Thermodynamic ground states of platinum metal nitrides

The thermodynamic stabilities of various phases of the nitrides of the platinum metal elements are systematically studied using density functional theory. It is shown that for the nitrides of Rh, Pd, Ir and Pt two new crystal structures, in which the metal ions occupy simple tetragonal lattice sites, have lower formation enthalpies at ambient conditions than any previously proposed structures. The region of stability with respect to those structures extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal structures at this pressure are thermodynamically stable also with respect to phase separation. The fact that the local density and generalized gradient approximations predict different values of the absolute formation enthalpies as well different relative stabilities between simple tetragonal and the pyrite or marcasite structures are further discussed.Comment: 5 pages, 4 figure

Spatio-temporal behavior of magnetohydrodynamic fluctuations with cross-helicity and background magnetic field

We study the spatiotemporal behavior of the Elsässer variables describing magnetic and velocity field fluctuations, using direct numerical simulations of three-dimensional magnetohydrodynamic turbulence. We consider cases with relatively small, intermediate, and large values of a mean background magnetic field and with null, small, and high cross-helicity (correlations between the velocity and the magnetic field). Wavenumber-dependent time correlation functions are computed for the different simulations. From these correlation functions, the decorrelation time is computed and compared with different theoretical characteristic times: the local nonlinear time, the random-sweeping time, and the Alfvénic time. It is found that decorrelation times are dominated by sweeping effects for low values of the mean magnetic field and for low values of the cross-helicity, while for large values of the background field or of the cross-helicity and for wave vectors sufficiently aligned with the guide field, decorrelation times are controlled by Alfvénic effects. Finally, we observe counterpropagation of Alfvénic fluctuations due to reflections produced by inhomogeneities in the total magnetic field. This effect becomes more prominent in flows with large cross-helicity, strongly modifying the propagation of waves in turbulent magnetohydrodynamic flows.Fil: Lugones, Rodrigo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Dmitruk, Pablo Ariel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Mininni, Pablo Daniel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Pouquet, A.. University of Colorado; Estados UnidosFil: Matthaeus, W. H.. University of Colorado; Estados Unido

A shooting algorithm for problems with singular arcs

In this article we propose a shooting algorithm for a class of optimal control problems for which all control variables appear linearly. The shooting system has, in the general case, more equations than unknowns and the Gauss-Newton method is used to compute a zero of the shooting function. This shooting algorithm is locally quadratically convergent if the derivative of the shooting function is one-to-one at the solution. The main result of this paper is to show that the latter holds whenever a sufficient condition for weak optimality is satisfied. We note that this condition is very close to a second order necessary condition. For the case when the shooting system can be reduced to one having the same number of unknowns and equations (square system) we prove that the mentioned sufficient condition guarantees the stability of the optimal solution under small perturbations and the invertibility of the Jacobian matrix of the shooting function associated to the perturbed problem. We present numerical tests that validate our method.Comment: No. RR-7763 (2011); Journal of Optimization, Theory and Applications, published as 'Online first', January 201

Turbulence in the Solar Atmosphere: Manifestations and Diagnostics via Solar Image Processing

Intermittent magnetohydrodynamical turbulence is most likely at work in the magnetized solar atmosphere. As a result, an array of scaling and multi-scaling image-processing techniques can be used to measure the expected self-organization of solar magnetic fields. While these techniques advance our understanding of the physical system at work, it is unclear whether they can be used to predict solar eruptions, thus obtaining a practical significance for space weather. We address part of this problem by focusing on solar active regions and by investigating the usefulness of scaling and multi-scaling image-processing techniques in solar flare prediction. Since solar flares exhibit spatial and temporal intermittency, we suggest that they are the products of instabilities subject to a critical threshold in a turbulent magnetic configuration. The identification of this threshold in scaling and multi-scaling spectra would then contribute meaningfully to the prediction of solar flares. We find that the fractal dimension of solar magnetic fields and their multi-fractal spectrum of generalized correlation dimensions do not have significant predictive ability. The respective multi-fractal structure functions and their inertial-range scaling exponents, however, probably provide some statistical distinguishing features between flaring and non-flaring active regions. More importantly, the temporal evolution of the above scaling exponents in flaring active regions probably shows a distinct behavior starting a few hours prior to a flare and therefore this temporal behavior may be practically useful in flare prediction. The results of this study need to be validated by more comprehensive works over a large number of solar active regions.Comment: 26 pages, 7 figure

On Solving the Coronal Heating Problem

This article assesses the current state of understanding of coronal heating, outlines the key elements of a comprehensive strategy for solving the problem, and warns of obstacles that must be overcome along the way.Comment: Accepted by Solar Physics; Published by Solar Physic

Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics

We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions

Hazardous organic and trace element occurrence in bottom sediments of dam reservoirs

