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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

A bootstrap approach to bump hunting

An important question in cluster analysis and pattern recognition is the determination of the number of clusters into which a given population should be divided. Frequently, particularly when certain specific clustering methods are being used, the number of clusters is taken to be equal to the number of modes, or local maxima, in the probability density function underlying the given data set. The use of kernal density estimates in mode estimation is discussed. The test statistic to be used is defined and a bootstrap technique for assessing significance is given. An illustrative application is followed by an examination of the asymptotic behavior of the test statistic.

Silverman, B. W.↗
NASA NTRS1983

A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows

Reaction rates in turbulent, reacting flows are reviewed. Assumed probability density functions (pdf) modeling of reaction rates is being investigated in relation to a three variable pdf employing a 'most likely pdf' model. Chemical kinetic mechanisms treating hydrogen air combustion is studied. Perfectly stirred reactor modeling of flame stabilizing recirculation regions was used to investigate the stable flame regions for silane, hydrogen, methane, and propane, and for certain mixtures thereof. It is concluded that in general, silane can be counted upon to stabilize flames only when the overall fuel air ratio is close to or greater than unity. For lean flames, silane may tend to destabilize the flame. Other factors favoring stable flames are high initial reactant temperatures and system pressure.

Goldstein, D.↗
NASA NTRS1983

Monte Carlo Modeling in Elliptic Flows

A Monte Carlo method capable of predicting scalar (e.g., concentration) probability density functions (pdf's) in nonreacting and reacting elliptic flows was developed. A research combustor specifically designed to provide measurements of concentration and temperature pdf's in nonreacting and reacting flows is emphasized.

Radhakrishnan, K.↗
NASA NTRS1983

A mathematical characterization of vegetation effect on microwave remote sensing from the Earth

In passive microwave remote sensing of the earth, a theoretical model that utilizes the radiative transfer equations was developed to account for the volume scattering effects of the vegetation canopy. Vegetation canopies such as alfalfa, sorghum, and corn are simulated by a layer of ellipsoidal scatterers and cylindrical structures. The ellipsoidal scatterers represent the leaves of vegetation and are randomly positioned and oriented. The orientation of ellipsoids is characterized by a probability density function of Eulerian angles of rotation. The cylindrical structures represent the stalks of vegetation and their radii are assumed to be much smaller than their lengths. The underlying soil is represented by a half-space medium with a homogeneous permittivity and uniform temperature profile. The radiative transfer quations are solved by a numerical method using a Gaussian quadrature formula to compute both the vertical and horizontal polarized brightness temperature as a function of observation angle. The theory was applied to the interpretation of experimental data obtained from sorghum covered fields near College Station, Texas.

Choe, Y.↗
NASA NTRS1983

B-57B gust gradient program

Analysis of data obtained from the Joint Airport Weather Studies Flights 6 and 7 is underway. Data from Flight 7 indicate that the B-57B encountered the upper portion of an outflow feature (microburst) at an altitude of 400 meters above ground level. Horizontal wind vector plots along the flight path have provided clues concerning the meteorological setting of the flights. In addition to suspected outflow features, wavelike variations of the horizontal wind vectors were observed. Statistical studies of gust gradients were undertaken with the goal of fitting probability density functions to the data. As expected, the density functions were highly non-Gaussian. Spectral analyses are proceeding and several spectral models for the gust gradient data are being investigated.

Campbell, W.↗
NASA NTRS1982

Solution to the backward-Kolmogorov equation for a nonstationary oscillation problem

The transition probability density function of a Markovian approximation of the response amplitude of an oscillator under nonstationary excitation is determined in an analytical form. A solution is presented for the associated backward-Kolmogorov equation by transforming the equation into a form amenable to solution by the method of the separation of variables. This procedure results in a boundary value problem which is then solved by using an infinite series of Laguerre polynomials. It is found that the infinite series solution is equivalent to a closed-form solution involving a Bessel function.

