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At least 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Extreme Ultraviolet Explorer observations of the magnetic cataclysmic variable RE 1938-461

The magnetic cataclysmic variable RE 1938-461 was observed by the Extreme Ultraviolet Explorer (EUVE) Deep Survey instrument on 1992 July 8-9 during in-orbit calibration. It was detected in the Lexan/ boron (65-190 A) band, with a quiescent count rate of 0.0062 +/- 0.0017/s, and was not detected in the aluminum/carbon (160-360 A) band. The Lexan/boron count rate is lower than the corresponding ROSAT wide-field camera Lexan/boron count rate. This is consistent with the fact that the source was in a low state during an optical observation performed just after the EUVE observation, whereas it was in an optical high state during the ROSAT observation. The quiescent count rates are consistent with a virtual cessation of accretion. Two transient events lasting about 1 hr occurred during the Lexan/boron pointing, the second at a count rate of 0.050 +/- 0.006/s. This appears to be the first detection of an EUV transient during the low state of a magnetic cataclysmic variable. We propose two possible explanations for the transient events.

Warren, John K.↗
NASA NTRS1953

Effect of variable viscosity and thermal conductivity on high-speed slip flow between concentric cylinders

The differential equations of slip flow, including the Burnett terms, were first solved by Schamberg assuming that the coefficients of viscosity and heat conduction of the gas were constants. The problem is solved herein for variable coefficients of viscosity and thermal conductivity by applying a transformation leading to an iteration method. The method, starting with the solution for constant coefficients, enables one to approximate the solution for variable coefficients very closely after one or two steps. Satisfactory results are shown to follow from Schamberg's solution by using his values of the constant coefficients multiplied by a constant factor 'N', leading to what are denoted as the effective coefficients of viscosity and thermal conductivity.

RESEARCH TECHNIQUE, AERODYNAMICS↗
NASA NTRS1954

Preliminary Investigation of Performance of Variable-throat Extended-plug-type Nozzles over Wide Range of Nozzle Pressure Ratios

As part of an overall program for the experimental investigation of large-scale jet nozzles, a preliminary evaluation of the internal performance characteristics of several variable-throat extended-plug-type nozzles was obtained over a range of nozzle pressure rations from 2 to 15 with nozzle throat variations as great as 2:1. The extended-plug nozzle attained peak thrust coefficients as high as those which have been attained with fixed-geometry convergent-divergent nozzles. The thrust coefficients of the extended-plug nozzles were relatively insensitive to both nozzle pressure ratio and throat area over the range investigated. The lack of sensitivity of thrust coefficient to nozzle pressure ratio and throat area was due primarily to the absence of severe decreases in thrust coefficient, which are normally observed at low nozzle pressure ratios for convergent-divergent nozzles because of overexpansion. For choked flow, the flow coefficients of the extended-plug nozzles varied from 0.95 to 0.997. A ratio of the outer-shell exit area to nozzle throat area was found to be an important variable affecting nozzle performance. Most of the configurations reported herein showed a small decrease in thrust coefficient at low nozzle pressure ratios; however, it is believed that this characteristic can nearly be eliminated by proper nozzle design.

NOZZLES↗
NASA NTRS1938

Aerodynamic characteristics of a large number of airfoils tested in the variable-density wind tunnel

The aerodynamic characteristics of a large number of miscellaneous airfoils tested in the variable-density tunnel have been reduced to a comparable form and are published in this report for convenient reference. Plots of the standard characteristics are given in tabular form. Included is a tabulation of important characteristics for the related airfoils reported in NACA report 460. This report, in conjunction with NACA report 610, makes available in comparable and convenient form the aerodynamic data for airfoils tested in the variable-density tunnel since January 1, 1931.

Pinkerton, Robert M↗
NASA NTRS1920

The Parker Variable Camber Wing

This report deals with the problem of increasing the speed range of an airplane by varying the camber of a wing surface. The variable camber wing offers many advantages over the variable incidence type of speed range.

