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NASA NTRS1980

Time spectral analysis of midlatitude disturbances in the Martian atmosphere

Time-spectral and cross-spectral analyses are used to investigate synoptic pressure variations in the Viking-2 pressure, wind, and temperature data for selected portions of the Mars fall, winter, and spring seasons. Estimates of the phase relationships between the highly coherent pressure, wind, and temperature oscillations are obtained, and found to be very similar to those expected for baroclinic waves and to those obtained from terrestrial surface data. Phase speeds and zonal wavenumbers are inferred by interpreting the pressure and meridional wind variations in terms of eastward-traveling quasi-geostrophic waves.

Barnes, J. R.↗

NASA NTRS1990

Numerical prediction of turbulent flame stability in premixed/prevaporized (HSCT) combustors

A numerical analysis of combustion instabilities that induce flashback in a lean, premixed, prevaporized dump combustor is performed. KIVA-II, a finite volume CFD code for the modeling of transient, multidimensional, chemically reactive flows, serves as the principal analytical tool. The experiment of Proctor and T'ien is used as a reference for developing the computational model. An experimentally derived combustion instability mechanism is presented on the basis of the observations of Proctor and T'ien and other investigators of instabilities in low speed (M less than 0.1) dump combustors. The analysis comprises two independent procedures that begin from a calculated stable flame: The first is a linear increase of the equivalence ratio and the second is the linear decrease of the inflow velocity. The objective is to observe changes in the aerothermochemical features of the flow field prior to flashback. It was found that only the linear increase of the equivalence ratio elicits a calculated flashback result. Though this result did not exhibit large scale coherent vortices in the turbulent shear layer coincident with a flame flickering mode as was observed experimentally, there were interesting acoustic effects which were resolved quite well in the calculation. A discussion of the k-e turbulence model used by KIVA-II is prompted by the absence of combustion instabilities in the model as the inflow velocity is linearly decreased. Finally, recommendations are made for further numerical analysis that may improve correlation with experimentally observed combustion instabilities.

Winowich, Nicholas S.↗

NASA NTRS1980

Evaluation of space shuttle main engine fluid dynamic frequency response characteristics

In order to determine the POGO stability characteristics of the space shuttle main engine liquid oxygen (LOX) system, the fluid dynamic frequency response functions between elements in the SSME LOX system was evaluated, both analytically and experimentally. For the experimental data evaluation, a software package was written for the Hewlett-Packard 5451C Fourier analyzer. The POGO analysis software is documented and consists of five separate segments. Each segment is stored on the 5451C disc as an individual program and performs its own unique function. Two separate data reduction methods, a signal calibration, coherence or pulser signal based frequency response function blanking, and automatic plotting features are included in the program. The 5451C allows variable parameter transfer from program to program. This feature is used to advantage and requires only minimal user interface during the data reduction process. Experimental results are included and compared with the analytical predictions in order to adjust the general model and arrive at a realistic simulation of the POGO characteristics.

Gardner, T. G.↗

NASA NTRS2001

Rotor Broadband Noise Prediction with Comparison to Model Data

This paper reports an analysis and prediction development of rotor broadband noise. The two primary components of this noise are Blade-Wake Interaction (BWI) noise, due to the blades' interaction with the turbulent wakes of the preceding blades, and "Self" noise, due to the development and shedding of turbulence within the blades' boundary layers. Emphasized in this report is the new code development for Self noise. The analysis and validation employs data from the HART program, a model BO-105 rotor wind tunnel test conducted in the German-Dutch Wind Tunnel (DNW). The BWI noise predictions are based on measured pressure response coherence functions using cross-spectral methods. The Self noise predictions are based on previously reported semiempirical modeling of Self noise obtained from isolated airfoil sections and the use of CAMRAD.Modl to define rotor performance and local blade segment flow conditions. Both BWI and Self noise from individual blade segments are Doppler shifted and summed at the observer positions. Prediction comparisons with measurements show good agreement for a range of rotor operating conditions from climb to steep descent. The broadband noise predictions, along with those of harmonic and impulsive Blade-Vortex Interaction (BVI) noise predictions, demonstrate a significant advance in predictive capability for main rotor noise.

