1,026 research outputs found
Clustering and collisions of heavy particles in random smooth flows
Finite-size impurities suspended in incompressible flows distribute
inhomogeneously, leading to a drastic enhancement of collisions. A description
of the dynamics in the full position-velocity phase space is essential to
understand the underlying mechanisms, especially for polydisperse suspensions.
These issues are here studied for particles much heavier than the fluid by
means of a Lagrangian approach. It is shown that inertia enhances collision
rates through two effects: correlation among particle positions induced by the
carrier flow and uncorrelation between velocities due to their finite size. A
phenomenological model yields an estimate of collision rates for particle pairs
with different sizes. This approach is supported by numerical simulations in
random flows.Comment: 12 pages, 9 Figures (revTeX 4) final published versio
Microfósiles calcáreos de la Formación Ranquiles (Cretácico Inferior) en la provincia de Neuquén, Argentina
Fil: Musacchio, Eduardo. Facultad de Ciencias Naturales y Museo; ArgentinaFil: Palamarczuk, Susana C.. Facultad de Ciencias Naturales y Museo; Argentin
Spindle assembly checkpoint: the third decade
The spindle assembly checkpoint controls cell cycle progression during mitosis, synchronizing it with the attachment of chromosomes to spindle microtubules. After the discovery of the mitotic arrest deficient (MAD) and budding uninhibited by benzymidazole (BUB) genes as crucial checkpoint components in 1991, the second decade of checkpoint studies (2001–2010) witnessed crucial advances in the elucidation of the mechanism through which the checkpoint effector, the mitotic checkpoint complex, targets the anaphase-promoting complex (APC/C) to prevent progression into anaphase. Concomitantly, the discovery that the Ndc80 complex and other components of the microtubule-binding interface of kinetochores are essential for the checkpoint response finally asserted that kinetochores are crucial for the checkpoint response. Nevertheless, the relationship between kinetochores and checkpoint control remains poorly understood. Crucial advances in this area in the third decade of checkpoint studies (2011–2020) are likely to be brought about by the characterization of the mechanism of kinetochore recruitment, activation and inactivation of checkpoint proteins, which remains elusive for the majority of checkpoint components. Here, we take a molecular view on the main challenges hampering this task
Endowments, Fiscal Federalism, and the Cost of Capital for States: Evidence from Brazil, 1891-1930
In the last few years there has been an explosion in the number of papers that aim to explain what determines country risk (defined as the difference between the yield of a sovereign’s bonds and the risk free rate). In this paper, we contribute to the discussion using by showing that Brazilian states with natural endowments that allowed them to export commodities that were in high demand (e.g., rubber and coffee) between 1891 and 1930 ended up having higher revenues per capita and, thus, lower cost of capital. The link between exports and state government revenues works in the Brazilian case because of the extreme form of fiscal federalism that the Brazilian government adopted in the Constitution of 1891, giving state governments the sole right to tax exports. We create a panel of state debt risk premia and a series of state level fiscal variables and we show, using OLS, that having specific commodities gave states access capital in better terms (i.e., lower risk premium) in international markets. We also confirm our results that states with better commodities had lower risk premia when we use export price indices for each of the states as instruments for state revenue per capita.
Heavy particle concentration in turbulence at dissipative and inertial scales
Spatial distributions of heavy particles suspended in an incompressible
isotropic and homogeneous turbulent flow are investigated by means of high
resolution direct numerical simulations. In the dissipative range, it is shown
that particles form fractal clusters with properties independent of the
Reynolds number. Clustering is there optimal when the particle response time is
of the order of the Kolmogorov time scale . In the inertial range,
the particle distribution is no longer scale-invariant. It is however shown
that deviations from uniformity depend on a rescaled contraction rate, which is
different from the local Stokes number given by dimensional analysis. Particle
distribution is characterized by voids spanning all scales of the turbulent
flow; their signature in the coarse-grained mass probability distribution is an
algebraic behavior at small densities.Comment: 4 RevTeX pgs + 4 color Figures included, 1 figure eliminated second
part of the paper completely revise
Emergence of chaos in a viscous solution of rods
It is shown that the addition of small amounts of microscopic rods in a
viscous fluid at low Reynolds number causes a significant increase of the flow
resistance. Numerical simulations of the dynamics of the solution reveal that
this phenomenon is associated to a transition from laminar to chaotic flow.
