Results for “Value function” — Page 58

NASA NTRS1986

The effect of turbulence on lidar backscattered signal fluctuations

CO2 lidar measurements being subjected to uncertainties caused by strong fluctuations were explained to appear primarily due to speckle effects whereas turbulence was to be negligible. Spectral cross correlation measurements performed in the near IR and in the visible show that the main contribution to the fluctuation for the above lidar wavelengths originates from the atmospheric turbulence. The standard deviation of the fluctuation signal was measured for the two wavelengths simultaneously. A typical curve representing mid-summer day changes in the value of sigma as a function of time is shown. The results, indicating the strong dependence on the turbulence, is analyzed and discussed.

Fastig, S.↗

NASA NTRS1986

A study of the stochastic energization of charged particles with and without synchrotron energy loss

A momentum-diffusion formulation that takes the initial values of the particle distribution functions and momentum-space boundary conditions is presently employed for the computational and analytical study of the stochastic energization of charged particles by plasma waves. The nature of the particle distribution functions resulting from stochastic energization and the validity of the momentum-diffusion formulation for the energization are determined. The validity of the momentum-diffusion formulation with power law diffusion coefficients is assessed.

Borovsky, J. E.↗

NASA NTRS1986

On the possibility of diffusionally driven oscillations in two component gas bubbles in fluids

The problem of an isolated, stationary, two-component gas bubble in a fluid is analyzed. The appropriate governing equations, and an approximate version of these equations, for this model system are reviewed. The qualitative differences in bubble dissolution behavior between single- and two-component gas bubbles are elucidated. In particular, it is demonstrated that in the latter case the gas bubble radius may exhibit extrema as a function of time for certain values of the controlling parameters. The conditions under which these extrema may occur, and the maximum number of extrema which are permitted are elucidated.

Weinberg, Michael C.↗

NASA NTRS1988

Antimisting kerosene: Low temperature degradation and blending

The inline filtration characteristics of freshly blended and degraded antimisting fuels (AMK) at low temperature are examined. A needle valve degrader was modified to include partial recirculation of degraded fuel and heat addition in the bypass loop. A pressure drop across the needle valve of up to 4,000 psi was used. The pressure drop across a 325 mesh filter screen placed inline with the degrader and directly downstream of the needle valve was measured as a function of time for different values of pressure drop across the needle valve. A volume flux of 1 gpm/sq in was employed based on the frontal area of the screen. It was found that, at ambient temperatures, freshly blended AMK fuel could be degraded using a single pass degradation at 4,000 psi pressure drop across the needle valve to give acceptable filterability performance. At fuel temperatures below -20 C, degradation becomes increasingly difficult and a single pass technique results in unacceptable filtration performance. Recirculation of a fraction of the degraded fuel and heat addition in the bypass loop improved low temperature degradation performance. The problem is addressed of blending the AMK additive with Jet A at various base fuel temperatures.

Yavrouian, A.↗

NASA NTRS1988

An expert system for natural language processing

A solution to the natural language processing problem that uses a rule based system, written in OPS5, to replace the traditional parsing method is proposed. The advantage to using a rule based system are explored. Specifically, the extensibility of a rule based solution is discussed as well as the value of maintaining rules that function independently. Finally, the power of using semantics to supplement the syntactic analysis of a sentence is considered.

Hennessy, John F.↗

NASA NTRS1987

Measurement and analysis of the far infrared absorption spectrum of the gaseous mixture H2-CH4

The collision-induced absorption of H2-CH4 mixtures was measured from 20 to 900/cm at 195 and 297 K. By subtracting the absorption due to H2-H2 and CH4-CH4 collisions from that of the mixture, the absorption due to H2-CH4 collisions was obtained. This spectrum was analyzed using the BC model line shape to provide a way of estimating the far-IR spectrum of H2-CH4 for various concentrations of H2 and CH4. Theoretical spectral moments were computed with different potential functions and compared with experimental values.

Birnbaum, George↗

NASA NTRS1989

A weak Hamiltonian finite element method for optimal control problems

A temporal finite element method based on a mixed form of the Hamiltonian weak principle is developed for dynamics and optimal control problems. The mixed form of Hamilton's weak principle contains both displacements and momenta as primary variables that are expanded in terms of nodal values and simple polynomial shape functions. Unlike other forms of Hamilton's principle, however, time derivatives of the momenta and displacements do not appear therein; instead, only the virtual momenta and virtual displacements are differentiated with respect to time. Based on the duality that is observed to exist between the mixed form of Hamilton's weak principle and variational principles governing classical optimal control problems, a temporal finite element formulation of the latter can be developed in a rather straightforward manner. Several well-known problems in dynamics and optimal control are illustrated. The example dynamics problem involves a time-marching problem. As optimal control examples, elementary trajectory optimization problems are treated.

