601 research outputs found
Turbulent thermalization of weakly coupled non-abelian plasmas
We study the dynamics of weakly coupled non-abelian plasmas within the
frameworks of classical-statistical lattice gauge-theory and kinetic theory. We
focus on a class of systems which are highly occupied, isotropic at all times
and initially characterized by a single momentum scale. These represent an
idealized version of the situation in relativistic heavy ion-collisions in the
color-glass condensate picture, where on a time scale after the
collision of heavy nuclei a longitudinally expanding plasma characterized by
the saturation scale is formed. Our results indicate that the system
evolves according to a turbulent Kolmogorov cascade in the classical regime.
Taking this into account, the kinetic description is able to reproduce
characteristic features of the evolution correctly.Comment: 8 pages, 6 figure
Quantification of coarse-graining error in Langevin and overdamped Langevin dynamics
In molecular dynamics and sampling of high dimensional Gibbs measures
coarse-graining is an important technique to reduce the dimensionality of the
problem. We will study and quantify the coarse-graining error between the
coarse-grained dynamics and an effective dynamics. The effective dynamics is a
Markov process on the coarse-grained state space obtained by a closure
procedure from the coarse-grained coefficients. We obtain error estimates both
in relative entropy and Wasserstein distance, for both Langevin and overdamped
Langevin dynamics. The approach allows for vectorial coarse-graining maps.
Hereby, the quality of the chosen coarse-graining is measured by certain
functional inequalities encoding the scale separation of the Gibbs measure. The
method is based on error estimates between solutions of (kinetic) Fokker-Planck
equations in terms of large-deviation rate functionals
Wind and boundary layers in Rayleigh-Benard convection. I: analysis and modeling
The aim of this paper is to contribute to the understanding and to model the
processes controlling the amplitude of the wind of Rayleigh-Benard convection.
We analyze results from direct simulation of an L/H = 4 aspect-ratio domain
with periodic sidewalls at Ra = 1e5; 1e6; 1e7; 1e8 and at Pr = 1 by decomposing
independent realizations into wind and fluctuations. It is shown that deep
inside the thermal boundary layer, horizontal heat-fuxes exceed the average
vertical heat-fux by a factor 3 due to the interaction between the wind and the
mean temperature field. These large horizontal heat-fluxes are responsible for
spatial temperature differences that drive the wind by creating pressure
gradients. The wall fluxes and turbulent mixing in the bulk provide damping.
Using the DNS results to parameterise the unclosed terms, a simple model
capturing the essential processes governing the wind structure is derived. The
model consists of two coupled differential equations for wind velocity and
temperature amplitude. The equations indicate that the formation of a wind
structure is inevitable due to the positive feedback resulting from the
interaction between the wind and temperature field. Furthermore, the wind
velocity is largely determined by the turbulence in the bulk rather than by the
wall-shear stress. The model reproduces the Ra dependence of wind Reynolds
number and temperature amplitude
Shear Effects in Non-Homogeneous Turbulence
Motivated by recent experimental and numerical results, a simple unifying
picture of intermittency in turbulent shear flows is suggested. Integral
Structure Functions (ISF), taking into account explicitly the shear intensity,
are introduced on phenomenological grounds. ISF can exhibit a universal scaling
behavior, independent of the shear intensity. This picture is in satisfactory
agreement with both experimental and numerical data. Possible extension to
convective turbulence and implication on closure conditions for Large-Eddy
Simulation of non-homogeneous flows are briefly discussed.Comment: 4 pages, 5 figure
Dynamics of vibrofluidized granular gases in periodic structures
The behavior of a driven granular gas in a container consisting of
connected compartments is studied employing a microscopic kinetic model. After
obtaining the governing equations for the occupation numbers and the granular
temperatures of each compartment we consider the various dynamical regimes. The
system displays interesting analogies with the ordering processes of phase
separating mixtures quenched below the their critical point. In particular, we
show that below a certain value of the driving intensity the populations of the
