Pinch Mode Research Articles

Magnetostatic Analysis of a Pinch Mode Magnetorheological Valve

Abstract The study deals with the pinch mode of magnetorheological (MR) fluids’ operation and its application in MR valves. By applying the principle in MR valves a highly non-uniform magnetic field can be generated in flow channels in such a way to solidify the portion of the material that is the nearest to the flow channel’s walls. This is in contrary to well-known MR flow mode valves. The authors investigate a basic pinch mode valve in several fundamental configurations, and then examine their magnetic circuits through magnetostatic finite-element (FE) analysis. Flux density contour maps are revealed and basic performance figures calculated and analysed. The FE analysis results yield confidence in that the performance of MR pinch mode devices can be effectively controlled through electromagnetic means.

On the linear stability of magnetized jets without current sheets – relativistic case

In our prior papers, we considered the non-relativistic linear stability analysis of magnetized jets that do not have current sheet at the boundary. In this paper, we extend our analysis to relativistic jets. In order to find the unstable modes of current sheet-free, magnetized relativistic jets, we linearize full relativistic magnetohydrodynamics equations and solve them numerically. We find the dispersion relation of the pinch and kink mode instabilities. By comparing the dispersion relations of mildly relativistic jet (Lorentz factor 2) with moderately relativistic jet (Lorentz factor 10), we find that the jet with higher Lorentz factor is significantly more stable in both pinch and kink modes. We show that inclusion of the current sheet-free magnetic field in the jet further enhances the stability. Both pinch and kink mode instabilities become progressively more stable with increasing magnetization. We also show a scaling relation between the maximum temporal growth rate of the unstable mode and the Lorentz factor of the jet. The maximum temporal growth rates of the unstable modes are inversely proportion to the Lorentz factors for most of the modes that we study. However, for the fundamental pinch mode it is inversely proportional to the square of the Lorentz factor. This very beneficial scaling relation holds regardless of the presence of a magnetic field.

THE INTERNAL STRUCTURE OF OVERPRESSURED, MAGNETIZED, RELATIVISTIC JETS

ABSTRACT This work presents the first characterization of the internal structure of overpressured, steady superfast-magnetosonic relativistic jets in connection with their dominant type of energy. To this aim, relativistic magnetohydrodynamic simulations of different jet models threaded by a helical magnetic field have been analyzed covering a wide region in the magnetosonic Mach number–specific internal energy plane. The merit of this plane is that models dominated by different types of energy (internal energy: hot jets; rest-mass energy: kinetically dominated jets; magnetic energy: Poynting-flux-dominated jets) occupy well-separated regions. The analyzed models also cover a wide range of magnetizations. Models dominated by the internal energy (i.e., hot models, or Poynting-flux-dominated jets with magnetizations larger than but close to one) have a rich internal structure characterized by a series of recollimation shocks and present the largest variations in the flow Lorentz factor (and internal energy density). Conversely, in kinetically dominated models, there is not much internal or magnetic energy to be converted into kinetic, and the jets are featureless with small variations in the flow Lorentz factor. The presence of a significant toroidal magnetic field threading the jet produces large gradients in the transversal profile of the internal energy density. Poynting-flux-dominated models with high magnetization (≈10 or larger) are prone to be unstable against magnetic pinch modes, which sets limits on the expected magnetization in parsec-scale active galactic nucleus jets or constrains their magnetic field configuration.

