61 research outputs found
Observation of acoustic turbulence in a system of nonlinear second sound waves in superfluid 4He.
We discuss the results of recent studies of acoustic turbulence in a system of nonlinear second sound waves in a high-quality resonator filled with superfluid 4He. It was found that, when the driving amplitude was sufficiently increased, a steady-state direct wave cascade is formed involving a flux of energy towards high frequencies. The wave amplitude distribution follows a power law over a wide range of frequencies. Development of a decay instability at high driving amplitudes results in the formation of subharmonics of the driving frequency, and to a backflow of energy towards the low-frequency spectral domain, in addition to the direct cascade
Nanotube-based source of charges for experiments with solid helium at low temperatures
Methods of preparation of the field-emission sources of charges from carbon nanotubes suitable for study of injected
charges in solid helium at low temperatures T < 1 K are presented. The sources have been prepared by arc
discharge deposition of nanotubes onto a flat copper substrate or by mechanical rubbing of nanotubes into porous
metal surface. The test study of the voltage-current characteristics of a diode cell with the nanotube source in superfluid
He II have shown that at voltages above 120 V one can observe a relatively large current I ā„ 10ā»Ā¹Ā³ A of negative
charges in liquid helium. The field and temperature dependences of positive and negative currents in solid ā“He
were studied in samples grown by the blocked capillary technique. Usage of the nanotube based source of injected
charges had permitted us for the first time to observe motion of the positive charges in solid helium at temperatures
below 0.1 Š. The current-voltage dependence could be described by a power law I ~ UĪ±, with the value of the exponent Ī± >> 2, much higher than what one would expect for the regime of space charge limited currents
Experiments on wave turbulence : the evolution and growth of second sound acoustic turbulence in superfluid 4He confirm self-similarity.
We report our experiments on the formation of second sound acoustic turbulence in superfluid 4He. The initial growth in spectral amplitude follows power laws that steepen rapidly with increasing harmonic number n, corresponding to a propagating front in frequency space. The lower growth exponents agree well with analytic predictions and numerical modeling. The observed increase in the formation delay with n validates the concept of selfsimilarity in the growth of wave turbulence
Decay of the turbulent cascade of capillary waves on the charged surface of liquid hyrdrogen.
We study the free decay of capillary turbulence on the charged surface of liquid hydrogen. We find that the decay begins from the high frequency spectral domains of the surface oscillations and is of a quasi-adiabatic character. The characteristic relaxation time of the whole turbulent cascade is close to the viscous damping time for capillary waves of frequency equal to the driving frequency
Structural and phase transitions in nanocluster ethanol samples at low temperatures
Results of neutron (SANS study) and x-ray diffraction experiments with nanocluster samples of deuteroethanol
(CāDā
OD) or ordinary pure ethanol (CāHā
OH) are presented. A deuterated ethanol sample, formed via
quick cooling of ethanolāhelium mixture down to 1.6 K, had clusters with the size of d ~ 20ā30 nm at liquid helium
temperatures. After warming up to liquid nitrogen temperatures the gel decays into an amorphous white
powder. It was observed that these powder samples remained in the amorphous state even after keeping
at T ā¤ 90 K for a long time (a few months). The neutron studies were supported by further x-ray investigations
of the structure and the phase transitions in the highly dispersed powder samples, which were created via the decay
of the gel samples of ordinary ethanol at temperatures above liquid nitrogen up to 150 K at saturated nitrogen
gas pressure. Annealing of the āgelā sample during half an hour at a temperature of T ~ 110 K resulted
in a phase transition to a monoclinic phase with the crystallite sizes ~30ā40 nm. For comparison we studied
the structure and phase transitions in ābulkā samples, prepared via quick freezing of liquid ethanol down
to liquid nitrogen temperature. The ābulkā sample had a similar transition at T ~ 125 K, which is by 15 K higher
than the temperature of the intensive phase transition in the āgelā sample. The mean grain size in the bulk material
was d ā„ 60 nm
Observation of an Inverse Energy Cascade in Developed Acoustic Turbulence in Superfluid Helium
We report observation of an inverse energy cascade in second sound acoustic
turbulence in He II. Its onset occurs above a critical driving energy and it is
accompanied by giant waves that constitute an acoustic analogue of the rogue
waves that occasionally appear on the surface of the ocean. The theory of the
phenomenon is developed and shown to be in good agreement with the experiments.Comment: 4 pages, 5 figures. The final version just prior to publicatio
Development of intuitive rules: Evaluating the application of the dual-system framework to understanding children's intuitive reasoning
This is an author-created version of this article. The original source of publication is Psychon Bull Rev. 2006 Dec;13(6):935-53
The final publication is available at www.springerlink.com
Published version: http://dx.doi.org/10.3758/BF0321390
Acoustic Turbulence in Superfluid 4He.
Recent work on nonlinear second sound wave propagation and acoustic turbulence in superfluid 4He is reviewed. Observations of direct and inverse turbulent energy cascades are described. The direct cascade arises due to the huge nonlinear dependence of the second sound wave velocity on its amplitude. The flux of energy injected at the driving frequency is transformed via successively higher harmonics until it is eventually attenuated by viscous dissipation at the short wavelength edge of the spectrum. The onset of the inverse cascade occurs above a critical driving energy density, and it is accompanied by giant waves that constitute an acoustic analogue of the rogue waves that occasionally appear on the surface of the ocean. The theory of the phenomena is outlined and shown to be in good agreement with the experiments
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