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Supersonic Shear Instabilities in Astrophysical Boundary Layers
Disk accretion onto weakly magnetized astrophysical objects often proceeds
via a boundary layer that forms near the object's surface, in which the
rotation speed of the accreted gas changes rapidly. Here we study the initial
stages of formation for such a boundary layer around a white dwarf or a young
star by examining the hydrodynamical shear instabilities that may initiate
mixing and momentum transport between the two fluids of different densities
moving supersonically with respect to each other. We find that an initially
laminar boundary layer is unstable to two different kinds of instabilities. One
is an instability of a supersonic vortex sheet (implying a discontinuous
initial profile of the angular speed of the gas) in the presence of gravity,
which we find to have a growth rate of order (but less than) the orbital
frequency. The other is a sonic instability of a finite width, supersonic shear
layer, which is similar to the Papaloizou-Pringle instability. It has a growth
rate proportional to the shear inside the transition layer, which is of order
the orbital frequency times the ratio of stellar radius to the boundary layer
thickness. For a boundary layer that is thin compared to the radius of the
star, the shear rate is much larger than the orbital frequency. Thus, we
conclude that sonic instabilities play a dominant role in the initial stages of
nonmagnetic boundary layer formation and give rise to very fast mixing between
disk gas and stellar fluid in the supersonic regime.Comment: 35 pages, 6 figures, submitted to Ap
The Properties of G-modes in Layered Semi-Convection
We study low frequency waves that propagate in a region of layered
semi-convection. Layered semi-convection is predicted to be present in stellar
and planetary interiors and can significantly modify the rate of thermal and
compositional mixing. We derive a series of analytical dispersion relations for
plane-parallel layered semi-convection in the Boussinesq approximation using a
matrix transfer formalism. We find that like a continuously stratified medium,
a semi-convective staircase -- in which small convective regions are separated
by sharp density jumps -- supports internal gravity waves (g-modes). When the
wavelength is much longer than the distance between semi-convective steps,
these behave nearly like g-modes in a continuously stratified medium. However,
the g-mode period spacing in a semi-convective region is systematically {\em
smaller} than in a continuously stratified medium, and it decreases with
decreasing mode frequency. When the g-mode wavelength becomes comparable to the
distance between semi-convective steps, the g-mode frequencies deviate
significantly from those of a continuously stratified medium (the frequencies
are higher). G-modes with vertical wavelengths smaller than the distance
between semi-convective steps are evanescent and do not propagate in the
staircase. Thus, there is a lower cutoff frequency for a given horizontal
wavenumber. We generalize our results to gravito-inertial waves relevant for
rapidly rotating stars and planets. Finally, we assess the prospects for
detecting layered semi-convection using astero/planetary seismology.Comment: 13 pages, 5 figures, accepted to MNRA
Threshold of microvascular occlusion: injury size defines the thrombosis scenario
Damage to the blood vessel triggers formation of a hemostatic plug, which is
meant to prevent bleeding, yet the same phenomenon may result in a total
blockade of a blood vessel by a thrombus, causing severe medical conditions.
Here, we show that the physical interplay between platelet adhesion and
hemodynamics in a microchannel manifests in a critical threshold behavior of a
growing thrombus. Depending on the size of injury, two distinct dynamic
pathways of thrombosis were found: the formation of a nonocclusive plug, if
injury length does not exceed the critical value, and the total occlusion of
the vessel by the thrombus otherwise. We develop a mathematical model that
demonstrates that switching between these regimes occurs as a result of a
saddle-node bifurcation. Our study reveals the mechanism of self-regulation of
thrombosis in blood microvessels and explains experimentally observed
distinctions between thrombi of different physical etiology. This also can be
useful for the design of platelet-aggregation-inspired engineering solutions.Comment: 7 pages, 5 figures + Supplementary informatio
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