A slender drop in a nonlinear extensional flow

The deformation of a slender drop in a nonlinear axisymmetric extensional and creeping flow has been theoretically studied. This problem, which was first suggested by Sherwood (J. Fluid Mech., vol. 144, 1984, pp. 281–295), is being revisited, and new results are presented. The problem is governed by three dimensionless parameters: the capillary number ($\mathit{Ca}\gg 1$), the viscosity ratio ($\unicode[STIX]{x1D706}\ll 1$), and the nonlinear intensity of the flow ($E\ll 1$). Contrary to linear extensional flow ($E=0$), where the local radius of the drop decreases monotonically (in the positive $z$ direction), in a nonlinear extensional flow ($E\neq 0$), two possible steady shapes exist: steady shapes (stable or unstable) with the local radius decreasing monotonically, and steady shapes (unstable) where the local radius of the drop has a local maximum, besides the one at the centre of the drop. Similar to linear extensional flow, the addition of nonlinear extensional effects does not change the end shape of the steady drop, which has pointed ends. A stability analysis has been done to distinguish between stable and unstable steady shapes and to determine the breakup point. Time-dependent studies reveal three types of breakup mechanism: a centre pinching mode, indefinite elongation, and a mechanism that remind us of tip-streaming, where a cusp is developed at the end of the drop.

Optical evidence for chemical interaction of the polyaniline/fullerene composites with N-methyl-2-pyrrolidinone

Surface enhanced Raman scattering (SERS) spectroscopic studies reveal a particular chemical interaction of the polyaniline/fullerene (PANI/C60) composite with N-methyl-2-pyrrolidinone (NMP). The chemical polymerization of aniline in the presence of sulfuric acid, potassium dichromate and fullerene has been used for the preparation of the PANI/C60 composite. The polymerization reaction involves a doping of PANI with C60 anion radicals. The interaction of the composite with NMP leads to a de-doping of PANI that involves a transformation of leucoemeraldine salt (LS) repeating units into leucoemeraldine base (LB). Additionally, a gradual increasing in the intensity of the Raman line at 1452 cm−1 associated to the Ag(2) pentagonal pinch mode of fullerene and a decrease in the intensity of the Raman lines of PANI are reported. This change arises from the formation of a charge-transfer complex C60-NMP. The subsequent chemical treatment of PANI-LB with FeCl3 leads to the formation PANI-emeraldine salt. An inhibition of the transformation of PANI doped with C60 anion radicals into a PANI-LB and the C60-NMP charge transfer complex in the presence of CdS particles dispersed in NMP is demonstrated by SERS spectroscopy.

Raman study of the temperature-induced decomposition of the fullerene dimers C120

Raman spectra of the C120 crystalline fullerene dimers, synthesized by high-pressure/high-temperature (HPHT) treatment of the C60 fullerite, were measured at ambient and elevated temperatures (T>130°C) in order to study the kinetics of their thermal decomposition. Measurements exhibit an intensity decrease of the dimer-related pentagon pinch (PP) mode and an increase of the monomer’s one. The relative intensity of the dimer’s PP mode decreases exponentially with the thermal treatment time and becomes faster at elevated temperatures. The activation energy EA of the dimer decomposition, obtained from the Arrhenius dependence of the exponential decay time constant on temperature, is (1.71±0.06)eV.

On the linear stability of sheared and magnetized jets without current sheets – non-relativistic case

In a prior paper (Kim et al. 2015) we considered the linear stability of magnetized jets that carry no net electric current and do not have current sheets. In this paper, in addition to physically well-motivated magnetic field structures, we also include the effects of jet shear. The jets we study have finite thermal pressure in addition to having realistic magnetic field structures and velocity shear. We find that shear has a strongly stabilizing effect on various modes of jet instability. Increasing shear stabilizes the fundamental pinch modes at long wavelengths and short wavelengths. Increasing shear also stabilizes the first reflection pinch modes at short wavelengths. Increasing shear has only a very modest stabilizing effect on the fundamental kink modes at long wavelengths; however, increasing shear does have a strong stabilizing effect on the fundamental kink modes at short wavelengths. The first reflection kink modes are strongly stabilized by increasing shear at shorter wavelengths. Overall, we find that the combined effect of magnetic field and shear stabilizes jets more than shear alone. In addition to the results from a formal linear stability analysis, we present a novel way of visualizing and understanding jet stability. This gives us a deeper understanding of the enhanced stability of sheared, magnetized jets. We also emphasize the value of our numerical approach in understanding the linear stability of jets with realistic structure.

