3,154 research outputs found
Coherent Raman spectroscopy for supersonic flow measurments
In collaboration with NASA/Langley Research Center, a truly nonintrusive and nonseeding method for measuring supersonic molecular flow parameters was proposed and developed at Colorado State University. The feasibility of this Raman Doppler Velocimetry (RDV), currently operated in a scanning mode, was demonstrated not only in a laboratory environment at Colorado State University, but also in a major wind tunnel at NASA/Langley Research Center. The research progress of the RDV development is summarized. In addition, methods of coherent Rayleigh-Brillouin spectroscopy and single-pulse coherent Raman spectroscopy are investigated, respectively, for measurements of high-pressure and turbulent flows
Coherent Raman spectroscopies for measuring molecular flow velocity
Various types of coherent Raman spectroscopy are characterized and their application to molecular flow velocity and direction measurement and species concentration and temperature determination is discussed
IR pumped third-harmonic generation and sum-frequency generation in diatomic molecules
The potential efficiency of using nonlinear up-conversion techniques for the high efficiency type lasers (CO,CO2, and chemical) is assessed. Results indicate that: the small pump photon energy necessitates the use of molecular media for conversion if resonance enhancement is to be used and that molecular systems present several problems. These difficulties include: their levels are complex; their transition probabilities are often unknown; and the oscillator strengths among vibrational levels in the ground electronic state of a molecule are much smaller than those among electronic states of an atom, thus limiting the magnitude of nonlinear interactions. It is shown that this problem can be eliminated by making use of vibronic transitions which, being primarily electronic transitions have much larger matrix elements and efficient conversion can be achieved with molecular systems
Velocity measurements by laser resonance fluorescence
The photonburst correlation method was used to detect single atoms in a buffer gas. Real time flow velocity measurements with laser induced resonance fluorescence from single or multiple atoms was demonstrated and this method was investigated as a tool for wind tunnel flow measurement. Investigations show that single atoms and their real time diffusional motion on a buffer gas can be measured by resonance fluorescence. By averaging over many atoms, flow velocities up to 88 m/s were measured in a time of 0.5 sec. It is expected that higher flow speeds can be measured and that the measurement time can be reduced by a factor of 10 or more by careful experimental design. The method is clearly not ready for incorporation in high speed wind tunnels because it is not yet known whether the stray light level will be higher or lower, and it is not known what detection efficiency can be obtained in a wind tunnel situation
Autofocus for ISAR Imaging Using Higher Order Statistics
Autofocus is imperative for inverse synthetic aperture radar (ISAR) imaging. In this letter, a new approach for ISAR autofocus is developed by using fourth-order statistics properties of the radar’s return signal. After the ISAR signal model is established, the approach is described. The results of processing real data confirm the effectiveness of the proposed approach and show its capability for suppressing noise. The developed approach has a numerical stability and a smaller computational load compared with the maximum image contrast and the minimum image entropy methods
Outlier Detection Using Nonconvex Penalized Regression
This paper studies the outlier detection problem from the point of view of
penalized regressions. Our regression model adds one mean shift parameter for
each of the data points. We then apply a regularization favoring a sparse
vector of mean shift parameters. The usual penalty yields a convex
criterion, but we find that it fails to deliver a robust estimator. The
penalty corresponds to soft thresholding. We introduce a thresholding (denoted
by ) based iterative procedure for outlier detection (-IPOD). A
version based on hard thresholding correctly identifies outliers on some hard
test problems. We find that -IPOD is much faster than iteratively
reweighted least squares for large data because each iteration costs at most
(and sometimes much less) avoiding an least squares estimate.
We describe the connection between -IPOD and -estimators. Our
proposed method has one tuning parameter with which to both identify outliers
and estimate regression coefficients. A data-dependent choice can be made based
on BIC. The tuned -IPOD shows outstanding performance in identifying
outliers in various situations in comparison to other existing approaches. This
methodology extends to high-dimensional modeling with , if both the
coefficient vector and the outlier pattern are sparse
Winding String Condensation and Noncommutative Deformation of Spacelike Singularity
In a previous paper (hep-th/0509067) using matrix model, we showed that
closed string tachyons can resolve spacelike singularity in one particular
class of Misner space (with anti-periodic boundary conditions for fermions
around the spatial circle). In this note, we show that for Misner space without
closed string tachyons, there also exists a mechanism to resolve the
singularity in the context of the matrix model, namely cosmological winding
string production. We show that here space and time also become noncommutative
due to these winding strings. Employing optical theorem, we study the bulk
boundary coupling by calculating the four-open-string cylinder amplitudes.Comment: 16 pages, no figures, harvmac; references added; added a section of
discussion on disk and cylinder amplitude
Towards a comprehensive view of dust events from multiple satellite and ground measurements: exemplified by the May 2017 East Asian dust storm
One or several aspects of the
source, distribution, transport, and optical properties of airborne dust have
been characterized using different types of satellite and ground
measurements, each with unique advantages. In this study, a dust event that occurred over the
East Asia area in May 2017 was exemplified to demonstrate how all the above-mentioned aspects of a dust event can be pictured by combining the advantages
of different satellite and ground measurements. The data used included the
Himawari-8 satellite Advanced Himawari Imager (AHI) true-colour images, the
Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)
Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aerosol vertical
profiles, the Aura satellite Ozone Monitoring Instrument (OMI) aerosol index
images, and the ground-based Aerosol Robotic Network (AERONET) aerosol
properties and the ground station particulate matter (PM) measurements. From
the multi-satellite/sensor (AHI, CALIOP, and OMI) time series observations,
the dust storm was found to originate from the Gobi Desert on the morning of
3 May 2017 and transport north-eastward to the Bering Sea, eastward to the
Korean Peninsula and Japan, and southward to south-central China. The air
quality in China deteriorated drastically: the PM10 (PM < 10 µm in aerodynamic diameter) concentrations measured at some air quality
stations located in northern China reached 4333 µg m−3. At the
AOE_Baotou, Beijing, Xuzhou-CUMT, and Ussuriysk AERONET sites,
the maximum aerosol optical depth values reached 2.96, 2.13, 2.87, and 0.65
and the extinction Ångström exponent dropped to 0.023, 0.068, 0.03,
and 0.097, respectively. The dust storm also induced unusual aerosol spectral
single-scattering albedo and volume size distribution.</p
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