7,906 research outputs found
Liquid acrobatics
We experiment with injecting a continuous stream of gas into a shallow
liquid, similar to how one might blow into a straw placed at the bottom of a
near-empty drink. By varying the angle of the straw (here a metal needle), we
observe a variety of dynamics, which we film using a high-speed camera. Most
noteworthy is an intermediate regime in which cyclical jets erupt from the
air-liquid interface and breakup into air-born droplets. These droplets trace
out a parabolic trajectory and bounce on the air-liquid interface before
eventually coalescing. The shape of each jet, as well as the time between jets,
is remarkably similar and leads to droplets with nearly identical trajectories.
The following article accompanies the linked fluid dynamics video submitted to
the Gallery of Fluid Motion in 2008.Comment: Accompanies video submission to APS DFD 2008 Gallery of Fluid Motion,
low
http://ecommons.library.cornell.edu/bitstream/1813/11469/3/Bird_DFD2008_mpeg1.mpg
, and high resolution
http://ecommons.library.cornell.edu/bitstream/1813/11469/2/Bird_DFD2008_mpeg2.mp
The NASA Electric Propulsion Program
The NASA OAST Propulsion, Power, and Energy Division supports an electric propulsion program aimed at providing benefits to a broad class of missions. Concepts which have the potential to enable or significantly benefit space exploration and exploitation are identified and advanced toward application in the near and far term. This paper summarizes recent program progress in mission/system analysis; in electrothermal, electrostatic, and electromagnetic propulsion technologies; and in propulsion/spacecraft integration
Dissipation in Compressible MHD Turbulence
We report results of a three dimensional, high resolution (up to 512^3)
numerical investigation of supersonic compressible magnetohydrodynamic
turbulence. We consider both forced and decaying turbulence. The model
parameters are appropriate to conditions found in Galactic molecular clouds. We
find that the dissipation time of turbulence is of order the flow crossing time
or smaller, even in the presence of strong magnetic fields. About half the
dissipation occurs in shocks. Weak magnetic fields are amplified and tangled by
the turbulence, while strong fields remain well ordered.Comment: 5 pages, 3 Postscript figures, LaTeX, accepted by Ap.J.Let
The Effect of Resistivity on the Nonlinear Stage of the Magnetorotational Instability in Accretion Disks
We present three-dimensional magnetohydrodynamic simulations of the nonlinear
evolution of the magnetorotational instability (MRI) with a non-zero Ohmic
resistivity. The properties of the saturated state depend on the initial
magnetic field configuration. In simulations with an initial uniform vertical
field, the MRI is able to support angular momentum transport even for large
resistivities through the quasi-periodic generation of axisymmetric radial
channel solutions rather than through the maintenance of anisotropic
turbulence. Simulations with zero net flux show that the angular momentum
transport and the amplitude of magnetic energy after saturation are
significantly reduced by finite resistivity, even at levels where the linear
modes are only slightly affected. This occurs at magnetic Reynolds numbers
expected in low, cool states of dwarf novae, these results suggest that finite
resistivity may account for the low and high angular momentum transport rates
inferred for these systems.Comment: 8 figures, accepted for publication in Ap
Hydrodynamical Non-radiative Accretion Flows in Two-Dimensions
Two-dimensional (axially symmetric) numerical hydrodynamical calculations of
accretion flows which cannot cool through emission of radiation are presented.
The calculations begin from an equilibrium configuration consisting of a thick
torus with constant specific angular momentum. Accretion is induced by the
addition of a small anomalous azimuthal shear stress which is characterized by
a function \nu. We study the flows generated as the amplitude and form of \nu
are varied. A spherical polar grid which spans more than two orders of
magnitude in radius is used to resolve the flow over a wide range of spatial
scales. We find that convection in the inner regions produces significant
outward mass motions that carry away both the energy liberated by, and a large
fraction of the mass participating in, the accretion flow. Although the
instantaneous structure of the flow is complex and dominated by convective
eddies, long time averages of the dynamical variables show remarkable
correspondence to certain steady-state solutions. Near the equatorial plane,
the radial profiles of the time-averaged variables are power-laws with an index
that depends on the radial scaling of the shear stress. We find that regardless
of the adiabatic index of the gas, or the form or magnitude of the shear
stress, the mass inflow rate is a strongly increasing function of radius, and
is everywhere nearly exactly balanced by mass outflow. The net mass accretion
rate through the disc is only a fraction of the rate at which mass is supplied
to the inflow at large radii, and is given by the local, viscous accretion rate
associated with the flow properties near the central object.Comment: 33 pages, 12 figures, accepted by MNRA
Runaway Coalescence at the Onset of Common Envelope Episodes
Luminous red nova transients, presumably from stellar coalescence, exhibit
long-term precursor emission over hundreds of binary orbits, leading to
impulsive outbursts with durations similar to a single orbital period. In an
effort to understand these signatures, we present and analyze a hydrodynamic
model of unstable mass transfer from a giant-star donor onto a more compact
accretor in a binary system. Our simulation begins with mass transfer at the
Roche limit separation and traces a phase of runaway decay leading up to the
plunge of the accretor within the envelope of the donor. We characterize the
fluxes of mass and angular momentum through the system and show that the
orbital evolution can be reconstructed from measurements of these quantities.