Measurement and Modeling of Extra-Column Effects Due to Injection and Connections in Capillary Liquid Chromatography

As column volumes continue to decrease, extra-column band broadening has become an increasingly important consideration when determining column performance. Combined contributions due to the injector and connecting tubing in a capillary LC system were measured and found to be larger than expected by Taylor-Aris theory. Variance from sigma-type and tau-type broadening was isolated from eluted peaks using the Foley-Dorsey Exponentially Modified Gaussian peak fitting model and confirmed with computational fluid dynamics. It was found that the tau-type contributions were the main cause for the excessive broadening because of poorly-swept volumes at the connection between the injector and tubing. To reduce tau-type contributions (and peak tailing), a timed pinch mode could be used for analyte injection.

On the linear stability of magnetized jets without current sheets – non-relativistic case

In this paper we consider stability of magnetized jets that carry no net electric current and do not have current sheets (Gourgouliatos et al. 2012). The non-relativistic MHD equations are linearized around the background velocity and the magnetic field structure of the jet. The resulting linear equations are solved numerically inside the jet. We find that introduction of current-sheet-free magnetic field significantly improves jet stability relative to unmagnetized jets or magnetized jets with current sheets at their surface. This particularly applies to the fundamental pinch and kink modes - they become completely suppressed in a wide range of long wavelengths that are known to become most pernicious to jet stability when the evolution enters the non-linear regime. The reflection modes, both for the pinch and kink instability, also become progressively more stable with increased magnetization.

Mechanical Ball-Milling Preparation of Fullerene/Cobalt Core/Shell Nanocomposites with High Electrochemical Hydrogen Storage Ability

The design and synthesis of new hydrogen storage nanomaterials with high capacity at low cost is extremely desirable but remains challenging for today's development of hydrogen economy. Because of the special honeycomb structures and excellent physical and chemical characters, fullerenes have been extensively considered as ideal materials for hydrogen storage materials. To take the most advantage of its distinctive symmetrical carbon cage structure, we have uniformly coated C60's surface with metal cobalt in nanoscale to form a core/shell structure through a simple ball-milling process in this work. The X-ray diffraction (XRD), scanning electron microscope (SEM), Raman spectra, high-solution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectrometry (EDX) elemental mappings, and X-ray photoelectron spectroscopy (XPS) measurements have been conducted to evaluate the size and the composition of the composites. In addition, the blue shift of C60 pentagonal pinch mode demonstrates the formation of Co-C chemical bond, and which enhances the stability of the as-obtained nanocomposites. And their electrochemical experimental results demonstrate that the as-obtained C60/Co composites have excellent electrochemical hydrogen storage cycle reversibility and considerably high hydrogen storage capacities of 907 mAh/g (3.32 wt % hydrogen) under room temperature and ambient pressure, which is very close to the theoretical hydrogen storage capacities of individual metal Co (3.33 wt % hydrogen). Furthermore, their hydrogen storage processes and the mechanism have also been investigated, in which the quasi-reversible C60/Co↔C60/Co-Hx reaction is the dominant cycle process.

A 160 kJ dual plasma focus (DuPF) for fusion-relevant materials testing and nano-materials fabrication

This paper summarizes PF-160 Dual Plasma Focus (DuPF) numerical experiments using the Lee Model code and preliminary 3D design drawings using SolidWorks software. This DuPF consists of two interchangeable electrodes enabling it to be optimized for both Slow Pinch Mode (SFM) and Fast Pinch Mode (FFM); the latter using a speed factor (SF) of 90 kA cm-1 Torr-0.5 for FFM in deuterium [S Lee et al, IEEE Trans Plasma Science 24, 1101-1105 (1996)]; and the former with SF of less than half that value for SFM. Starting with available 6 × 450 µF capacitors rated at 11kV (10% reversal), numerical experiments indicate safe operation at 9 kV, 6 Torr deuterium with FFM anode of 5 cm radius; producing intense ion beam and streaming plasma pulses which would be useful for studies of potential fusion reactor wall materials. On the other hand operating at 5 kV, 10 Torr deuterium with SFM anode of 10 cm radius leads to long-duration, uniform large-area flow which could be more suitable for synthesis of nano-materials. The dual plasma focus design is illustrated here with two figures showing FFM and SFM electrodes.

