COMPUTATIONAL ASPECTS OF THE CHOICE OF OPERATOR AND SAMPLING INTERVAL FOR NUMERICAL DIFFERENTIATION IN LARGE-SCALE SIMULATION OF WAVE PHENOMENA*

Conventional finite-difference operators for numerical differentiation become progressively inaccurate at higher frequencies and therefore require very fine computational grids. This problem is avoided when the derivatives are computed by multiplication in the Fourier domain. However, because matrix transpositions are involved, efficient application of this method is restricted to computational environments where the complete data volume required by each computational step can be kept in random access memory. To circumvent these problems a generalized numerical dispersion analysis for wave equation computations is developed. Operators for spatial differentiation can then be designed by minimizing the corresponding peak relative error in group velocity within a spatial frequency band. For specified levels of maximum relative error in group velocity ranging from 0.03% to 3%, differentiators have been designed that have the largest possible bandwidth for a given operator length. The relation between operator length and the required number of grid points per shortest wavelength, for a required accuracy, provides a useful starting point for the design of cost-effective numerical schemes. To illustrate this, different alternatives for numerical simulation of the time evolution of acoustic waves in three-dimensional inhomogeneous media are investigated. It is demonstrated that algorithms can be implemented that require fewer arithmetic and I/O operations by orders of magnitude compared to conventional second-order finite-difference schemes to yield results with a specified minimum accuracy.     

Anthropogenic load on the estuarine area of the Northern Dvina River

The results of calculation and analysis of the inflow of dissolved chemical substances to the estuarine area of the Northern Dvina River are considered. Based on the long-term regime hydrochemical information of the State Service of Observation and Monitoring of the Natural Environmental State, the anthropogenic load of dissolved chemical substances on the estuarine area and on the estuarine coastal water is estimated per inflow volumes. The extent of the water environment pollution of the near-delta part of the Northern Dvina River and its delta is estimated. The distribution of pollutant inflow among arms and channels in delta is under consideration.     

Changes in the Period of Negative Superhumps of Type SU UMa Dwarf Novae. II. NY Her (2017 and 2020)

Astrophysics - Photometric studies of the type SU UMa dwarf nova NY Her were made during 2017 and 2020 at the Crimean Astrophysical Observatory and at the Sanglokh Observatory over a total of 77...     

Multi-level assessment of chronic toxicity of estuarine sediments with the amphipod Gammarus locusta: II. Organism and population-level endpoints

This study aimed to test the performance of the amphipod Gammarus locusta (L.) in chronic sediment toxicity tests. It constitutes part of a multi-level assessment of chronic toxicity of estuarine sediments, integrating organism and population-level endpoints with biochemical markers responses. Here we account for organism and population-level effects, while biomarker responses were reported in a companion article. Five moderately contaminated sediments from Sado and Tagus estuaries were tested, comprising 3 muddy and 2 sandy sediments. These sediments either did not show acute toxicity or were diluted with control sediment as much as required to remove acute toxicity. Subsequent chronic tests consisted of 28-day exposures with survival, individual growth and reproductive traits as endpoints. Two of the muddy sediments induced higher growth rates in the amphipods, and improved reproductive traits. This was understood to be a consequence of the amount of organic matter in the sediment, which was nutritionally beneficial to the amphipods, while concurrently decreasing contaminant bioavailability. Biomarker responses did not reveal toxicant-induced stress in amphipods exposed to these sediments. One of the sandy sediments was acutely toxic at 50% dilution, but in contrast stimulated amphipod growth when diluted 75%. This was presumed to be an indication of a hormetic response. Finally the two remaining contaminated sediments showed pronounced chronic toxicity, affecting survival and reproduction. The sex ratio of survivors was highly biased towards females, and offspring production was severely impaired. The particulars of the responses of this amphipod were examined, as well as strengths versus limitations of the sediment test. This study illustrates the utility of this chronic test for toxicity assessment of contaminated estuarine sediments, with potential application all along Atlantic Europe.     

Analysis of Photoelectric Occultations and Development of a Digital Model of the Lunar Libration Zone

Astronomy Reports - One of the priority tasks of modern astronomy is the observation and study of transient celestial processes, which also concerns photoelectric observations of lunar occultations...     

Untersuchung der Sorptionseigenschaften des Kationenaustauschers KB-2M mit Makronetzstruktur zur Abtrennung von Zinkionen aus Ab- und Spülwässern

Study of Sorptional Properties of the Cation Exchanger KB-2M with Macroreticular Structure for Recovery of Zinc Ions from Sewage and Rinsing Water Although a number of ion-exchange methods have been employed for the recovery of some transition metals from industrial effluents, knowledge about ion-exchange resins with macroreticular structure is poor. The present paper describes the mechanism of sorption on such exchangers and their application for recovery of zinc from sewage rinsing water. Ion exchanger of macroreticular structure are polymers with long-chained cross-linking agents. We have synthesized carboxylic ion-exchange resins by hydrolysis of copolymerisates of methyl acrylate with different cross-linking agents: divinylbenzene, divinyl sulfide, divinyl ester of ethylene glycol and divinyl ester of di- or triethylene glycol. The sorption process on modifications of the carboxylic resins KB-2 of various structure was studied with different methods: potentiometric titration, infrared spectroscopy, electron microscopy, X-ray structural analysis. The initial zinc concentration in rinsing water was 0.05 mol/L at pH from 3 to 6. For the sorption, 0.2…1.0 g of resin were equilibrated with 100 mL of zinc solution. After equilibrium (12 h), the resin was separated from solution. The zinc ions were determined by atomic absorption spectrometry after stripping with 100 mL of 10% sulfuric acid. The distribution ratio D was calculated (D: mmole of Zn sorbed per gram of resin divided by mmole of Zn per millilitre of solution). By means of infrared spectroscopy, the mechanism of sorption of zinc ions from rinsing water was determined. There may be a possibility of the formation of complexes in the cation-exchange resin phase. It was found out in this paper that the cation-exchanger KB-2M of macroreticular structure is the most effective for the sorption of the Zn²⁺-ions from sewage and rinsing water.     

