376 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
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
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
Zonal flow generation in the improved confinement mode plasma and its role in confinement bifurcations
Unstable fluctuations develop in the initially quiescent plasma in the improved confinement mode of the H-1 heliac when the radial electric field (Er) shear exceeds some critical value. These unstable Er shear-driven modes are shown to generate zonal-flow-like poloidally symmetric potential structures, similar to those generated in the low confinement mode (Shats M G and Solomon W M 2002 Phys. Rev. Lett. 88 045001). The structures modulate their parent waves, the background Er shear and the fluctuation-driven radial transport. The onset of zonal flows is observed as a precursor to the plasma confinement bifurcation to an even higher confinement regime.We discuss the interband light tunneling in a two-dimensional periodic photonic structure, as studied recently in experiments for optically induced photonic lattices [Trompeter et al., Phys. Rev. Lett. 96, 053903 (2006)]. We identify the Zener tunneling regime at the crossing of two Bloch bands, which occurs in the generic case of a Bragg reflection when the Bloch index crosses the edge of the irreducible Brillouin zone. Similarly, higher-order Zener tunneling involves four Bloch bands when the Bloch index passes through a high-symmetry point on the edge of the Brillouin zone. We derive simple analytical models that describe the tunneling effect, and calculate the corresponding tunneling probabilities
Experimental evidence of self-regulation of fluctuations by time-varying flows
We report the first extended experimental results indicating that radially localized time-varying potential structures, which possess many of the characteristics of zonal flows, are generated by strong fluctuations. Experiments performed in the H-1 heliac show that these poloidally symmetric flows are nonlinearly coupled to other fluctuations and are responsible for significant modifications in fluctuations and in the fluctuation-driven transport
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