108 research outputs found
Space Structures: Issues in Dynamics and Control
A selective technical overview is presented on the vibration and control of large space structures, the analysis, design, and construction of which will require major technical contributions from the civil/structural, mechanical, and extended engineering communities. The immediacy of the U.S. space station makes the particular emphasis placed on large space structures and their control appropriate. The space station is but one part of the space program, and includes the lunar base, which the space station is to service. This paper attempts to summarize some of the key technical issues and hence provide a starting point for further involvement. The first half of this paper provides an introduction and overview of large space structures and their dynamics; the latter half discusses structural control, including controlâsystem design and nonlinearities. A crucial aspect of the large space structures problem is that dynamics and control must be considered simultaneously; the problems cannot be addressed individually and coupled as an afterthought
Errors of linear multistep methods and Runge-Kutta methods for singular perturbation problems with delays
AbstractThis paper is concerned with the error analysis of linear multistep methods and Runge-Kutta methods applied to some classes of one-parameter stiff singularly perturbed problems with delays. We derive the global error estimates of A(α)-stable linear multistep methods and algebraically and diagonally stable Runge-Kutta methods with Lagrange interpolation procedure. Numerical experiments confirm our theoretical analysis
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SciCADE 95: International conference on scientific computation and differential equations
This report consists of abstracts from the conference. Topics include algorithms, computer codes, and numerical solutions for differential equations. Linear and nonlinear as well as boundary-value and initial-value problems are covered. Various applications of these problems are also included
Diagonally Implicit Runge-Kutta Methods for Ordinary Differential Equations. A Review
A review of diagonally implicit Runge-Kutta (DIRK) methods applied to rst-order ordinary di erential equations (ODEs) is undertaken. The goal of this review is to summarize the characteristics, assess the potential, and then design several nearly optimal, general purpose, DIRK-type methods. Over 20 important aspects of DIRKtype methods are reviewed. A design study is then conducted on DIRK-type methods having from two to seven implicit stages. From this, 15 schemes are selected for general purpose application. Testing of the 15 chosen methods is done on three singular perturbation problems. Based on the review of method characteristics, these methods focus on having a stage order of two, sti accuracy, L-stability, high quality embedded and dense-output methods, small magnitudes of the algebraic stability matrix eigenvalues, small values of aii, and small or vanishing values of the internal stability function for large eigenvalues of the Jacobian. Among the 15 new methods, ESDIRK4(3)6L[2]SA is recommended as a good default method for solving sti problems at moderate error tolerances
Relay Feedback and Multivariable Control
This doctoral thesis treats three issues in control engineering related to relay feedback and multivariable control systems. Linear systems with relay feedback is the first topic. Such systems are shown to exhibit several interesting behaviors. It is proved that there exist multiple fast relay switches if and only if the sign of the first non-vanishing Markov parameter of the linear system is positive. It is also shown that these fast switches can appear as part of a stable limit cycle. A linear system with pole excess one or two is demonstrated to be particularly interesting. Stability conditions for these cases are derived. It is also discussed how fast relay switches can be approximated by sliding modes. Performance limitations in linear multivariable control systems is the second topic. It is proved that if the top left submatrices of a stable transfer matrix have no right half-plane zeros and a certain high-frequency condition holds, then there exists a diagonal stabilizing feedback that makes a weighted sensitivity function arbitrarily small. Implications on control structure design and sequential loop-closure are given. A novel multivariable laboratory process is also presented. Its linearized dynamics have a transmission zero that can be located anywhere on the real axis by simply adjusting two valves. This process is well suited to illustrate many issues in multivariable control, for example, control design limitations due to right half-plane zeros. The third topic is a combination of relay feedback and multivariable control. Tuning of individual loops in an existing multivariable control system is discussed. It is shown that a specific relay feedback experiment can be used to obtain process information suitable for performance improvement in a loop, without any prior knowledge of the system dynamics. The influence of the loop retuning on the overall closed-loop performance is derived and interpreted in several ways
Large Eddy Simulation Studies of Island Effects in the Caribbean Trade Wind Region
In dieser Dissertation wird das kompressible, nicht-hydrostatische und dreidimensionale Modell
All Scale Atmospheric Model (ASAM) fĂŒr Grobstruktur- bzw.
Large-Eddy-Simulationen (LES) angewendet, um lokale
Inseleffekte in der karibischen Passatwindzone zu untersuchen.
Da das Modell bis dato noch keine Anwendung im Bereich von LES feuchter atmosphÀrischer
Grenzschichten und heterogener OberflÀchen fand, wurden einige Bestandteile zum Modellcode
hinzugefĂŒgt oder ĂŒberarbeitet. Ein Hauptaugenmerk liegt dabei auf das Einbeziehen orographischer
Strukturen mittels angeschnittener Zellen (engl. cut cells). Sowohl die rÀumliche und zeitliche
Diskretisierung der Modellgleichungen als auch die nötigen physikalischen Parameterisierungen werden
in einer umfassenden Modellbeschreibung zusammengefasst. Die Robustheit und StabilitÀt der Modellformulierung
wird durch eine Reihe von Simulationen idealisierter TestfÀlle bestÀtigt.