Brooks, Thomas F.↗

NASA NTRS2018

NICER Discovers the Ultracompact Orbit of the Accreting Millisecond Pulsar IGR J17062-6143

We present results of recent Neutron Star Interior Composition Explorer (NICER) observations of the accreting millisecond X-ray pulsar (AMXP) IGR J17062-6143 that show that it resides in a circular, ultracompact binary with a 38-minute orbital period. NICER observed the source for 26 kiloseconds over a 5.3-day span in 2017 August, and again for 14 and 11 kiloseconds in 2017 October and November, respectively. A power spectral analysis of the August exposure confirms the previous detection of pulsations at 163.656 Hertz in Rossi X-ray Timing Explorer (RXTE) data, and reveals phase modulation due to orbital motion of the neutron star. A coherent search for the orbital solution using the Z squared method finds a best-fitting circular orbit with a period of 2278.21 seconds (37.97 minutes), a projected semimajor axis of 0.00390 lt-s (Localization Test Statistic), and a barycentric pulsar frequency of 163.6561105 Hertz. This is currently the shortest known orbital period for an AMXP. The mass function is 9.12 times 10 (sup minus 8) solar mass, presently the smallest known for a stellar binary. The minimum donor mass ranges from approximately 0.005 to 0.007 times the solar mass for a neutron star mass from 1.2 to 2 times the solar mass. Assuming mass transfer is driven by gravitational radiation, we find donor mass and binary inclination bounds of 0.0175-0.0155 times the solar mass and 19 degrees less than i less than 27.5 degrees, where the lower and upper bounds correspond to 1.4 and 2 times the solar mass neutron stars, respectively. Folding the data accounting for the orbital modulation reveals a sinusoidal profile with fractional amplitude 2.04 plus or minus 0.11 percent (0.3-3.2 kiloelectronvolts).

Strohmayer, T. E.↗

NASA NTRS2023

On the Physical Nature of the So-Called Prominence Tornadoes

The term ‘tornado’ has been used in recent years to describe several solar phenomena, from large-scale eruptive prominences to small-scale photospheric vortices. It has also been applied to the generally stable quiescent prominences, sparking a renewed interest in what historically was called ‘prominence tornadoes’. This paper carries out an in-depth review of the physical nature of ‘prominence tornadoes’, where their name subconsciously makes us think of violent rotational dynamics. However, after careful consideration and analysis of the published observational data and theoretical models, we conclude that ‘prominence tornadoes’ do not differ in any substantial way from other stable solar prominences. There is simply no unequivocal observational evidence of sustained and coherent rotational movements in quiescent prominences that would justify a distinct category of prominences sharing the name with the well-known atmospheric phenomenon. The visual impression of the column-like silhouettes, the perceived helical motions, or the suggestive Doppler-shift patterns all have a simpler, more likely explanation. They are a consequence of projection effects combined with the presence of oscillations and/or counter-streaming flows. ‘Prominence tornadoes’ are thus just manifestations of the complex nature of solar prominences when observed in specific projections. These coincidental viewing angles, together with the presence of fine-structure dynamics and simple yet profoundly distorting projection effects, may sometimes play havoc with our intuitive understanding of perceived shapes and motions, leading to the incorrect analogy with atmospheric tornadoes.

Stanislav Gunar↗

NASA NTRS1977

Subsonic wind-tunnel performance

Analysis of conditional sampling averages for turbulent flows shows how conditional averages can be related to conventional averages incorporated into existing flow models. Technique allows vast quantities of data generated by conditional sampling to be analyzed in coherent way.

