8,728 research outputs found
On the unsteady behavior of turbulence models
Periodically forced turbulence is used as a test case to evaluate the
predictions of two-equation and multiple-scale turbulence models in unsteady
flows. The limitations of the two-equation model are shown to originate in the
basic assumption of spectral equilibrium. A multiple-scale model based on a
picture of stepwise energy cascade overcomes some of these limitations, but the
absence of nonlocal interactions proves to lead to poor predictions of the time
variation of the dissipation rate. A new multiple-scale model that includes
nonlocal interactions is proposed and shown to reproduce the main features of
the frequency response correctly
Turbulence and turbulent pattern formation in a minimal model for active fluids
Active matter systems display a fascinating range of dynamical states,
including stationary patterns and turbulent phases. While the former can be
tackled with methods from the field of pattern formation, the spatio-temporal
disorder of the active turbulence phase calls for a statistical description.
Borrowing techniques from turbulence theory, we here establish a quantitative
description of correlation functions and spectra of a minimal continuum model
for active turbulence. Further exploring the parameter space, we also report on
a surprising type of turbulence-driven pattern formation far beyond linear
onset: the emergence of a dynamic hexagonal vortex lattice state after an
extended turbulent transient, which can only be explained taking into account
turbulent energy transfer across scales.Comment: Supplemental videos available at https://youtu.be/gbf6cRho03w
https://youtu.be/n0qUUhAUJFQ https://youtu.be/LGmamkM012
Extreme Lagrangian acceleration in confined turbulent flow
A Lagrangian study of two-dimensional turbulence for two different
geometries, a periodic and a confined circular geometry, is presented to
investigate the influence of solid boundaries on the Lagrangian dynamics. It is
found that the Lagrangian acceleration is even more intermittent in the
confined domain than in the periodic domain. The flatness of the Lagrangian
acceleration as a function of the radius shows that the influence of the wall
on the Lagrangian dynamics becomes negligible in the center of the domain and
it also reveals that the wall is responsible for the increased intermittency.
The transition in the Lagrangian statistics between this region, not directly
influenced by the walls, and a critical radius which defines a Lagrangian
boundary layer, is shown to be very sharp with a sudden increase of the
acceleration flatness from about 5 to about 20
Zonal flow generation and its feedback on turbulence production in drift wave turbulence
Plasma turbulence described by the Hasegawa-Wakatani equations has been
simulated numerically for different models and values of the adiabaticity
parameter C. It is found that for low values of C turbulence remains isotropic,
zonal flows are not generated and there is no suppression of the meridional
drift waves and of the particle transport. For high values of C, turbulence
evolves toward highly anisotropic states with a dominant contribution of the
zonal sector to the kinetic energy. This anisotropic flow leads to a decrease
of a turbulence production in the meridional sector and limits the particle
transport across the mean isopycnal surfaces. This behavior allows to consider
the Hasegawa-Wakatani equations a minimal PDE model which contains the
drift-wave/zonal-flow feedback loop prototypical of the LH transition in plasma
devices.Comment: 14 pages, 7 figure
Reynolds number effect on the velocity increment skewness in isotropic turbulence
Second and third order longitudinal structure functions and wavenumber
spectra of isotropic turbulence are computed using the EDQNM model and compared
to results of the multifractal formalism. At the highest Reynolds number
available in windtunnel experiments, , both the multifractal
model and EDQNM give power-law corrections to the inertial range scaling of the
velocity increment skewness. For EDQNM, this correction is a finite Reynolds
number effect, whereas for the multifractal formalism it is an intermittency
correction that persists at any high Reynolds number. Furthermore, the two
approaches yield realistic behavior of second and third order statistics of the
velocity fluctuations in the dissipative and near-dissipative ranges.
Similarities and differences are highlighted, in particular the Reynolds number
dependence
Sclera solar diameter observations
Focus is given to possible variations in solar luminosity and accurate methods of monitoring it. Aside from direct bolometry, one methodology for this type of research makes use of measurements of the solar diameter and limb darkening function as indirect indicators of the solar luminosity. This approach was reviewed
Origin of Lagrangian Intermittency in Drift-Wave Turbulence
The Lagrangian velocity statistics of dissipative drift-wave turbulence are
investigated. For large values of the adiabaticity (or small collisionality),
the probability density function of the Lagrangian acceleration shows
exponential tails, as opposed to the stretched exponential or algebraic tails,
generally observed for the highly intermittent acceleration of Navier-Stokes
turbulence. This exponential distribution is shown to be a robust feature
independent of the Reynolds number. For small adiabaticity, algebraic tails are
observed, suggesting the strong influence of point-vortex-like dynamics on the
acceleration. A causal connection is found between the shape of the probability
density function and the autocorrelation of the norm of the acceleration
The role of coherent vorticity in turbulent transport in resistive drift-wave turbulence
The coherent vortex extraction method, a wavelet technique for extracting
coherent vortices out of turbulent flows, is applied to simulations of
resistive drift-wave turbulence in magnetized plasma (Hasegawa-Wakatani
system). The aim is to retain only the essential degrees of freedom,
responsible for the transport. It is shown that the radial density flux is
carried by these coherent modes. In the quasi-hydrodynamic regime, coherent
vortices exhibit depletion of the polarization-drift nonlinearity and vorticity
strongly dominates strain, in contrast to the quasiadiabatic regime
Spectral imbalance and the normalized dissipation rate of turbulence
The normalized turbulent dissipation rate is studied in decaying
and forced turbulence by direct numerical simulations, large-eddy simulations,
and closure calculations. A large difference in the values of is
observed for the two types of turbulence. This difference is found at moderate
Reynolds number, and it is shown that it persists at high Reynolds number,
where the value of becomes independent of the Reynolds number, but
is still not unique. This difference can be explained by the influence of the
nonlinear cascade time that introduces a spectral disequilibrium for
statistically nonstationary turbulence. Phenomenological analysis yields simple
analytical models that satisfactorily reproduce the numerical results. These
simple spectral models also reproduce and explain the increase of
at low Reynolds number that is observed in the simulations
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