378 research outputs found
Experimental system for drilling simulated lunar rock in ultrahigh vacuum
An experimental apparatus designed for studying drillability of hard volcanic rock in a simulated lunar vacuum of 5 x 10 to the minus 10th power torr is described. The engineering techniques used to provide suitable drilling torque inside the ultrahigh vacuum chamber while excluding all hydrocarbon are detailed. Totally unlubricated bearings and gears were used to better approximate the true lunar surface conditions within the ultrahigh vacuum system. The drilling system has a starting torque of 30 in-lb with an unloaded running torque of 4 in-lb. Nominal torque increase during drilling is 4.5 in-lb or a total drilling torque of 8.5 in-lb with a 100-lb load on the drill bit at 210 rpm. The research shows conclusively that it is possible to design operational equipment for moderate loads operating under UHV conditions without the use of sealed bearings or any need of lubricants whatsoever
Surface friction of rock in terrestrial and simulated lunar environments
The conventional probe-on-the rotating-disk concept was used to determine the surface friction in mineral probe/specimen interfaces. Nine rocks or minerals and two stainless steels were tested in both new (NT) and same track (ST) tests under three different pressure environments-atmospheric, UHV, and dry nitrogen. Each environment was further subdivided into two testing conditions, that is, ambient and elevated (135 C) temperatures. In NT tests, friction was the lowest in an atmospheric pressure condition for all rock types and increased to the largest in UHV ambient condition except for pyroxene and stainless steel. Friction values measured in dry nitrogen ambient condition lie between the two extremes. Heating tends to increase friction in atmospheric and dry nitrogen environment but decreases in UHV environment with the exception of stainless steel, basalt, and pyroxene. In ST tests, friction was the lowest in the first run and increased in subsequent runs except for stainless steel where the reverse was true. The increases leveled off after a few runs ranging from the second to the seventh depending on rock types
A localised subgrid scale model for fluid dynamical simulations in astrophysics II: Application to type Ia supernovae
The dynamics of the explosive burning process is highly sensitive to the
flame speed model in numerical simulations of type Ia supernovae. Based upon
the hypothesis that the effective flame speed is determined by the unresolved
turbulent velocity fluctuations, we employ a new subgrid scale model which
includes a localised treatment of the energy transfer through the turbulence
cascade in combination with semi-statistical closures for the dissipation and
non-local transport of turbulence energy. In addition, subgrid scale buoyancy
effects are included. In the limit of negligible energy transfer and transport,
the dynamical model reduces to the Sharp-Wheeler relation. According to our
findings, the Sharp-Wheeler relation is insuffcient to account for the
complicated turbulent dynamics of flames in thermonuclear supernovae. The
application of a co-moving grid technique enables us to achieve very high
spatial resolution in the burning region. Turbulence is produced mostly at the
flame surface and in the interior ash regions. Consequently, there is a
pronounced anisotropy in the vicinity of the flame fronts. The localised
subgrid scale model predicts significantly enhanced energy generation and less
unburnt carbon and oxygen at low velocities compared to earlier simulations.Comment: 13 pages, 10 figures, accepted for publication in Astron. Astrophys.;
3D visualisations not included; complete PDF version can be downloaded from
http://www.astro.uni-wuerzburg.de/%7Eschmidt/Paper/SGSModel_II_AA.pd
Off-center ignition in type Ia supernova: I. Initial evolution and implications for delayed detonation
The explosion of a carbon-oxygen white dwarf as a Type Ia supernova is known
to be sensitive to the manner in which the burning is ignited. Studies of the
pre-supernova evolution suggest asymmetric, off-center ignition, and here we
explore its consequences in two- and three-dimensional simulations. Compared
with centrally ignited models, one-sided ignitions initially burn less and
release less energy. For the distributions of ignition points studied, ignition
within two hemispheres typically leads to the unbinding of the white dwarf,
while ignition within a small fraction of one hemisphere does not. We also
examine the spreading of the blast over the surface of the white dwarf that
occurs as the first plumes of burning erupt from the star. In particular, our
studies test whether the collision of strong compressional waves can trigger a
detonation on the far side of the star as has been suggested by Plewa et al.
