13,038 research outputs found
Plasma-based Control of Supersonic Nozzle Flow
The flow structure obtained when Localized Arc Filament Plasma Actuators
(LAFPA) are employed to control the flow issuing from a perfectly expanded Mach
1.3 nozzle is elucidated by visualizing coherent structures obtained from
Implicit Large-Eddy Simulations. The computations reproduce recent experimental
observations at the Ohio State University to influence the acoustic and mixing
properties of the jet. Eight actuators were placed on a collar around the
periphery of the nozzle exit and selectively excited to generate various modes,
including first and second mixed (m = +/- 1 and m = +/- 2) and axisymmetric (m
= 0). In this fluid dynamics video
http://ecommons.library.cornell.edu/bitstream/1813/13723/2/Alljoinedtotalwithmodetextlong2-Datta%20MPEG-1.m1v,
http://ecommons.library.cornell.edu/bitstream/1813/13723/3/Alljoinedtotalwithmodetextlong2-Datta%20MPEG-2.m2v},
unsteady and phase-averaged quantities are displayed to aid understanding of
the vortex dynamics associated with the m = +/- 1 and m = 0 modes excited at
the preferred column-mode frequency (Strouhal number 0.3). The unsteady flow in
both contains a broad spectrum of coherent features. For m = +/- 1, the
phase-averaged flow reveals the generation of successive distorted elliptic
vortex rings with axes in the flapping plane, but alternating on either side of
the jet axis. This generates a chain of structures where each interacts with
its predecessor on one side and its successor on the other. Through self and
mutual interaction, the leading segment of each loop is pinched and passes
through the previous ring before rapidly breaking up, and the mean jet flow
takes on an elliptic shape. The m = 0 mode exhibits relatively stable roll-up
events, with vortex ribs in the braid regions connecting successive large
coherent structures.Comment: 3 pages. Video submission to Gallery of Fluid Motion, American
Physical Society, Division of Fluid Dynamics, 62nd Annual Meeting, November
22-24, 2009, Minneapolis, MN. Replacement deletes TeX commands to correct web
link
The Deconfinement Transition in SO(3) Gauge Theory
The SO(3) lattice gauge theory with a Villain form of action was investigated
by Monte Carlo techniques on asymmetric lattices with Nt = 2 and 4, where Nt is
the number of sites in the temporal extent. Unlike the results for higher Nt,
only one transition of second order was found for Nt = 2 . An extended action
with an irrelevant term to suppress Z_2 monopoles enabled us to get a better
view of the deconfinement transition as the effects of bulk transition could be
suppressed as well. Although the action has no global Z_2 symmetry for the
SO(3) theory, unlike the SU(2) theory at finite temperature, our study revealed
a second order deconfinement transition, with properties similar to the
deconfinement transition of SU(2).Comment: 19 pages latex, incl. figure
Possible signatures for strange stars in stellar X-ray binaries
Kilohertz quasi-periodic brightness oscillations (kHz QPOs) observed in
certain X-ray burst sources may represent Keplerian frequencies in the inner
regions of the accretion disk in such systems. If this assumption is strictly
adhered to, we show here that if the central accretor in stellar X-ray burst
sources is a strange star (made up of u, d and s quarks in beta equilibrium,
referred to as strange matter) then the calculated QPO frequencies are
reconcilable with the observed QPO frequencies (corresponding to the highest
frequency of 1.22 kHz, observed so far from the source 4U 1636-53) only for
particular values of the QCD-related parameters which describe the equation of
state of strange matter. We demonstrate that QPO frequencies in the very high
range (1.9-3.1) kHz can be understood in terms of a (non- magnetized) strange
star X-ray binary (SSXB) rather than a neutron star X-ray binary (NSXB). Future
discovery of such high frequency QPOs from X-ray burst sources will constitute
a new astrophysical di- agnostic for identifying solar mass range stable
strange stars in our galaxy.Comment: 4 pages, 2 figs., uses psbox.tex, submitted to A&
A smoother approach to scaling by suppressing monopoles and vortices
Suppressing monopoles and vortices by introducing large chemical potentials
for them in the Wilson action for the SU(2) lattice gauge theory, we study the
nature of the deconfinement phase transition on N_sigma^3 X N_tau lattices for
N_tau =4, 5, 6 and 8 and N_sigma = 8-16. Using finite size scaling theory, we
obtain \omega = 1.93 +/- 0.03 for N_tau = 4, in excellent agreement with
universality. The critical couplings for N_tau= 4, 5, 6 and 8 lattices exhibit
large shifts towards the strong coupling region when compared with the usual
Wilson action, and suggest a lot smoother approach to scaling.Comment: Lattice 2000 (Topology and Vaccum II); LaTeX 4 pages, 2 figure
Efficient implementation of the nonequilibrium Green function method for electronic transport calculations
An efficient implementation of the nonequilibrium Green function (NEGF)
method combined with the density functional theory (DFT) using localized
pseudo-atomic orbitals (PAOs) is presented for electronic transport
calculations of a system connected with two leads under a finite bias voltage.
In the implementation, accurate and efficient methods are developed especially
for evaluation of the density matrix and treatment of boundaries between the
scattering region and the leads. Equilibrium and nonequilibrium contributions
in the density matrix are evaluated with very high precision by a contour
integration with a continued fraction representation of the Fermi-Dirac
function and by a simple quadratureon the real axis with a small imaginary
part, respectively. The Hartree potential is computed efficiently by a
combination of the two dimensional fast Fourier transform (FFT) and a finite
difference method, and the charge density near the boundaries is constructed
with a careful treatment to avoid the spurious scattering at the boundaries.
The efficiency of the implementation is demonstrated by rapid convergence
properties of the density matrix. In addition, as an illustration, our method
is applied for zigzag graphene nanoribbons, a Fe/MgO/Fe tunneling junction, and
a LaMnOSrMnO superlattice, demonstrating its applicability to a wide
variety of systems.Comment: 20 pages, 11 figure
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