51,110 research outputs found
New technique for determination of cross-power spectral density with damped oscillators
New cross-power spectral density computation technique has been developed, as well as a technique for discrimination between periodic and random signals. This development is applicable to analysis of any stationary random process, and can be used in the aerospace and transportation fields
Formation of magnetic skyrmions with tunable properties in PdFe bilayer deposited on Ir(111)
We perform an extensive study of the spin-configurations in a PdFe bilayer on
Ir(111) in terms of ab initio and spin-model calculations. We use the
spin-cluster expansion technique to obtain spin model parameters, and solve the
Landau-Lifshitz-Gilbert equations at zero temperature. In particular, we focus
on effects of layer relaxations and the evolution of the magnetic ground state
in external magnetic field. In the absence of magnetic field, we find a
spin-spiral ground state, while applying external magnetic field skyrmions are
generated in the system. Based on energy calculations of frozen spin
configurations with varying magnetic field we obtain excellent agreement for
the phase boundaries with available experiments. We find that the wave length
of spin-spirals and the diameter of skyrmions decrease with increasing inward
Fe layer relaxation which is correlated with the increasing ratio of the
nearest-neighbor Dzyaloshinskii-Moriya interaction and the isotropic exchange
coupling, . Our results also indicate that the applied field needed to
stabilize the skyrmion lattice increases when the diameter of individual
skyrmions decreases. Based on our observations, we suggest that the formation
of the skyrmion lattice can be tuned by small structural modification of the
thin film.Comment: 7 pages, 5 figures, 2 table
Mapping a star with transits: orbit precession effects in the Kepler-13 system
Kepler-13b (KOI-13.01) is a most intriguing exoplanet system due to the rapid
precession rate, exhibiting several exotic phenomena. We analyzed
Short Cadence data up to Quarter 14, with a total time-span of 928 days, to
reveal changes in transit duration, depth, asymmetry, and identify the possible
signals of stellar rotation and low-level activity. We investigated long-term
variations of transit light curves, testing for duration, peak depth and
asymmetry. We also performed cluster analysis on quarters. We computed
the autocorrelation function of the out-of-transit light variations. Transit
duration, peak depth, and asymmetry evolve slowly, due to the slowly drifting
transit path through the stellar disk. The detected transit shapes will map the
stellar surface on the time scale of decades. We found a very significant
clustering pattern with 3-orbit period. Its source is very probably the
rotating stellar surface, in the 5:3 spin-orbit resonance reported in a
previous study. The autocorrelation function of the out-of-transit light
variations, filtered to 25.4 hours and harmonics, shows slow variations and a
peak around 300--360 day period, which could be related to the activity cycle
of the host star.Comment: 7 pages, 7 figures, accepted in MNRA
Implementation of an intelligent control system
A laboratory testbed facility which was constructed at NASA LeRC for the development of an Intelligent Control System (ICS) for reusable rocket engines is described. The framework of the ICS consists of a hierarchy of various control and diagnostic functions. The traditional high speed, closed-loop controller resides at the lowest level of the ICS hierarchy. Above this level resides the diagnostic functions which identify engine faults. The ICS top level consists of the coordination function which manages the interaction between an expert system and a traditional control system. The purpose of the testbed is to demonstrate the feasibility of the OCS concept by implementing the ICS as the primary controller in a simulation of the Space Shuttle Main Engine (SSME). The functions of the ICS which are implemented in the testbed are as follows: an SSME dynamic simulation with selected fault mode models, a reconfigurable controller, a neural network for sensor validation, a model-based failure detection algorithm, a rule based failure detection algorithm, a diagnostic expert system, an intelligent coordinator, and a user interface which provides a graphical representation of the event occurring within the testbed. The diverse nature of the ICS has led to the development of a distributed architecture consisting of specialized hardware and software for the implementation of the various functions. This testbed is made up of five different computer systems. These individual computers are discussed along with the schemes used to implement the various ICS components. The communication between computers and the timing and synchronization between components are also addressed
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