308 research outputs found
Nonaxisymmetric MHD instabilities of Chandrasekhar states in Taylor-Couette geometry
We consider axially periodic Taylor-Couette geometry with insulating boundary
conditions. The imposed basic states are so-called Chandrasekhar states, where
the azimuthal flow and magnetic field have the same radial
profiles. Mainly three particular profiles are considered: the Rayleigh limit,
quasi-Keplerian, and solid-body rotation. In each case we begin by computing
linear instability curves and their dependence on the magnetic Prandtl number
Pm. For the azimuthal wavenumber m=1 modes, the instability curves always scale
with the Reynolds number and the Hartmann number. For sufficiently small Pm
these modes therefore only become unstable for magnetic Mach numbers less than
unity, and are thus not relevant for most astrophysical applications. However,
modes with m>10 can behave very differently. For sufficiently flat profiles,
they scale with the magnetic Reynolds number and the Lundquist number, thereby
allowing instability also for the large magnetic Mach numbers of astrophysical
objects. We further compute fully nonlinear, three-dimensional equilibration of
these instabilities, and investigate how the energy is distributed among the
azimuthal (m) and axial (k) wavenumbers. In comparison spectra become steeper
for large m, reflecting the smoothing action of shear. On the other hand
kinetic and magnetic energy spectra exhibit similar behavior: if several
azimuthal modes are already linearly unstable they are relatively flat, but for
the rigidly rotating case where m=1 is the only unstable mode they are so steep
that neither Kolmogorov nor Iroshnikov-Kraichnan spectra fit the results. The
total magnetic energy exceeds the kinetic energy only for large magnetic
Reynolds numbers Rm>100.Comment: 12 pages, 14 figures, submitted to Ap
Dynamo action in a quasi-Keplerian Taylor-Couette flow
We numerically compute the flow of an electrically conducting fluid in a Taylor-Couette geometry where the rotation rates of the inner and outer cylinders satisfy \Omega_o/\Omega_i=(r_o/r_i)-3/2. In this quasi-Keplerian regime a non-magnetic system would be Rayleigh-stable for all Reynolds numbers Re, and the resulting purely azimuthal flow incapable of kinematic dynamo action for all magnetic Reynolds numbers Rm. For Re=10^4 and Rm=10^5 we demonstrate the existence of a finite-amplitude dynamo, whereby a suitable initial condition yields mutually sustaining turbulence and magnetic fields, even though neither could exist without the other. This dynamo solution results in significantly increased outward angular momentum transport, with the bulk of the transport being by Maxwell rather than Reynolds stresses
Transition to magnetorotational turbulence in Taylor-Couette flow with imposed azimuthal magnetic field
The magnetorotational instability (MRI) is thought to be a powerful source of turbulence and momentum transport in astrophysical accretion discs, but obtaining observational evidence of its operation is challenging. Recently, laboratory experi-ments of TaylorâCouette flow with externally imposed axial and azimuthal magnetic fields have revealed the kinematic and dynamic properties of the MRI close to the instability onset. While good agreement was found with linear stability analyses, little is known about the transition to turbulence and transport properties of the MRI. We here report on a numerical investigation of the MRI with an imposed azimuthal magnetic field. We show that the laminar TaylorâCouette flow becomes unstable to a wave rotating in the azimuthal direction and standing in the axial direction via a supercritical Hopf bifurcation. Subsequently, the flow features a catastrophic transition to spatio-temporal defects which is mediated by a subcritical subharmonic Hopf bifurcation. Our results are in qualitative agreement with the PROMISE ex-periment and dramatically extend their realizable parameter range. We find that as the Reynolds number increases defects accumulate and grow into turbulence, yet the momentum transport scales weakly
The benefits of using a walking interface to navigate virtual environments
Navigation is the most common interactive task performed in three-dimensional virtual environments (VEs), but it is also a task that users often find difficult. We investigated how body-based information about the translational and rotational components of movement helped participants to perform a navigational search task (finding targets hidden inside boxes in a room-sized space). When participants physically walked around the VE while viewing it on a head-mounted display (HMD), they then performed 90% of trials perfectly, comparable to participants who had performed an equivalent task in the real world during a previous study. By contrast, participants performed less than 50% of trials perfectly if they used a tethered HMD (move by physically turning but pressing a button to translate) or a desktop display (no body-based information). This is the most complex navigational task in which a real-world level of performance has been achieved in a VE. Behavioral data indicates that both translational and rotational body-based information are required to accurately update one's position during navigation, and participants who walked tended to avoid obstacles, even though collision detection was not implemented and feedback not provided. A walking interface would bring immediate benefits to a number of VE applications
Density-shear instability in electron magneto-hydrodynamics
We discuss a novel instability in inertia-less electron magneto-hydrodynamics (EMHD), which arises from a combination of electron velocity shear and electron density gradients. The unstable modes have a lengthscale longer than the transverse density scale, and a growth-rate of the order of the inverse Hall timescale. We suggest that this density-shear instability may be of importance in magnetic reconnection regions on scales smaller than the ion skin depth, and in neutron star crusts. We demonstrate that the so-called Hall drift instability, previously argued to be relevant in neutron star crusts, is a resistive tearing instability rather than an instability of the Hall term itself. We argue that the density-shear instability is of greater significance in neutron stars than the tearing instability, because it generally has a faster growth-rate and is less sensitive to geometry and boundary conditions. We prove that, for uniform electron density, EMHD is "at least as stable" as regular, incompressible MHD, in the sense that any field configuration that is stable in MHD is also stable in EMHD. We present a connection between the density-shear instability in EMHD and the magneto-buoyancy instability in anelastic MHD
