253 research outputs found
Aeroacoustic Measurements of the Bell 699 Rotor on the Tiltrotor Test Rig in the National Full-Scale Aerodynamics Complex 40- by 80-Foot Wind Tunnel
The Tiltrotor Test Rig (TTR) with the Bell 699 Rotor was tested in the National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-Foot Wind Tunnel from 2017 to 2018. The primary goal of the test was to understand the operational capabilities of the TTR while also acquiring research data, including acoustic data. A data quality study revealed that the NFAC 40- by 80-Foot Wind Tunnel is an adequate acoustic environment to test the Bell 699 Rotor for helicopter, conversion, and airplane configurations. Representative acoustic data are presented, and selected acoustic data and corresponding test conditions are included
Extraction of Dynamic Inflow Models for Coaxial and Tandem Rotors from CFD Simulations
The dynamic inflow coupling with rotor/body dynamics is crucial in the analysis of stability and control law design for helicopters. Over the past several decades, finite-state inflow models for single rotor configurations in hover, forward flight, and maneuver have developed (Ref.1-3). By capturing the interference effects between rotors, the extension of pressure potential finite state inflow model has promising result for coaxial rotor configuration (Ref.4-6). Recently, the focus of the dynamic inflow modeling has shifted to tandem rotor configurations (Ref.7, 8). The development of the dynamic inflow models for tandem rotor configuration still have some limitations due to the lack of knowledge of rotor-to-rotor interference, and rotor-wake interference. Experimental methods, and computational fluid dynamics methods are commonly used to understand the rotor performance and rotor airload variations, and measure or predict inflow velocity distributions at the rotor desk. The inflow distributions are subsequently used to improve the dynamic inflow models. Tandem rotor configurations have been studied experimentally and computationally for several decades (Ref.9-12). Sweet (Ref.10) observed that a tandem rotor with 76-percent-radius overlap required 14% more induced power at hovering condition, relative to an isolated rotor of equivalent disk area. Sweet also found that, above a shaft-to-shaft distance of 1.03 diameter, the performance of the tandem rotor was nearly the same as two isolated rotors. The objective of the present study is to apply computational fluid dynamics simulations of tandem rotors for the extraction of dynamic inflow models. The extended methodology is first validated by comparing the computed induced power against test data. Subsequently inflow distributions and wake structures are analyzed
Spectroscopic abundance analysis of dwarfs in young open cluster IC 4665
We report a detailed spectroscopic abundance analysis for a sample of 18 F-K
dwarfs of the young open cluster IC 4665. Stellar parameters and element
abundances of Li, O, Mg, Si, Ca, Ti, Cr, Fe and Ni have been derived using the
spectroscopic synthesis tool SME (Spectroscopy Made Easy). Within the
measurement uncertainties the iron abundance is uniform with a standard
deviation of 0.04 dex. No correlation is found between the iron abundance and
the mass of the stellar convective zone, and between the Li abundance and the
Fe abundance. In other words, our results do not reveal any signature of
accretion and therefore do not support the scenario that stars with planets
(SWPs) acquire their on the average higher metallicity compared to field stars
via accretion of metal-rich planetary material. Instead the higher metallicity
of SWPs may simply reflect the fact that planet formation is more efficient in
high metallicity environs. However, since that many details of the planet
system formation processes remain poorly understood, further studies are needed
for a final settlement of the problem of the high metallicity of SWPs.
The standard deviation of [Fe/H] deduced from our observations, taken as an
upper limit on the metallicity dispersion amongst the IC 4665 member stars, has
been used to constrain proto-planetary disk evolution, terrestrial and giant
planets formation and evolution processes. Our results do not support the
possibility that the migration of gas giants and the circularization of
terrestrial planets' orbits are regulated by their interaction with a residual
population of planetesimals and dust particles.Comment: 18 pages, 6 figures, accepted for publication in Ap
Symmetry of Traveling Wave Solutions to the Allen-Cahn Equation in \Er^2
In this paper, we prove even symmetry of monotone traveling wave solutions to
the balanced Allen-Cahn equation in the entire plane. Related results for the
unbalanced Allen-Cahn equation are also discussed
The evolution of rotating stars
First, we review the main physical effects to be considered in the building
of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The
internal rotation law evolves as a result of contraction and expansion,
meridional circulation, diffusion processes and mass loss. In turn,
differential rotation and mixing exert a feedback on circulation and diffusion,
so that a consistent treatment is necessary.
