16,944 research outputs found
Local Solutions for Generic Multidimensional Resonant Wave Conversion
In more than one spatial dimension, resonant linear conversion from one wave
type to another can have a more complex geometry than the familiar 'avoided
crossing' of one-dimensional problems. In previous work we have shown that
helical ray shapes are generic in a mathematical sense. Here we briefly
describe how the local field structure can be computed.Comment: 4 pages, to appear in the AIP Proceedings of the 15th Topical
Conference on RF Power in Plasma
Elastic-plastic finite-element analyses of thermally cycled double-edge wedge specimens
Elastic-plastic stress-strain analyses were performed for double-edge wedge specimens subjected to thermal cycling in fluidized beds at 316 and 1088 C. Four cases involving different nickel-base alloys (IN 100, Mar M-200, NASA TAZ-8A, and Rene 80) were analyzed by using the MARC nonlinear, finite element computer program. Elastic solutions from MARC showed good agreement with previously reported solutions obtained by using the NASTRAN and ISO3DQ computer programs. Equivalent total strain ranges at the critical locations calculated by elastic analyses agreed within 3 percent with those calculated from elastic-plastic analyses. The elastic analyses always resulted in compressive mean stresses at the critical locations. However, elastic-plastic analyses showed tensile mean stresses for two of the four alloys and an increase in the compressive mean stress for the highest plastic strain case
Cyclic structural analysis of air-cooled gas turbine blades and vanes
The creep fatigue behavior of a fully impingement cooled blade for four cyclic cases was analyzed by using the Elas 55, finite element, nonlinear structural computer program. Expected cyclic lives were calculated by using the method of strainrange partitioning for reversed inelastic strains and time fractions for ratcheted tensile creep strains. Strainrange partitioning was also applied to previous results from a one dimensional cyclic analysis of a film impingement cooled vane. The analyses indicated that strainrange partitioning is more applicable to a constrained airfoil such as the film impingement cooled vane than to the relatively unconstrained fully impingement cooled airfoil
Materials constitutive models for nonlinear analysis of thermally cycled structures
Effects of inelastic materials models on computed stress-strain solutions for thermally loaded structures were studied by performing nonlinear (elastoplastic creep) and elastic structural analyses on a prismatic, double edge wedge specimen of IN 100 alloy that was subjected to thermal cycling in fluidized beds. Four incremental plasticity creep models (isotropic, kinematic, combined isotropic kinematic, and combined plus transient creep) were exercised for the problem by using the MARC nonlinear, finite element computer program. Maximum total strain ranges computed from the elastic and nonlinear analyses agreed within 5 percent. Mean cyclic stresses, inelastic strain ranges, and inelastic work were significantly affected by the choice of inelastic constitutive model. The computing time per cycle for the nonlinear analyses was more than five times that required for the elastic analysis
Nonlinear, three-dimensional finite-element analysis of air-cooled gas turbine blades
Cyclic stress-strain states in cooled turbine blades were calculated for a simulated mission of an advanced-technology commercial aircraft engine. The MARC, nonlinear, finite-element computer program was used for the analysis of impingement-cooled airfoils, with and without leading-edge film cooling. Creep was the predominant damage mode (ignoring hot corrosion), particularly artund film-cooling holes. Radially angled holes exhibited less creep than holes with axes normal to the surface. Beam-theory analyses of all-impingement-cooled airfoils gave fair agreement with MARC results for initial creep
Joining techniques for fabrication of composite air-cooled turbine blades and vanes
Activated diffusion brazing studies of joining methods for composite air-cooled turbine blade and vane fabricatio
Performance optimization for rotors in hover and axial flight
Performance optimization for rotors in hover and axial flight is a topic of continuing importance to rotorcraft designers. The aim of this Phase 1 effort has been to demonstrate that a linear optimization algorithm could be coupled to an existing influence coefficient hover performance code. This code, dubbed EHPIC (Evaluation of Hover Performance using Influence Coefficients), uses a quasi-linear wake relaxation to solve for the rotor performance. The coupling was accomplished by expanding of the matrix of linearized influence coefficients in EHPIC to accommodate design variables and deriving new coefficients for linearized equations governing perturbations in power and thrust. These coefficients formed the input to a linear optimization analysis, which used the flow tangency conditions on the blade and in the wake to impose equality constraints on the expanded system of equations; user-specified inequality contraints were also employed to bound the changes in the design. It was found that this locally linearized analysis could be invoked to predict a design change that would produce a reduction in the power required by the rotor at constant thrust. Thus, an efficient search for improved versions of the baseline design can be carried out while retaining the accuracy inherent in a free wake/lifting surface performance analysis
Variable gamma-ray emission from the Be/X-ray transient A0535+26?
We present a study of the unidentified gamma-ray source 3EG J0542+2610. This
source is spatially superposed to the supernova remnant G180.0-1.7, but its
time variability makes unlikely a physical link. We have searched into the
EGRET location error box for compact radio sources that could be the low energy
counterpart of the gamma-ray source. Although 29 point-like radio sources were
detected and measured, none of them is strong enough as to be considered the
counterpart of a background gamma-ray emitting AGN. We suggest that the only
object within the 95 % error box capable of producing the required gamma-ray
flux is the X-ray transient A0535+26. We show that this Be/accreting pulsar can
produce variable hadronic gamma-ray emission through the mechanism originally
proposed by Cheng & Ruderman (1989), where a proton beam accelerated in a
magnetospheric electrostatic gap impacts the transient accretion disk.Comment: 6 pages, 3 figures, Astronomy & Astrophysics in pres
Laser Doppler dust devil measurements
A scanning laser doppler velocimeter (SLDV) system was used to detect, track, and measure the velocity flow field of naturally occurring tornado-like flows (dust devils) in the atmosphere. A general description of the dust devil phenomenon is given along with a description of the test program, measurement system, and data processing techniques used to collect information on the dust devil flow field. The general meteorological conditions occurring during the test program are also described, and the information collected on two selected dust devils are discussed in detail to show the type of information which can be obtained with a SLDV system. The results from these measurements agree well with those of other investigators and illustrate the potential for the SLDV in future endeavors
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