12,116 research outputs found
Turbomachinery technology for high-speed civil flight
NASA Lewis' research and technology efforts applicable to turbomachinery for high-speed flight are discussed. The potential benefits and cycle requirements for advanced variable cycle engines and the supersonic throughflow fan engine for a high-speed civil transport application are presented. The supersonic throughflow fan technology program is discussed. Technology efforts in the basic discipline areas addressing the severe operating conditions associated with high-speed flight turbomachinery are reviewed. Included are examples of work in internal fluid mechanics, high-temperature materials, structural analysis, instrumentation and controls
Statistical properties of fractures in damaged materials
We introduce a model for the dynamics of mud cracking in the limit of of
extremely thin layers. In this model the growth of fracture proceeds by
selecting the part of the material with the smallest (quenched) breaking
threshold. In addition, weakening affects the area of the sample neighbour to
the crack. Due to the simplicity of the model, it is possible to derive some
analytical results. In particular, we find that the total time to break down
the sample grows with the dimension L of the lattice as L^2 even though the
percolating cluster has a non trivial fractal dimension. Furthermore, we obtain
a formula for the mean weakening with time of the whole sample.Comment: 5 pages, 4 figures, to be published in Europhysics Letter
Emergency escape system uses self-braking mechanism on fixed cable
Slide-wire system with a twist level slide device incorporates automatic descent and braking for the safe and rapid evacuation of personnel from tall structures. This device is used on any tall structure that might require emergency evacuation. It is also used to transfer materials and equipment
Accuracy, Scalability, and Efficiency of Mixed-Element USM3D for Benchmark Three-Dimensional Flows
The unstructured, mixed-element, cell-centered, finite-volume flow solver USM3D is enhanced with new capabilities including parallelization, line generation for general unstructured grids, improved discretization scheme, and optimized iterative solver. The paper reports on the new developments to the flow solver and assesses the accuracy, scalability, and efficiency. The USM3D assessments are conducted using a baseline method and the recent hierarchical adaptive nonlinear iteration method framework. Two benchmark turbulent flows, namely, a subsonic separated flow around a three-dimensional hemisphere-cylinder configuration and a transonic flow around the ONERA M6 wing are considered
Evidence for J and H-band excess in classical T Tauri stars and the implications for disk structure and estimated ages
We argue that classical T Tauri stars (cTTs) possess significant non-
photospheric excess in the J and H bands. We first show that normalizing the
spectral energy distributions (SEDs) of cTTs to the J-band leads to a poor fit
of the optical fluxes, while normalizing the SEDs to the Ic-band produces a
better fit to the optical bands and in many cases reveals the presence of a
considerable excess at J and H. NIR spectroscopic veiling measurements from the
literature support this result. We find that J and H-band excesses correlate
well with the K-band excess, and that the J-K and H-K colors of the excess
emission are consistent with that of a black body at the dust sublimation
temperature (~ 1500-2000 K). We propose that this near-IR excess originates at
a hot inner rim, analogous to those suggested to explain the near-IR bump in
the SEDs of Herbig Ae/Be stars. To test our hypothesis, we use the model
presented by Dullemond et al. (2001) to fit the photometry data between 0.5 um
and 24 um of 10 cTTs associated with the Chamaeleon II molecular cloud. The
models that best fit the data are those where the inner radius of the disk is
larger than expected for a rim in thermal equilibrium with the photospheric
radiation field alone. In particular, we find that large inner rims are
necessary to account for the mid infrared fluxes (3.6-8.0 um) obtained by the
Spitzer Space Telescope. Finally, we argue that deriving the stellar
luminosities of cTTs by making bolometric corrections to the J-band fluxes
systematically overestimates these luminosities. The overestimated luminosities
translate into underestimated ages when the stars are placed in the H-R
diagram. Thus, the results presented herein have important implications for the
dissipation timescale of inner accretion disks.Comment: 45 pages, 13 figure
The time to extinction for an SIS-household-epidemic model
We analyse a stochastic SIS epidemic amongst a finite population partitioned
into households. Since the population is finite, the epidemic will eventually
go extinct, i.e., have no more infectives in the population. We study the
effects of population size and within household transmission upon the time to
extinction. This is done through two approximations. The first approximation is
suitable for all levels of within household transmission and is based upon an
Ornstein-Uhlenbeck process approximation for the diseases fluctuations about an
endemic level relying on a large population. The second approximation is
suitable for high levels of within household transmission and approximates the
number of infectious households by a simple homogeneously mixing SIS model with
the households replaced by individuals. The analysis, supported by a simulation
study, shows that the mean time to extinction is minimized by moderate levels
of within household transmission
Angular Momentum Transport by MHD Turbulence in Accretion Disks: Gas Pressure Dependence of the Saturation Level of the Magnetorotational Instability
The saturation level of the magnetorotational instability (MRI) is
investigated using three-dimensional MHD simulations. The shearing box
approximation is adopted and the vertical component of gravity is ignored, so
that the evolution of the MRI is followed in a small local part of the disk. We
focus on the dependence of the saturation level of the stress on the gas
pressure, which is a key assumption in the standard alpha disk model. From our
numerical experiments it is found that there is a weak power-law relation
between the saturation level of the Maxwell stress and the gas pressure in the
nonlinear regime; the higher the gas pressure, the larger the stress. Although
the power-law index depends slightly on the initial field geometry, the
relationship between stress and gas pressure is independent of the initial
field strength, and is unaffected by Ohmic dissipation if the magnetic Reynolds
number is at least 10. The relationship is the same in adiabatic calculations,
where pressure increases over time, and nearly-isothermal calculations, where
pressure varies little with time. Our numerical results are qualitatively
consistent with an idea that the saturation level of the MRI is determined by a
balance between the growth of the MRI and the dissipation of the field through
reconnection. The quantitative interpretation of the pressure-stress relation,
however, may require advances in the theoretical understanding of non-steady
magnetic reconnection.Comment: 45 pages, 5 tables, 17 figures, accepted for publication in Ap
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