22,137 research outputs found
Comment on "Existence of Internal Modes of Sine-Gordon Kinks"
In Ref.[1] [Phys. Rev. B. {\bf 42}, 2290 (1990)] we used a rigorous
projection operator collective variable formalism for nonlinear Klein-Gordon
equations to prove the continuum Sine-Gordon (SG) equation has a long lived
quasimode whose frequency = 1.004 is in the continuum just
above the lower phonon band edge with a lifetime () = 0.0017
. We confirmed the analytic calculations by simulations which agreed
very closely with the analytic results. In Ref.[3] [Phys. Rev. E. {\bf 62}, R60
(2000)] the authors performed two numerical investigations which they asserted
``show that neither intrinsic internal modes nor quasimodes exist in contrast
to previous results.'' In this paper we prove their first numerical
investigation could not possibly observe the quasimode in principle and their
second numerical investigation actually demonstrates the existence of the SG
quasimode. Our analytic calculations and verifying simulations were performed
for a stationary Sine-Gordon soliton fixed at the origin. Yet the authors in
Ref.[3] state the explanation of our analytic simulations and confirming
simulations are due to the Doppler shift of the phonons emitted by our
stationary Sine-Gordon soliton which thus has a zero Doppler shift.Comment: 5 pages, submitted to Phys. Rev.
Coherent structures in localised and global pipe turbulence
The recent discovery of unstable travelling waves (TWs) in pipe flow has been
hailed as a significant breakthrough with the hope that they populate the
turbulent attractor. We confirm the existence of coherent states with internal
fast and slow streaks commensurate in both structure and energy with known TWs
using numerical simulations in a long pipe. These only occur, however, within
less energetic regions of (localized) `puff' turbulence at low Reynolds numbers
(Re=2000-2400), and not at all in (homogeneous) `slug' turbulence at Re=2800.
This strongly suggests that all currently known TWs sit in an intermediate
region of phase space between the laminar and turbulent states rather than
being embedded within the turbulent attractor itself. New coherent fast streak
states with strongly decelerated cores appear to populate the turbulent
attractor instead.Comment: As accepted for PRL. 4 pages, 6 figures. Alterations to figs. 4,5.
Significant changes to tex
A subsonic probe for the measurement of d-region charged particle densities
Subsonic probe for measurement of charged particle densities in D layer of ionospher
The American species of the annulatipes group of the subgenus Lepidohelea, genus Forcipomyia (Diptera: Ceratopogonidae)
The annulatipes group of the genus Forcipomyia Meigen, subgenus Lepidohelea Kieffer, is represented in the Western Hemisphere by 12 species. Keys are presented for their identification, and to distinguish them from other groups of the subgenus Lepidohelea. The three previously known species, annulatipes Macfie, brasiliensis Macfie, and kuanoskeles Macfie, from southern Brazil, as well as the following nine new species, are described and illustrated: bahiensis, basifemoralis, bifida, convexipenis, euthystyla, gravesi, herediae, hobbsi, and weemsi
Reply to Comment on 'Critical behaviour in the relaminarization of localized turbulence in pipe flow'
This is a Reply to Comment arXiv:0707.2642 by Hof et al. on Letter
arXiv:physics/0608292 which was subsequently published in Phys Rev Lett, 98,
014501 (2007).
In our letter it was reported that in pipe flow the median time for
relaminarisation of localised turbulent disturbances closely follows the
scaling . This conclusion was based on data from
collections of 40 to 60 independent simulations at each of six different
Reynolds numbers, Re. In the Comment, Hof et al. estimate differently
for the point at lowest Re. Although this point is the most uncertain, it forms
the basis for their assertion that the data might then fit an exponential
scaling , for some constant A, supporting Hof et al.
(2006) Nature, 443, 59. The most certain point (at largest Re) does not fit
their conclusion and is rejected. We clarify why their argument for rejecting
this point is flawed. The median is estimated from the distribution of
observations, and it is shown that the correct part of the distribution is
used. The data is sufficiently well determined to show that the exponential
scaling cannot be fit to the data over this range of Re, whereas the fit is excellent, indicating critical behaviour and supporting
experiments by Peixinho & Mullin 2006.Comment: 1 page, 1 figur
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