Solomos, G. P.↗
NASA NTRS1983

Classification by thresholding

A procedure is given which substantially reduces the processing time needed to perform maximum likelihood classification on large data sets. The given method uses a set of fixed thresholds which, if exceeded by one probability density function, makes it unnecessary to evaluate a competing density function. Proofs are given of the existence and optimality of these thresholds for the class of continuous, unimodal, and quasi-concave density functions (which includes the multivariable normal), and a method for computing the thresholds is provided for the specific case of multivariate normal densities. An example with remote sensing data consisting of some 20,000 observations of four-dimensional data from nine ground-cover classes shows that by using thresholds, one could cut the processing time almost in half.

Feiveson, A. H.↗
NASA NTRS1983

Prediction of an axisymmetric combusting flow

A numerical model for turbulent, recirculating combusting flow is developed and applied to a research combustor. The model is based on the time-averaged Navier-Stokes equations with k-epsilon turbulence closure and the compositional fluctuations at each point are given probabilistically in terms of the mixture fraction. The probability density function is derived from transport equations for its first two moments along with an assumption regarding its shape. The resulting equations are solved using a standard line relaxation algorithm. It is found that the predictions of the model are in good agreement with data from an axisymmetric, bluff-body stabilized research combustor, while the major discrepancies are similar to those found in isothermal flow comparisons. The peculiar features of this flow which contribute to the errors are examined. The agreement between the theory and data deteriorates as the central jet velocity is increased which indicates an enhanced role for unsteady effects.

Correa, S. M.↗
NASA NTRS1983

An analysis of spanwise gust gradient data

Plots of winds encountered during flight are presented for severe turbulence cases from flights 6 and 7 of the Joint Airport Weather Studies (JAWS) Project (McCarthy et al., 1982). During run 10 of flight 7, analyzed here in considerable detail, the B-57B Cambera aircraft showed a sharp increase in airspeed of 15 m/sec over a distance of about 130 m and then a more gradual decrease of 20 to 25 m/sec over a distance of about 2 km. This suspected outflow feature was associated with a downdraft exceeding 10 m/sec. The horizontal wind direction changed nearly 180 deg during the pass through the feature. Unexpectedly, the intensity of the velocity differences decreased for all three components within the downdraft. Calculated probability density functions for u, v, and w show a jagged character, whereas delta-u, delta-v, and delta-w show a much smoother, unimodal behavior. The distributions for delta-u, delta-v, and delta-w are distinctly non-Gaussian.

Campbell, W.↗
NASA NTRS1982

A statistical model for radar images of agricultural scenes

The presently derived and validated statistical model for radar images containing many different homogeneous fields predicts the probability density functions of radar images of entire agricultural scenes, thereby allowing histograms of large scenes composed of a variety of crops to be described. Seasat-A SAR images of agricultural scenes are accurately predicted by the model on the basis of three assumptions: each field has the same SNR, all target classes cover approximately the same area, and the true reflectivity characterizing each individual target class is a uniformly distributed random variable. The model is expected to be useful in the design of data processing algorithms and for scene analysis using radar images.

Frost, V. S.↗
NASA NTRS1984

A stochastic model for photon noise induced by charged particles in multiplier phototubes of the space telescope fine guidance sensors

The Space Telescope (ST) is subjected to charged particle strikes in its space environment. ST's onboard fine guidance sensors utilize multiplier phototubes (PMT) for attitude determination. These tubes, when subjected to charged particle strikes, generate spurious photons in the form of Cerenkov radiation and fluorescence which give rise to unwanted disturbances in the pointing of the telescope. A stochastic model for the number of these spurious photons which strike the photocathode of the multiplier phototube which in turn produce the unwanted photon noise are presented. The model is applicable to both galactic cosmic rays and charged particles trapped in the Earth's radiation belts. The model which was programmed allows for easy adaption to a wide range of particles and different parameters for the phototube of the multiplier. The probability density functions for photons noise caused by protons, alpha particles, and carbon nuclei were using thousands of simulated strikes. These distributions are used as part of an overall ST dynamics simulation. The sensitivity of the density function to changes in the window parameters was also investigated.

