177,471 research outputs found
Consequences of Symmetries on the Analysis and Construction of Turbulence Models
Since they represent fundamental physical properties in turbulence
(conservation laws, wall laws, Kolmogorov energy spectrum, ...), symmetries are
used to analyse common turbulence models. A class of symmetry preserving
turbulence models is proposed. This class is refined such that the models
respect the second law of thermodynamics. Finally, an example of model
belonging to the class is numerically tested.Comment: Published in SIGMA (Symmetry, Integrability and Geometry: Methods and
Applications) at http://www.emis.de/journals/SIGMA
Towards a Simple Model of Compressible Alfvenic Turbulence
A simple model collisionless, dissipative, compressible MHD (Alfvenic)
turbulence in a magnetized system is investigated. In contrast to more familiar
paradigms of turbulence, dissipation arises from Landau damping, enters via
nonlinearity, and is distributed over all scales. The theory predicts that two
different regimes or phases of turbulence are possible, depending on the ratio
of steepening to damping coefficient (m_1/m_2). For strong damping
(|m_1/m_2|<1), a regime of smooth, hydrodynamic turbulence is predicted. For
|m_1/m_2|>1, steady state turbulence does not exist in the hydrodynamic limit.
Rather, spikey, small scale structure is predicted.Comment: 6 pages, one figure, REVTeX; this version to be published in PRE. For
related papers, see http://sdphpd.ucsd.edu/~medvedev/papers.htm
HTTP Turbulence
In this paper, we consider a set of HTTP flows using TCP over a common drop-tail link to download files. After each download, a flow waits for a random think time before requesting the download of another file, whose size is also random. When a flow is active its throughput is increasing with time according to the additive increase rule, but if it suffers losses created when the total transmission rate of the flows exceeds the link rate, its transmission rate is decreased. The throughput obtained by a flow, and the consecutive time to download one file are then given as the consequence of the interaction of all the flows through their total transmission rate and the link's behavior. We study the mean-field model obtained by letting the number of flows go to infinity. This mean-field limit may have two stable regimes : one without congestion in the link, in which the density of transmission rate can be explicitly described, the other one with periodic congestion epochs, where the inter-congestion time can be characterized as the solution of a fixed point equation, that we compute numerically, leading to a density of transmission rate given by as the solution of a Fredholm equation. It is shown that for certain values of the parameters (more precisely when the link capacity per user is not significantly larger than the load per user), each of these two stable regimes can be reached depending on the initial condition. This phenomenon can be seen as an analogue of turbulence in fluid dynamics: for some initial conditions, the transfers progress in a fluid and interaction-less way; for others, the connections interact and slow down because of the resulting fluctuations, which in turn perpetuates interaction forever, in spite of the fact that the load per user is less than the capacity per user. We prove that this phenomenon is present in the Tahoe case and both the numerical method that we develop and simulations suggest that it is also be present in the Reno case. It translates into a bi-stability phenomenon for the finite population model within this range of parameters. This research was supported in part by the "Opération Stratégique Conjointe" Alcatel-INRIA entitled "End to End Analysis of IP Traffic"
Cavitating Langmuir Turbulence in the Terrestrial Aurora
Langmuir cavitons have been artificially produced in the earth's ionosphere,
but evidence of naturally-occurring cavitation has been elusive. By measuring
and modeling the spectra of electrostatic plasma modes, we show that natural
cavitating, or strong, Langmuir turbulence does occur in the ionosphere, via a
process in which a beam of auroral electrons drives Langmuir waves, which in
turn produce cascading Langmuir and ion-acoustic excitations and cavitating
Langmuir turbulence. The data presented here are the first direct evidence of
cavitating Langmuir turbulence occurring naturally in any space or
astrophysical plasma.Comment: 4 pages, 4 figures, published in PRL on 9 March 2012
http://link.aps.org/doi/10.1103/PhysRevLett.108.10500
Kinetic Energy Decay Rates of Supersonic and Super-Alfvenic Turbulence in Star-Forming Clouds
We present numerical studies of compressible, decaying turbulence, with and
without magnetic fields, with initial rms Alfven and Mach numbers ranging up to
five, and apply the results to the question of the support of star-forming
interstellar clouds of molecular gas. We find that, in 1D, magnetized
turbulence actually decays faster than unmagnetized turbulence. In all the
regimes that we have studied 3D turbulence-super-Alfvenic, supersonic,
sub-Alfvenic, and subsonic-the kinetic energy decays as (t-t0)^(-x), with 0.85
< x < 1.2. We compared results from two entirely different algorithms in the
unmagnetized case, and have performed extensive resolution studies in all
cases, reaching resolutions of 256^3 zones or 350,000 particles. We conclude
that the observed long lifetimes and supersonic motions in molecular clouds
must be due to external driving, as undriven turbulence decays far too fast to
explain the observations.Comment: Submitted to Phys. Rev. Letters, 29 Nov. 1997. 10 pages, 2 figures,
also available from http://www.mpia-hd.mpg.de/theory/preprints.html#maclo
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