146 research outputs found
Capillary rogue waves
We report the first observation of extreme wave events (rogue waves) in
parametrically driven capillary waves. Rogue waves are observed above a certain
threshold in forcing. Above this threshold, frequency spectra broaden and
develop exponential tails. For the first time we present evidence of strong
four-wave coupling in non-linear waves (high tricoherence), which points to
modulation instability as the main mechanism in rogue waves. The generation of
rogue waves is identified as the onset of a distinct tail in the probability
density function of the wave heights. Their probability is higher than expected
from the measured wave background.Comment: 4 pages, 5 figure
Modulation instability and capillary wave turbulence
Formation of turbulence of capillary waves is studied in laboratory
experiments. The spectra show multiple exponentially decreasing harmonics of
the parametrically excited wave which nonlinearly broaden with the increase in
forcing. Spectral broadening leads to the development of the spectral continuum
which scales as , in agreement with the weak turbulence
theory (WTT) prediction. Modulation instability of capillary waves is shown to
be responsible for the transition from discrete to broadband spectrum. The
instability leads to spectral broadening of the harmonics, randomization of
their phases, it isolates the wave field from the wall, eventually allows the
transition from 4- to 3-wave interactions as the dominant nonlinear process,
thus creating the prerequisites assumed in WTT.Comment: 6 pages, 5 figure
Turbulence damping as a measure of the flow dimensionality
The dimensionality of turbulence in fluid layers determines their properties.
We study electromagnetically driven flows in finite depth fluid layers and show
that eddy viscosity, which appears as a result of three-dimensional motions,
leads to increased bottom damping. The anomaly coefficient, which characterizes
the deviation of damping from the one derived using a quasi-two-dimensional
model, can be used as a measure of the flow dimensionality. Experiments in
turbulent layers show that when the anomaly coefficient becomes high, the
turbulent inverse energy cascade is suppressed. In the opposite limit
turbulence can self-organize into a coherent flow.Comment: 4 pages, 4 figure
Multichannel visible spectroscopy diagnostic for particle transport studies in the H-1 heliac
A multichannel spectroscopy diagnostic has been developed to study cross-field particle transport in the radiation-dominated low-temperature plasmas (Te<100 eV) in the H-1 heliac. The optical setup covers the full plasma minor radius in the poloidal plane collecting light from ten parallel chords arranged tangentially to the flux surfaces. The light collected from the plasma is coupled into optical fibers and through interference filters into photomultipliers. Two such ten-fiber arrays are aligned parallel to one another to allow the simultaneous monitoring of two different spectral lines. The net radial electron particle flux is determined from the continuity equation by integrating over the ionization source term in the steady-state partially ionized plasma. The diagnostic measures the neutral line intensities and their ratios (in case of helium using the line ratio technique) and also measures excited neutral and ion spectral lines (in case of the argon plasma transport studies). A comparative analysis of the radial particle transport in the low- and high-confinement regimes is presented
Three-dimensional fluid motion in Faraday waves: creation of vorticity and generation of two-dimensional turbulence
We study the generation of 2D turbulence in Faraday waves by investigating
the creation of spatially periodic vortices in this system. Measurements which
couple a diffusing light imaging technique and particle tracking algorithms
allow the simultaneous observation of the three-dimensional fluid motion and of
the temporal changes in the wave field topography. Quasi-standing waves are
found to coexist with a spatially extended fluid transport. More specifically,
the destruction of regular patterns of oscillons coincides with the emergence
of a complex fluid motion whose statistics are similar to that of
two-dimensional turbulence. We reveal that a lattice of oscillons generates
vorticity at the oscillon scale in the horizontal flow. The interaction of
these vortices explain how 2D turbulence is fueled by almost standing waves.
Remarkably, the curvature of Lagrangian trajectories reveals a "footprint" of
the forcing scale vortices in fully developed turbulence. 2D Navier-Stokes
turbulence should be considered a source of disorder in Faraday waves. These
findings also provide a new paradigm for vorticity creation in 2D flows
Strong ExB shear flows in the pedestal region in H-mode plasma
We report the first experimental observation of stationary zonal flows in the
pedestal region of the H-mode plasma in the H-1 toroidal heliac. Strong peaks
in E_r shear mark the top and foot of the density pedestal. Strong m=n=0
low-frequency (f < 0.6 kHz) zonal flows are observed in regions of increased
E_r, suggesting substantial contribution of zonal flows to the spatial
modulation of E_r radial profiles. Radial localization of zonal flows is
correlated with a region of zero magnetic shear and low-order (7/5) rational
surfaces.Comment: 4 pages, 5 figure
Fluctuation studies using combined Mach/triple probe
A probe consisting of two poloidally separated triple probes and a Mach probe (TMT probe) has been designed and installed on the H-1 heliac to study fluctuations. Mach probes are shown to be sensitive to the fluctuations in the electron density, electron and ion temperatures, and ion drift velocity. If the ion Larmor radius is much larger than the characteristic probe dimension, then the Mach probe is insensitive to the magnetic field. When the Mach probe is oriented such that the two tips are separated radially, it becomes sensitive to the radial velocity of the ions. A model has been devised to allow the above mentioned time-resolved plasma parameters to be reconstructed from the data obtained using the TMT probe. One of the important results of these studies is that ion temperature fluctuations cannot be neglected
Robust inverse energy cascade and turbulence structure in three-dimensional layers of fluid
Here we report the first evidence of the inverse energy cascade in a flow
dominated by 3D motions. Experiments are performed in thick fluid layers where
turbulence is driven electromagnetically. It is shown that if the free surface
of the layer is not perturbed, the top part of the layer behaves as quasi-2D
and supports the inverse energy cascade regardless of the layer thickness. Well
below the surface the cascade survives even in the presence of strong 3D eddies
developing when the layer depth exceeds half the forcing scale. In a bounded
flow at low bottom dissipation, the inverse energy cascade leads to the
generation of a spectral condensate below the free surface. Such coherent flow
can destroy 3D eddies in the bulk of the layer and enforce the flow planarity
over the entire layer thickness.Comment: 8 pages, 6 figure
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