Symmetric and asymmetric action integration during cooperative object manipulation in virtual environments
Cooperation between multiple users in a virtual environment (VE) can take place at one of three levels. These
are defined as where users can perceive each other (Level 1), individually change the scene (Level 2), or
simultaneously act on and manipulate the same object (Level 3). Despite representing the highest level of
cooperation, multi-user object manipulation has rarely been studied. This paper describes a behavioral
experiment in which the piano movers' problem (maneuvering a large object through a restricted space) was
used to investigate object manipulation by pairs of participants in a VE. Participants' interactions with the object
were integrated together either symmetrically or asymmetrically. The former only allowed the common
component of participants' actions to take place, but the latter used the mean. Symmetric action integration was
superior for sections of the task when both participants had to perform similar actions, but if participants had to
move in different ways (e.g., one maneuvering themselves through a narrow opening while the other traveled
down a wide corridor) then asymmetric integration was superior. With both forms of integration, the extent to
which participants coordinated their actions was poor and this led to a substantial cooperation overhead (the
reduction in performance caused by having to cooperate with another person)
Experimental observation and characterization of the magnetorotational instability
Differential rotation occurs in conducting flows in accretion disks and
planetary cores. In such systems, the magnetorotational instability can arise
from coupling Lorentz and centrifugal forces to cause large radial angular
momentum fluxes. We present the first experimental observation of the
magnetorotational instability. Our system consists of liquid sodium between
differentially rotating spheres, with an imposed coaxial magnetic field. We
characterize the observed patterns, dynamics and torque increases, and
establish that this instability can occur from a hydrodynamic turbulent
background.Comment: 4 pages, 5 figures. Final version, accepted for publication in
Physical Review Letter
Correlative geochemical study of crude oils from southeastern and southern parts of the Pannonian Basin
Several crude oils from the southeastern Yugoslav part of the Pannonian Basin are correlated for the first time on the basis of many bulk and specific parameters. The investigations involved 15 oils from oil-gas fields A and B and oil field C from the Banat depression and oil field D from South BaÄka depression, and four oils from recently discovered oil shows E (the Kostolac depression). On the basis of structural group analysis and other bulk parameters such as API gravity, contents of asphaltenes and sulfur, as well as content of n-alkanes and pristane to phytane ratio, the examined oils are classified into four groups. However, the distributions and relative abundances of n-alkanes, isoprenoids (C19, C20), steranes, tricyclic terpanes and pentacyclic triterpanes, obtained by capillary GC and computerized GC-MS analysis, suggest classification of the examined oils into only two genetic types: A-D and E1-E4. The oils are of relatively high maturity. They are not biodegraded, except for both oils from field D and the oil A9. © 1987
Cetuximab plus gemcitabine/oxaliplatin (GEMOXCET) in first-line metastatic pancreatic cancer: a multicentre phase II study
Targeting the epidermal growth factor receptor pathway in pancreatic cancer seems to be an attractive therapeutic approach. This study assessed the efficacy of cetuximab plus the combination of gemcitabine/oxaliplatin in metastatic pancreatic cancer. Eligible subjects had histological or cytological diagnosis of metastatic pancreatic adenocarcinoma. The primary end point was response according to RECIST. Patients received cetuximab 400âmgâmâ2 at first infusion followed by weekly 250âmgâmâ2 combined with gemcitabine 1000âmgâmâ2 as a 100âmin infusion on day 1 and oxaliplatin 100âmgâmâ2 as a 2-h infusion on day 2 every 2 weeks. Between January 2005 and August 2006, a total of 64 patients (22 women (34%), 42 men (66%); median age 64 years (range 31â78)) were enrolled at seven study centres. On October 2007, a total of 17 patients were alive. Sixty-two patients were evaluable for baseline and 61 for assessment of response to treatment in an intention-to-treat analysis. Six patients had an incomplete drug combination within the first cycle of the treatment plan (n=4 hypersensitivity reactions to the first cetuximab infusion, n=2 refused to continue therapy). Reported grade 3/4 toxicities (% of patients) were leukopaenia 15%, anaemia 8%, thrombocytopaenia 10%, diarrhoea 7%, nausea 18%, infection 18% and allergy 7%. Cetuximab-attributable skin reactions occurred as follows: grade 0: 20%, grade 1: 41%, grade 2: 30% and grade 3: 10%. The intention-to-treat analysis of 61 evaluable patients showed an overall response rate of 33%, including 1 (2%) complete and 19 (31%) partial remissions. There were 31% patients with stable and 36% with progressive disease or discontinuation of the therapy before re-staging. The presence of a grade 2 or higher skin rash was associated with a higher likelihood of achieving objective response. Median time to progression was 118 days, with a median overall survival of 213 days. A clinical benefit response was noted in 24 of the evaluable 61 patients (39%). The addition of cetuximab to the combination of gemcitabine and oxaliplatin is well tolerated but does not increase response or survival in patients with metastatic pancreatic cancer
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Scientific basis and engineering design to accommodate disruption and halo current loads for the DIII-D tokamak
Plasma disruptions and halo current events apply sudden impulsive forces to the interior structures and vacuum vessel walls of tokamaks. These forces arise when induced toroidal currents and attached poloidal halo currents in plasma facing components interact with the poloidal and toroidal magnetic fields respectively. Increasing understanding of plasma disruptions and halo current events has been developed from experiments on DIII-D and other machines. Although the understanding has improved, these events must be planned for in system design because there is no assurance that these events can be eliminated in the operation of tokamaks. Increased understanding has allowed an improved focus of engineering designs
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