Studies on turbulent diffusion processes and evaluation of diffusivity values from hydrodynamic observations in Corpus Christi Bay

The physical process of dispersion which can be attributed to turbulence (turbulent diffusion) or shear (shear-augmented diffusion) within the flow field is very important as it ultimately governs the distribution of constituents of interest within the environment. A series of diffusion experiments were conducted in Corpus Christi Bay, TX with the purpose of characterizing turbulent diffusion through dispersion coefficients or turbulent diffusivity, K_i (i=x, y, z) dependent on the degree of randomness or turbulence intensity, I.Measured with a boat-mounted acoustic doppler current profiler (ADCP), the Eulerian velocity time-series of fluid particles in random motion, u_i was used in the evaluation of the Eulerian time-scale of turbulence, T^E based on the velocity correlation function, R^E with T^E being related to the Lagrangian time-scale T^L through a scaling parameter, β(=T^L/T^E). Surface currents were obtained with high frequency (HF) Radar equipment deployed over the study area from which the horizontal velocity gradients were determined.Within the spatial scale of the experiment (1000 m), the observed low horizontal gradients (10⁻⁴ s⁻¹) allowed for the generation of velocity time-series from an ADCP mounted on a moving platform. A numerical scheme for evaluating turbulent diffusivity values was developed on the basis of Eulerian current measurements and calibrated through the statistics of an evolving dye patch for the scaling parameter β which in this scheme was found to be in the range 1–3.     

THE ELECTRICAL CURRENT PATTERN INDUCED BY AN OSCILLATING MAGNETIC DIPOLE IN A SEMI INFINITE CONDUCTIVE THIN PLATE*

In a paper by Koefoed and Kegge (1968), which was based on previous work of Wesley (1958), the electrical current pattern has been derived that is induced by an oscillating magnetic dipole in a semi-infinite thin plate of infinitesimal resistivity. In the present paper, the range of validity of the assumptions, on which the work of Wesley is based, is subjected to a theoretical analysis. It is found that the decisive factor for the validity of Wesley's derivation is the quotient of the square of the penetration depth of the electrical current over the product of the thickness of the plate and a distance that is indicative of the size of the current loops in the plate. Wesley's derivation is shown to be valid only when this factor is negligible. It is also shown that in this condition the imaginary component of the anomaly must be negligible. Model experiments are described in which the electrical current pattern is studied also in the range in which the derivation of Wesley is not valid. The procedure used in these model experiments was to measure the tangential component of the magnetic field strength very close to the metal plate that simulated the conductive dyke. In order to express the results of the measurements in terms of the imaginary to real ratio, these results are compared with an interpretation graph for field measurements that was published by Hedström and Parasnis (1958). It is found that the current pattern in the plate is essentially the same as that which follows from Wesley's derivation, provided that the imaginary to real ratio is less than one third. The measurements do not permit to draw conclusions regarding the current pattern in the plate in conditions that correspond to larger values of the imaginary to real ratio.     

Detailed architecture of a compound incised valley system and correlation with forced regressive wedges: Example of Late Quaternary Têt and Agly rivers,western Gulf of Lions,Mediterranean Sea,France

Quaternary incised valley systems are usually characterized by the preservation of a single valley-fill attributed to the last post-glacial period. Moreover, there are very few cases of correlation between incised valley system developed on inner shelf and sedimentary units observed on the mid to outer shelf, mainly forced regressive wedges. The Roussillon shelf, in the western part of the Gulf of Lion, is a particular example of preserved Quaternary compound incised valley system also characterized by a direct correlation with the forced regressive lowstand wedges on the mid-outer shelf. High-resolution seismic data and a borehole, 60 m deep, located on the beach barrier permit an accurate study of the geometry and lithology of the system. Six imbricated and more or less preserved incised valleys and valley-fills are observed up to the inner to mid-shelf. The key surfaces associated to the incised valleys are correlated to the boundaries of the forced regressive wedges. They are assumed to be reworked surfaces. At the borehole location, only few thin layers, less than 1 m thick, of coarse grain and/or floating pebbles, are observed and should correspond to preserved fluvial lowstand deposits reworked under marine influence. The valley fills are mainly composed of estuarine muddy silts. From AMS ¹⁴C age dating it is inferred that the uppermost incised valley system is younger than 45 ky cal BP. Based on those observations, the six preserved incised valley systems are assumed to be controlled by the last six 4th order sea-level cycles — 100 ky — of the middle to late Quaternary. The paleo-topography of the underlying Plio-Quaternary deposits controls the compound incised valley system location. The deep topography of the Messinian Erosionnal Surface is a controlling factor at a lower degree. The partial preservation of the successive valley fill is attributed not only to the differential subsidence but also to the lateral migration of each incision and to the hydrodynamic regime.