Howell, L. W.↗
NASA NTRS1984

Non-Gaussian statistical models of surface wave fields for remote sensing applications

Based on the complete Stokes wave model with the bias term and using a simple mapping approach and an iteration solution method, we established a formula for the joint probability density function of the surface slope elevation of a nonlinear random wave field. The formula requires three parameters to define the whole density function: the rms surface elevation and slope values and the significant slope. This model represents the dynamics of the wave in a more direct way than the Gram-Charlier approximation. Based on this new statistical model and laboratory experiments, formula and numerical values of EM bias and dynamics bias are derived. The results indicate that various biases should be considered seriously if accuracy of the altimeter measurement is required in centimeter range.

Huang, N. E.↗
NASA NTRS1984

An investigation into the probabilistic combination of quasi-static and random accelerations

The development of design load factors are investigated for aerospace and aircraft components and experiment support structures, which are subject to a simultaneous vehicle dynamic vibration and acoustically generated random vibration. The characteristics of the combined acceleration probability density function is determined, and an appropriate percentile level for the combined acceleration is selected. This mechanism is developed and graphical data is provided to select combined accelerations for most popular percentile levels.

Schock, R. W.↗
NASA NTRS1984

Single-pulse, laser-saturated fluorescence measurements of OH in turbulent nonpremixed flames

A single-pulse, laser-saturated fluorescence technique has been developed for absolute OH concentration measurements with a temporal resolution of 2 nsec, a spatial resolution of less than 0.1 cu mm, and an estimated accuracy of + or - 30 percent. It has been applied in laminar, transitional, and turbulent hydrogen-air diffusion flames, providing the first reported quantitative measurements of average values, rms fluctuations, and probability-density functions of OH-radical concentration in nonpremixed flames.

Lucht, R. P.↗
NASA NTRS1984

Mass and momentum turbulent transport experiments with confined swirling coaxial jets. I

An experimental study of mixing downstream of swirling coaxial jets discharging into an expanded duct was conducted to obtain data for the evaluation and improvement of turbulent transport models currently used in a variety of computational procedures throughout the combustion community. A combination of laser velocimeter and laser induced fluorescence techniques was employed to obtain mean and fluctuating velocity and concentration distributions which were used to derive mass and momentum turbulent transport parameters currently incorporated into various combustor flow models. Flow visualization techniques were also employed to determine qualitatively the time dependent characteristics of the flow and the scale of turbulence. Mean and fluctuating velocity profiles and probability density functions were obtained at selected axial and radial locations throughout the flow field. Mixing occurred in several steps of axial and radial mass transport and was coupled with a large radial mean convective flux. Major mixing regions were observed to occur (1) at the interface between the inner stream and the centerline recirculation zone, and (2) at the interface between the inner jet and the annular jet streams. Mixing for swirling flow was completed in one-third the length required for nonswirling flow.

Johnson, B. V.↗
NASA NTRS1984

Computing approximate random Delta v magnitude probability densities

This paper describes the development and use of an algorithm to compute approximate statistics of the magnitude of a single random trajectory correction maneuver (TCM) Delta v vector. The TCM Delta v vector is modeled as a three component Cartesian vector each of whose components is a random variable having a normal (Gaussian) distribution with zero mean and possibly unequal standard deviations. The algorithm uses these standard deviations as input to produce approximations to (1) the mean and standard deviation of the magnitude of Delta v, (2) points of the probability density function of the magnitude of Delta v, and (3) points of the cumulative and inverse cumulative distribution functions of Delta v. The approximates are based on Monte Carlo techniques developed in a previous paper by the author and extended here. The algorithm described is expected to be useful in both pre-flight planning and in-flight analysis of maneuver propellant requirements for space missions.

Chadwick, C.↗
NASA NTRS1985

Analysis of data from NASA B-57B gust gradient program

Statistical analysis of the turbulence measured in flight 6 of the NASA B-57B over Denver, Colorado, from July 7 to July 23, 1982 included the calculations of average turbulence parameters, integral length scales, probability density functions, single point autocorrelation coefficients, two point autocorrelation coefficients, normalized autospectra, normalized two point autospectra, and two point cross sectra for gust velocities. The single point autocorrelation coefficients were compared with the theoretical model developed by von Karman. Theoretical analyses were developed which address the effects spanwise gust distributions, using two point spatial turbulence correlations.

Frost, W.↗