18,398 research outputs found
The effects of forcing and dissipation on phase transitions in thin granular layers
Recent experimental and computational studies of vibrated thin layers of
identical spheres have shown transitions to ordered phases similar to those
seen in equilibrium systems. Motivated by these results, we carry out
simulations of hard inelastic spheres forced by homogenous white noise. We find
a transition to an ordered state of the same symmetry as that seen in the
experiments, but the clear phase separation observed in the vibrated system is
absent. Simulations of purely elastic spheres also show no evidence for phase
separation. We show that the energy injection in the vibrated system is
dramatically different in the different phases, and suggest that this creates
an effective surface tension not present in the equilibrium or randomly forced
systems. We do find, however, that inelasticity suppresses the onset of the
ordered phase with random forcing, as is observed in the vibrating system, and
that the amount of the suppression is proportional to the degree of
inelasticity. The suppression depends on the details of the energy injection
mechanism, but is completely eliminated when inelastic collisions are replaced
by uniform system-wide energy dissipation.Comment: 10 pages, 5 figure
Variations of the solar granulation motions with height using the GOLF/SoHO experiment
Below 1 mHz, the power spectrum of helioseismic velocity measurements is
dominated by the spectrum of convective motions (granulation and
supergranulation) making it difficult to detect the low-order acoustic modes
and the gravity modes. We want to better understand the behavior of solar
granulation as a function of the observing height in the solar atmosphere and
with magnetic activity during solar cycle 23. We analyze the Power Spectral
Density (PSD) of eleven years of GOLF/SOHO velocity-time series using a
Harvey-type model to characterize the properties of the convective motions in
the solar oscillation power spectrum. We study then the evolution of the
granulation with the altitude in the solar atmosphere and with the solar
activity. First, we show that the traditional use of a lorentzian profile to
fit the envelope of the p modes is not well suitable for GOLF data. Indeed, to
properly model the solar spectrum, we need a second lorentzian profile. Second,
we show that the granulation clearly evolves with the height in the photosphere
but does not present any significant variation with the activity cycle.Comment: Paper accepted in A&A. 7 pages, 4 figures, 2 table
Small bowel and liver/small bowel transplantation in children.
A clinical trial of intestinal transplantation was initiated at the University of Pittsburgh in May 1990. Eleven children received either a combined liver/small bowel graft (n = 8) or an isolated small bowel graft (n = 3). Induction as well as maintenance immunosuppression was with FK-506 and steroids. Four patients were male, and seven were female; the age range was 6 months to 10.2 years. There were 3 deaths (all in recipients of the combined liver/small bowel graft), which were attributed to graft-versus-host disease (n = 1), posttransplant lymphoproliferative disease (n = 1), and biliary leak (n = 1). Transplantation of the intestine has evolved into a feasible operation, with an overall patient and graft survival rate of 73%. These survivors are free of total parenteral nutrition, and the majority are home. These encouraging results justify further clinical trials
Sensitivity analysis of the solar rotation to helioseismic data from GONG, GOLF and MDI observations
Accurate determination of the rotation rate in the radiative zone of the sun
from helioseismic observations requires rotational frequency splittings of
exceptional quality as well as reliable inversion techniques. We present here
inferences based on mode parameters calculated from 2088-days long MDI, GONG
and GOLF time series that were fitted to estimate very low frequency rotational
splittings (nu < 1.7 mHz). These low frequency modes provide data of
exceptional quality, since the width of the mode peaks is much smaller than the
rotational splitting and hence it is much easier to separate the rotational
splittings from the effects caused by the finite lifetime and the stochastic
excitation of the modes. We also have implemented a new inversion methodology
that allows us to infer the rotation rate of the radiative interior from mode
sets that span l=1 to 25. Our results are compatible with the sun rotating like
a rigid solid in most of the radiative zone and slowing down in the core (R_sun
< 0.2). A resolution analysis of the inversion was carried out for the solar
rotation inverse problem. This analysis effectively establishes a direct
relationship between the mode set included in the inversion and the sensitivity
and information content of the resulting inferences. We show that such an
approach allows us to determine the effect of adding low frequency and low
degree p-modes, high frequency and low degree p-modes, as well as some g-modes
on the derived rotation rate in the solar radiative zone, and in particular the
solar core. We conclude that the level of uncertainties that is needed to infer
the dynamical conditions in the core when only p-modes are included is unlikely
to be reached in the near future, and hence sustained efforts are needed
towards the detection and characterization of g-modes.Comment: Accepted for publication in Astrophysical journal. 15 pages, 19
figure
- …