On the structure and normal modes of hydrogenated Ti-fullerene compounds

When titanium covers a C60 core, the metal atoms may suppress the fullerene's capacity of storing hydrogen, depending on the number of Ti atoms covering the C60 framework, the Ti–C binding energy, and diffusion barriers. In this article, we study the structural and vibrational properties of the C60TiH n (n = 2, 4, 6, and 8) and C60Ti6H48 compounds. The IR spectra of C60TiH n compounds have a maximum attributable to the Ti–H stretching mode, which shifts to lower values in the structures with n = 4, 8, while their Raman spectra show two peaks corresponding to the stretching modes of H2 molecules at apical and azimuthal positions. On the other hand, the IR spectrum of C60Ti6H48 shows an intense peak due to the Ti–H in-phase stretching mode, while its Raman spectrum has a maximum attributed to the pentagonal pinch of the C60 core. Finally, we have found that the presence of one apical H2 molecule enhances the pentagonal pinch mode, becoming the maximum in the Raman spectrum.

On the evolution and breakup of slender drops in an extensional flow

The evolution of the shape of an elongated drop embedded in an extensional flow is studied in the framework of slender body theory. The external flow has a weak but not neglected inertia. The problem is governed by three dimensionless parameters: the capillary number, the external Reynolds number, and the viscosity ratio between the drop and the external fluid, and exhibits a multiplicity of stationary shapes with only one being stable. Evolution of the drop surface from initial shapes was studied when the flow intensity was either kept constant or subjected to a sudden change. It was shown that the dynamics of the shape evolution can lead to a breakup of the drop or to a stable stationary shape. Two modes of breakup are revealed: an indefinite elongation and a center pinching mode. The former appears when the viscous forces dominate the inertia effects, with the typical case being that of a creeping flow. The latter breakup mode takes over in the presence of inertia when the drop viscosity diminishes with the extreme example being that of an inviscid drop.

Electronic charge transfer in cobalt doped fullerene thin films and effect of energetic ion impacts by x-ray absorption spectroscopy

We report on the electronic charge transfer in cobalt doped fullerene thin films by means of near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy measurement. Co-doped fullerene films were prepared by co-deposition technique and subjected to energetic ion irradiation (120 MeV Au) for possibly alignment or interconnect of randomly distributed metal particles. Polarization dependent NEXAFS spectra revealed the alignment of Co and C atoms along the irradiated ionic path. The structural changes in Co-doped as-deposited and ion irradiated fullerene films were investigated by means of Raman spectroscopy measurements. Downshift of pentagonal pinch mode A g(2) in Raman spectroscopy indicated the electronic charge transfer from Co atom to fullerene molecules, which is further confirmed by NEXAFS at C K-edge for Co-doped fullerene films.

Photo‐polymerization of hole‐doped C60 grown by liquid‐liquid interfacial precipitation method

AbstractThe aim of our study is to synthesize an amorphous 3D C60 polymer at the bulk scale for various application fields. To polymerize the C60 molecules, powders of C60 and I2 were mixed for hole doping and the free electron laser (FEL) photopolymerization was carried out in vacuum. FEL with the wavelength of 500 nm was irradiated. In order to promote the polymerization degree, I2 was doped in alcohol while the liquid‐liquid interfacial precipitaiton (LLIP) growth of C60. It is concerned that the mixture process in air was probably accompanied with a reaction with oxygen, and reduced the polimerization degree. Specimens were analyzed by micro‐Raman spectra. To analyze a polymerization degree, one of the Raman‐active Ag modes, called pentagonal pinch mode Ag (2), is regularly investigated. From the results of Raman spectra, the important parameters to obtaine and promote polymerization phase was rather high pressure more than at least 600 MPa, I2 doping, and the FEL irradiation. So‐called F phase was obtained probably due to the reduction of energy barrier for transformation from sp2 to sp3 and high energy power density of micropulses of the FEL (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman study of the temperature-induced decomposition of the two-dimensional rhombohedral polymer of C60 and the intermediate states formed