(2004). The maximum temperature reached in these collisions is sensitive to how
much burning and expansion has already gone on, and to the dimensionality of
the calculation. Though detonations are sometimes observed in 2D models, none
ever happens in the corresponding 3D calculations. Collisions between the
expansion fronts of multiple bubbles also seem, in the usual case, unable to
ignite a detonation. "Gravitationally confined detonation" is therefore not a
robust mechanism for the explosion. Detonation may still be possible in these
models however, either following a pulsation or by spontaneous detonation if
the turbulent energy is high enough.Comment: 13 pages, 10 figures (resolution of some figures reduced to comply
with astro-ph file size restriction); submitted to the Astrophysical Journal
on 8/3/200
The Cellular Burning Regime in Type Ia Supernova Explosions - I. Flame Propagation into Quiescent Fuel
We present a numerical investigation of the cellular burning regime in Type
Ia supernova explosions. This regime holds at small scales (i.e. below the
Gibson scale), which are unresolved in large-scale Type Ia supernova
simulations. The fundamental effects that dominate the flame evolution here are
the Landau-Darrieus instability and its nonlinear stabilization, leading to a
stabilization of the flame in a cellular shape. The flame propagation into
quiescent fuel is investigated addressing the dependence of the simulation
results on the specific parameters of the numerical setup. Furthermore, we
investigate the flame stability at a range of fuel densities. This is directly
connected to the questions of active turbulent combustion (a mechanism of flame
destabilization and subsequent self-turbulization) and a
deflagration-to-detonation transition of the flame. In our simulations we find
no substantial destabilization of the flame when propagating into quiescent
fuels of densities down to ~10^7 g/cm^3, corroborating fundamental assumptions
of large-scale SN Ia explosion models. For these models, however, we suggest an
increased lower cutoff for the flame propagation velocity to take the cellular
burning regime into account.Comment: 12 pages, 2 tables, 10 figures, resolution of figures degraded due to
archive file size restrictions, submitted to A&
Modeling the Diversity of Type Ia Supernova Explosions
Type Ia supernovae (SNe Ia) are a prime tool in observational cosmology. A
relation between their peak luminosities and the shapes of their light curves
allows to infer their intrinsic luminosities and to use them as distance
indicators. This relation has been established empirically. However, a
theoretical understanding is necessary in order to get a handle on the
systematics in SN Ia cosmology. Here, a model reproducing the observed
diversity of normal SNe Ia is presented. The challenge in the numerical
implementation arises from the vast range of scales involved in the physical
mechanism. Simulating the supernova on scales of the exploding white dwarf
requires specific models of the microphysics involved in the thermonuclear
combustion process. Such techniques are discussed and results of simulations
are presented.Comment: 6 pages, ASTRONUM-2009 "Numerical Modeling of Space Plasma Flows",
Chamonix, France, July 2009, to appear in ASP Conf. Pro
A Common Explosion Mechanism for Type Ia Supernovae
Type Ia supernovae, the thermonuclear explosions of white dwarf stars
composed of carbon and oxygen, were instrumental as distance indicators in
establishing the acceleration of the universe's expansion. However, the physics
of the explosion are debated. Here we report a systematic spectral analysis of
a large sample of well observed type Ia supernovae. Mapping the velocity
distribution of the main products of nuclear burning, we constrain theoretical
scenarios. We find that all supernovae have low-velocity cores of stable
iron-group elements. Outside this core, nickel-56 dominates the supernova
ejecta. The outer extent of the iron-group material depends on the amount of
nickel-56 and coincides with the inner extent of silicon, the principal product
of incomplete burning. The outer extent of the bulk of silicon is similar in
all SNe, having an expansion velocity of ~11000 km/s and corresponding to a
mass of slightly over one solar mass. This indicates that all the supernovae
considered here burned similar masses, and suggests that their progenitors had
the same mass. Synthetic light curve parameters and three-dimensional explosion
simulations support this interpretation. A single explosion scenario, possibly
a delayed detonation, may thus explain most type Ia supernovae.Comment: 8 pages, 2 figure
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