43,153 research outputs found
Instabilities of geared couplings: Theory and practice
The use of couplings for high speed turbocompressors or pumps is essential to transmit power from the driver. Typical couplings are either of the lubricated gear or dry diaphragm type design. Gear couplings have been the standard design for many years and recent advances in power and speed requirements have pushed the standard design criteria to the limit. Recent test stand and field data on continuous lube gear type couplings have forced a closer examination of design tolerances and concepts to avoid operational instabilities. Two types of mechanical instabilities are reviewed in this paper: (1) entrapped fluid, and (2) gear mesh instability resulting in spacer throw-out onset. Test stand results of these types of instabilities and other directly related problems are presented together with criteria for proper coupling design to avoid these conditions. An additional test case discussed shows the importance of proper material selection and processing and what can happen to an otherwise good design
Effect of microstructure and temperature on the erosion rates and mechanisms of modified EB PVD TBCs
Thermal barrier coatings (TBCs) have now been used in gas turbine engines for a
number of decades and are now considered to be an accepted technology. As there
is a constant drive to increase the turbine entry temperature, in order to
increase engine efficiency, the coatings operate in increasingly hostile
environments. Thus there is a constant drive to both increase the temperature
capabilities of TBCs while at the same time reducing their thermal
conductivities. The thermal conductivity of standard 7 wt% yttria stabilized
zirconia (7YSZ) electron beam (EB) physical vapour deposited (PVD) TBCs can be
reduced in two ways: the first by modification of the microstructure of the TBC
and the second by addition of ternary oxides. By modifying the microstructure of
the TBC such that there are more fine pores, more photon scattering centres are
introduced into the coatings, which reduce the heat transfer by radiation. While
ternary oxides will introduce lattice defects into the coating, which increases
the phonon scattering, thus reducing the thermal conductivity via lattice
vibrations. Unfortunately, both of these methods can have a negative effect on
the erosion resistance of EB PVD TBCs. This paper compares the relative erosion
rates of ten different EB PVD TBCs tested at 90à ° impact at room temperature and
at high temperature and discusses the results in term of microstructural and
temperature effects. It was found that by modifying the coating deposition, such
that a low density coating with a highly â  featheredâ  microstructure formed,
generally resulted in an increase in the erosion rate at room temperature. When
there was a significant change between the room temperature and the high
temperature erosion mechanism it was accompanied by a significant decrease in
the erosion rate, while additions of dopents was found to significantly increase
the erosion rate at room and high temperature. However, all the modified
coatings still had a lower erosion rate than a plasma sprayed coatings. So,
although, relative to a standard 7YSZ coating, the modified coatings have a
lower erosion resistance, they still perform better than PS TBCs and their lower
thermal conductivities could make them viable alternatives to 7YSZ for use in
gas turbine en
3-d resistive MHD simulations of magnetic reconnection and the tearing mode instability in current sheets
Magnetic reconnection plays a critical role in many astrophysical processes
where high energy emission is observed, e.g. particle acceleration,
relativistic accretion powered outflows, pulsar winds and probably in
dissipation of Poynting flux in GRBs. The magnetic field acts as a reservoir of
energy and can dissipate its energy to thermal and kinetic energy via the
tearing mode instability. We have performed 3d nonlinear MHD simulations of the
tearing mode instability in a current sheet. Results from a temporal stability
analysis in both the linear regime and weakly nonlinear (Rutherford) regime are
compared to the numerical simulations. We observe magnetic island formation,
island merging and oscillation once the instability has saturated. The growth
in the linear regime is exponential in agreement with linear theory. In the
second, Rutherford regime the island width grows linearly with time. We find
that thermal energy produced in the current sheet strongly dominates the
kinetic energy. Finally preliminary analysis indicates a P(k) 4.8 power law for
the power spectral density which suggests that the tearing mode vortices play a
role in setting up an energy cascade.Comment: 4 pages, 8 figures, accepted for publication in the International
Journal of Modern Physics D, proceedings of HEPRO meeting, held in Dublin, in
September 200
Numerical solutions of the one-dimensional nucleon-meson cascade equations
Numerical integration of meson-nucleon cascade equations for accelerator shielding calculation
A Program for the Collection, Storage, and Analysis of Baseline Environmental Data for Cook Inlet, Alaska
The scope of this report is to provide a general, yet comprehensive,
description of the Cook Inlet System which will serve as a
basis for understanding the interrelated natural and man-made factors
governing its future; to present a program of field research studies
for the estuarine environment that will describe the existing state of
the Inlet with respect to the water quality and biota; to provide a
framework whereby the program of studies can be evaluated and redirected
in light of the preliminary results; and, to provide a method of storing
and analyzing the data from the investigations so that it can be made
available to interested parties in the most efficient manner possible.This report was prepared by the Institute of Water Resources of the
University of Alaska for the Alaska Water Laboratory, Federal Water
Pollution Control Administration under Contract No. 14-12-449
A unique facility for V/STOL aircraft hover testing
The Langley Impact Dynamics Research Facility (IDRF) was modified to obtain static force and moment data and to allow assessment of aircraft handling qualities during dynamic tethered hover flight. Test probe procedures were also established. Static lift and control measurements obtained are presented along with results of limited dynamic tethered hover flight
Toeplitz algebras and spectral results for the one-dimensional Heisenberg model
We determine the structure of the spectrum and obtain non-propagation
estimates for a class of Toeplitz operators acting on a subset of the lattice
. This class contains the Hamiltonian of the one-dimensional Heisenberg
model.Comment: 13 page
Analysis and identification of subsynchronous vibration for a high pressure parallel flow centrifugal compressor
The summary of a complete analytical design evaluation of an existing parallel flow compressor is presented and a field vibration problem that manifested itself as a subsynchronous vibration that tracked at approximately 2/3 of compressor speed is reviewed. The comparison of predicted and observed peak response speeds, frequency spectrum content, and the performance of the bearing-seal systems are presented as the events of the field problem are reviewed. Conclusions and recommendations are made as to the degree of accuracy of the analytical techniques used to evaluate the compressor design
Teacher and student perceptions of the development of learner autonomy : a case study in the biological sciences
Biology teachers in a UK university expressed a majority view that student learning autonomy increases with progression through university. A minority suggested that pre-existing diversity in learning autonomy was more important and that individuals not cohorts differ in their learning autonomy. They suggested that personal experience prior to university and age were important and that mature students are more autonomous than 18-20 year olds. Our application of an autonomous learning scale (ALS) to four year-groups of biology students confirmed that the learning autonomy of students increases through their time at university but not that mature students are necessarily more autonomous than their younger peers. It was evident however that year of study explained relatively little
A semantic web approach for built heritage representation
In a built heritage process, meant as a structured system of activities
aimed at the investigation, preservation, and management of architectural
heritage, any task accomplished by the several actors involved in it is deeply
influenced by the way the knowledge is represented and shared. In the current
heritage practice, knowledge representation and management have shown several
limitations due to the difficulty of dealing with large amount of extremely heterogeneous
data. On this basis, this research aims at extending semantic web
approaches and technologies to architectural heritage knowledge management in
order to provide an integrated and multidisciplinary representation of the artifact
and of the knowledge necessary to support any decision or any intervention and
management activity. To this purpose, an ontology-based system, representing
the knowledge related to the artifact and its contexts, has been developed through
the formalization of domain-specific entities and relationships between them
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