39 research outputs found
Specific Heat of Liquid Helium in Zero Gravity very near the Lambda Point
We report the details and revised analysis of an experiment to measure the
specific heat of helium with subnanokelvin temperature resolution near the
lambda point. The measurements were made at the vapor pressure spanning the
region from 22 mK below the superfluid transition to 4 uK above. The experiment
was performed in earth orbit to reduce the rounding of the transition caused by
gravitationally induced pressure gradients on earth. Specific heat measurements
were made deep in the asymptotic region to within 2 nK of the transition. No
evidence of rounding was found to this resolution. The optimum value of the
critical exponent describing the specific heat singularity was found to be a =
-0.0127+ - 0.0003. This is bracketed by two recent estimates based on
renormalization group techniques, but is slightly outside the range of the
error of the most recent result. The ratio of the coefficients of the leading
order singularity on the two sides of the transition is A+/A- =1.053+ - 0.002,
which agrees well with a recent estimate. By combining the specific heat and
superfluid density exponents a test of the Josephson scaling relation can be
made. Excellent agreement is found based on high precision measurements of the
superfluid density made elsewhere. These results represent the most precise
tests of theoretical predictions for critical phenomena to date.Comment: 27 Pages, 20 Figure
Formation of Large-Scale Semi-Organized Structures in Turbulent Convection
A new mean-field theory of turbulent convection is developed. This theory
predicts the convective wind instability in a shear-free turbulent convection
which causes formation of large-scale semi-organized fluid motions in the form
of cells or rolls. Spatial characteristics of these motions, such as the
minimum size of the growing perturbations and the size of perturbations with
the maximum growth rate, are determined. This study predicts also the existence
of the convective shear instability in a sheared turbulent convection which
results in generation of convective shear waves with a nonzero hydrodynamic
helicity. Increase of shear promotes excitation of the convective shear
instability. Applications of the obtained results to the atmospheric turbulent
convection and the laboratory experiments on turbulent convection are
discussed. This theory can be applied also for the describing a mesogranular
turbulent convection in astrophysics.Comment: 16 pages, 10 figures, REVTEX4, PHYSICAL REVIEW E, v. 67, in press
(2003