105 research outputs found
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
Dynamics of spectrally truncated inviscid turbulence
The evolution of the turbulent energy spectrum for the inviscid spectrally
truncated Euler equations is studied by closure calculations. The observed
behavior is similar to the one found in direct numerical simulations
[Cichowlas, Bona\"ititi, Debbasch, and Brachet, Phys. Rev. Lett. 95, 264502
(2005)]. A Kolmogorov spectral range and an equipartition range are observed
simultaneously. Between these two ranges a "quasi-dissipative" zone is present
in the kinetic energy spectrum. The time evolution of the wave number that
marks the beginning of the equipartition range is analyzed and it is shown that
spectral nonlocal interactions are governing this evolution
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
Space-local Navier--Stokes turbulence
We investigate the physical-space locality of interactions in
three-dimensional incompressible turbulent flow. To that, we modify the
nonlinear terms of the vorticity equation such that the vorticity field is
advected and stretched by the locally induced velocity. This space-local
velocity field is defined by the truncated Biot--Savart law, where only the
neighboring vorticity field in a sphere of radius is integrated. We conduct
direct numerical simulations of the space-local system to investigate its
statistics in the inertial range. We observe a standard
scaling of the energy spectrum associated with an energy cascade for scales
smaller than the space-local domain size . This result is
consistent with the assumption Kolmogorov's 1941 paper made for the
space-locality of the nonlinear interactions. The enstrophy production is
suppressed for larger scales , and for these scales, the system
exhibits a scaling consistent with a conservative enstrophy cascade.Comment: 22 pages, 8 figure
Minimal modeling of the intrinsic cycle of turbulence driven by steady forcing
Quasi-Cyclic Behavior (QCB) is a common feature of various laminar and
turbulent flows. We conduct Direct Numerical Simulations (DNS) of
three-dimensional flow driven by the steady Taylor--Green forcing to find a
silent similarity between a stable periodic flow at a small Reynolds number
() and turbulent QCB at higher . These two temporal
dynamics are continuously connected by varying . A close
examination of the periodic flow allows the formulation of a simple
three-equation model, representing the evolution of Fourier modes in three
distinct scales. The model reproduces the continuously connected periodic
solution and QCB when is varied. We find that non-local triad
interactions are necessary to maintain the periodic solution and QCB.
Bifurcation analyses illustrate that the model can also reproduce several
critical features of turbulence, such as sudden relaminarization of transient
chaos. These findings suggest that the model is not specific to the studied
flow in a periodic domain but is of more general importance in investigating
turbulence in different flow configurations
Rapid generation of angular momentum in bounded magnetized plasma
Direct numerical simulations of two-dimensional decaying MHD turbulence in
bounded domains show the rapid generation of angular momentum in
nonaxisymmetric geometries. It is found that magnetic fluctuations enhance this
mechanism. On a larger time scale, the generation of a magnetic angular
momentum, or angular field, is observed. For axisymmetric geometries, the
generation of angular momentum is absent; nevertheless, a weak magnetic field
can be observed. The derived evolution equations for both the angular momentum
and angular field yield possible explanations for the observed behavior
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