3,171 research outputs found
Indications for the onset of deconfinement in nucleus nucleus collisions
The hadronic final state of central Pb+Pb collisions at 20, 30, 40, 80, and 158 AGeV has been measured by the CERN NA49 collaboration. The mean transverse mass of pions and kaons at midrapidity stays nearly constant in this energy range, whereas at lower energies, at the AGS, a steep increase with beam energy was measured. Compared to p+p collisions as well as to model calculations, anomalies in the energy dependence of pion and kaon production at lower SPS energies are observed. These findings can be explained, assuming that the energy density reached in central A+A collisions at lower SPS energies is sufficient to force the hot and dense nuclear matter into a deconfined phase
Scaling laws for convection and jet speeds in the giant planets
Three-dimensional studies of convection in deep spherical shells have been
used to test the hypothesis that the strong jet streams on Jupiter, Saturn,
Uranus, and Neptune result from convection throughout the molecular envelopes.
Due to computational limitations, these simulations must adopt viscosities and
heat fluxes many orders of magnitude larger than the planetary values. Several
numerical investigations have identified trends for how the mean jet speed
varies with heat flux and viscosity, but no previous theories have been
advanced to explain these trends. Here, we show using simple arguments that if
convective release of potential energy pumps the jets and viscosity damps them,
the mean jet speeds split into two regimes. When the convection is weakly
nonlinear, the equilibrated jet speeds should scale approximately with F/nu,
where F is the convective heat flux and nu is the viscosity. When the
convection is strongly nonlinear, the jet speeds are faster and should scale
approximately as (F/nu)^{1/2}. We demonstrate how this regime shift can
naturally result from a shift in the behavior of the jet-pumping efficiency
with heat flux and viscosity. Moreover, the simulations hint at a third regime
where, at sufficiently small viscosities, the jet speed becomes independent of
the viscosity. We show based on mixing-length estimates that if such a regime
exists, mean jet speeds should scale as heat flux to the 1/4 power. Our
scalings provide a good match to the mean jet speeds obtained in previous
Boussinesq and anelastic, three-dimensional simulations of convection within
giant planets over a broad range of parameters. When extrapolated to the real
heat fluxes, these scalings suggest that the mass-weighted jet speeds in the
molecular envelopes of the giant planets are much weaker--by an order of
magnitude or more--than the speeds measured at cloud level.Comment: 23 pages, 10 figures, in press at Icaru
Resonant Activation of Population Extinctions
Understanding the mechanisms governing population extinctions is of key
importance to many problems in ecology and evolution. Stochastic factors are
known to play a central role in extinction, but the interactions between a
population's demographic stochasticity and environmental noise remain poorly
understood. Here, we model environmental forcing as a stochastic fluctuation
between two states, one with a higher death rate than the other. We find that
in general there exists a rate of fluctuations that minimizes the mean time to
extinction, a phenomenon previously dubbed "resonant activation." We develop a
heuristic description of the phenomenon, together with a criterion for the
existence of resonant activation. Specifically the minimum extinction time
arises as a result of the system approaching a scenario wherein the severity of
rare events is balanced by the time interval between them. We discuss our
findings within the context of more general forms of environmental noise, and
suggest potential applications to evolutionary models.Comment: 12 pages, 7 Figures, Accepted for publication in Physical Review
Jovian vortices and jets
We explore the conditions required for isolated vortices to exist in sheared
zonal flows and the stability of the underlying zonal winds. This is done using
the standard 2-layer quasigeostrophic model with the lower layer depth becoming
infinite; however, this model differs from the usual layer model because the
lower layer is not assumed to be motionless but has a steady configuration of
alternating zonal flows [1]. Steady state vortices are obtained by a simulated
annealing computational method introduced in [2], generalized and applied in
[3] in fluid flow, and used in the context of magnetohydrodynamics in [4-6].
Various cases of vortices with a constant potential vorticity anomaly atop
zonal winds and the stability of the underlying winds are considered using a
mix of computational and analytical techniques
Convectively driven shear and decreased heat flux
We report on direct numerical simulations of two-dimensional, horizontally
periodic Rayleigh-B\'enard convection, focusing on its ability to drive
large-scale horizontal flow that is vertically sheared. For the Prandtl numbers
() between 1 and 10 simulated here, this large-scale shear can be induced
by raising the Rayleigh number () sufficiently, and we explore the
resulting convection for up to . When present in our simulations,
the sheared mean flow accounts for a large fraction of the total kinetic
energy, and this fraction tends towards unity as . The shear helps
disperse convective structures, and it reduces vertical heat flux; in parameter
regimes where one state with large-scale shear and one without are both stable,
the Nusselt number of the state with shear is smaller and grows more slowly
with . When the large-scale shear is present with , the
convection undergoes strong global oscillations on long timescales, and heat
transport occurs in bursts. Nusselt numbers, time-averaged over these bursts,
vary non-monotonically with for . When the shear is present with
, the flow does not burst, and convective heat transport is
sustained at all times. Nusselt numbers then grow roughly as powers of ,
but the growth rates are slower than any previously reported for
Rayleigh-B\'enard convection without large-scale shear. We find the Nusselt
numbers grow proportionally to when and to when
. Analogies with tokamak plasmas are described.Comment: 25 pages, 12 figures, 5 video
Making an IMPACT: Empowering Student via Information Use
Presentation on Purdue\u27s IMPACT program for the First Nations Knowledge Services without Borders Institute gathering at Maskwacis, Alberta in April, 2016
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