Weakly non-linear stability of a hydrodynamic accretion disc

When the cold accretion disc coupling between neutral gas and a magnetic field is so weak that the magnetorotational instability is less effective or even stops working, it is of prime interest to investigate the pure hydrodynamic origin of turbulence and transport phenomena. As the Reynolds number increases, the relative importance of the non-linear term in the hydrodynamic equation increases. In an accretion disc where the molecular viscosity is too small, the Reynolds number is large enough for the non-linear term to have new effects. We investigate the scenario of the `weakly non-linear' evolution of the amplitude of the linear mode when the flow is bounded by two parallel walls. The unperturbed flow is similar to the plane Couette flow, but with the Coriolis force included in the hydrodynamic equation. Although there is no exponentially growing eigenmode, because of the self-interaction, the least stable eigenmode will grow in an intermediate phase. Later, this will lead to higher-order non-linearity and plausible turbulence. Although the non-linear term in the hydrodynamic equation is energy-conserving, within the weakly non-linear analysis it is possible to define a lower bound of the energy (alpha A(c)(2), where A(c) is the threshold amplitude) needed for the flow to transform to the turbulent phase. Such an unstable phase is possible only if the Reynolds number >= 10(3-4). The numerical difficulties in obtaining such a large Reynolds number might be the reason for the negative result of numerical simulations on a pure hydrodynamic Keplerian accretion disc.

Mathematical model of COREX melter gasifier: Part I. Steady-state model

The COREX melter gasifier is a countercurrent reactor to produce liquid iron. Directly reduced iron (DRI), noncoking coal, and other additives are charged to the melter gasifier at their respective temperatures, and O-2 is blown through the tuyeres. Functionally, a melter gasifier is divided into three zones: a moving bed, fluidized bed, and free board. A model has been developed for the moving bed, where the tuyere region is two-dimensional (2-D) and the rest is one-dimensional (1-D). It is based on multiphase conservation of mass, momentum, and heat. The fluidized bed has been treated as 1-D. Partial equilibrium is calculated for the free board. The calculated temperature of the hot metal, the top gas, and the chemistry of the top gas agree with the reported plant data. The model has been used to study the effects of bed height, injection of impure O-2, coal chemistry, and reactivity on the process performance.

Fluid dynamics study of a new vacuum degassing process

Recently, the demand of the steel having superior chemical and physical properties has increased for which the content of carbon must be in ultra low range. There are many processes which can produce low carbon steel such as Tank degasser and RH (Rheinstahl-Heraeus) processes. It has been claimed that using a new process, called REDA (Revolutionary Degassing Activator), one can achieve the carbon content below 10ppm in less time. REDA process in terms of installment cost is in between tank degasser and RH processes. As such, REDA process has not been studied thoroughly. Fluid flow phenomena affect the decarburization rate the most besides the chemical reaction rate. Therefore, momentum balance equations along with k-ε turbulent model have been solved for gas and liquid phases in two-dimension (2D) for REDA process. The fluid flow phenomena have been studied in details for this process by varying gas flow rate, depth of immersed snorkel in the steel, diameter of the snorkel and change in vacuum pressure. It is found that design of snorkel affects the mixing process of the bath significantly.

Anderson transitions in Ln1-x Sr x CoO3 (Ln═Pr or Nd)

Electrical conductivity measurements show that Ln1-x Sr x CoO3, (Ln = Pr or Nd) undergoes a non-metal-metal transition when x≈0 3. The d.c. conductivity of compositions with 0and the a.c. conductivity data follow the ωδ law with δ in the range 0.6–0.8. Surprisingly, however, δ seems to increase with increasing temperature. Thermopower measurements show clear evidence for a change in the mechanism of conduction (around 350 K) from variable range hopping to transport at E c. All these transport properties suggest that Co4+ ions in Ln1-x Sr x CoO3 (0gas and that the states at the Fermi energy are Anderson localized.

Effect of natural convection on oscillating flow in a pipe with cryogenic temperature difference across the ends

The effect of natural convection on the oscillatory flow in an open-ended pipe driven by a timewise sinusoidally varying pressure at one end and subjected to an ambient-to-cryogenic temperature difference across the ends, is numerically studied. Conjugate effects arising out of the interaction of oscillatory flow with heat conduction in the pipe wall are taken into account by considering a finite thickness wall with an insulated exterior surface. Two cases, namely, one with natural convection acting downwards and the other, with natural convection acting upwards, are considered. The full set of compressible flow equations with axissymmetry are solved using a pressure correction algorithm. Parametric studies are conducted with frequencies in the range 5-15 Hz for an end-to-end temperature difference of 200 and 50 K. Results are obtained for the variation of velocity, temperature. Nusselt number and the phase relationship between mass flow rate and temperature. It is found that the Rayleigh number has a minimal effect on the time averaged Nusselt number and phase angle. However, it does influence the local variation of velocity and Nusselt number over one cycle. The natural convection and pressure amplitude have influence on the energy flow through the gas and solid. (C) 2011 Elsevier Ltd. All rights reserved.

