Large Velocity Component Research Articles

Blowup of Dyadic MHD Models with Forward Energy Cascade

Abstract A particular type of dyadic model for the magnetohydrodynamics (MHD) with dominating forward energy cascade is studied. The model includes intermittency dimension $\delta $ in the nonlinear scales. It is shown that when $\delta $ is small, positive solution with large initial data for either the dyadic MHD or the dyadic Hall MHD model develops blowup in finite time. Moreover, for a class of positive initial data with large velocity components and small magnetic field components, we prove that there exists a positive solution that blows up at a finite time.

Comparative analysis of two kinds of pneumatic compact spinning using finite element method

PurposeThe purpose of this paper is to know airflow field and its distribution of pneumatic compact spinning systems. Complete compact spinning (CCS) and four-line rollers compact spinning (FRCS) are both two kinds of pneumatic compact spinning systems, which utilizes airflow in condensing equipment to condense fiber bundle and improve yarn properties.Design/methodology/approachThe paper opted for an exploratory study using finite element method, the airflow field in the condensing area of CCS and FRCS were simulated. First, a periodic movement of the fibers in bundle in condensing area was detected, and the yarn tracks were described veritably under the high-speed-video-camera and AutoCAD Software. Then the physical models of the condensing zone were constructed according to the physical parameters of the practical system. The simulation of airflow velocities were extracted along the yarn tracks using ANSYS Software. Finally, the numerical results were verified by spinning experiments.FindingsThe results show that the negative velocity component along the Y-axis helps keeping beneficial hairiness. CCS has higher negative velocity value and more abundant beneficial hairiness than FRCS. The velocity component in the X-axis direction has a direct effect on yarn evenness. For the same liner density of CCS and FRCS, the larger the value of the velocity component on X-axis is, the better the yarn evenness is. For 9.7tex, CCS has larger velocity component in the X-axis direction and better yarn evenness than FRCS, showing that CCS is more suitable for spinning fine count yarn. The velocity component in the Z-axis direction has a direct effect on breaking strength. CCS has little velocity component in the Z-axis direction and little breaking strength than FRCS.Originality/valueTo know airflow field and its distribution by finite element method is helpful to investigate the condensing principles of the fiber bundle and improve yarn properties.

Stability of relativistic surfatron acceleration.

In this paper we consider the surfatron acceleration of relativistic charged particles by a strong electrostatic wave propagating in a transverse direction relative to the background magnetic field. We investigate how high-frequency fluctuations of the background magnetic field affect the process of the resonant acceleration. We show that the presence of fluctuations leads to particle escape from the surfatron resonance and illustrate that fluctuations of different components of the magnetic field have quite a distinct effect on the energy gained by particles. In the case of the same power density, the strongest effect corresponds to fluctuations of the component directed along the background magnetic field, while the effect of the component along the wave front is substantially weaker. This is more important for particles with a large velocity component along the background magnetic field. We demonstrate that the dynamics of particles can statistically be described in terms of the adiabatic invariant diffusion. We derive the corresponding diffusion equation and compare solutions of that equation with results obtained by the explicit particle tracing.

Interferometric mapping of the 3.3-mm continuum emission of comet 17P/Holmes after its 2007 outburst

Comet 17P/Holmes underwent a dramatic outburst in October 2007, caused by the sudden fragmentation of its nucleus and the production of a large quantity of grains scattering sunlight. We report on 90 GHz continuum observations carried out with the IRAM Plateau de Bure interferometer on 27.1 and 28.2 October 2007 UT, i.e., 4-5 days after the outburst. These observations probed the thermal radiation of large dust particles, and therefore provide the best constraints on the mass in the ejecta debris. The thermal emission of the debris was modelled and coupled to a time-dependent description of their expansion after the outburst. The analysis was performed in the Fourier plane. Visibilities were computed for the two observing dates and compared to the data to measure their velocity and mass. Optical data and 250-GHz continuum measurements published in the literature were used to further constrain the dust kinematics and size distribution. Two distinct dust components in terms of kinematic properties are identified in the data. The large-velocity component, with typical velocities V0 of 50-100 m/s for 1 mm particles, displays a steep size distribution with a size index estimated to q = -3.7 (\pm0.1), assuming a minimum grain size of 0.1 \mum. It corresponds to the fast expanding shell observed in optical images. The slowly-moving "core" component (V0 = 7-9 m/s) detected near the nucleus has a size index |q| < 3.4 and contains a higher proportion of large particles than the shell. The dust mass in the core is in the range 0.1-1 that of the shell. Using optical constants pertaining to porous grains (50% porosity) made of astronomical silicates mixed with water ice (48% in mass), the total dust mass Mdust injected by the outburst is estimated to 4-14 x 10**11 kg, corresponding to 3-9% the nucleus mass.

A Magnetohydrodynamic Boost for Relativistic Jets

We have performed relativistic magnetohydrodynamic simulations of the hydrodynamic boosting mechanism for relativistic jets explored by Aloy & Rezzolla (2006) using the RAISHIN code. Simulation results show that the presence of a magnetic field may change the properties of the shock interface between the tenuous, overpressured jet (V(sub j) (sup z)) flowing tangentially to a dense external medium. Magnetic fields can lead to more efficient acceleration of the jet, in comparison to the pure-hydrodynamic case. A poloidal magnetic field (B(sup z)), tangent to the interface and parallel to the jet flow, produces both a stronger outward moving shock and inward moving rarefaction wave. This leads to a large velocity component normal to the interface in addition to acceleration tangent to the interface, and the jet is thus accelerated to a larger Lorentz factors than those obtained in the pure-hydrodynamic case. In contrast, a strong toroidal magnetic field (B(sup y)), tangent to the interface but perpendicular to the jet flow, also leads to stronger acceleration tangent to the shock interface relative to the pure-hydrodynamic case, but to a lesser extent than found for the poloidal case due to the fact that the velocity component normal to the shock interface is now much smaller. Overall, the acceleration efficiency in the toroidal case is less than that of the poloidal case but both geometries still result in higher Lorentz factors than the pure-hydrodynamic case. Thus, the presence and relative orientation of a magnetic field in relativistic jets can have a significant influence on the hydrodynamic boost mechanism studied by Aloy & Rezzolla (2006).

Li in Chromospherically Active Stars with Large Velocity Components

We present lithium abundances for nine chromospherically young, kinematically old late-type stars. The data support the interpretation that these objects can be formed during the coalescence of a short-period binary.

On the spectrum of longitudinal plasma oscillations

Longitudinal plasma oscillations are investigated in the presence of a beam of electrons possessing a large velocity component V⊥0 perpendicular to the external magnetic field B0. The angle between the wave vector k and the external magnetic field is taken equal to π/2. If the wave frequency ω is large compared with the gyration frequency of the beam electrons and the wavelength is small compared with their gyration radius, the instability appears under the resonance condition ω = kv⊥0. With ω kv⊥0 in a rather dense plasma, the Langmuir frequency Ωp exceeds considerably the gyration frequency ωc, and the oscillations are excited with frequencies close to the half-integral harmonics of the gyration frequency. In the case when the wavelength is of the order of the beam electron gyration radius, it is oscillations with frequencies near the hybrid one, √(Ωp2 + ωc2) that appear unstable (herein, √(Ωp2 + ωc2) = Lωc, L = 2, 3,...).