Evolution of filamentary molecular clouds in the presence of magnetic fields

The purpose of this paper is to explore the effect of magnetic fields on the dynamics of magnetized filamentary molecular clouds.We suppose there is a filament with cylindrical symmetry and two components of axial and toroidal magnetic fields.In comparison to previous works,the novelty in the present work involves a similarity solution that does not define a function of the magnetic fields or density.We consider the effect of the magnetic field on the collapse of the filament in both axial and toroidal directions and show that the magnetic field has a braking effect,which means that the increasing intensity of the magnetic field reduces the velocity of collapse.This is consistent with other studies.We find that the magnetic field in the central region tends to be aligned with the filament axis.Also,the magnitude and the direction of the magnetic field depend on the magnitude and direction of the initial magnetic field in the outer region.Moreover,we show that more energy dissipation from the filament causes a rise in the infall velocity.     

Gravitational instability of discs with dissipative coronae around supermassive black holes

We study the dynamical structure of a self-gravitating disc with coronae around a supermassive black hole. Assuming that the magnetorotational instability responsible for generating the turbulent stresses inside the disc is also the source for a magnetically dominated corona, a fraction of the power released when the disc matter accretes is transported to and dissipated in the corona. This has a major effect on the structure of the disc and its gravitational (in)stability according to our analytical and self-consistent solutions. We determine the radius where the disc crosses the inner radius of gravitational instability and forms the first stars. Not only the location of this radius which may extend to very large distances from the central black hole, but also the mass of the first stars highly depends on the input parameters, notably the viscosity coefficient, the mass of the central object and the accretion rate. For accretion discs around quasi-stellar objects (QSOs) and the Galactic Centre, we determine the self-gravitating radius and the mass of the first clumps. Comparing the cases with a corona and without a corona for typical discs around QSOs or the Galactic Centre, when the viscosity coefficient is around 0.3, we show that the self-gravitating radius decreases by a factor of approximately 2, but the mass of the fragments increases with more or less the same factor. The existence of a corona implies a more gravitationally unstable disc according to our results. The effect of a corona on the instability of the disc is more effective when the viscosity coefficient increases.     

Dynamic 3-D soil–railway track interaction by BEM–FEM

A study on the dynamic response of a railway track is presented via a 3-D formulation based on the frequency domain Boundary Element Method (BEM) and the Finite Element Method (FEM). The railway track consists of a group of surface, massive, rigid footings resting on a viscoelastic half-space and connected by an overlaying rail structure. The BEM, employing the full-space fundamental solutions and quadrilateral elements, is used for the simulation of the elastic half-space while the FEM is used to model the rigid footings and the rail superstructure. The loading function consists of a set of externally applied, harmonic or transient loads. Frequency as well as transient, by way of FFT, results are presented for various modes of vibration. Various numerical studies assess the through-the-soil interaction of the adjacent footings, the influence of soil damping, the effect of the overlaying structure on the frequency content of the system, and the effective simulation of an infinitely long railway track by a truncated one.     

Phosphorus status and sorption characteristics of some calcareous soils of Hamadan,western Iran

Phosphorus (P) application in excess of plant requirement may result in contamination of drinking water and eutrophication of surface water bodies. The phosphorous buffer capacity (PBC) of soil is important in plant nutrition and is an important soil property in the determination of the P release potential of soils. Phosphorus sorption greatly affects both plant nutrition and environmental pollution. For better and accurate P fertilizer recommendations, it is necessary to quantify P sorption. This study was conducted to investigate available P and P sorption by calcareous soils in a semi-arid region of Hamadan, western Iran. The soil samples were mainly from cultivated land. Olsen’s biocarbonate extractable P (Olsen P) varied among soils and ranged from 10 to 80 mg kg⁻¹ with a mean of 36 mg kg⁻¹. Half of the soils had an Olsen P > 40 mg kg⁻¹ and >70% of them had a concentration >20 mg kg⁻¹, whereas the critical concentration for most crops is <15 mg P kg⁻¹. Greater average Olsen P in soils occurred under garlic (56 mg kg⁻¹) and potato (44 kg kg⁻¹) fields than in dry-land wheat farming (24 mg kg⁻¹), pasture (30 mg kg⁻¹), and wheat (24 mg P kg⁻¹) fields. A marked increase in fertilizer P rates applied to agricultural soils has caused P to be accumulated in the surface soil. Phosphate sorption curves were well fitted to the Freundlich equation. The standard P requirement (SPR) of soils, defined as the amount of P sorbed at an equilibrium concentration of 0.2 mg l⁻¹ ranged from 4 to 102 mg kg⁻¹. Phosphorus buffer capacity was relatively high and varied from 16 to 123 l kg⁻¹ with an average of 58 l kg⁻¹. In areas of intensive crop production, continual P applications as P fertilizer and farmyard manure have been used at levels exceeding crop requirements. Surface soil accumulations of P are high enough that loss of P in surface runoff and a high risk for P transfer into groundwater have become priority management concerns.     