The morphology of outflow from the binary changes significantly as the binary
orbit tightens. At wide separations, a thin stream of relatively high-entropy
gas trails from the outer Lagrange points. As the orbit tightens, the orbital
motion desynchronizes from the donor's rotation, and low-entropy ejecta trace a
broad fan of largely ballistic trajectories. An order-of-magnitude increase in
mass ejection rate accompanies the plunge of the accretor with the envelope of
the donor. We argue that this transition marks the precursor-to-outburst
transition observed in stellar coalescence transients.Comment: Revised following peer-review. ApJ accepted. Animated version of
Figure 5 will be available via the Journal's online publicatio
Cosmological Radiation Hydrodynamics with ENZO
We describe an extension of the cosmological hydrodynamics code ENZO to
include the self-consistent transport of ionizing radiation modeled in the
flux-limited diffusion approximation. A novel feature of our algorithm is a
coupled implicit solution of radiation transport, ionization kinetics, and gas
photoheating, making the timestepping for this portion of the calculation
resolution independent. The implicit system is coupled to the explicit
cosmological hydrodynamics through operator splitting and solved with scalable
multigrid methods. We summarize the numerical method, present a verification
test on cosmological Stromgren spheres, and then apply it to the problem of
cosmological hydrogen reionization.Comment: 14 pages, 3 figures, to appear in Recent Directions in Astrophysical
Quantitative Spectroscopy and Radiation Hydrodynamics, Ed. I. Hubeny,
American Institute of Physics (2009
Bound Outflows, Unbound Ejecta, and the Shaping of Bipolar Remnants during Stellar Coalescence
Recent observations have revealed that the remnants of stellar-coalescence
transients are bipolar. This raises the questions of how these bipolar
morphologies arise and what they teach us about the mechanisms of mass ejection
during stellar mergers and common-envelope phases. In this paper, we analyze
hydrodynamic simulations of the lead-in to binary coalescence, a phase of
unstable Roche lobe overflow that takes the binary from the Roche limit
separation to the engulfment of the more compact accretor within the envelope
of the extended donor. As mass transfer runs away at increasing rates, gas
trails away from the binary. Contrary to previous expectations, early mass loss
from the system remains bound to the binary and forms a circumbinary torus.
Later ejecta, generated as the accretor grazes the surface of the donor, have
very different morphologies and are unbound. These two components of mass loss
from the binary interact as later, higher-velocity ejecta collide with the
circumbinary torus formed by earlier mass loss. Unbound ejecta are redirected
toward the poles, and escaping material creates a bipolar outflow. Our findings
show that the transition from bound to unbound ejecta from coalescing binaries
can explain the bipolar nature of their remnants, with implications for our
understanding of the origin of bipolar remnants of stellar-coalescence
transients and, perhaps, some preplanetary nebulae.Comment: ApJ accepte
Shearing Box Simulations of the MRI in a Collisionless Plasma
We describe local shearing box simulations of turbulence driven by the
magnetorotational instability (MRI) in a collisionless plasma. Collisionless
effects may be important in radiatively inefficient accretion flows, such as
near the black hole in the Galactic Center. The MHD version of ZEUS is modified
to evolve an anisotropic pressure tensor. A fluid closure approximation is used
to calculate heat conduction along magnetic field lines. The anisotropic
pressure tensor provides a qualitatively new mechanism for transporting angular
momentum in accretion flows (in addition to the Maxwell and Reynolds stresses).
We estimate limits on the pressure anisotropy due to pitch angle scattering by
kinetic instabilities. Such instabilities provide an effective ``collision''
rate in a collisionless plasma and lead to more MHD-like dynamics. We find that
the MRI leads to efficient growth of the magnetic field in a collisionless
plasma, with saturation amplitudes comparable to those in MHD. In the saturated
state, the anisotropic stress is comparable to the Maxwell stress, implying
that the rate of angular momentum transport may be moderately enhanced in a
collisionless plasma.Comment: 20 pages, 9 figures, submitted to Ap
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