16,005 research outputs found
Vortex density fluctuations in quantum turbulence
We compute the frequency spectrum of turbulent superfluid vortex density
fluctuations and obtain the same Kolmogorov scaling which has been observed in
a recent experiment in Helium-4. We show that the scaling can be interpreted in
terms of the spectrum of reconnecting material lines. The calculation is
performed using a vortex tree algorithm which considerably speeds up the
evaluation of Biot-Savart integrals.Comment: 7 Pages, 7 figure
Universal dissipation scaling for non-equilibrium turbulence
It is experimentally shown that the non-classical high Reynolds number energy
dissipation behaviour, ,
observed during the decay of fractal square grid-generated turbulence is also
manifested in decaying turbulence originating from various regular grids. For
sufficiently high values of the global Reynolds numbers , .Comment: 5 pages, 6 figure
A digital computer program for the dynamic interaction simulation of controls and structure (DISCOS), volume 1
A theoretical development and associated digital computer program system for the dynamic simulation and stability analysis of passive and actively controlled spacecraft are presented. The dynamic system (spacecraft) is modeled as an assembly of rigid and/or flexible bodies not necessarily in a topological tree configuration. The computer program system is used to investigate total system dynamic characteristics, including interaction effects between rigid and/or flexible bodies, control systems, and a wide range of environmental loadings. In addition, the program system is used for designing attitude control systems and for evaluating total dynamic system performance, including time domain response and frequency domain stability analyses
Consequences of a Change in the Galactic Environment of the Sun
The interaction of the heliosphere with interstellar clouds has attracted
interest since the late 1920's, both with a view to explaining apparent
quasi-periodic climate "catastrophes" as well as periodic mass extinctions.
Until recently, however, models describing the solar wind - local interstellar
medium (LISM) interaction self-consistently had not been developed. Here, we
describe the results of a two-dimensional (2D) simulation of the interaction
between the heliosphere and an interstellar cloud with the same properties as
currently, except that the neutral H density is increased from the present
value of n(H) ~ 0.2 cm^-3 to 10 cm^-3. The mutual interaction of interstellar
neutral hydrogen and plasma is included. The heliospheric cavity is reduced
considerably in size (approximately 10 - 14 AU to the termination shock in the
upstream direction) and is highly dynamical. The interplanetary environment at
the orbit of the Earth changes markedly, with the density of interstellar H
increasing to ~2 cm^-3. The termination shock itself experiences periods where
it disappears, reforms and disappears again. Considerable mixing of the shocked
solar wind and LISM occurs due to Rayleigh-Taylor-like instabilities at the
nose, driven by ion-neutral friction. Implications for two anomalously high
concentrations of 10Be found in Antarctic ice cores 33 kya and 60 kya, and the
absence of prior similar events, are discussed in terms of density enhancements
in the surrounding interstellar cloud. The calculation presented here supports
past speculation that the galactic environment of the Sun moderates the
interplanetary environment at the orbit of the Earth, and possibly also the
terrestrial climate.Comment: 23 pages, 2 color plates (jpg), 3 figures (eps
Microstructure of the Local Interstellar Cloud and the Identification of the Hyades Cloud
We analyze high-resolution UV spectra of the Mg II h and k lines for 18
members of the Hyades Cluster to study inhomogeneity along these proximate
lines of sight. The observations were taken by the Space Telescope Imaging
Spectrograph (STIS) instrument on board the Hubble Space Telescope (HST). Three
distinct velocity components are observed. All 18 lines of sight show
absorption by the Local Interstellar Cloud (LIC), ten stars show absorption by
an additional cloud, which we name the Hyades Cloud, and one star exhibits a
third absorption component. The LIC absorption is observed at a lower radial
velocity than predicted by the LIC velocity vector derived by Lallement &
Bertin (1992) and Lallement et al. (1995), (v(predicted LIC) - v(observed LIC)
= 2.9 +/- 0.7 km/s), which may indicate a compression or deceleration at the
leading edge of the LIC. We propose an extention of the Hyades Cloud boundary
based on previous HST observations of other stars in the general vicinity of
the Hyades, as well as ground-based Ca II observations. We present our fits of
the interstellar parameters for each absorption component. The availability of
18 similar lines of sight provides an excellent opportunity to study the
inhomogeneity of the warm, partially ionized local interstellar medium (LISM).
