30,266 research outputs found
Shock accelerated cylindrical gas inhomogeneities. Part 2 - A heavy gas cylinder
Experiments have been carried out in which a cylindrical volume of a heavy gas is impulsively
accelerated by a weak shock wave. A laminar jet of sulphur hexafluoride (SF_6) is used to produce
the heavy gas cylinder. Planar laser induced fluorescence (PLIF) is used to visualize the flow. In
viewing the PLIF images it is discovered that the vorticity that early on resides on the boundary
between the two gasses, separates from the cylinder to form a pair of vortices. Subsequently these
vortices wrap the heavy gas around them. This process is quite different from what is observed
when the cylinder is lighter than its surroundings. Similar experiments with helium (part 1 of this
series) showed that a small amount light gas stays with the vorticity, eventually becoming part of
the vortex cores. A simple model capable of explaining these differences is presented. In addition,
the displacement of the jet cross section is measured and agrees reasonably well with previous
experimental and computational results
Shock-induced mixing of a light-gas cylinder
Experiments have been carried out to quantify the mixing induced by the interaction of a weak shock wave with a cylindrical volume of a gas (helium) that is lighter than its surroundings (air). In these experiments a round laminar jet was used to produce the light-gas cylinder, and planar laser-induced fluorescence (PLIF), utilizing a fluorescent tracer (biacetyl) mixed with the helium, was used to visualize the flow. These techniques provide a higher quality of flow visualization than that obtained in previous investigations. In addition, the PLIF technique could be used for the measurement of species concentration. The distortion of the helium cylinder produced by the passing shock wave was found to be similar to that displayed by images from previous experimental and computational investigations. The downstream
displacement of several points on the boundary of the light-gas cylinder are measured and agree reasonably well with the results of earlier experimental and theoretical studies as well. Because the mixing process causes the helium originally contained within the cylinder to be dispersed into the surrounding air, the PLIF image area inside the contour at one half the maximum concentration of the
fluorescent tracer decreases as the two gases mixed. The change in this area is used as a measure of the mixing rate, and it is found that the time rate of change of this
area divided by the area of the initial jet is approximately - 0.7 X 10^3 S^(-1)
Quality of Variational Trial States
Besides perturbation theory (which clearly requires the knowledge of the
exact unperturbed solution), variational techniques represent the main tool for
any investigation of the eigenvalue problem of some semibounded operator H in
quantum theory. For a reasonable choice of the employed trial subspace of the
domain of H, the lowest eigenvalues of H usually can be located with acceptable
precision whereas the trial-subspace vectors corresponding to these eigenvalues
approximate, in general, the exact eigenstates of H with much less accuracy.
Accordingly, various measures for the accuracy of the approximate eigenstates
derived by variational techniques are scrutinized. In particular, the matrix
elements of the commutator of the operator H and (suitably chosen) different
operators with respect to degenerate approximate eigenstates of H obtained by
variational methods are proposed as new criteria for the accuracy of
variational eigenstates. These considerations are applied to precisely that
Hamiltonian for which the eigenvalue problem defines the well-known spinless
Salpeter equation. This bound-state wave equation may be regarded as (the most
straightforward) relativistic generalization of the usual nonrelativistic
Schroedinger formalism, and is frequently used to describe, e.g., spin-averaged
mass spectra of bound states of quarks.Comment: LaTeX, 7 pages, version to appear in Physical Review
Statically checking confidentiality via dynamic labels
This paper presents a new approach for verifying confidentiality
for programs, based on abstract interpretation. The
framework is formally developed and proved correct in the
theorem prover PVS. We use dynamic labeling functions
to abstractly interpret a simple programming language via
modification of security levels of variables. Our approach
is sound and compositional and results in an algorithm for
statically checking confidentiality
RIES: Internet voting in action
RIES stands for Rijnland Internet Election System. It is an online voting system that was developed by one of the Dutch local authorities on water management. The system has been used twice in the fall of 2004 for in total approximately two million potential voters. In this paper we describe how this system works. Furthermore we do not only describe how the outcome of the elections can be verified but also how it has been verified by us. To conclude the paper we describe some possible points for improvement
Passive propellant system
The system utilizes a spherical tank structure A separated into two equal volume compartments by a flat bulkhead B. Each compartment has four similar gallery channel legs located in the principal vehicle axes, ensuring that bulk propellant will contact at least one gallery leg during vehicle maneuvers. The forward compartment gallery channel legs collect propellant and feed it into the aft compartment through communication screens which protrude into the aft compartment. The propellant is then collected by the screened gallery channels in the aft compartment and supplied to the propellant outlet. The invention resides in the independent gallery assembly and screen structure by means of which propellant flow from forward to aft compartments is maintained. Liquid surface tension of the liquid on the screens is used to control liquid flow. The system provides gas-free propellants in low or zero-g environments regardless of axial accelerations and propellant orientation in bulk regions of the vessel
ATM-CMG control system stability
Stability analyses and simulation data and results are presented for an initial Control Moment Gyroscope system proposed for the Apollo Telescope Mount cluster (later named Skylab) using momentum vector feedback. A compensation filtering technique is presented which significantly improved analytical and simulation performance of the system. This technique is quite similar to the complementary filtering technique and represents an early NASA application
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