Eckert, W. T.↗

NASA NTRS1977

Statistical properties of the sound and source fields of an axisymmetric jet

It is shown, by measurements and analysis, that the sound field radiated from subsonic jets as well as the source field are statistically axially symmetric at lower frequencies. In this range, the source field behaves like a line-quadrupole. The sources are coherent with respect to phase and compact with respect to acoustic wavelength. The results are developed in terms of correlation, coherence, and phase. A new and simpler form of the source integral is obtained in the range where the phases are linear functions of frequency through symmetry, translation, and rotation of the axis.

Maestrello, L.↗

NASA NTRS1988

A mesoscale gravity wave event observed during CCOPE. III - Wave environment and probable source mechanisms

The multiscale environment of gravity wave events and the probable mechanisms of their origin are examined on the basis of observations taken during the Cooperative Convective Precipitation Experiment in extreme eastern Montana, during the period from 1200 UTC July 11, 1981, to 0500 UTC July 12. During this time, two distinct gravity wave episodes were diagnosed. The results of the analysis of the evolving structures in the subsynoptic-scale and mesoscale environments indicate that the observed mesoscale gravity waves were generated by geostrophic adjustment processes, with additional energy supplied through interaction with the critical level; their coherence was maintained through a ducting mechanism.

Koch, Steven E.↗

NASA NTRS2015

On the Connection Between Flap Side-Edge Noise and Tip Vortex Dynamics

The goal of the present work is to investigate how the dynamics of the vortical flow about the flap side edge of an aircraft determine the acoustic radiation. A validated lattice- Boltzmann CFD solution of the unsteady flow about a detailed business jet configuration in approach conditions is used for the present analysis. Evidence of the connection between the noise generated by several segments of the inboard flap tip and the aerodynamic forces acting on the same segments is given, proving that the noise generation mechanism has a spatially coherent and acoustically compact character on the scale of the flap chord, and that the edge-scattering effects are of secondary importance. Subsequently, evidence of the connection between the kinematics of the tip vortex system and the aerodynamic force is provided. The kinematics of the dual vortex system are investigated via a core detection technique. Emphasis is placed on the mutual induction effects between the two main vortices rolling up from the pressure and suction sides of the flap edge. A simple heuristic formula that relates the far-field noise spectrum and the cross-spectrum of the unsteady vortical positions is developed.

Casalino, D.↗

NASA NTRS1973

An investigation and conceptual design of a holographic starfield and landmark tracker

The analysis, experiments, and design effort of this study have supported the feasibility of the basic holographic tracker concept. Image intensifiers and photoplastic recording materials were examined, along with a Polaroid rapid process silver halide material. Two reference beam, coherent optical matched filter technique was used for multiplexing spatial frequency filters for starfields. A 1 watt HeNe laser and an electro-optical readout are also considered.

Welch, J. D.↗

NASA NTRS1992

Sea surface velocities from visible and infrared multispectral atmospheric mapping sensor imagery

High resolution (100 m), sequential Multispectral Atmospheric Mapping Sensor (MAMS) images were used in a study to calculate advective surface velocities using the Maximum Cross Correlation (MCC) technique. Radiance and brightness temperature gradient magnitude images were formed from visible (0.48 microns) and infrared (11.12 microns) image pairs, respectively, of Chandeleur Sound, which is a shallow body of water northeast of the Mississippi delta, at 145546 GMT and 170701 GMT on 30 Mar. 1989. The gradient magnitude images enhanced the surface water feature boundaries, and a lower cutoff on the gradient magnitudes calculated allowed the undesirable sunglare and backscatter gradients in the visible images, and the water vapor absorption gradients in the infrared images, to be reduced in strength. Requiring high (greater than 0.4) maximum cross correlation coefficients and spatial coherence of the vector field aided in the selection of an optimal template size of 10 x 10 pixels (first image) and search limit of 20 pixels (second image) to use in the MCC technique. Use of these optimum input parameters to the MCC algorithm, and high correlation and spatial coherence filtering of the resulting velocity field from the MCC calculation yielded a clustered velocity distribution over the visible and infrared gradient images. The velocity field calculated from the visible gradient image pair agreed well with a subjective analysis of the motion, but the velocity field from the infrared gradient image pair did not. This was attributed to the changing shapes of the gradient features, their nonuniqueness, and large displacements relative to the mean distance between them. These problems implied a lower repeat time for the imagery was needed in order to improve the velocity field derived from gradient imagery. Suggestions are given for optimizing the repeat time of sequential imagery when using the MCC method for motion studies. Applying the MCC method to the infrared brightness temperature imagery yielded a velocity field which did agree with the subjective analysis of the motion and that derived from the visible gradient imagery. Differences between the visible and infrared derived velocities were 14.9 cm/s in speed and 56.7 degrees in direction. Both of these velocity fields also agreed well with the motion expected from considerations of the ocean bottom topography and wind and tidal forcing in the study area during the 2.175 hour time interval.