A Series of Trifacial Pd6 Molecular Barrels with Porphyrin Walls

Three new nanoscopic trigonal prisms, (tmen)6Pd6(H2L)3](NO3)12 (1), (Meen)6Pd6(H2L)3](NO3)12 (2), and (2,2'-bipy)6Pd6(H2L)3](NO3)12 (3), have been synthesized in excellent yields through single-step metalligand-coordination-driven self-assembly using 5,10,15,20-tetrakis(3-pyridyl)porphyrin (H2L) as a donor and cis-blocked PdII 90 degrees acceptors. These complexes were fully characterized by spectroscopic studies and single-crystal X-ray diffraction. All of these barrels quantitatively bind ZnII ions in the N4 pockets of the porphyrin walls at room temperature. Their corresponding zinc-embedded complexes, (tmen)6Pd6(ZnL)3](NO3)12 (1?a), (Meen)6Pd6(ZnL)3](NO3)12 (2?a), and (2,2'-bipy)6Pd6(ZnL)3](NO3)12 (3?a), were synthesized under ambient conditions by the post-synthetic binding of ZnII ions into the H2N4 pockets of the porphyrin walls of these complexes. These zinc-embedded complexes were characterized by electronic absorption, fluorescence emission, 1H NMR spectroscopy, as well as elemental analysis. Complexes 13 exhibited considerable microporosity in their solid state. Complex 1 was an efficient adsorbent for nitrogen gas and EtOH, MeOH, and water vapors.

Mechanochemical Synthesis of Amide Functionalized Porous Organic Polymers

Two porous organic polymers decorated with the amide functionality were synthesized mechanochemically and their properties were compared with the ones prepared by conventional solution mediated method. All the POPs were subjected to gas and water vapor sorption studies. The mechanochemically synthesized POPs have less surface area and show moderate adsorption properties compared to the solution mediated POPs. The amide based POPs show remarkable stability in water and concentrated acids.

Kinetic theory for sheared granular flows

Rapid granular flows are far-from-equilibrium-driven dissipative systems where the interaction between the particles dissipates energy, and so a continuous supply of energy is required to agitate the particles and facilitate the rearrangement required for the flow. This is in contrast to flows of molecular fluids, which are usually close to equilibrium, where the molecules are agitated by thermal fluctuations. Sheared granular flows form a class of flows where the energy required for agitating the particles in the flowing state is provided by the mean shear. These flows have been studied using the methods of kinetic theory of gases, where the particles are treated in a manner similar to molecules in a molecular gas, and the interactions between particles are treated as instantaneous energy-dissipating binary collisions. The validity of the assumptions underlying kinetic theory, and their applicability to the idealistic case of dilute sheared granular flows are first discussed. The successes and challenges for applying kinetic theory for realistic dense sheared granular flows are then summarised. (C) 2014 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Simulation studies on Performance parameters of a TPC Polarimeter

X-ray polarimeters based on Time Projection Chamber (TPC) geometry are currently being studied and developed to make sensitive measurement of polarization in 2-10keV energy range. TPC soft X-ray polarimeters exploit the fact that emission direction of the photoelectron ejected via photoelectric effect in a gas proportional counter carries the information of the polarization of the incident X-ray photon. Operating parameters such as pressure, drift field and drift-gap affect the performance of a TPC polarimeter. Simulations presented here showcase the effect of these operating parameters on the modulation factor of the TPC polarimeter. Models of Garfield are used to study photoelectron interaction in gas and drift of electron cloud towards Gas Electron Multiplier (GEM). The emission direction is reconstructed from the image and modulation factor is computed. Our study has shown that Ne/DME (50/50) at lower pressure and drift field can be used for a TPC polarimeter with modulation factor of 50-65%.

Drift parameters optimization of a TPC polarimeter: a simulation study

Time Projection Chamber (TPC) based X-ray polarimeters using Gas Electron Multiplier (GEM) are currently being developed to make sensitive measurement of polarization in 2-10 keV energy range. The emission direction of the photoelectron ejected via photoelectric effect carries the information of the polarization of the incident X-ray photon. Performance of a gas based polarimeter is affected by the operating drift parameters such as gas pressure, drift field and drift-gap. We present simulation studies carried out in order to understand the effect of these operating parameters on the modulation factor of a TPC polarimeter. Models of Garfield are used to study photoelectron interaction in gas and drift of electron cloud towards GEM. Our study is aimed at achieving higher modulation factors by optimizing drift parameters. Study has shown that Ne/DME (50/50) at lower pressure and drift field can lead to desired performance of a TPC polarimeter.