516 research outputs found
Numerical Study of Velocity Statistics in Steady Counterflow Quantum Turbulence
We investigate the velocity statistics by calculating the Biot--Savart
velocity induced by vortex filaments in steady counterflow turbulence
investigated in a previous study [Phys. Rev. B {\bf 81}, 104511 (2010)]. The
probability density function (PDF) obeys a Gaussian distribution in the
low-velocity region and a power-law distribution in the high-velocity
region. This transition between the two distributions occur at the velocity
characterized by the mean inter-vortex distance. Counterflow turbulence causes
anisotropy of the vortex tangle, which leads to a difference in the PDF for the
velocities perpendicular to and parallel to the counterflow.Comment: 4 pages, 7 figure
Vortex density spectrum of quantum turbulence
The fluctuations of the vortex density in a turbulent quantum fluid are
deduced from local second-sound attenuation measurements. These measurements
are performed with a micromachined open-cavity resonator inserted across a flow
of turbulent He-II near 1.6 K. The power spectrum of the measured vortex line
density is compatible with a (-5/3) power law. The physical interpretation,
still open, is discussed.Comment: Submitted to Europhys. Let
Kolmogorov spectrum of superfluid turbulence: numerical analysis of the Gross-Pitaevskii equation with the small scale dissipation
The energy spectrum of superfluid turbulence is studied numerically by
solving the Gross-Pitaevskii equation. We introduce the dissipation term which
works only in the scale smaller than the healing length, to remove short
wavelength excitations which may hinder the cascade process of quantized
vortices in the inertial range. The obtained energy spectrum is consistent with
the Kolmogorov law.Comment: 4 pages, 4 figures and 1 table. Submitted to American Journal of
Physic
Vortex length, vortex energy and fractal dimension of superfluid turbulence at very low temperature
By assuming a self-similar structure for Kelvin waves along vortex loops with
successive smaller scale features, we model the fractal dimension of a
superfluid vortex tangle in the zero temperature limit. Our model assumes that
at each step the total energy of the vortices is conserved, but the total
length can change. We obtain a relation between the fractal dimension and the
exponent describing how the vortex energy per unit length changes with the
length scale. This relation does not depend on the specific model, and shows
that if smaller length scales make a decreasing relative contribution to the
energy per unit length of vortex lines, the fractal dimension will be higher
than unity. Finally, for the sake of more concrete illustration, we relate the
fractal dimension of the tangle to the scaling exponents of amplitude and
wavelength of a cascade of Kelvin waves.Comment: 12 pages, 1 figur
Spectrum of turbulent Kelvin-waves cascade in superfluid helium
To explain the observed decay of superfluid turbulence at very low
temperature, it has been proposed that a cascade of Kelvin waves (analogous to
the classical Kolmogorov cascade) transfers kinetic energy to length scales
which are small enough that sound can be radiated away. We report results of
numerical simulations of the interaction of quantized vortex filaments. We
observe the development of the Kelvin-waves cascade, and compute the statistics
of the curvature, the amplitude spectrum (which we compare with competing
theories) and the fractal dimension.Comment: 32 pages, 22 figure
Nonlinear Dynamics in a Trapped Atomic Bose--Einstein Condensate Induced by an Oscillating Gaussian Potential
We consider a trapped atomic Bose--Einstein condensate penetrated by a
repulsive Gaussian potential and theoretically investigate the dynamics induced
by oscillating the Gaussian potential. Our study is based on the numerical
calculation of the two-dimensional Gross--Pitaevskii equation. Our calculation
reveals the dependence of the characteristic behavior of the condensate on the
amplitude and frequency of the oscillating potential. These dynamics are deeply
related to the nucleation and dynamics of quantized vortices and solitons. When
the potential oscillates with a large amplitude, it nucleates many vortex pairs
that move away from the potential. When the amplitude of the oscillation is
small, it nucleates solitons through annihilation of vortex pairs. We discuss
three issues concerning the nucleation of vortices. The first is the phase
diagram for the nucleation of vortices and solitons near the oscillating
potential. The second is the mechanism and critical velocity of the nucleation.