Large-Eddy-Simulationen werden fĂŒr das Gebiet der Karibikinsel Barbados
zur Untersuchung von Inseleffekten bezĂŒglich Grenzschichtmodifikation, Wolkenbildung
und vertikaler Durchmischung von Aerosolen durchgefĂŒhrt. Durch das Vorhandensein
einer topographisch strukturierten InseloberflÀche in der Mitte des Modellgebietes
muss das Modellsetup offene seitliche Randbedingungen beinhalten. Damit das einströmende
Windfeld konsistent mit der Dynamik einer turbulenten, marinen Grenzschicht ist,
wird eine neue Methode implementiert und angewendet, welche auf Störungen des potentiellen Temperaturfeldes mittels finiter Amplituden basiert.
Beobachtungen aus der SALTRACE-Messkampagne werden benutzt, um die ModelllÀufe anzutreiben.
Die Ergebnisse einiger SensitivitÀtstests zeigen Probleme der Modellierung im Bereich der \"Terra incognita\" auf. Dabei handelt es sich um die Modellierung auf rÀumlichen Skalen, welche
zwischen denen von LES und wolkenauflösenden Modellen liegen. AuĂerdem werden Auswirkungen von
entweder turbulent oder laminar anströmenden Windfeldern auf die Simulationsergebnisse untersucht.
Besonders die Wolkeneigenschaften im Lee von Barbados werden in diesen Simulationen merklich
beeinflusst. Ergebnisse einer weiteren Simulation mit einer sehr starken Passatinversion bringt
deren Einfluss auf die Dicke und Höhe der simulierten Wolkenschichten zum Vorschein. Die VerÀnderung von
Saharastaubschichten, welche Barbados ĂŒber weitrĂ€umigen Transport ĂŒber den Atlantik erreichen,
wird analysiert. Die Auswirkungen beinhalten sowohl eine AusdĂŒnnung und ein Absinken dieser Schichten
als auch turbulenter Transport in Richtung ErdoberflÀche.
Die genaue Position der beeinflussten Schichten und die StÀrke des turbulenten Mischens
werden hauptsÀchlich von der atmosphÀrischen Schichtung, der InversionsstÀrke und Windscherung
gesteuert. Vergleiche zwischen den LES-Modellergebnissen und Daten aus Doppler-Windlidarmessungen
zeigen gute Ăbereinstimmungen in der Formierung der konvektiven Strukturen tagsĂŒber und
des Vertikalwindfeldes.In this thesis, the fully compressible, three-dimensional,
nonhydrostatic atmospheric model called All Scale Atmospheric Model
(ASAM) is utilized for large eddy simulations (LES)
to investigate local island effects at the Caribbean.
Since the model has not been applied to LES for moist boundary layers
and heterogeneous surfaces so far, several parts are added to the model code
or reworked. A special focus lies on the inclusion of orographical structures
via the cut cell method. Spatial and temporal discretization as well as
necessary physical parameterizations are summarized in a thorough model description.
The robustness of the model formulation is confirmed by a set of idealized
test case simulations.
Large eddy simulations are performed for the area of the Caribbean island Barbados to investigate
island effects on boundary layer modification, cloud generation and vertical mixing of aerosols.
Due to the presence of a topographically structured island surface in the domain center,
the model setup has to be designed with open lateral boundaries.
In order to generate inflow turbulence consistent with the upstream marine boundary layer forcing,
the newly developed cell perturbation method based on finite amplitude perturbations is applied.
Observations from the SALTRACE field campaign are used to initialize the model runs.
Several numerical sensitivity tests are carried out
to demonstrate the problems related to \"gray zone modeling\" beyond LES scales
or when the turbulent marine boundary layer flow
is replaced by laminar winds. Especially cloud properties west of Barbados (downwind)
are markedly affected in these simulations.
Results of an additional simulation with a strong trade-wind inversion
reveal its effect on cloud layer depth and height.
The modification of Saharan dust layers reaching Barbados via
long-range transport over the North Atlantic is analyzed.
Effects of layer thinning, subsidence and turbulent downward transport
near the layer bottom become apparent.
The position of these layers and strength of downward mixing is found
to be mainly controlled atmospheric stability, inversion strength and wind shear.
Comparisons of LES model output with wind lidar data show similarities in the
formation of the daytime convective plume and the vertical wind structure
Systems reliability issues for future aircraft
The reliability of adaptive controls for future aircraft are discussed. The research, formulation, and experimentation for improved aircraft performance are considered
MATLAB
This excellent book represents the final part of three-volumes regarding MATLAB-based applications in almost every branch of science. The book consists of 19 excellent, insightful articles and the readers will find the results very useful to their work. In particular, the book consists of three parts, the first one is devoted to mathematical methods in the applied sciences by using MATLAB, the second is devoted to MATLAB applications of general interest and the third one discusses MATLAB for educational purposes. This collection of high quality articles, refers to a large range of professional fields and can be used for science as well as for various educational purposes
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