Hodges, Dewey H.↗

NASA NTRS1989

A penalty approach for nonlinear optimization with discrete design variables

Introduced here is a simple approach to minimization problems with discrete design variables by modifying the penaly function approach of converting the constrained problems into sequential unconstrained minimization technique (SUMT) problems. It was discovered, during the course of the present work, that a similar idea was suggested by Marcal and Gellatly. However, no further work has been encountered. A brief description of the SUMT is presented. The form of the penalty function for the discrete-valued design variables and strategy used for the implementation of the procedure is discussed next. Finally, several design examples are used to demonstrate the procedure, and results are compared with the ones available in the literature.

Shin, Dong K.↗

NASA NTRS1991

Weak Hamiltonian finite element method for optimal control problems

A temporal finite element method based on a mixed form of the Hamiltonian weak principle is developed for dynamics and optimal control problems. The mixed form of Hamilton's weak principle contains both displacements and momenta as primary variables that are expanded in terms of nodal values and simple polynomial shape functions. Unlike other forms of Hamilton's principle, however, time derivatives of the momenta and displacements do not appear therein; instead, only the virtual momenta and virtual displacements are differentiated with respect to time. Based on the duality that is observed to exist between the mixed form of Hamilton's weak principle and variational principles governing classical optimal control problems, a temporal finite element formulation of the latter can be developed in a rather straightforward manner. Several well-known problems in dynamics and optimal control are illustrated. The example dynamics problem involves a time-marching problem. As optimal control examples, elementary trajectory optimization problems are treated.

Hodges, Dewey H.↗

NASA NTRS1990

A weak Hamiltonian finite element method for optimal control problems

A temporal finite element method based on a mixed form of the Hamiltonian weak principle is developed for dynamics and optimal control problems. The mixed form of Hamilton's weak principle contains both displacements and momenta as primary variables that are expanded in terms of nodal values and simple polynomial shape functions. Unlike other forms of Hamilton's principle, however, time derivatives of the momenta and displacements do not appear therein; instead, only the virtual momenta and virtual displacements are differentiated with respect to time. Based on the duality that is observed to exist between the mixed form of Hamilton's weak principle and variational principles governing classical optimal control problems, a temporal finite element formulation of the latter can be developed in a rather straightforward manner. Several well-known problems in dynamics and optimal control are illustrated. The example dynamics problem involves a time-marching problem. As optimal control examples, elementary trajectory optimization problems are treated.

Hodges, Dewey H.↗

NASA NTRS1991

Probabilistic analysis of structures involving random stress-strain behavior

The present methodology for analysis of structures with random stress strain behavior characterizes the uniaxial stress-strain curve in terms of (1) elastic modulus, (2) engineering stress at initial yield, (3) initial plastic-hardening slope, (4) engineering stress at point of ultimate load, and (5) engineering strain at point of ultimate load. The methodology is incorporated into the Numerical Evaluation of Stochastic Structures Under Stress code for probabilistic structural analysis. The illustrative problem of a thick cylinder under internal pressure, where both the internal pressure and the stress-strain curve are random, is addressed by means of the code. The response value is the cumulative distribution function of the equivalent plastic strain at the inner radius.

Millwater, H. R.↗

NASA NTRS1990

The Galactic far ultraviolet background

Investigations of the FUV background are discussed in the light of the methods employed and significant results of data interpretation. Attention is given to the data that confirm the generally accepted theory that the FUV flux originates in the Galaxy. The relationship between the flux measurements and the types of scattering by dust are investigated with attention given to the discrepancies inherent in reported scattering properties. Recent evidence shows wide ranges of albedo values and scattering-phase functions, and UV scattering grains are noted in all directions of the Galaxy that have low albedos and scatter isotropically. Recent detections of spectral emission features support the Galactic fountain model for the scattering material, and molecular hydrogen emission suggests that the emitting material tends to clump in hydrogen clouds.