Polymer stresses give rise to flow instabilities which, in turn, perturb the
alignment of the rods. This coupled dynamics results in the activation of a
wide range of scales, which enhances the mixing efficiency of viscous flows.Comment: 4 pages, 5 figures, 1 tabl
Evaluation of Mater Bi and Polylactic Acid as materials for biodegradable innovative mini-radiosondes to track small scale fluctuations within clouds
Turbulence plays an important part in determining the chemical and physical processes, on both the micro- and macro-scales, whereby clouds are formed and behave. However, exactly how these are linked together and how turbulence impacts each of these processes is not yet fully understood. This is partly due to a lack of in-situ small scale fluctuation measurements due to a limitation in the available technology. It is in this context that the radiosondes, for which the material characterisation is presented in this paper, are being developed to generate a Lagrangian set of data which can be used to improve the ever-expanding knowledge of atmospheric processes and, in particular, the understanding of the interaction between turbulence and micro-physical phenomenologies inside clouds (www.complete-h2020network.eu). Specifically, the materials developed for the balloons are discussed in further detail within this paper. Mater Bi and polylactic acid are the two common biodegradable thermoplastics that were used initially to make the balloons. To tailor their properties, the balloons were then coated with carnauba wax blended with either pine resin or SiO2 nanoparticles. The properties such as hydrophobicity, toughness, elasticity and helium gas permeability are investigated and improved in order to keep the density of the radiosondes as constant as possible for a couple of hours. This will allow them to float inside and outside clouds on an isopycnic surface, to measure various properties such as velocity, temperature, pressure and humidity by means of solid state sensors and to transmit them to receivers on Earth. Tests have been made under a rigorous metrological approach comparing the 6 new materials with two reference balloon materials, latex and mylar. It was found that Mater Bi with the two carnauba wax coatings is the most suited though its roughness and water vapour permeability should be improved
Evaluation of Mater Bi and Polylactic Acid as materials for biodegradable innovative mini-radiosondes to track small scale fluctuations within clouds
Turbulence plays an important part in determining the chemical and physical
processes, on both the micro- and macro-scales, whereby clouds are formed and
behave. However, exactly how these are linked together and how turbulence
impacts each of these processes is not yet fully understood. This is partly due
to a lack of in-situ small scale fluctuation measurements due to a limitation
in the available technology. It is in this context that the radiosondes, for
which the material characterisation is presented in this paper, are being
developed to generate a Lagrangian set of data which can be used to improve the
ever-expanding knowledge of atmospheric processes and, in particular, the
understanding of the interaction between turbulence and micro-physical
phenomenologies inside clouds (www.complete-h2020network.eu). Specifically, the
materials developed for the balloons are discussed in further detail within
this paper. Mater Bi and polylactic acid are the two common biodegradable
thermoplastics that were used initially to make the balloons. To tailor their
properties, the balloons were then coated with carnauba wax blended with either
pine resin or SiO_2 nanoparticles. The properties such as hydrophobicity,
toughness, elasticity and helium gas permeability are investigated and improved
in order to keep the density of the radiosondes as constant as possible for a
couple of hours. This will allow them to float inside and outside clouds on an
isopycnic surface, to measure various properties such as velocity, temperature,
pressure and humidity by means of solid state sensors and to transmit them to
receivers on Earth. Tests have been made under a rigorous metrological approach
comparing the 6 new materials with two reference balloon materials, latex and
mylar. It was found that Mater Bi with the two carnaubua wax coatings is the
most suited.
- …