Pope, P. A.↗

NASA NTRS2000

The Origin of the X-Ray and Ultraviolet Emission in NGC 7469

We present a spectral analysis of a approx 30 day, near-continuous observation of the Seyfert 1 galaxy NGC 7469 with RXTE. Daily integrations show strong spectral changes during the observation. Our main result is that we find the X-ray spectral index to be correlated with the UV flux. Furthermore, the broadband X-ray photon flux is also correlated with the UV continuum. These correlations point toward a model in which the X-rays originate via thermal Comptonization of UV seed photons. Furthermore, the UV is also correlated with the extrapolation of the X-ray power law into the soft X-ray/EUV region. Our data analysis therefore reopens the possibility that the UV photons and their variability arise from reprocessing, as long as the primary source of heating is photoelectric absorption in the re-processor, rather than Compton down-scattering. A coherent picture of the X-ray/UV variability can therefore be constructed whereby absorption and reprocessing of EUV/soft X-rays in a standard accretion disk produce a variable seed photon distribution, which is in turn up-scattered into the X-ray band. We also find a significant correlation between the 2-10 keV flux and the 6.4 keV iron K-alpha line, suggesting that at least some portion of the line originates within approx. 1 light day of the X-ray continuum source. Neither the power-law photon index nor the Compton reflection component are correlated with the 2-10 keV flux. The latter is not correlated with the iron K-alpha line flux either. We do find an apparent correlation between the X-ray spectral index and the strength of the Compton reflection component. In an Appendix we show, however, that this can be produced by a combination of statistical and systematic errors. We conclude that the apparent variations in the Compton reflection component may be an artifact of these effects.

Nandra, K.↗

NASA NTRS2013

Methods for Retrievals of CO2 Mixing Ratios from JPL Laser Absorption Spectrometer Flights During a Summer 2011 Campaign

The JPL airborne Laser Absorption Spectrometer instrument has been flown several times in the 2007-2011 time frame for the purpose of measuring CO2 mixing ratios in the lower atmosphere. This instrument employs CW laser transmitters and coherent detection receivers in the 2.05- micro m spectral region. The Integrated Path Differential Absorption (IPDA) method is used to retrieve weighted CO2 column mixing ratios. We present key features of the evolving LAS signal processing and data analysis algorithms and the calibration/validation methodology. Results from 2011 flights in various U.S. locations include observed mid-day CO2 drawdown in the Midwest and high spatial resolution plume detection during a leg downwind of the Four Corners power plant in New Mexico.

laser remote sensing↗

NASA NTRS2020

Electromagnetic scattering by discrete random media illuminated by a Gaussian beam I: Derivation of the radiative transfer equation

In this paper we present the vector radiative transfer theory for a discrete random medium illuminated by a Gaussian beam. The analysis is based on a plane wave spectrum representation for a Gaussian beam and uses an approach developed previously for a discrete random medium illuminated by a plane electromagnetic wave. Specifically, we establish an integral representation for the coherent field, define an approximate coherent field that satisfies the differential equation fulfilled by the coherent field corresponding to a plane electromagnetic wave and matches the Gaussian beam at the interface of the particulate medium, and finally, derive the vector radiative transfer equation. For weakly focused Gaussian beams, the resulting equation is the traditional radiative transfer equation

