960 research outputs found
Transition to hydrodynamics in colliding fermion clouds
We study the transition from the collisionless to the hydrodynamic regime in
a two-component spin-polarized mixture of 40K atoms by exciting its dipolar
oscillation modes inside harmonic traps. The time evolution of the mixture is
described by the Vlasov-Landau equations and numerically solved with a fully
three-dimensional concurrent code. We observe a master/slave behaviour of the
oscillation frequencies depending on the dipolar mode that is excited.
Regardless of the initial conditions, the transition to hydrodynamics is found
to shift to lower values of the collision rate as temperature decreases.Comment: 11 pages, iop style. submitted to the proceedings of the Levico 2003
worksho
Multiscale anisotropic fluctuations in sheared turbulence with multiple states
We use high resolution direct numerical simulations to study the anisotropic
contents of a turbulent, statistically homogeneous flow with random transitions
among multiple energy containing states. We decompose the velocity correlation
functions on different sectors of the three dimensional group of rotations,
SO(3), using a high-precision quadrature. Scaling properties of anisotropic
components of longitudinal and transverse velocity fluctuations are accurately
measured at changing Reynolds numbers. We show that independently of the
anisotropic content of the energy containing eddies, small-scale turbulent
fluctuations recover isotropy and universality faster than previously reported
in experimental and numerical studies. The discrepancies are ascribed to the
presence of highly anisotropic contributions that have either been neglected or
measured with less accuracy in the foregoing works. Furthermore, the anomalous
anisotropic scaling exponents are devoid of any sign of saturation with
increasing order. Our study paves the way to systematically assess persistence
of anisotropy in high Reynolds number flows.Comment: 6 pages, 5 figure
Shear-Improved Smagorinsky Model for Large-Eddy Simulation of Wall-Bounded Turbulent Flows
A shear-improved Smagorinsky model is introduced based on recent results
concerning shear effects in wall-bounded turbulence by Toschi et al. (2000).
The Smagorinsky eddy-viscosity is modified subtracting the magnitude of the
mean shear from the magnitude of the instantaneous resolved strain-rate tensor.
This subgrid-scale model is tested in large-eddy simulations of plane-channel
flows at two different Reynolds numbers. First comparisons with the dynamic
Smagorinsky model and direct numerical simulations, including mean velocity,
turbulent kinetic energy and Reynolds stress profiles, are shown to be
extremely satisfactory. The proposed model, in addition of being physically
sound, has a low computational cost and possesses a high potentiality of
generalization to more complex non-homogeneous turbulent flows.Comment: 10 pages, 6 figures, added some reference
Effects of collisions against thermal impurities in the dynamics of a trapped fermion gas
We present a theoretical study of the dynamical behavior of a gas made of
ultracold fermionic atoms, which during their motions can collide with a much
smaller number of thermal bosonic impurities. The atoms are confined inside
harmonic traps and the interactions between the two species are treated as due
to s-wave scattering with a negative scattering length modeling the 40K-87Rb
fermion-boson system. We set the fermions into motion by giving a small shift
to their trap center and examine two alternative types of initial conditions,
referring to (i) a close-to-equilibrium situation in which the two species are
at the same temperature (well below the Fermi temperature and well above the
Bose-Einstein condensation temperature); and (ii) a far-from-equilibrium case
in which the impurities are given a Boltzmann distribution of momenta while the
fermions are at very low temperatures. The dynamics of the gas is evaluated by
the numerical solution of the Vlasov-Landau equations for the one-body
distribution functions, supported by some analytical results on the collisional
properties of a fermion gas. We find that the trapped gaseous mixture is close
to the collisionless regime for values of the parameters corresponding to
current experiments, but can be driven towards a collisional regime even
without increasing the strength of the interactions, either by going over to
heavier impurity masses or by matching the width of the momentum distribution
of the impurities to the Fermi momentum of the fermion gas.Comment: 7 pages, 4 figures, RevTeX 4, accepted in PR
“Ruminant Placental Adaptation in Early Maternal Undernutrition: An Overview”
Correct placental development during early gestation is considered the main determinant of fetal growth in late pregnancy. A reduction in maternal nourishment occurring across the early developmental window has been linked to a wide range of pregnancy disorders affecting placental transport capacity and consequently the fetal nutrient supply line, with long-term implications for offspring health and productivity. In livestock, ruminant species specifically experience maternal undernutrition in extensive systems due to seasonal changes in food availability, with significant economic losses for the farmer in some situations. In this review, we aim to discuss the effects of reduced maternal nutrition during early pregnancy on placental development with a specific focus on ruminant placenta physiology. Different types of placental adaptation strategies were examined, also considering the potential effects on the epigenetic landscape, which is known to undergo extensive reprogramming during early mammalian development. We also discussed the involvement of autophagy as a cellular degradation mechanism that may play a key role in the placental response to nutrient deficiency mediated by mammalian target of rapamycin, named the mTOR intracellular pathway
- …