various compartments become unequal and the system clusterizes. Such a
phenomenon is not instantaneous, but is characterized by a time scale, ,
which follows a Vogel-Vulcher exponential behavior. On the other hand, the
reverse phenomenon which involves the ``evaporation'' of a cluster due to the
driving force is also characterized by a second time scale which diverges at
the limit of stability of the cluster.Comment: 11 pages, 17 figure
Roughness-induced critical phenomena in a turbulent flow
I present empirical evidence that turbulent flows are closely analogous to
critical phenomena, from a reanalysis of friction factor measurements in rough
pipes. The data collapse found here corresponds to Widom scaling near critical
points, and implies that a full understanding of turbulence requires explicit
accounting for boundary roughness
Travelling waves in pipe flow
A family of three-dimensional travelling waves for flow through a pipe of
circular cross section is identified. The travelling waves are dominated by
pairs of downstream vortices and streaks. They originate in saddle-node
bifurcations at Reynolds numbers as low as 1250. All states are immediately
unstable. Their dynamical significance is that they provide a skeleton for the
formation of a chaotic saddle that can explain the intermittent transition to
turbulence and the sensitive dependence on initial conditions in this shear
flow.Comment: 4 pages, 5 figure
Macroscopic effects of the spectral structure in turbulent flows
Two aspects of turbulent flows have been the subject of extensive, split
research efforts: macroscopic properties, such as the frictional drag
experienced by a flow past a wall, and the turbulent spectrum. The turbulent
spectrum may be said to represent the fabric of a turbulent state; in practice
it is a power law of exponent \alpha (the "spectral exponent") that gives the
revolving velocity of a turbulent fluctuation (or "eddy") of size s as a
function of s. The link, if any, between macroscopic properties and the
turbulent spectrum remains missing. Might it be found by contrasting the
frictional drag in flows with differing types of spectra? Here we perform
unprecedented measurements of the frictional drag in soap-film flows, where the
spectral exponent \alpha = 3 and compare the results with the frictional drag
in pipe flows, where the spectral exponent \alpha = 5/3. For moderate values of
the Reynolds number Re (a measure of the strength of the turbulence), we find
that in soap-film flows the frictional drag scales as Re^{-1/2}, whereas in
pipe flows the frictional drag scales as Re^{-1/4} . Each of these scalings may
be predicted from the attendant value of \alpha by using a new theory, in which
the frictional drag is explicitly linked to the turbulent spectrum. Our work
indicates that in turbulence, as in continuous phase transitions, macroscopic
properties are governed by the spectral structure of the fluctuations.Comment: 6 pages, 3 figure
Climate change in the oceans: evolutionary versus phenotypically plastic responses of marine animals and plants
I summarize marine studies on plastic versus adaptive responses to global change. Due to the lack of time series, this review focuses largely on the potential for adaptive evolution in marine animals and plants. The approaches were mainly synchronic comparisons of phenotypically divergent populations, substituting spatial contrasts in temperature or CO2 environments for temporal changes, or in assessments of adaptive genetic diversity within populations for traits important under global change. The available literature is biased towards gastropods, crustaceans, cnidarians and macroalgae. Focal traits were mostly environmental tolerances, which correspond to phenotypic buffering, a plasticity type that maintains a functional phenotype despite external disturbance. Almost all studies address coastal species that are already today exposed to fluctuations in temperature, pH and oxygen levels. Recommendations for future research include (i) initiation and analyses of observational and experimental temporal studies encompassing diverse phenotypic traits (including diapausing cues, dispersal traits, reproductive timing, morphology) (ii) quantification of nongenetic trans-generational effects along with components of additive genetic variance (iii) adaptive changes in microbe–host associations under the holobiont model in response to global change (iv) evolution of plasticity patterns under increasingly fluctuating environments and extreme conditions and (v) joint consideration of demography and evolutionary adaptation in evolutionary rescue approaches
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