3,853 research outputs found
Turbulence transition and the edge of chaos in pipe flow
The linear stability of pipe flow implies that only perturbations of
sufficient strength will trigger the transition to turbulence. In order to
determine this threshold in perturbation amplitude we study the \emph{edge of
chaos} which separates perturbations that decay towards the laminar profile and
perturbations that trigger turbulence. Using the lifetime as an indicator and
methods developed in (Skufca et al, Phys. Rev. Lett. {\bf 96}, 174101 (2006))
we show that superimposed on an overall -scaling predicted and studied
previously there are small, non-monotonic variations reflecting folds in the
edge of chaos. By tracing the motion in the edge we find that it is formed by
the stable manifold of a unique flow field that is dominated by a pair of
downstream vortices, asymmetrically placed towards the wall. The flow field
that generates the edge of chaos shows intrinsic chaotic dynamics.Comment: 4 pages, 5 figure
Correlations of electromagnetic fields in chaotic cavities
We consider the fluctuations of electromagnetic fields in chaotic microwave
cavities. We calculate the transversal and longitudinal correlation function
based on a random wave assumption and compare the predictions with measurements
on two- and three-dimensional microwave cavities.Comment: Europhys style, 8 pages, 3 figures (included
High-temperature LM cathode ion thrusters Quarterly progress report, 5 May - 4 Aug. 1968
Design and operation of high temperature liquid mercury cathode ion thruster
High-temperature LM cathode ion thrusters Quarterly progress report, 5 Feb. - 4 May 1968
Poiseuille flow measurements for high temperature liquid metal cathode ion thruster
Asymmetry of temporal cross-correlations in turbulent shear flows
We investigate spatial and temporal cross-correlations between streamwise and
normal velocity components in three shear flows: a low-dimensional model for
vortex-streak interactions, direct numerical simulations for a nearly
homogeneous shear flow and experimental data for a turbulent boundary layer. A
driving of streamwise streaks by streamwise vortices gives rise to a temporal
asymmetry in the short time correlation. Close to the wall or the bounding
surface in the free-slip situations, this asymmetry is identified. Further away
from the boundaries the asymmetry becomes weaker and changes character,
indicating the prevalence of other processes. The systematic variation of the
asymmetry measure may be used as a complementary indicator to separate
different layers in turbulent shear flows. The location of the extrema at
different streamwise displacements can be used to read off the mean advection
speed; it differs from the mean streamwise velocity because of asymmetries in
the normal extension of the structures.Comment: 10 pages, 7 Postscript figures (low quality due to downsizing
On statistically stationary homogeneous shear turbulence
A statistically stationary turbulence with a mean shear gradient is realized
in a flow driven by suitable body forces. The flow domain is periodic in
downstream and spanwise directions and bounded by stress free surfaces in the
normal direction. Except for small layers near the surfaces the flow is
homogeneous. The fluctuations in turbulent energy are less violent than in the
simulations using remeshing, but the anisotropy on small scales as measured by
the skewness of derivatives is similar and decays weakly with increasing
Reynolds number.Comment: 4 pages, 5 figures (Figs. 3 and 4 as external JPG-Files
Echoes in classical dynamical systems
Echoes arise when external manipulations to a system induce a reversal of its
time evolution that leads to a more or less perfect recovery of the initial
state. We discuss the accuracy with which a cloud of trajectories returns to
the initial state in classical dynamical systems that are exposed to additive
noise and small differences in the equations of motion for forward and backward
evolution. The cases of integrable and chaotic motion and small or large noise
are studied in some detail and many different dynamical laws are identified.
Experimental tests in 2-d flows that show chaotic advection are proposed.Comment: to be published in J. Phys.
Reply to ‘‘Comment on ‘Brillouin-scattering study of the elastic constants of phenothiazine through the phase transition’ ’’
Disagreement in the ordering of relative magnitudes of the elastic constants of phenothiazine with those of Ecolivet et al. is resolved. Dubious methodologies and assumptions employed by Ecolivet et al. in their measurements, approximations, and arguments [C. Ecolivet et al., Phys. Rev. B 44, 4185 (1991)] are noted. Samples employed by these investigators in both their ultrasonic and Brillouin measurements are shown to exhibit properties consistent with impure samples and inconsistent with those of Sartwell and Eckhardt
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