8,879 research outputs found
Limitations on wind-tunnel pressure signature extrapolation
Analysis of some recent experimental sonic boom data has revived the hypothesis that there is a closeness limit to the near-field separation distance from which measured wind tunnel pressure signatures can be extrapolated to the ground as though generated by a supersonic-cruise aircraft. Geometric acoustic theory is used to derive an estimate of this distance and the sample data is used to provide a preliminary indication of practical separation distance values
Wind-tunnel investigation of the validity of a sonic-boom-minimization concept
The Langley unitary plan unitary plan wind tunnel was used to determine the validity of a sonic-boom-minimization theory. Five models - two reference and three low-boom constrained - were tested at design Mach numbers of 1.5 and 2.7. Results show that the pressure signatures generated by the low-boom models had significantly lower overpressure levels than those produced by the reference models and that small changes in the Mach number and/or the lift caused relatively small changes in the signature shape and overpressure level. Boundary-layer effects were found in the signature shape and overpressure level. Boundary-layer effects were found to be sizable on the low-boom models, and when viscous corrections were included in the analysis, improved agreement between the predicted and the measured signatures was noted. Since this agreement was better at Mach 1.5 than at Mach 2.7, it was concluded that the minimization method was definitely valid at Mach 1.5 and was probably valid at Mach 2.7, with further work needed to resolve the uncertainty
Current research in sonic-boom minimization
A review is given of several questions as yet unanswered in the area of sonic-boom research. Efforts, both here at Langley and elsewhere, in the area of minimization, human response, design techniques and in developing higher order propagation methods are discussed. In addition, a wind-tunnel test program being conducted to assess the validity of minimization methods based on a forward spike in the F-function is described
Large-Scale Radio Structure in the Universe: Giant Radio Galaxies
Giant radio galaxies (GRGs), with linear sizes larger than 1 Mpc (H0=50
km/s/Mpc), represent the biggest single objects in the Universe. GRGs are rare
among the entire population of radio galaxies (RGs) and their physical
evolution is not well understood though for many years they have been of
special interest for several reasons. The lobes of radio sources can compress
cold gas clumps and trigger star or even dwarf galaxy formation, they can also
transport gas from a host galaxy to large distances and seed the IGM with
magnetic fields. Since GRGs have about 10 to 100 times larger sizes than normal
RGs, their influence on the ambient medium is correspondingly wider and is
pronounced on scales comparable to those of clusters of galaxies or larger.
Therefore `giants' could play an important role in the process of large-scale
structure formation in the Universe. Recently, thanks to the new all sky radio
surveys, significant progress in searching for new GRGs has been made.Comment: To appear in Multiwavelength AGN Surveys, ed. R. Maiolino and R.
Mujica, Singapore: World Scientific, 2004, 2 page
Theoretical Analysis of Acceptance Rates in Multigrid Monte Carlo
We analyze the kinematics of multigrid Monte Carlo algorithms by
investigating acceptance rates for nonlocal Metropolis updates. With the help
of a simple criterion we can decide whether or not a multigrid algorithm will
have a chance to overcome critial slowing down for a given model. Our method is
introduced in the context of spin models. A multigrid Monte Carlo procedure for
nonabelian lattice gauge theory is described, and its kinematics is analyzed in
detail.Comment: 7 pages, no figures, (talk at LATTICE 92 in Amsterdam
A wind-tunnel investigation of sonic-boom pressure distributions of bodies of revolution at Mach 2.96, 3.83, and 4.63
Measurements of sonic boom pressure distribution of bodies of revolution at Mach 2.96, 3.83, and 4.63 in Unitary Plan wind tunne
Nexus solitons in the center vortex picture of QCD
It is very plausible that confinement in QCD comes from linking of Wilson
loops to finite-thickness vortices with magnetic fluxes corresponding to the
center of the gauge group. The vortices are solitons of a gauge-invariant QCD
action representing the generation of gluon mass. There are a number of other
solitonic states of this action. We discuss here what we call nexus solitons,
in which for gauge group SU(N), up to N vortices meet a a center, or nexus,
provided that the total flux of the vortices adds to zero (mod N). There are
fundamentally two kinds of nexuses: Quasi-Abelian, which can be described as
composites of Abelian imbedded monopoles, whose Dirac strings are cancelled by
the flux condition; and fully non-Abelian, resembling a deformed sphaleron.
Analytic solutions are available for the quasi-Abelian case, and we discuss
variational estimates of the action of the fully non-Abelian nexus solitons in
SU(2). The non-Abelian nexuses carry Chern-Simons number (or topological charge
in four dimensions). Their presence does not change the fundamentals of
confinement in the center-vortex picture, but they may lead to a modified
picture of the QCD vacuum.Comment: LateX, 24 pages, 2 .eps figure
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