6,786 research outputs found
Nonmodal energy growth and optimal perturbations in compressible plane Couette flow
Nonmodal transient growth studies and estimation of optimal perturbations
have been made for the compressible plane Couette flow with three-dimensional
disturbances. The maximum amplification of perturbation energy over time,
, is found to increase with increasing Reynolds number ,
but decreases with increasing Mach number . More specifically, the optimal
energy amplification (the supremum of over both the
streamwise and spanwise wavenumbers) is maximum in the incompressible limit and
decreases monotonically as increases. The corresponding optimal streamwise
wavenumber, , is non-zero at M=0, increases with increasing
, reaching a maximum for some value of and then decreases, eventually
becoming zero at high Mach numbers. While the pure streamwise vortices are the
optimal patterns at high Mach numbers, the modulated streamwise vortices are
the optimal patterns for low-to-moderate values of the Mach number. Unlike in
incompressible shear flows, the streamwise-independent modes in the present
flow do not follow the scaling law , the reasons
for which are shown to be tied to the dominance of some terms in the linear
stability operator. Based on a detailed nonmodal energy analysis, we show that
the transient energy growth occurs due to the transfer of energy from the mean
flow to perturbations via an inviscid {\it algebraic} instability. The decrease
of transient growth with increasing Mach number is also shown to be tied to the
decrease in the energy transferred from the mean flow () in
the same limit
Linear stability, transient energy growth and the role of viscosity stratification in compressible plane Couette flow
Linear stability and the non-modal transient energy growth in compressible
plane Couette flow are investigated for two prototype mean flows: (a) the {\it
uniform shear} flow with constant viscosity, and (b) the {\it non-uniform
shear} flow with {\it stratified} viscosity. Both mean flows are linearly
unstable for a range of supersonic Mach numbers (). For a given , the
critical Reynolds number () is significantly smaller for the uniform shear
flow than its non-uniform shear counterpart. An analysis of perturbation energy
reveals that the instability is primarily caused by an excess transfer of
energy from mean-flow to perturbations. It is shown that the energy-transfer
from mean-flow occurs close to the moving top-wall for ``mode I'' instability,
whereas it occurs in the bulk of the flow domain for ``mode II''. For the
non-modal analysis, it is shown that the maximum amplification of perturbation
energy, , is significantly larger for the uniform shear case compared
to its non-uniform counterpart. For , the linear stability operator
can be partitioned into , and the
-dependent operator is shown to have a negligibly small
contribution to perturbation energy which is responsible for the validity of
the well-known quadratic-scaling law in uniform shear flow: . A reduced inviscid model has been shown to capture all salient
features of transient energy growth of full viscous problem. For both modal and
non-modal instability, it is shown that the {\it viscosity-stratification} of
the underlying mean flow would lead to a delayed transition in compressible
Couette flow
Probing Electroweak Top Quark Couplings at Hadron Colliders
We consider QCD t\bar{t}\gamma and t\bar{t}Z production at hadron colliders
as a tool to measure the tt\gamma and ttZ couplings. At the Tevatron it may be
possible to perform a first, albeit not very precise, test of the tt\gamma
vector and axial vector couplings in t\bar{t}\gamma production, provided that
more than 5 fb^{-1} of integrated luminosity are accumulated. The t\bar{t}Z
cross section at the Tevatron is too small to be observable. At the CERN Large
Hadron Collider (LHC) it will be possible to probe the tt\gamma couplings at
the few percent level, which approaches the precision which one hopes to
achieve with a next-generation e^+e^- linear collider. The LHC's capability of
associated QCD t\bar{t}V (V=\gamma, Z) production has the added advantage that
the tt\gamma and ttZ couplings are not entangled. For an integrated luminosity
of 300 fb^{-1}, the ttZ vector (axial vector) coupling can be determined with
an uncertainty of 45-85% (15-20%), whereas the dimension-five dipole form
factors can be measured with a precision of 50-55%. The achievable limits
improve typically by a factor of 2-3 for the luminosity-upgraded (3 ab^{-1})
LHC.Comment: Revtex3, 30 pages, 9 Figures, 6 Table
Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows
The nature of dynamo action in shear flows prone to magnetohydrodynamic
instabilities is investigated using the magnetorotational dynamo in Keplerian
shear flow as a prototype problem. Using direct numerical simulations and
Newton's method, we compute an exact time-periodic magnetorotational dynamo
solution to the three-dimensional dissipative incompressible
magnetohydrodynamic equations with rotation and shear. We discuss the physical
mechanism behind the cycle and show that it results from a combination of
linear and nonlinear interactions between a large-scale axisymmetric toroidal
magnetic field and non-axisymmetric perturbations amplified by the
magnetorotational instability. We demonstrate that this large scale dynamo
mechanism is overall intrinsically nonlinear and not reducible to the standard
mean-field dynamo formalism. Our results therefore provide clear evidence for a
generic nonlinear generation mechanism of time-dependent coherent large-scale
magnetic fields in shear flows and call for new theoretical dynamo models.