Parker, H F↗
NASA NTRS1936

Characteristics of the NACA 23012 Airfoil from Tests in the Full-Scale and Variable-Density Tunnels

This report gives the results of tests in the NACA full-scale and variable-density tunnels of a new wing section, the NACA 23012, which is one of the more promising of an extended series of related airfoils recently developed. The tests were made at several values of the Reynolds number between 1,000,000 and 8,000,000. The new airfoil develops a reasonably high maximum lift and a low profile drag, which results in an unusually high value of the speed-range index. In addition, the pitching-moment coefficient is very small. The superiority of the new section over well-known and commonly used sections of small camber and moderate thickness is indicated by making a direct comparison with variable-density tests of the NACA 2212, the well-known NACA family airfoil that most nearly resembles it. The superiority is further indicated by comparing the characteristics with those obtained from full-scale-tunnel tests of the Clark y airfoil.

Jacobs, Eastman N↗
NASA NTRS1939

Airfoil section data obtained in the NACA variable-density tunnel as affected by support interference and other corrections

The results of an investigation of the effect of support interference on airfoil drag data obtained in the variable-density tunnel are presented. As a result of the support interference, previously published airfoil data from the variable-density tunnel have shown too large drag coefficients and too large a rate of increase of drag coefficients and too large a rate increase of drag coefficients with airfoil thickness. The practical effect of the corrections on the choice of the optimum section is briefly considered and corrected data for a selected list of airfoils are presented as a convenience to the designer. Methods of correcting published data for other airfoils are presented.

Jacobs, Eastman N↗
NASA NTRS1954

Effect of variable viscosity and thermal conductivity on high-speed slip flow between concentric cylinders

Schamberg was the first to solve the differential equations of slip flow, including the Burnett terms, for concentric circular cylinders assuming constant coefficients of viscosity and thermal conductivity. The problem is solved for variable coefficients of viscosity and thermal conductivity in this paper by applying a transformation which leads to an iteration method. Starting with the solution for constant coefficients, this method enables one to approximate the solution for variable coefficients very closely after one or two steps. Satisfactory results are shown to follow from Schamberg's solution by using his values of constant coefficients multiplied by a constant factor, leading to what are denoted as the effective coefficients of viscosity and thermal conductivity.

Lin, T C↗
NASA NTRS1951

Stability of the cylindrical shell of variable curvature

This report is a first attempt to devise a calculation method for representing the buckling behavior of cylindrical shells of variable curvature. The problem occurs, for instance, in dimensioning wing noses, the stability of which is decisively influenced by the variability of curvature. The calculation is made possible by simplifying the stability equations (permissible for the shell of small curvature) and by assuming that the curvature 1/R as a function of the arc lengths can be represented by a very few Fourier terms. The formulas for the special case of an ellipse-like half oval with an axis ratio 1/3 ?= e ?= 1 under compression in longitudinal direction,shear, and a combination of shear and compression were evaluated. However, the results can also be applied approximately to an unsymmetrical oval-shell segment under compression, shear, and bending so that the numerical values contained in the diagrams 10 to 12 represent directly dimensioning data for the wing nose.

LOADS AND STRESSES, STRUCTURAL - BENDING↗
NASA NTRS1993

Necessary conditions for the optimality of variable rate residual vector quantizers

Residual vector quantization (RVQ), or multistage VQ, as it is also called, has recently been shown to be a competitive technique for data compression. The competitive performance of RVQ reported in results from the joint optimization of variable rate encoding and RVQ direct-sum code books. In this paper, necessary conditions for the optimality of variable rate RVQ's are derived, and an iterative descent algorithm based on a Lagrangian formulation is introduced for designing RVQ's having minimum average distortion subject to an entropy constraint. Simulation results for these entropy-constrained RVQ's (EC-RVQ's) are presented for memory less Gaussian, Laplacian, and uniform sources. A Gauss-Markov source is also considered. The performance is superior to that of entropy-constrained scalar quantizers (EC-SQ's) and practical entropy-constrained vector quantizers (EC-VQ's), and is competitive with that of some of the best source coding techniques that have appeared in the literature.