Formation of Small-Scale Condensations in the Molecular Clouds via Thermal Instability

A systematic study of the linear thermal instability of a self-gravitating magnetic molecular cloud is carried out for the case when the unperturbed background is subject to local expansion or contraction. We consider the ambipolar diffusion, or ion-neutral friction on the perturbed states. In this way, we obtain a non-dimensional characteristic equation that reduces to the prior characteristic equation in the non-gravitating stationary background. By parametric manipulation of this characteristic equation, we conclude that there are, not only oblate condensation forming solutions, but also prolate solutions according to local expansion or contraction of the background. We obtain the conditions for existence of the Field lengths that thermal instability in the molecular clouds can occur. If these conditions establish, small-scale condensations in the form of spherical, oblate, or prolate shape may be produced via thermal instability.     

Modeling in situ benzene bioremediation in the contaminated Liwa aquifer (UAE) using the slow-release oxygen source technique

Dissolved benzene was detected in the shallow unconfined Liwa aquifer, UAE, which represents the main freshwater source for the nearby residence Bu Hasa camp area. The main source of this contamination is believed to be the rejected water released from Bu Hasa liquid recovery plant. In this paper, a finite element model (METABIOTRANS) is used to simulate the fate and transport of the dissolved benzene plume in Liwa aquifer. Different remediation scenarios were simulated in which the slow-release oxygen source (SOS) technique is utilized to minimize benzene concentrations at the nearest camp supply wells downstream of the contamination zone. Results of the remediation scenarios show that the highest biodegradation rates occur when the oxygen source is placed near the plume center; where higher benzene concentrations exist. The nearest oxygen release source to the contamination zone caused higher stimulation to bacterial growth than further down-gradient oxygen sources. It also exhibited longer resident time of oxygen in the aquifer; and therefore, yielded higher reductions in benzene concentrations. However, using one central SOS proved to be insufficient as contaminant escaped laterally. Additional four transverse oxygen sources were necessary to capture benzene that laterally spread away from the contamination zone. These lateral SOSs were essential to reduce benzene concentrations at the supply wells that are located at the plume fringes. Finally, it was found that increasing oxygen release from one source did not always improve remediation; and that using several SOSs with lower release rates could be a more practical approach to enhance benzene biodegradation in the aquifer.     

New approaches for non‐classically damped system eigenanalysis

This paper presents three new approaches for solving eigenvalue problems of non‐classically damped linear dynamics systems with fewer calculations than the conventional state vector approach. In the latter, the second‐order differential equation of motion is converted into a first‐order system by doubling the size of the matrices. The new approaches simplify the approach and reduce the number of calculations. The mathematical formulations for the proposed approaches are presented and the numerical results compared with the existing method by solving a sample problem with different damping properties. Of the three proposed approaches, the expansion approach was found to be the simplest and fastest to compute. Copyright © 2005 John Wiley & Sons, Ltd.     

Investigating thermal evolution of the self-gravitating one dimensional molecular cloud by smoothed particle hydrodynamics

The heating of the ion-neutral (or ambipolar) diffusion may affect the thermal phases of the molecular clouds. We present an investigation on the effect of this heating mechanism in the thermal instability of the molecular clouds. A weakly ionized one-dimensional slab geometry, which is allowed for self-gravity and ambipolar diffusion, is chosen to study its thermal phases. We use the thermodynamic evolution of the slab to obtain the regions where slab cloud becomes thermally unstable. We investigate this evolution using the model of ambipolar diffusion with two-fluid smoothed particle hydrodynamics, as outlined by Hosking and Whitworth. Firstly, some parts of the technique are improved to test the pioneer works on behavior of the ambipolar diffusion in an isothermal self-gravitating slab. Afterwards, the improved two-fluid technique is used for thermal evolution of the slab. The results show that the thermal instability may persist inhomogeneities with a large density contrast at the intermediate parts of the cloud. We suggest that this feature may be responsible for the planet formation in the intermediate regions of a collapsing molecular cloud and/or may also be relevant to the formation of star forming dense cores in the clumps.