These findings may offer important clues to understand the transitional and
statistical properties of subcritical magnetorotational turbulence.Comment: 10 pages, 6 figures, accepted for publication in Physical Review
On the nature of XTE J0421+560/CI Cam
We present the results of the analysis of RXTE, BATSE and optical/infrared
data of the 1998 outburst of the X-ray transient system XTE J0421+560 (CI Cam).
The X-ray outburst shows a very fast decay (initial e-folding time ~0.5 days,
slowing down to ~2.3 days). The X-ray spectrum in the 2-25 keV band is complex,
softening considerably during decay and with strongly variable intrinsic
absorption. A strong iron emission line is observed. No fast time variability
is detected (<0.5 % rms in the 1-4096 Hz band at the outburst peak). The
analysis of the optical/IR data suggests that the secondary is a B[e] star
surrounded by cool dust and places the system at a distance of >~ 2 kpc. At
this distance the peak 2-25 keV luminosity is ~4 x 10^37 erg/s. We compare the
properties of this peculiar system with those of the Be/NS LMC transient A
0538-66 and suggest that CI Cam is of similar nature. The presence of strong
radio emission during outburst indicates that the compact object is likely to
be a black hole or a weakly magnetized neutron star.Comment: Accepted for publication on The Astrophysical Journal, July 199
Structural and electrical transport properties of superconducting Au{0.7}In{0.3} films: A random array of superconductor-normal metal-superconductor (SNS) Josephson junctions
The structural and superconducting properties of Au{0.7}In{0.3} films, grown
by interdiffusion of alternating Au and In layers, have been studied. The films
were found to consist of a uniform solid solution of Au{0.9}In{0.1}, with
excess In precipitated in the form of In-rich grains of various Au-In phases
(with distinct atomic compositions), including intermetallic compounds. As the
temperature was lowered, these individual grains became superconducting at a
particular transition temperature (Tc), determined primarily by the atomic
composition of the grain, before a fully superconducting state of zero
resistance was established. From the observed onset Tc, it was inferred that up
to three different superconducting phases could have formed in these
Au{0.7}In{0.3} films, all of which were embedded in a uniform Au{0.9}In{0.1}
matrix. Among these phases, the Tc of a particular one, 0.8 K, is higher than
any previously reported for the Au-In system. The electrical transport
properties were studied down to low temperatures. The transport results were
found to be well correlated with those of the structural studies. The present
work suggests that Au{0.7}In{0.3} can be modeled as a random array of
superconductor-normal metal-superconductor (SNS) Josephson junctions. The
effect of disorder and the nature of the superconducting transition in these
Au{0.7}In{0.3} films are discussed.Comment: 8 text pages, 10 figures in one separate PDF file, submitted to PR
One-neutron removal reactions on light neutron-rich nuclei
A study of high energy (43--68 MeV/nucleon) one-neutron removal reactions on
a range of neutron-rich psd-shell nuclei (Z = 5--9, A = 12--25) has been
undertaken. The inclusive longitudinal and transverse momentum distributions
for the core fragments, together with the cross sections have been measured for
breakup on a carbon target. Momentum distributions for reactions on tantalum
were also measured for a subset of nuclei. An extended version of the Glauber
model incorporating second order noneikonal corrections to the JLM
parametrisation of the optical potential has been used to describe the nuclear
breakup, whilst the Coulomb dissociation is treated within first order
perturbation theory. The projectile structure has been taken into account via
shell model calculations employing the psd-interaction of Warburton and Brown.
Both the longitudinal and transverse momentum distributions, together with the
integrated cross sections were well reproduced by these calculations and
spin-parity assignments are thus proposed for B, C, N,
O, F. In addition to the large spectroscopic amplitudes for
the s intruder configuration in the N=9 isotones,B and
C, significant s admixtures appear to occur in the
ground state of the neighbouring N=10 nuclei B and C. Similarly,
crossing the N=14 subshell, the occupation of the s orbital is
observed for O, F. Analysis of the longitudinal and transverse
momentum distributions reveals that both carry spectroscopic information, often
of a complementary nature. The general utility of high energy nucleon removal
reactions as a spectroscopic tool is also examined.Comment: 50 pages, 19 figures, submitted to Phys. Rev.
The origins of X-ray emission from the hotspots of FRII radio sources
We use new and archival Chandra data to investigate the X-ray emission from a
large sample of compact hotspots of FRII radio galaxies and quasars from the 3C
catalogue. We find that only the most luminous hotspots tend to be in good
agreement with the predictions of a synchrotron self-Compton model with
equipartition magnetic fields. At low hotspot luminosities inverse-Compton
predictions are routinely exceeded by several orders of magnitude, but this is
never seen in more luminous hotspots. We argue that an additional synchrotron
component of the X-ray emission is present in low-luminosity hotspots, and that
the hotspot luminosity controls the ability of a given hotspot to produce
synchrotron X-rays, probably by determining the high-energy cutoff of the
electron energy spectrum. It remains plausible that all hotspots are close to
the equipartition condition.Comment: 49 pages, 16 figures. ApJ accepted. Revised version fixes a typo in
one of the Tables and corrects a statement about 3C27
- âŠ