculations.

astro-ph.SR↗

arXiv2009

The influence of non-minimally coupled scalar fields on the dynamics of interacting galaxies

We study bar formation in galactic disks as a consequence of the collision of two spiral galaxies under the influence of a potential which is obtained from the Newtonian limit of a scalar--tensor theory of gravity. We found that dynamical effects depend on parameters ($α$, $λ$) of the theory. In particular, we observe that the bar is shorter for weaker tidal perturbations, which in turn corresponds to smaller values of $λ$ used in our numerical experiments.

astro-ph.GA↗

arXiv2009

The influence of scalar fields in protogalactic interactions

We present simulations within the framework of scalar-tensor theories, in the Newtonian limit, to investigate the influence of massive scalar fields on the dynamics of the collision of two equal spherical clouds. We employ a SPH code modified to include the scalar field to simulate two initially non-rotating protogalaxies that approach each other, and as a result of the tidal interaction, intrinsic angular momentum is generated. We have obtained sufficient large values of J/M to suggest that intrinsic angular momentum can be the result of tidal interactions.

astro-ph.CO↗