4,361 research outputs found
Application of dynamical systems theory to the high angle of attack dynamics of the F-14
Dynamical systems theory has been used to
study the nonlinear dynamics of the F-14. An eight
degree of freedom model that does not include the
control system present in operational F-14's has
been analyzed. The aerodynamic model, supplied
by NASA, includes nonlinearities as functions of the
angles of attack and sideslip, the rotation rate, and
the elevator deflection. A continuation method has
been used to calculate the steady states of the F-14
as continuous functions of the control surface deflections.
Bifurcations of these steady states have been used to predict the onset of wing rock, spiral divergence, and jump phenomena which cause the aircraft to enter a spin. A simple feedback control system was designed to eliminate the wing rock and spiral divergence instabilities. The predictions were verified with numerical simulations
Modeling for Active Control of Combustion and Thermally Driven Oscillations
Organized oscillations excited and sustained by high densities of energy release in combustion chambers have long caused serious problems in development of propulsion systems. The amplitudes often become sufficiently large to cause unacceptable structural vibrations. Because the oscillations are self-excited, they reach limiting amplitudes (limit cycles) only because of the action of nonlinear processes. Traditionally, satisfactory behavior
has been achieved through a combination of trial-and-error
design and testing, with control always involving passive means: geometrical modifications, changes of propellant composition, or devices to enhance dissipation of acoustic energy. Active control has been applied only to small-scale laboratory devices, but the limited success suggests the possibility of serious applications to full-scale propulsion systems. Realization of that potential rests on further experimental work, combined with deeper understanding of the mechanisms causing the oscillations and of the physical behavior of the systems. Effective design of active control systems will require faithful modeling of the relevant processes over broad frequency ranges covering the spectra of natural modes. This paper will cover the general character of the linear and nonlinear behavior of combustion systems, with special attention to acoustics and the mechanisms of excitation.
The discussion is intended to supplement the paper by Doyle et al. concerned primarily with controls issues and the observed behavior of simple laboratory devices
Gravitational lens candidates in the E-CDFS
We report ten lens candidates in the E-CDFS from the GEMS survey. Nine of the
systems are new detections and only one of the candidates is a known lens
system. For the most promising five systems including the known lens system, we
present results from preliminary lens mass modelling, which tests if the
candidates are plausible lens systems. Photometric redshifts of the candidate
lens galaxies are obtained from the COMBO-17 galaxy catalog. Stellar masses of
the candidate lens galaxies within the Einstein radius are obtained by using
the -band luminosity and the color-based stellar mass-to-light ratios.
As expected, the lensing masses are found to be larger than the stellar masses
of the candidate lens galaxies. These candidates have similar dark matter
fractions as compared to lenses in SLACS and COSMOS. They also roughly follow
the halo mass-stellar mass relation predicted by the subhalo abundance matching
technique. One of the candidate lens galaxies qualifies as a LIRG and may not
be a true lens because the arc-like feature in the system is likely to be an
active region of star formation in the candidate lens galaxy. Amongst the five
best candidates, one is a confirmed lens system, one is a likely lens system,
two are less likely to be lenses and the status of one of the candidates is
ambiguous. Spectroscopic follow-up of these systems is still required to
confirm lensing and/or for more accurate determination of the lens masses and
mass density profiles.Comment: 12 pages, 5 figures, 3 tables, ApJ accepte
Information Systems and Health Care - V: A MultiModal Approach to Health Care Decision Support Systems
Decision Support Systems (DSS) in health care must be designed in consideration of the empirical context and problem space where they are being applied. However differences such as workflow, clinical expertise and organizational norms make it hard to define the context where a DSS will be used. What is needed is a DSS that is able to enhance health care delivery in different contexts. In this paper we present a multi-modal DSS that supports decision making in different contexts. The paper describes the theoretical basis for the DSS, explores a problem space in palliative care and describes a prototype implementation of the DSS to address the issues from the problem space
The Impact of Agricultural Research in Tropical Africa: A Study of the Collaboration between the International and National Research Systems
CGIAR Study Paper on the problems and opportunities of international agricultural research in Sub Saharan Africa, and the impact of CGIAR Centers on African national agricultural research systems and on agricultural production and food security in Africa. Written by Hans E. Jahnke, Dieter Kirschke, and Johannes Lagermann and published as CGIAR Study Paper No. 21, part of the series comprising the CGIAR Impact Study of the 1980s
Application of dynamical systems theory to nonlinear aircraft dynamics
Dynamical systems theory has been used to study nonlinear aircraft dynamics. A six degree of freedom model that neglects gravity has been analyzed. The aerodynamical model, supplied by NASA, is for a generic swept wing fighter and includes nonlinearities as functions of the angle of attack. A continuation method was use to calculate the steady states of the aircraft, and bifurcations of these steady states, as functions of the control deflections. Bifurcations were used to predict jump phenomena and the onset of periodic motion for roll coupling instabilities and high angle of attack maneuvers. The predictions were verified with numerical simulations
Application of Bifurcation Theory to the High-Angle-of-Attack Dynamics of the F-14
Bifurcation theory has been used to study Ihe nonlinear dynamics of the F-14. An 8 degree-of-freedom model
that does not include the control system present in operational F-14's has been analyzed. The aerodynamic
model, supplied by NASA, includes nonlinearlties as functions of the angles of attack and sideslip, the rotation
rate about the velocity vector, and the elevator deflection. A continuation method has been used to calculate
the steady states of the F -14 as continuous functions of the elevator deflection. Bifurcations of these steady states
have been used to predict the onset of wing rock, spiral divergence, and jump phenomena that cause the aircraft
to enter a spin. A simple feedback control system was designed to eliminate the wing rock and spiral divergence
instabilities. The predictions were verified with numerical simulations
Quantitative Indicators for Priorities in International Agricultural Research
FAO commissioned study by the German consulting firm Gesellschaft fur Agrarprojekte on the selection and use of quantitative indicators for establishing priorities in international agricultural research. The paper discusses the problems of research policy evaluation and the complexities of selecting suitable priority indicators. It describes the uses and limitations of commodity-oriented, resource-oriented, agroecological, and development indicators. Agenda document, TAC 32nd Meeting, October 1983
On quasar host galaxies as tests of non-cosmological redshifts
Despite a general consensus in the astronomical community that all quasars
are located at the distances implied by their redshifts, a number of
observations still challenge this interpretation, possibly indicating that some
subpopulation of quasars may harbour significant redshift components not
related to the expansion of the universe. It has been suggested that these
objects may have been ejected from local galaxies and are likely to evolve into
new galaxies themselves. Here, a test of such exotic scenarios is proposed,
based on the spectral energy distribution of the galaxies hosting quasars with
suspected ejection origin. Provided that the time scales over which the ejected
objects manifest themselves as quasars is short, one would in the framework of
ejection scenarios expect to find either no quasar host galaxy, a pseudo-host
consisting of gas ionized by the quasar, or a host galaxy consisting of young
stars only. It is argued that the spectral energy distributions corresponding
to the latter two options should differ significantly from those of most quasar
host galaxies detected at low redshift so far, thus providing a potential test
of the claimed existence of ejected quasars. A minimal implementation of this
test, involving optical and near-IR broadband photometry, is suggested.Comment: 9 pages, 4 figure
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