We review recent results on the evolution of internal rotation and the
surface rotational velocities for stars on the Upper MS, for red giants,
supergiants and W-R stars. A fast rotation is enhancing the mass loss by
stellar winds and reciprocally high mass loss is removing a lot of angular
momentum. The problem of the ``break-up'' or -limit is critically
examined in connection with the origin of Be and LBV stars. The effects of
rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the
blue to red supergiant ratios, the formation of W-R stars, the chemical
abundances in massive stars as well as in red giants and AGB stars, are
reviewed in relation to recent observations for stars in the Galaxy and
Magellanic Clouds. The effects of rotation on the final stages and on the
chemical yields are examined, as well as the constraints placed by the periods
of pulsars. On the whole, this review points out that stellar evolution is not
only a function of mass M and metallicity Z, but of angular velocity
as well.Comment: 78 pages, 7 figures, review for Annual Review of Astronomy and
Astrophysics, vol. 38 (2000
Inhibition by melittin of phospholipid-sensitive and calmodulin-sensitive Ca2+ -dependent protein kinases
Dielectric Properties of the Quasi-Two-Dimensional Electron Liquid in Heterojunctions
A quasi-two-dimensional (Q2D) electron liquid (EL) is formed at the interface
of a semiconductor heterojunction. For an accurate characterization of the Q2D
EL, many-body effects need to be taken into account beyond the random phase
approximation. In this theoretical work, the self-consistent static local-field
correction known as STLS is applied for the analysis of the Q2D EL. The
penetration of the charge distribution to the barrier-acting material is taken
into consideration through a variational approach. The Coulomb from factor that
describes the effective 2D interaction is rigorously treated. The longitudinal
dielectric function and the plasmon dispersion of the Q2D EL are presented for
a wide range of electron and ionized acceptor densities choosing GaAs/AlGaAs as
the physical system. Analytical expressions fitted to our results are also
supplied to enable a widespread use of these results.Comment: 39 pages (in LaTeX), including 8 PostScript figure
Why did I not prepare for this? The politics of negotiating fieldwork access, identity, and methodology in researching microfinance institutions
It has been increasingly recognized that undertaking qualitative research can pose many challenges for researchers. However, scanty literature focuses directly on the experiences of doctoral research students from developing countries studying in Western Europe and other similar geographic regions, and the challenges of doing fieldwork when they return âback homeâ. In this article, I use my experiences in the process of undertaking PhD fieldwork on two donor-funded microfinance institutions located in Zambia to demonstrate that doctoral students from specific regions (Africa in particular) undertaking research in their native countries can struggle to manage and make sense of the challenges and identity issues raised in their âfamiliarâ environments. I also present a detailed discussion of how various gatekeepers and participants facilitated access, identity alteration, and the impact of insiderâoutsider positionality on collected data. It is concluded that organizational âpoliticsâ and local context can have significant bearing on power relationships, identities of researchers, and methodological preferences
Language and anxiety: an ethnographic study of international postgraduate students
This paper presents some findings from an ethnographic study of international postgraduate students at a university in the South of England, which involved interviews and participant observation over a twelve-month academic year. One of the major themes that emerged from this research was studentsâ anxiety over their level of English language. Although all students entered their course with a minimum level of IELTS 6, the majority felt disadvantaged by particularly poor spoken English, and suffered feelings of anxiety, shame and inferiority. Low self-confidence meant that they felt ill-equipped to engage in class discussion and in social interaction which used English as the medium of communication. A common reaction to stress caused by language problems was to retreat into monoethnic communication with students from the same country, further inhibiting progress in language. Whilst some linguistic progress was made by nearly all students during the academic sojourn, the anxiety suffered by students in the initial stage must not be underestimated, and appropriate support systems must be put in place to alleviate their distress
Reinventing grounded theory: some questions about theory, ground and discovery
Grounded theoryâs popularity persists after three decades of broad-ranging critique. In this article three problematic notions are discussedââtheory,â âgroundâ and âdiscoveryââwhich linger in the continuing use and development of grounded theory procedures. It is argued that far from providing the epistemic security promised by grounded theory, these notionsâembodied in continuing reinventions of grounded theoryâconstrain and distort qualitative inquiry, and that what is contrived is not in fact theory in any meaningful sense, that âgroundâ is a misnomer when talking about interpretation and that what ultimately materializes following grounded theory procedures is less like discovery and more akin to invention. The procedures admittedly provide signposts for qualitative inquirers, but educational researchers should be wary, for the significance of interpretation, narrative and reflection can be undermined in the procedures of grounded theory
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