40,656 research outputs found
A mathematical simulation model of the CH-47B helicopter, volume 1
A nonlinear simulation model of the CH-47B helicopter was adapted for use in the NASA Ames Research Center (ARC) simulation facility. The model represents the specific configuration of the ARC variable stability CH-47B helicopter and will be used in ground simulation research and to expedite and verify flight experiment design. Modeling of the helicopter uses a total force approach in six rigid body degrees of freedom. Rotor dynamics are simulated using the Wheatlely-Bailey equations including steady-state flapping dynamics. Also included in the model is the option for simulation of external suspension, slung-load equations of motion
Novel characterization method of impedance cardiography signals using time-frequency distributions
The purpose of this document is to describe a methodology to select the most adequate time-frequency distribution (TFD) kernel for the characterization of impedance cardiography signals (ICG). The predominant ICG beat was extracted from a patient and was synthetized using time-frequency variant Fourier approximations. These synthetized signals were used to optimize several TFD kernels according to a performance maximization. The optimized kernels were tested for noise resistance on a clinical database. The resulting optimized TFD kernels are presented with their performance calculated using newly proposed methods. The procedure explained in this work showcases a new method to select an appropriate kernel for ICG signals and compares the performance of different time-frequency kernels found in the literature for the case of ICG signals. We conclude that, for ICG signals, the performance (P) of the spectrogram with either Hanning or Hamming windows (P¿=¿0.780) and the extended modified beta distribution (P¿=¿0.765) provided similar results, higher than the rest of analyzed kernels.Peer ReviewedPostprint (published version
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Chemical characterization of water-soluble organic carbon aerosols at a rural site in the Pearl River Delta, China, in the summer of 2006
Online measurements of water-soluble organic carbon (WSOC) aerosols were made using a particle-into-liquid sampler (PILS) combined with a total organic carbon (TOC) analyzer at a rural site in the Pearl River Delta region, China, in July 2006. A macroporous nonionic (DAX-8) resin was used to quantify hydrophilic and hydrophobic WSOC, which are defined as the fractions of WSOC that penetrated through and retained on the DAX-8 column, respectively. Laboratory calibrations showed that hydrophilic WSOC (WSOCHPI) included low-molecular aliphatic dicarboxylic acids and carbonyls, saccharides, and amines, while hydrophobic WSOC (WSOCHPO) included longer-chain aliphatic dicarboxylic acids and carbonyls, aromatic acids, phenols, organic nitrates, cyclic acids, and fulvic acids. On average, total WSOC (TWSOC) accounted for 60% of OC, and WSOCHPO accounted for 60% of TWSOC. Both WSOC HIP and WSOCHPO increased with photochemical aging determined from the NOx/NOy ratio. In particular, the average WSOCHPO mass was found to increase by a factor of five within a timescale of ∼10 hours, which was substantially larger than that of WSOCHPI (by a factor of 2-3). The total increase in OC mass with photochemical aging was associated with the large increase in WSOCHPO mass. These results, combined with the laboratory calibrations, suggest that significant amounts of hydrophobic organic compounds (likely containing large carbon numbers) were produced by photochemical processing. By contrast, water-insoluble OC (WIOC) mass did not exhibit significant changes with photochemical aging, suggesting that chemical transformation of WIOC to WSOC was not a dominant process for the production of WSOC during the study period. Copyright 2009 by the American Geophysical Union
High-Pressure Induced Structural Phase Transition in CaCrO4: Evidence from Raman Scattering Studies
Raman spectroscopic studies have been carried out on CaCrO4 under pressure up
to 26GPa at ambient temperature. The Raman spectra showed CaCrO4 experienced a
continuous structural phase transition started at near 6GPa, and finished at
about 10GPa. It is found that the high-pressure phase could be quenched to
ambient conditions. Pressure dependence of the Raman peaks suggested there
existed four pressure regions related to different structural characters. We
discussed these characters and inferred that the nonreversible structural
transition in CaCrO4, most likely was from a zircon-type (I41/amd) ambient
phase to a scheelite-type high pressure structure (I41/a).Comment: submitte
Upper limits on stray force noise for LISA
We have developed a torsion pendulum facility for LISA gravitational
reference sensor ground testing that allows us to put significant upper limits
on residual stray forces exerted by LISA-like position sensors on a
representative test mass and to characterize specific sources of disturbances
for LISA. We present here the details of the facility, the experimental
procedures used to maximize its sensitivity, and the techniques used to
characterize the pendulum itself that allowed us to reach a torque sensitivity
below 20 fNm /sqrt{Hz} from 0.3 to 10 mHz. We also discuss the implications of
the obtained results for LISA.Comment: To be published in Classical and Quantum Gravity, special issue on
Amaldi5 2003 conference proceedings (10 pages, 6 figures
Analysis of aggregated tick returns: evidence for anomalous diffusion
In order to investigate the origin of large price fluctuations, we analyze
stock price changes of ten frequently traded NASDAQ stocks in the year 2002.
Though the influence of the trading frequency on the aggregate return in a
certain time interval is important, it cannot alone explain the heavy tailed
distribution of stock price changes. For this reason, we analyze intervals with
a fixed number of trades in order to eliminate the influence of the trading
frequency and investigate the relevance of other factors for the aggregate
return. We show that in tick time the price follows a discrete diffusion
process with a variable step width while the difference between the number of
steps in positive and negative direction in an interval is Gaussian
distributed. The step width is given by the return due to a single trade and is
long-term correlated in tick time. Hence, its mean value can well characterize
an interval of many trades and turns out to be an important determinant for
large aggregate returns. We also present a statistical model reproducing the
cumulative distribution of aggregate returns. For an accurate agreement with
the empirical distribution, we also take into account asymmetries of the step
widths in different directions together with crosscorrelations between these
asymmetries and the mean step width as well as the signs of the steps.Comment: 9 pages, 10 figures, typos correcte
Uplink performance investigations of the service area based beyond 3G system JOINT
The joint transmission and detection integrated network (JOINT) is a novel OFDM-based vNP00 air interface solution for beyond 3G (B3G) mobile radio communications systems WMSL02. JOINT aims at eliminating the multiple access interference (MAI) and improving the system capacity by the application of MIMO techniques applied in the service area (SA) based system architecture. In a SA based structure the intra-SA multiple access interference (MAI) can be easily combated by algorithms like, e.g., joint detection (JD) Kle96,Ver98 in the uplink. The parameters like, e.g., the SA size, the system load and reuse factor show great impacts on the system performances, in terms of the average bit error rate (BER) and the BER statistics. Spectrum efficiency of JOINT is also investigated based on the simulation results
Resonant vibrations, peak broadening and noise in single molecule contacts: beyond the resonant tunnelling picture
We carry out experiments on single-molecule junctions at low temperatures,
using the mechanically controlled break junction technique. Analyzing the
results received with more than ten different molecules the nature of the first
peak in the differential conductance spectra is elucidated. We observe an
electronic transition with a vibronic fine structure, which is most frequently
smeared out and forms a broad peak. In the usual parameter range we find strong
indications that additionally fluctuations become active even at low
temperatures. We conclude that the electrical field feeds instabilities, which
are triggered by the onset of current. This is underscored by noise
measurements that show strong anomalies at the onset of charge transport
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