Kossentini, Faouzi↗
NASA NTRS1986

Studies of an x ray selected sample of cataclysmic variables

Just prior to the thesis research, an all-sky survey in hard x rays with the HEAO-1 satellite and further observations in the optical resulted in a catalog of about 700 x-ray sources with known optical counterparts. This sample includes 43 cataclysmic variables, which are binaries consisting of a detached white-dwarf and a Roche lobe filling companion star. This thesis consists of studies of the x-ray selected sample of catalcysmic variables.

Silber, Andrew D.↗
NASA NTRS1993

Variable-permittivity linear inverse problem for the H(sub z)-polarized case

The H(sub z)-polarized inverse problem has rarely been studied before due to the complicated way in which the unknown permittivity appears in the wave equation. This problem is equivalent to the acoustic inverse problem with variable density. We have recently reported the solution to the nonlinear variable-permittivity H(sub z)-polarized inverse problem using the Born iterative method. Here, the linear inverse problem is solved for permittivity (epsilon) and permeability (mu) using a different approach which is an extension of the basic ideas of diffraction tomography (DT). The key to solving this problem is to utilize frequency diversity to obtain the required independent measurements. The receivers are assumed to be in the far field of the object, and plane wave incidence is also assumed. It is assumed that the scatterer is weak, so that the Born approximation can be used to arrive at a relationship between the measured pressure field and two terms related to the spatial Fourier transform of the two unknowns, epsilon and mu. The term involving permeability corresponds to monopole scattering and that for permittivity to dipole scattering. Measurements at several frequencies are used and a least squares problem is solved to reconstruct epsilon and mu. It is observed that the low spatial frequencies in the spectra of epsilon and mu produce inaccuracies in the results. Hence, a regularization method is devised to remove this problem. Several results are shown. Low contrast objects for which the above analysis holds are used to show that good reconstructions are obtained for both permittivity and permeability after regularization is applied.

Moghaddam, M.↗
NASA NTRS1993

A comparison of two multi-variable integrator windup protection schemes

Two methods are examined for limit and integrator wind-up protection for multi-input, multi-output linear controllers subject to actuator constraints. The methods begin with an existing linear controller that satisfies the specifications for the nominal, small perturbation, linear model of the plant. The controllers are formulated to include an additional contribution to the state derivative calculations. The first method to be examined is the multi-variable version of the single-input, single-output, high gain, Conventional Anti-Windup (CAW) scheme. Except for the actuator limits, the CAW scheme is linear. The second scheme to be examined, denoted the Modified Anti-Windup (MAW) scheme, uses a scalar to modify the magnitude of the controller output vector while maintaining the vector direction. The calculation of the scalar modifier is a nonlinear function of the controller outputs and the actuator limits. In both cases the constrained actuator is tracked. These two integrator windup protection methods are demonstrated on a turbofan engine control system with five measurements, four control variables, and four actuators. The closed-loop responses of the two schemes are compared and contrasted during limit operation. The issue of maintaining the direction of the controller output vector using the Modified Anti-Windup scheme is discussed and the advantages and disadvantages of both of the IWP methods are presented.

Mattern, Duane↗
NASA NTRS1992

Status of the variable diameter centerbody inlet program

The Variable Diameter Centerbody (VDC) inlet is an ongoing research program at LeRC. The VDC inlet is a mixed compression, axisymmetric inlet that has potential application on the next generation supersonic transport. This inlet was identified as one of the most promising axisymmetric concepts for supersonic cruise aircraft during the SCAR program in the late 1970's. Some of its features include high recovery, low bleed, good angle-of-attack tolerance, and excellent engine airflow matching. These features were demonstrated at LeRC in the past by the design and testing of fixed hardware models. A current test program in the LeRC 10' x 10' Supersonic Wind Tunnel (SWT) will attempt to duplicate these features on model hardware that actually incorporates a flight-like variable diameter centerbody mechanism.