The critical velocity of the nucleation is an important issue in quantum
fluids, and we propose a new expression for the velocity containing both the
coherence length and the size of the potential. The third is the divergence of
the nucleation time, which is the time it takes for the potential to nucleate
vortices, near the critical parameters for vortex nucleation.Comment: 15 pages, 16 figure
Thermal dissipation in quantum turbulence
The microscopic mechanism of thermal dissipation in quantum turbulence has
been numerically studied by solving the coupled system involving the
Gross-Pitaevskii equation and the Bogoliubov-de Gennes equation. At low
temperatures, the obtained dissipation does not work at scales greater than the
vortex core size. However, as the temperature increases, dissipation works at
large scales and it affects the vortex dynamics. We successfully obtained the
mutual friction coefficients of the vortex dynamics as functions of
temperature, which can be applied to the vortex dynamics in dilute
Bose-Einstein condensates.Comment: 4 pages, 6 figures, submitted to AP
Turbulent superfluid profiles in a counterflow channel
We have developed a two-dimensional model of quantised vortices in helium II
moving under the influence of applied normal fluid and superfluid in a
counterflow channel. We predict superfluid and vortex-line density profiles
which could be experimentally tested using recently developed visualization
techniques.Comment: 3 double figures, 9 page
Educating for urban sustainability: A transdisciplinary approach
An understanding of sustainability issues should be a key component of degree programmes. It is widely regarded as being a central attribute to professional practice and responsible global citizenship, arguably more so for the training of teachers since they potentially influence their students. This issue was brought to the fore when responsibility for delivering the 'design and the environment' course was transferred to the building discipline at the University of Newcastle in Australia as a result of restructuring. The attractiveness of the subject as an elective, the need to make it accessible to distance learning students and the desirability of applying transdisciplinary approaches to solving environmental problems presented the course designers with both challenges and opportunities, particularly in devising an assessment context within which students from multiple disciplines could be exposed to, and learn from each other's professional environmental evaluation norms. This paper describes an innovative holistic, multi-criteria problem-solving course design that allows a diverse mix of undergraduates to develop a transdisciplinary understanding of sustainability issues through the use of learning contracts. It reports the experiences of staff and students involved with the course, highlighting the beneficial outcomes
Retaining women in a prenatal care randomized controlled trial in Canada: implications for program planning
<p>Abstract</p> <p>Background:</p> <p>Challenges to retention in prenatal care seem to exist under both universal systems of care, as in Canada, and non-universal systems of care, as in the United States. However, among populations being served by a system of publicly funded health care, the barriers are less well understood and universal uptake of prenatal services has not been realized. Determining the characteristics of women who dropped out of a prenatal care randomized controlled trial can help identify those who may need alternate retention and service approaches.</p> <p>Methods:</p> <p>In this study, pregnant women were randomized to: a) current standard of care; b) 'a' plus nursing support; or c) 'b' plus a paraprofessional home visitor. 16% of 2,015 women did not complete all three telephone interviews (197 dropped out and 124 became unreachable). Responders were compared to non-responders on demographics, lifestyle, psychosocial factors, and life events using chi-squared tests. Logistic regression models were constructed using stepwise logistic regression to determine the probability of not completing the prenatal program.</p> <p>Results:</p> <p>Completion rates did not differ by intervention. In comparison to responders, non-responders were more likely to be younger, less educated, have lower incomes, smoke, have low social support, have a history of depression, and have separated or divorced parents (all p < 0.05). Unreachable women were more likely to be single, use drugs, report distress and adverse life events (all p < 0.05). Non-Caucasian women were more likely to drop out (p = 0.002). Logistic regression modeling indicated that independent key risk factors for dropping out were: less than high school education, separated or divorced parents, lower social support, and being non-Caucasian. Pregnant women who were single/separated/divorced, less than 25 years old, had less than high school education, earned less than $40,000 in annual household income, and/or smoked had greater odds of becoming unreachable at some point during pregnancy and not completing the study.</p> <p>Conclusion:</p> <p>Women at risk due to lifestyle and challenging circumstances were difficult to retain in a prenatal care study, regardless of the intervention. For women with complex health, lifestyle and social issues, lack of retention may reflect incongruence between their needs and the program.</p> <p>Trial registration:</p> <p>Current Controlled Trials ISRCTN64070727</p
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