Quality of bottom sediments was examined for 15 reservoirs classified as the Sudeten, Carpathian or lowland reservoirs, according to their location. These reservoirs serve multiple functions including flood control, providing hydroelectric power, supplying water for general public, industry or agriculture and recreational. The study was conducted in 2008 and 2010. The scope of the analysis included trace elements (As, Cr, Zn, Cd, Cu, Ni, Pb and Hg) and organic compounds: polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and chloroorganic pesticides. It was determined that concentration of trace element compounds ranged from <0.01 mg/kg to 422 mg/kg, while of PAHs from <0.0001 mg/kg to 7.5 mg/kg, PCBs from <0.0001 mg/kg to 0.78 mg/kg and of chloroorganic pesticides from <0.0002 mg/kg to 0.065 mg/kg. The sediments from the Sudeten reservoirs were found to be the most polluted, while the sediments from the lowland reservoirs the least.Wykonano badania jakości osadów dennych pobranych z 15 zbiorników zaporowych, które w zależności od lokalizacji przyporządkowano do grupy zbiorników sudeckich, karpackich lub nizinnych. Zbiorniki te służą do różnych celów, takich jak ochrona przeciwpowodziowa, energetyka wodna, zaopatrzenie w wodę ludności, przemysłu i rolnictwa oraz rekreacja. Badania wykonano w latach 2008 i 2010. Zakres analizy osadów dennych obejmował pierwiastki śladowe (As, Cr, Zn, Cd, Cu, Ni, Pb i Hg) i związki organiczne (wielopierścieniowe węglowodory aromatyczne – WWA, polichlorowane bifenyle – PCB i pestycydy chloroorganiczne). Stwierdzono, że zawartość pierwiastków śladowych w osadach wynosiła od <0,01 mg/kg do 422 mg/kg; WWA od <0,0001 mg/kg do 7,5 mg/kg, PCB od <0,0001 mg/kg do 0,78 mg/kg, a pestycydy chloroorganiczne od <0,0002 mg/kg do 0,065 mg/kg. Najbardziej zanieczyszczone okazały się osady denne pobrane ze zbiorników sudeckich a najmniej z nizinnych

Chlorophenols in water environment

Chlorofenole w wodzie oznaczono metodą chromatograficzną z użyciem chromatografu gazowego z detektorem wychwytu elektronów (ECD). Zbadano odzysk chlorofenoli w wodzie destylowanej oraz w wodzie powierzchniowej, który wynosił w zakresie 64-92 %. W próbkach wód powierzchniowych oznaczono następujące chlorofenole: 2-chlorofenol, 2,4-dichlorofenol, 2,4,6-trichlorofenol i pentachlorofenol. Zawartość chlorofenoli oznaczono w wodzie z Wisły na odcinku od Krakowa do Gdańska i w wodach z dwóch małych cieków w okolicy Warszawy (rzeka Jeziorka i Potok Służewiecki). Nie stwierdzono obecności 2-chlorofenolu, natomiast pentachlorofenol występował w prawie wszystkich próbkach wody. W wodzie z Wisły najwyższa była zawartość 2,4-dichlorofenolu - 0,275 mg/m3, natomiast najwyższa zawartość sumy czterech chlorofenoli wyniosła 0,501 mg/m3. Wyższą zawartość chlorofenoli zanotowano w wodach z Jeziorki i Potoku Służewieckiego, w których suma oznaczonych chlorofenoli wyniosła odpowiednio 1,32 mg/m3 i 3,61 mg/m3.Chlorophenols in the aquatic environment were analyzed by gas chromatography with an ECD. Distilled water and surface water samples were compared for chlorophenol recovery which ranged between 64% and 92%. In the surface water samples the presence of 2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol was detected. The samples were collected along the Vistula River between Cracow and Gdansk, as well as along two small streams in the proximity of Warsaw (Jeziorka and Potok Sluzewiecki). No 2-chlorophenol was detected in any of the surface water samples while nearly all of them contained pentachlorophenol. The highest concentration of a single chlorophenol in the Vistula amounted to 0.275 mg/m3 (2,4-dichlorophenol), the sum of the four chlorophenols examined being equal to 0.501 mg/m3. Much higher chlorophenol concentrations were determined in the two streams near Warsaw; the sum of the four chlorophenols totalled 1.32 mg/m3 and 3.61 mg/m3 in the Jeziorka and Potok Sluzewiecki, respectively

Determining AOX concentrations in water and wastewater

Wskaźnik AOX (adsorbowalne związki halogenoorganiczne) określa sumaryczną zawartość substancji halogenoorganicznych, tj. związków organicznych związanych z chlorem, bromem lub jodem w różnych elementach środowiska naturalnego, w tym w środowisku wodnym. W badaniach określono zawartość AOX w wybranych próbkach wód powierzchniowych i ścieków w województwie mazowieckim. Stwierdzono, że całkowita zawartość związków halogenoorganicznych w badanych wodach powierzchniowych nie przekraczała wartości 160 mgCl-/m3. Zgodnie z klasyfikacją oceny stanu jakości wód obowiązującą w Niemczech, rzeki w badanych punktach pomiarowych charakteryzowały się znaczącym lub podwyższonym zanieczyszczeniem związkami halogenoorganicznymi. Oznaczony zakres wartości wskaźnika AOX w oczyszczonych ściekach bytowo-gospodarczych wynosił 22÷262 mgCl-/m3, natomiast w przypadku ścieków przemysłowych wartości AOX wahały się od 82 mgCl-/m3 do 167 mgCl-/m3. Wartości te spełniały warunki obowiązujące w Polsce w przypadku odprowadzania ścieków do wód i ziemi, bądź do urządzeń kanalizacyjnych.AOX (adsorbable organically bound halogens) is a parameter determining the overall content of organohalogens, i.e. organic compounds bound with chlorine, bromine or iodine in different elements of the natural environment. In the study reported on in this paper, AOX concentrations were determined in samples of surface water and wastewater (both municipal and industrial), collected within the area of the Mazovian Voivodship. It has been found that the overall organohalogen content of the surface water samples did not exceed the value of 160 mgCl-/m3. According to the water quality classification that is in effect in Ger-many, riverine water at the measuring sites was characterized by significant or increased levels of pollution from organohalogens. The measured values of the AOX parameter ranged from 22 mgCl-/m3 to 262 mgCl-/m3 in domestic sewage and from 82 mgCl-/m3 to 167 mgCl-/m3 in industrial wastewater. These concentrations meet the requirements imposed in Poland on the quality of waste-waters being discharged into surface water, soil, or a sewer system