15,061 research outputs found
Cut-and-connect of two antiparallel vortex tubes
Motivated by an early conjecture that vortex cut-and-connect plays a key role in mixing and production of turbulence, helicity and aerodynamic noise, the cross-linking of two antiparallel viscous vortex tubes via direct numerical simulation is studied. The Navier-Stokes equations are solved by a dealiased pseudo-spectral method with 64 cubed grid points in a periodic domain for initial Reynolds numbers Re up to 1000. The vortex tubes are given an initial sinusoidal perturbation to induce a collision and keep the two tubes pressed against each other as annihilation continues. Cross-sectional and wire plots of various properties depict three stages of evolution: (1) Inviscid induction causing vortex cores to first approach and form a contact zone with a dipole cross-section, and then to flatten and stretch; (2) Vorticity annihilation in the contact zone accompanied by bridging between the two vortices at both ends of the contact zone due to a collection of cross-linked vortex lines, now orthogonal to the initial vortex tubes. The direction of dipole advection in the contact zone reverses; and (3) Threading of the remnants of the original vortices in between the bridges as they pull apart. The crucial stage 2 is shown to be a simple consequence of vorticity annihilation in the contact zone, link-up of the un-annihilated parts of vortex lines, and stretching and advection by the vortex tube swirl of the cross-linked lines, which accumulate at stagnation points in front of the annihilating vortex dipole. It is claimed that bridging is the essence of any vorticity cross-linking and that annihilation is sustained by stretching of the dipole by the bridges. Vortex reconnection details are found to be insensitive to asymmetry. Modeling of the reconnection process is briefly examined. The 3D spatial details of scalar transport (at unity Schmidt number), enstrophy production, dissipation and helicity are also examined
Unconventional domain wall magnetoresistance of patterned Ni/Nb bilayer structures below superconducting transition temperature of Nb
Scattering of spin-up and spin-down electrons while passing through a
ferromagnetic domain wall leads to an additional resistance for transport
current, usually observed prominently in constricted magnetic structures. In
this report, we use the resistance of the domain wall as a probe to find
indirect signatures of the theoretically predicted spin-singlet supercurrent to
spin-triplet supercurrent conversion effect of ferromagnetic domain walls. Here
we examine the domain wall induced resistance in Ni stripe in a bilayer Ni/Nb
geometry in the normal state and in the superconducting state of Nb. By making
a 6um wide gap in the top Nb layer we routed the transport current through the
Ni layer in the normal state and in the superconducting state of Nb. In the
normal state of Nb, in-field transport measurements showed a clear domain wall
magneto-resistance (DWMR) peak near the coercive field, where the domain wall
density is expected to be maximum. Interestingly, however, below the
superconducting transition temperature of Nb, the DWMR peak of the Ni layer
showed a sharp drop in the field range where the number of domain walls becomes
maximum. This observation may be a possible signature of magnetic domain wall
induced spin-triplet correlations in the Ni layer due to the direct injection
of spin-singlet Cooper pairs from Nb into the magnetic domain walls
Re-Scaling of Energy in the Stringy Charged Black Hole Solutions using Approximate Symmetries
This paper is devoted to study the energy problem in general relativity using
approximate Lie symmetry methods for differential equations. We evaluate
second-order approximate symmetries of the geodesic equations for the stringy
charged black hole solutions. It is concluded that energy must be re-scaled by
some factor in the second-order approximation.Comment: 18 pages, accepted for publication in Canadian J. Physic
Phage ϕC2 mediates transduction of Tn6215, encoding erythromycin resistance, between Clostridium difficile strains
UNLABELLED: In this work, we show that Clostridium difficile phage ϕC2 transduces erm(B), which confers erythromycin resistance, from a donor to a recipient strain at a frequency of 10(-6) per PFU. The transductants were lysogenic for ϕC2 and contained the erm(B) gene in a novel transposon, Tn6215. This element is 13,008 bp in length and contains 17 putative open reading frames (ORFs). It could also be transferred at a lower frequency by filter mating. IMPORTANCE: Clostridium difficile is a major human pathogen that causes diarrhea that can be persistent and difficult to resolve using antibiotics. C. difficile is potentially zoonotic and has been detected in animals, food, and environmental samples. C. difficile genomes contain large portions of horizontally acquired genetic elements. The conjugative elements have been reasonably well studied, but transduction has not yet been demonstrated. Here, we show for the first time transduction as a mechanism for the transfer of a novel genetic element in C. difficile. Transduction may also be a useful tool for the genetic manipulation of C. difficile.Peer reviewe
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An approach to Measure Transition Density of Binary Sequences for X-filling based Test Pattern Generator in Scan based Design
Switching activity and Transition density computation is an essential stage for dynamic power estimation and testing time reduction. The study of switching activity, transition densities and weighted switching activities of pseudo random binary sequences generated by Linear Feedback shift registers and Feed Forward shift registers plays a crucial role in design approaches of Built-In Self Test, cryptosystems, secure scan designs and other applications. This paper proposed an approach to find transition densities, which plays an important role in choosing of test pattern generator We have analyze conventional and proposed designs using our approache, This work also describes the testing time of benchmark circuits. The outcome of this paper is presented in the form of algorithm, theorems with proofs and analyses table which strongly support the same. The proposed algorithm reduces switching activity and testing time up to 51.56% and 84.61% respectively
The Coronal Structure of AB Doradus
We perform a numerical simulation of the corona of the young, rapidly
rotating K0 dwarf AB Doradus using a global MHD model. The model is driven by a
surface map of the radial magnetic field constructed using Zeeman-Doppler
Imaging. We find that the global structure of the stellar corona is dominated
by strong azimuthal tangling of the magnetic field due to the rapid rotation.
The MHD solution enables us to calculate realistic Alfv\'en surfaces and we can
therefore estimate the stellar mass loss rate and angular momentum loss rate
without making undue theoretical simplifications. We consider three cases,
parametrized by the base density of the corona, that span the range of possible
solutions for the system. We find that overall, the mass and angular-momentum
loss rates are higher than in the solar case; the mass loss rates are 10 to 500
times higher, and the angular momentum loss rate can be up to
higher than present day solar values. Our simulations show that this model can
be use to constrain the wide parameter space of stellar systems. It also shows
that an MHD approach can provide more information about the physical system
over the commonly used potential field extrapolation.Comment: 13 pages, 7 figure
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