We find that these structures are roughly homogeneous. The measured Mg II
column densities do not vary by more than a factor of 2 for angular separations
of < 8 degrees, which at the outer edge of the LIC correspond to physical
separations of < 0.6 pc.Comment: 35 pages, 11 figures, AASTEX v.5.0 plus EPSF extensions in mkfig.sty;
accepted by Ap
Estimates for the two-dimensional Navier-Stokes equations in terms of the Reynolds number
The tradition in Navier-Stokes analysis of finding estimates in terms of the
Grashof number \bG, whose character depends on the ratio of the forcing to
the viscosity , means that it is difficult to make comparisons with other
results expressed in terms of Reynolds number \Rey, whose character depends
on the fluid response to the forcing. The first task of this paper is to apply
the approach of Doering and Foias \cite{DF} to the two-dimensional
Navier-Stokes equations on a periodic domain by estimating
quantities of physical relevance, particularly long-time averages
\left, in terms of the Reynolds number \Rey = U\ell/\nu, where
U^{2}= L^{-2}\left and is the forcing scale. In
particular, the Constantin-Foias-Temam upper bound \cite{CFT} on the attractor
dimension converts to a_{\ell}^{2}\Rey(1 + \ln\Rey)^{1/3}, while the estimate
for the inverse Kraichnan length is (a_{\ell}^{2}\Rey)^{1/2}, where
is the aspect ratio of the forcing. Other inverse length scales,
based on time averages, and associated with higher derivatives, are estimated
in a similar manner. The second task is to address the issue of intermittency :
it is shown how the time axis is broken up into very short intervals on which
various quantities have lower bounds, larger than long time-averages, which are
themselves interspersed by longer, more quiescent, intervals of time.Comment: 21 pages, 1 figure, accepted for publication from J. Math. Phys. for
the special issue on mathematical fluid mechanic
Statistical model for intermittent plasma edge turbulence
The Probability Distribution Function of plasma density fluctuations at the
edge of fusion devices is known to be skewed and strongly non-Gaussian. The
causes of this peculiar behaviour are, up to now, largely unexplored. On the
other hand, understanding the origin and the properties of edge turbulence is a
key issue in magnetic fusion research. In this work we show that a stochastic
fragmentation model, already successfully applied to fluid turbulence, is able
to predict an asymmetric distribution that closely matches experimental data.
The asymmetry is found to be a direct consequence of intermittency. A
discussion of our results in terms of recently suggested BHP universal curve
[S.T. Bramwell, P.C.W. Holdsworth, J.-F. Pinton, Nature (London) 396, 552
(1998)], that should hold for strongly correlated and critical systems, is also
proposedComment: 13 pages. Physica Review E, accepte
Entire solutions of hydrodynamical equations with exponential dissipation
We consider a modification of the three-dimensional Navier--Stokes equations
and other hydrodynamical evolution equations with space-periodic initial
conditions in which the usual Laplacian of the dissipation operator is replaced
by an operator whose Fourier symbol grows exponentially as \ue ^{|k|/\kd} at
high wavenumbers . Using estimates in suitable classes of analytic
functions, we show that the solutions with initially finite energy become
immediately entire in the space variables and that the Fourier coefficients
decay faster than \ue ^{-C(k/\kd) \ln (|k|/\kd)} for any . The
same result holds for the one-dimensional Burgers equation with exponential
dissipation but can be improved: heuristic arguments and very precise
simulations, analyzed by the method of asymptotic extrapolation of van der
Hoeven, indicate that the leading-order asymptotics is precisely of the above
form with . The same behavior with a universal constant
is conjectured for the Navier--Stokes equations with exponential
dissipation in any space dimension. This universality prevents the strong
growth of intermittency in the far dissipation range which is obtained for
ordinary Navier--Stokes turbulence. Possible applications to improved spectral
simulations are briefly discussed.Comment: 29 pages, 3 figures, Comm. Math. Phys., in pres
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