Bowyer, Stuart↗

NASA NTRS1992

Estimation of excitation energy of diatomic molecules in expanding nonequilibrium flows

The energy contained in the highly excited vibrational and rotational states in a diatomic gas in a thermochemical nonequilibrium state during expansion is estimated. The estimation is made on the assumption that the populations of the vibrational and rotational states, when normalized by their respective equilibrium values, are describable by simple functions containing no more than four arbitrary parameters. A cubic polynomial, a logarithmic-cubic polynomial, and a bimodal step function are used for this purpose. The four parameters are determined by imposing conditions known at the ground state and the dissociation limit and the mass conservation law. The energy in excess of that accounted for by assuming a Boltzmann distribution of these states, defined here as excess excitation energy, is calculated for N2, O2, NO, CO, OH, and H2. A calculation made for a typical nozzle flow shows that the excess energy may reach 6 percent of the total enthalpy of the flow, and that the flow velocity may decrease by as much as 4 percent due to the nonequilibrium excitation phenomenon.

Park, Chul↗

NASA NTRS1992

Theoretical determination of the alkali-metal superoxide bond energies

The bond dissociation energies for the alkali-metal superoxides have been computed using extensive Gaussian basis sets and treating electron correlation at the modified coupled-pair functional level. Our computed D0 values are 61.4, 37.2, 40.6, and 38.4 kcal/mol for LiO2, NaO2, KO2, and RbO2, respectively. These values, which are expected to be lower bounds and accurate to 2 kcal/mol, agree well with some of the older flame data, but rule out several recent experimental measurements.

Partridge, Harry↗

NASA NTRS1992

Computational/experimental basis for conducting alkane droplet combustion experiments on space-based-platforms

An analysis is conducted of the requirement for the conduct of spherically symmetric droplet-combustion experiments on space platforms, on the basis of a novel time-dependent computational droplet combustion model that allows the time- and temperature-dependent transport characteristics to be incorporated. While at low oxygen indices the droplet burning extinction becomes a strong function of oxygen index, it becomes a weaker function at higher oxygen index values. The oxygen index that separates these two ranges are dependent on the diluent, being higher for He and lower for N.

Choi, Mun Y.↗

NASA NTRS1993

A simple fuzzy logic real-time camera tracking system

A fuzzy logic control of camera pan and tilt has been implemented to provide real-time camera tracking of a moving object. The user clicks a mouse button to identify the object that is to be tracked. A rapid centroid estimation algorithm is used to estimate the location of the moving object, and based on simple fuzzy membership functions, fuzzy x and y values are input into a six-rule fuzzy logic rule base. The output of this system is de-fuzzified to provide pan and tilt velocities required to keep the image of the object approximately centered in the camera field of view.

Magee, Kevin N.↗

NASA NTRS1950

Method for determining the frequency-response characteristics of an element or system from the system transient output response to a known input function

A method is presented for the determination of the frequency-response characteristics of an element or system by utilizing the transient output response to a known but arbitrary input to the system. Since the application of special inputs, such as step functions or sinusoids, is often imperfect or impractical, a method for utilizing arbitrary inputs is desirable. Simple flight-test data may be reduced by this method to give the frequency response of an aircraft. Examples are given as determinations of aircraft frequency responses; however, the method can be applied to any type of dynamic system, such as automatic-control components, vibration-absorption equipment, and many types of instruments. The method requires that the arbitrary input function tend to a finite value after a finite time and that the system or element output be measured as a representative quantity having a static sensitivity. (author)

Curfman, Howard J↗

NASA NTRS1993

The 24 GHz measurements of 2.2 lambda conical horn antennas illuminating a conducting sheet

Monostatic reflection-coefficient magnitude, absolute value of Gamma, measurements occurring between a radiating horn and a metal reflecting plate are presented for a family of three 2.2 lambda diameter conical horn antennas. The three horns have different aperture phase deviations: 6 deg, 22.5 deg, and 125 deg. Measurements of the magnitude of absolute value of Gamma as a function of horn-plate separation (d) extend from an effective antenna aperture short (d = O) to beyond the far-field boundary (d = 2D(sup 2)/lambda, where D is the antenna diameter). Measurement data are presented with various physical environments for each of the horns. Measured scalar data are compared with theoretical data from two models, a numerical model for a circular waveguide aperture in a ground plane and a scalar diffraction theory model. This work was conducted in support of the development effort for a spaceborne multifrequency microwave reflectometer designed to accurately determine the distance from a space vehicle's surface to a reflecting plasma boundary. The metal reflecting plate was used to simulate the RF reflectivity of a critically dense plasma. The resulting configuration, a ground plane mounted aperture facing a reflecting plane in close proximity, produces a strong interaction between the ground plane and the reflecting plate, especially at integral half-wavelength separations. The transition coefficient is characterized by large amplitude variations.

Cross, A. E.↗