Gaussian beam↗

NASA NTRS1994

(abstract) Observation by Clementine of a Hugh Opposition Surge on the Moon at Very Small Solar Phase Angles

The Clementine mission enabled the first quantative observations of the Moon at very small solar phase angles. It is well established from Earth-based observations that the Moon exhibits a non-linear increase in brightness as its face becomes fully illuminated to a terrestrial observer. Because the models of the opposition effect are sensitive indicators of surfical compaction state and particle size, observations at small solar phase angles are important to obtain. A recent model for optical coherent backscatter seeks to explain a narrow opposition spike at very small phase angles (< 1 degree) seen on several icy satellites , including Europa, Icarus 90, and Oberon . Over 90 images of the Moon's surface at phase angles less than 0.5(deg) (the minimum phase angle observable from Earth due to the Moon's angular size) were obtained by the Clementine spacecraft. Our analysis of these images shows the moon exhibits a surge in brightness of approximately 20% below 0.25(deg), comparable to the values observed on icy satellites. No color dependence was detected in the lunar phase curve below 2 degrees.

partical size icy satellites Europa Icarus 90 Ober↗

NASA NTRS1986

Analysis and measurement of electromagnetic scattering by pyramidal and wedge absorbers

By modifying the reflection coefficients in the Uniform Geometrical Theory of Diffraction a solution that approximates the scattering from a dielectric wedge is found. This solution agrees closely with the exact solution of Rawlins which is only valid for a few minor cases. This modification is then applied to the corner diffraction coefficient and combined with an equivalent current and geometrical optics solutions to model scattering from pyramid and wedge absorbers. Measured results from 12 inch pyramid absorbers from 2 to 18 GHz are compared to calculations assuming the returns add incoherently and assuming the returns add coherently. The measured results tend to be between the two curves. Measured results from the 8 inch wedge absorber are also compared to calculations with the return being dominated by the wedge diffraction. The procedures for measuring and specifying absorber performance are discussed and calibration equations are derived to calculate a reflection coefficient or a reflectivity using a reference sphere. Shaping changes to the present absorber designs are introduced to improve performance based on both high and low frequency analysis. Some prototypes were built and tested.

Dewitt, B. T.↗

NASA NTRS2004

Neural network uncertainty assessment using Bayesian statistics: a remote sensing application

Neural network (NN) techniques have proved successful for many regression problems, in particular for remote sensing; however, uncertainty estimates are rarely provided. In this article, a Bayesian technique to evaluate uncertainties of the NN parameters (i.e., synaptic weights) is first presented. In contrast to more traditional approaches based on point estimation of the NN weights, we assess uncertainties on such estimates to monitor the robustness of the NN model. These theoretical developments are illustrated by applying them to the problem of retrieving surface skin temperature, microwave surface emissivities, and integrated water vapor content from a combined analysis of satellite microwave and infrared observations over land. The weight uncertainty estimates are then used to compute analytically the uncertainties in the network outputs (i.e., error bars and correlation structure of these errors). Such quantities are very important for evaluating any application of an NN model. The uncertainties on the NN Jacobians are then considered in the third part of this article. Used for regression fitting, NN models can be used effectively to represent highly nonlinear, multivariate functions. In this situation, most emphasis is put on estimating the output errors, but almost no attention has been given to errors associated with the internal structure of the regression model. The complex structure of dependency inside the NN is the essence of the model, and assessing its quality, coherency, and physical character makes all the difference between a blackbox model with small output errors and a reliable, robust, and physically coherent model. Such dependency structures are described to the first order by the NN Jacobians: they indicate the sensitivity of one output with respect to the inputs of the model for given input data. We use a Monte Carlo integration procedure to estimate the robustness of the NN Jacobians. A regularization strategy based on principal component analysis is proposed to suppress the multicollinearities in order to make these Jacobians robust and physically meaningful.

Neural Networks (Computer)↗