2,418 research outputs found
The hydraulic bump: The surface signature of a plunging jet
When a falling jet of fluid strikes a horizontal fluid layer, a hydraulic
jump arises downstream of the point of impact provided a critical flow rate is
exceeded. We here examine a phenomenon that arises below this jump threshold, a
circular deflection of relatively small amplitude on the free surface, that we
call the hydraulic bump. The form of the circular bump can be simply understood
in terms of the underlying vortex structure and its height simply deduced with
Bernoulli arguments. As the incoming flux increases, a breaking of axial
symmetry leads to polygonal hydraulic bumps. The relation between this
polygonal instability and that arising in the hydraulic jump is discussed. The
coexistence of hydraulic jumps and bumps can give rise to striking nested
structures with polygonal jumps bound within polygonal bumps. The absence of a
pronounced surface signature on the hydraulic bump indicates the dominant
influence of the subsurface vorticity on its instability
Comparison of SUSY spectrum calculations and impact on the relic density constraints from WMAP
We compare results of four public supersymmetric (SUSY) spectrum codes,
Isajet, Softsusy, Spheno and Suspect to estimate the present-day uncertainty in
the calculation of the relic density of dark matter in mSUGRA models. We find
that even for mass differences of about 1% the spread in the obtained relic
densities can be 10%. In difficult regions of the parameter space, such as
large tan(beta) or large m_0, discrepancies in the relic density are much
larger. We also find important differences in the stau co-annihilation region.
We show the impact of these uncertainties on the bounds from WMAP for several
scenarios, concentrating on the regions of parameter space most relevant for
collider phenomenology. We also discuss the case of non-zero A_0 and the stop
co-annihilation region. Moreover, we present a web application for the online
comparison of the spectrum codes.Comment: 26 pages, 6 figures, 10 tables; version to appear in PR
General Relativistic Flux Modulations from Disk Instabilities in Sagittarius A*
Near-IR and X-ray flares have been detected from the supermassive black hole
Sgr A* at the center of our Galaxy with a (quasi)-period of ~17-20 minutes,
suggesting an emission region only a few Schwarzschild radii above the event
horizon. The latest X-ray flare, detected with XMM-Newton, is notable for its
detailed lightcurve, yielding not only the highest quality period thus far, but
also important structure reflecting the geometry of the emitting region. Recent
MHD simulations of Sgr A*'s disk have demonstrated the growth of a Rossby wave
instability, that enhances the accretion rate for several hours, possibly
accounting for the observed flares. In this Letter, we carry out ray-tracing
calculations in a Schwarzschild metric to determine as accurately as possible
the lightcurve produced by general relativistic effects during such a
disruption. We find that the Rossby wave induced spiral pattern in the disk is
an excellent fit to the data, implying a disk inclination angle of ~77 deg.
Note, however, that if this association is correct, the observed period is not
due to the underlying Keplerian motion but, rather, to the pattern speed. The
favorable comparison between the observed and simulated lightcurves provides
important additional evidence that the flares are produced in Sgr A*'s inner
disk.Comment: 5 Pages, 3 Figures, accepted for publication in ApJ Lette
The role of astroglia in neuroprotection
Astrocytes are the main neural cell type responsible for the maintenance of brain homeostasis. They form highly organized anatomical domains that are interconnected into extensive networks. These features, along with the expression of a wide array of receptors, transporters, and ion channels, ideally position them to sense and dynamically modulate neuronal activity. Astrocytes cooperate with neurons on several levels, including neurotransmitter trafficking and recycling, ion homeostasis, energy metabolism, and defense against oxidative stress. The critical dependence of neurons upon their constant support confers astrocytes with intrinsic neuroprotective properties which are discussed here. Conversely, pathogenic stimuli may disturb astrocytic function, thus compromising neuronal functionality and viability. Using neuroinflammation, Alzheimer's disease, and hepatic encephalopathy as examples, we discuss how astrocytic defense mechanisms may be overwhelmed in pathological conditions, contributing to disease progression
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