Saunders, John D.↗
NASA NTRS1994

Variable stiffness torsion springs

In a torsion spring the spring action is a result of the relationships between the torque applied in twisting the spring, the angle through which the torsion spring twists, and the modulus of elasticity of the spring material in shear. Torsion springs employed industrially have been strips, rods, or bars, generally termed shafts, capabable of being flexed by twisting their axes. They rely on the variations in shearing forces to furnish an internal restoring torque. In the torsion springs herein the restoring torque is external and therefore independent of the shearing modulus of elasticity of the torsion spring shaft. Also provided herein is a variable stiffness torsion spring. This torsion spring can be so adjusted as to have a given spring constant. Such variable stiffness torsion springs are extremely useful in gimballed payloads such as sensors, telescopes, and electronic devices on such platforms as a space shuttle or a space station.

Alhorn, Dean C.↗
NASA NTRS1993

Variability in the Gulf of Alaska from Geosat altimetry data

Satellite altimetry was used to examine annual and interannual variability in the Gulf of Alaska region. Crossover data from the Geosat Geodetic Mission (GM) and collinear data from the Exact Repeat Mission (ERM) were processed separately to form sea surface height anomalies at grid points. The time series from the GM and ERM were then combined to produce a 3.75-year data set. The time series from the ERM data set agreed fairly well with hydrographic dynamic heights at several locations, with an average correlation of 0.70 between the two data sets. The combined and ERM altimetric data sets were analyzed using empirical orthogonal functions (EOFs). These revealed variability that occurs primarily on annual and interannual time scales. A comparison with EOF analysis of the atmospheric pressure field during the same time periods showed that the annual variation in pressure seemed to be reflected in both the combined and ERM altimetric data sets. The amplitude time series of the first mode in the combined data set was very similar to the North Pacific pressure index during the 1985-1989 time frame. The maximum correlation was at a lag of 250 days. Finally, an interannual mode was present in all three data sets which was closely linked to the baroclinic variations measured by the hydrographic data.

Bhaskaran, Shyam↗
NASA NTRS1993

Modelling time variable and total eclipses of the millisecond pulsar PSR 1744-24A

The 11 ms pulsar PSR 1744-24A in the globular clusters Terzan 5 is the first and only known example of a binary eclipsing pulsar showing time-variable eclipses with irregular and nonsymmetric eclipse patterns. Occasionally, the pulsar eclipse becomes total, and no radio emission is detected for an extended period of time. We present here numerical results of a hydrodynamical model aimed at explaining the remarkable eclipse properties of PSR 1744-24A. We show that the hydrodynamic time scale for the partial or total pulsar enshrouding is of order of the orbital period for a relatively large mass loss rate, in agreement with observations. The time-variable eclipse egress positions imply that the mass loss rate from the companion varies within one orbital period. Hydrodynamic modelling of the mass outflow of PSR 1744-24A is of crucial importance for the understanding of a class of millisecond pulsars whose radio emission might be occasionally or permanently quenched by mass outflows from their companions. The properties of PSR 1744-24A suggest a new way of studying millisecond pulsars involving radio, X-ray, gamma-ray, and optical observations.

Tavani, Marco↗
NASA NTRS1993

The reddening and variability of XX Ophiuchi

We present polarization data on the XX Oph system which suggest that the interstellar extinction to this object has been overestimated in the past: our data imply A(V) = 1.6 mag. Our photometry and infrared spectroscopy suggest a spectral class of M7III for the late component, and a BOV companion that ionizes the wind of the cool component. XX Oph seems more like a Zeta Aur/VV Cep system than a 'symbiotic object'. The photometric variability of XX Oph seems to have a number of causes, ranging from shell-type variability in the U band to variations in the M component in the infrared.

Evans, A.↗