Raman spectra of single crystalline two-dimensional rhombohedral (2D-R) polymer of C60 were measured at ambient conditions after its high temperature treatment (HTT) in order to study the polymer decomposition process. The data obtained indicate that the 2D-R polymer remains stable after 0.5h treatment up to ∼503K, while at higher temperatures a material transformation takes place. New Raman lines appear in the Raman spectrum after HTT in the range of 513–553K, related to the Ag(2) pentagon pinch (PP) mode of 2D tetragonal-like (2D-T-like) and 1D orthorhombic-like (1D-O-like) oligomers as well as to C60 dimers and monomers, typical for an intermediate state of partially decomposed 2D-R polymer. Above 553K, the material changes completely and its new composition is dominated by C60 monomers with some possible inclusion of C60 dimers. The activation energy of the 2D-R polymer decomposition, obtained from the dependence of the decomposition time on the treatment temperature, is EA=1.76±0.07eV/molecule.

FT‐Raman characterization of the antipodal bis‐adduct of C60 and anthracene

AbstractWe studied the mono‐ and the antipodal bis‐adduct of C60 and anthracene with FT‐Raman spectroscopy. Due to the reduced symmetry, the adducts have a much larger number of Raman‐active modes than C60 with its icosahedral symmetry. Especially interesting is the fact that in the region of the C60 pentagonal pinch mode (PPM) the adducts show two strong peaks, of which the higher one is almost at the same frequency as the C60 mode and the lower one is downshifted by 5.4 and 10.5 cm−1 for the mono‐ and bis‐adduct, respectively. The higher frequency peak could be explained by a degradation of the adducts into C60. However, calculated Raman spectra of the pure adducts also show two strong peaks in the PPM region, of which the higher frequency one is not PPM related. Finally, the C–H vibrations are the only clear evidence for anthracene in the adduct spectra.

To distinguish fullerene C 60 nanotubes and C 60 nanowhiskers using Raman spectroscopy

Raman spectroscopic measurements were carried out for fullerene C 60 nanotubes (FNTs) and nanowhiskers (FNWs) with a laser of 514 nm at power density of 20 mW/mm 2 in their solution and air-dried states. It is found that the Raman spectroscopic features of FNWs are very similar to those of FNTs in the solution. However, the Raman peak of pentagonal pinch mode A g(2) of the C 60 was quite different between the air-dried FNTs and FNWs upon laser irradiation. The Raman results implied that compared to the FNWs, the FNTs were more easily transformed to a glassy graphitic phase under the laser irradiation in air atmosphere.

MRI channel flows and their parasites

Local simulations of the magnetorotational instability (MRI) in accretion disks can exhibit recurrent coherent structures called channel flows. The formation and destruction of these structures may play a role in the development and saturation of MRI-induced turbulence, and consequently help us understand the time-dependent accretion behaviour of certain astrophysical objects. Previous investigations have revealed that channel solutions are attacked by various parasitic modes, foremost of which is an analogue of the Kelvin-Helmholtz instability. We revisit these instabilities and show how they relate to the classical instabilities of plasma physics, the kink and pinch modes. However, we argue that in most cases channels emerge from developed turbulence and are eventually destroyed by turbulent mixing, not by the parasites. The exceptions are the clean isolated channels which appear in systems near criticality or which emerge from low amplitude initial conditions. These structures inevitably achieve large amplitudes and are only then destroyed, giving rise to eruptive behaviour.

On the dynamics of a pulsed high-current discharge in a dense gas: Criteria for characterization of the plasma expansion modes

Criteria for discrimination between the free and pinch modes of the expansion of pulsed high-current discharges in dense gases are determined on the basis of real experiments and literature data analysis.