1,492 research outputs found
NSI-1 Squib adapter development and final test report for usage on space shuttle gas sampler valve/bottle assembly 3270
The possibility of utilizing the NSI-1 squib in place cartridge assembly 2270 for the function of both events required for the Space Shuttle Gas Sampler Valve/Bottle Assembly 3270 was determined. Additionally, it was a requirement that the closure disk of the NSI-1 squib and explosive residue therefrom be retained from the valve cavity in so far as possible to prevent any significant particulate from scratching the valve bore and causing sample leakage following the postfire 2 event
Synchronized flow and wide moving jams from balanced vehicular traffic
Recently we proposed an extension to the traffic model of Aw, Rascle and
Greenberg. The extended traffic model can be written as a hyperbolic system of
balance laws and numerically reproduces the reverse shape of the
fundamental diagram of traffic flow. In the current work we analyze the steady
state solutions of the new model and their stability properties. In addition to
the equilibrium flow curve the trivial steady state solutions form two
additional branches in the flow-density diagram. We show that the
characteristic structure excludes parts of these branches resulting in the
reverse shape of the flow-density relation. The upper branch is
metastable against the formation of synchronized flow for intermediate
densities and unstable for high densities, whereas the lower branch is unstable
for intermediate densities and metastable for high densities. Moreover, the
model can reproduce the typical speed of the downstream front of wide moving
jams. It further reproduces a constant outflow from wide moving jams, which is
far below the maximum free flow. Applying the model to simulate traffic flow at
a bottleneck we observe a general pattern with wide moving jams traveling
through the bottleneck.Comment: 10 pages, 12 figure
Axisymmetric core collapse simulations using characteristic numerical relativity
We present results from axisymmetric stellar core collapse simulations in
general relativity. Our hydrodynamics code has proved robust and accurate
enough to allow for a detailed analysis of the global dynamics of the collapse.
Contrary to traditional approaches based on the 3+1 formulation of the
gravitational field equations, our framework uses a foliation based on a family
of outgoing light cones, emanating from a regular center, and terminating at
future null infinity. Such a coordinate system is well adapted to the study of
interesting dynamical spacetimes in relativistic astrophysics such as stellar
core collapse and neutron star formation. Perhaps most importantly this
procedure allows for the unambiguous extraction of gravitational waves at
future null infinity without any approximation, along with the commonly used
quadrupole formalism for the gravitational wave extraction. Our results
concerning the gravitational wave signals show noticeable disagreement when
those are extracted by computing the Bondi news at future null infinity on the
one hand and by using the quadrupole formula on the other hand. We have strong
indication that for our setup the quadrupole formula on the null cone does not
lead to physical gravitational wave signals. The Bondi gravitational wave
signals extracted at infinity show typical oscillation frequencies of about 0.5
kHz.Comment: 17 pages, 18 figures, submitted to Phys. Rev.
Tethered subsatellite study
The results are presented of studies performed relating to the feasibility of deploying a subsatellite from the shuttle by means of a tether. The dynamics, the control laws, the aerodynamics, the heating, and some communication considerations of the tethered subsatellite system are considered. Nothing was found that prohibits the use of a subsatellite joined to the shuttle by a long (100 km) tether. More detailed studies directed at specific applications are recommended
Simulating the dynamics of relativistic stars via a light-cone approach
We present new numerical algorithms for the coupled Einstein-perfect fluid system in axisymmetry. Our framework uses a foliation based on a family of light cones, emanating from a regular center, and terminating at future null infinity. This coordinate system is well adapted to the study of the dynamical spacetimes associated with isolated relativistic compact objects such as neutron stars. In particular, the approach allows the unambiguous extraction of gravitational waves at future null infinity and avoids spurious outer boundary reflections. The code can accurately maintain long-term stability of polytropic equilibrium models of relativistic stars. We demonstrate global energy conservation in a strongly perturbed neutron star spacetime, for which the total energy radiated away by gravitational waves corresponds to a significant fraction of the Bondi mass. As a first application we present results in the study of pulsations of axisymmetric relativistic stars, extracting the frequencies of the different fluid modes in fully relativistic evolutions of the Einstein-perfect fluid system and making a first comparison between the gravitational news function and the predicted wave using the approximations of the quadrupole formula
First-order quasilinear canonical representation of the characteristic formulation of the Einstein equations
We prescribe a choice of 18 variables in all that casts the equations of the
fully nonlinear characteristic formulation of general relativity in
first--order quasi-linear canonical form. At the analytical level, a
formulation of this type allows us to make concrete statements about existence
of solutions. In addition, it offers concrete advantages for numerical
applications as it now becomes possible to incorporate advanced numerical
techniques for first order systems, which had thus far not been applicable to
the characteristic problem of the Einstein equations, as well as in providing a
framework for a unified treatment of the vacuum and matter problems. This is of
relevance to the accurate simulation of gravitational waves emitted in
astrophysical scenarios such as stellar core collapse.Comment: revtex4, 7 pages, text and references added, typos corrected, to
appear in Phys. Rev.
Numerical Evolution of axisymmetric vacuum spacetimes: a code based on the Galerkin method
We present the first numerical code based on the Galerkin and Collocation
methods to integrate the field equations of the Bondi problem. The Galerkin
method like all spectral methods provide high accuracy with moderate
computational effort. Several numerical tests were performed to verify the
issues of convergence, stability and accuracy with promising results. This code
opens up several possibilities of applications in more general scenarios for
studying the evolution of spacetimes with gravitational waves.Comment: 11 pages, 6 figures. To appear in Classical and Quantum Gravit
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