Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Smooth approximation”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Aperture amplitude and phase control of offset dual reflectors

An approximate synthesis method is presented for solving the problem of offset dual-shaped reflectors. It is shown that the equations obtained by this method are partial differential equations that can be integrated numerically, except that these equations are generally not total and therefore do not have a 'smooth' solution. However, it is demonstrated that these equations form a nearly total differential that enables one to generate a smooth subreflector by integrating the differential equations and then synthesizing a main reflector which gives an exact solution for the specified aperture phase distribution. Several important illustrative examples are treated, which validate the adequacy of the proposed method.

Galindo-Israel, V.↗
NASA NTRS1980

The nonlinear equatorial Kelvin wave

Using the method of strained coordinates, a uniformly valid approximation to the nonlinear equatorial Kelvin wave is derived. It is shown that nonlinear effects are negligible for the Kelvin waves associated with the Gulf of Guinea upwelling. The Kelvin waves involved in El Nino, however, are significantly distorted both in shape and speed. The leading edge is smoothed and expanded rather than steepened, but the trailing edge will form sharp fronts and eventually break.

Boyd, J. P.↗
NASA NTRS1994

The Balmer-dominated northeast limb of the Cygnus loop supernova remnant

We present a comprehensive investigation of the Balmar-dominated northeast limb of the Cygnus Loop supernova remnant. Data presented include H alpha (O III), and X-ray images, UV and visible spectrophotometry, and high-resolution spectroscophy. The two relatively bright Balmer-dominated filaments visible on the POSS prints are seen to be part of a very smooth and regular complex of filaments. These filaments mark the current location of the blast wave and are seen to bound the sharply limb-brightened X-ray emission, including the previously reported X-ray, 'halo.' The (O III)/h beta ratio throughout the region is approximately 0.1, except for regions in which the shock is undergoing a transition from nonradiative to incomplete radiative to incomplete radiative conditions. At these locations (O III) emission from the cooling region is quite strong, while collisionally excited Balmer-line emission can be weak because of photoionization of the preshock medium by UV from the nascent cooling region. As a result (O III)/H beta is greater than 100 in some locations. The nonradiative/radiative transition is best studied along the length of the northwestern of the two brightest filaments, where the shock velocity and swept-up column go from approximately 180 km/s and 10(exp 17)/sq cm at one end to approximately 140 km/s and 8 x 10(exp 17)/cm at the other. There are also a number of locations of such incomplete radiative emission where the shock has recently encountered denser regions with characteristic sizes of approximately 10(exp 18) cm. There is a considerable amount of evidence that the shock has decelerated from approximately 400 km/s to less than 200 km/s in the last 1000 yr. We interpret this as the result of the blast wave hitting the wall of a cavity which surround the supernova precursor and succeed in matching a wide range of data with a reflected shock model in which the density ofthe cavity wall is approximately 1.2/cu cm and the density in the interior of the cavity is about 0.09/cu cm. This analysis supports previous suggestions that the Cygnus Loop was a cavity explosion. We opredict that the X-ray shell will continue to brighten for the next 1000 years or so, during which time the shock front will become radiative throughout the northeast region. The similarity of the time scales for post-shock cooling, shock deceleration, and the 1ressure evolution of the region violates assumptions of steady flow, and greatly complicated interpretation of shock specptra in the region. This may be a common and important effect in many supernova remnants. High-resolutoin spectroscopy of the H alpha line profile shows that the broad component is surprisingly narrow (approximately 130 km/s). Consistency between this line width and the shock velocity inferred from spectrophotometry requires either rapid postshock equilibration of T(sub i) and T(sub e), or that the shock front has declerated from 180 km/s to 150 km/s in the last approximately 200 yr. While we cannot unambiguously distinquish between these models, the required deceleration is consistent with extrapolation of the shock decleration rate inferred over the last 1000 years. The width of the narrow component of the h alpha line profile is approximately 3 km/s, which indicates that the preshock medium has been heated to approximately 25,000 K by a shock precursor. The most promising types of precursor are heating either by fast neutrals which overtake the shock or by cosmic rays. In the latter case the observed properties of the precursor constrain the cosmic-ray diffusion coefficient to be between about 10(exp 23) sq cm/s. The characteristics of the precursor seem to be about right to explain the presence of weak (S II) and (N II) emission previously reported by Fesen & Itoh (1985).

Hester, J. Jeff↗
NASA NTRS1999

Accuracy of the Independent Pixel Approximation at Absorbing Wavelengths

In order to correctly interpret shortwave cloud radiation measured by satellites and ground-based radiometers, or by two aircraft flying above and below clouds, we need to better understand interactions between inhomogeneous clouds and solar radiation. The discrepancies between shortwave absorption inferred from measurements and predicted by models, between cloud optical depths estimated from satellites and ground measurements, between single scattering albedo retrieved from in situ radiation measurements and computed from measured droplet size distribution, among others, are strongly affected by cloud horizontal inhomogeneity. Net horizontal photon transport (i. e., horizontal fluxes) are a direct consequence of the inhomoqeneity in cloud structure. Horizontal fluxes and their effect on the accuracy of the pixel-by-pixel one-dimensional (1 D) radiative transfer calculations has recently undergone close scrutiny for conservative scattering. However, the properties and magnitude of horizontal fluxes in absorbing wavelengths are still poorly understood. As far as we are aware, only Ackerman and Cox and Titov discussed correlations between horizontal fluxes at absorbing wavelengths, though these were far from comprehensive. This paper partly fills this gap. We discuss here of whether the accuracy of the Independent Pixel Approximation (IPA), a 1 D radiative transfer approximation for each pixel, is a better model for multiple scattering at conservative or at absorbing wavelengths. Issues addressed here are: (1) dependence of net horizontal fluxes on single scattering albedo; (2) connection between pixel-by-pixel accuracy of the IPA and horizontal fluxes and (3) radiative smoothing and horizontal fluxes at absorbing wavelengths. In contrast to the traditional understanding of IPA, we study IPA accuracies not only for reflectance but also for transmittance and absorptance at both conservative and absorbing wavelengths. In spite of the apparent similarity between the three processes, dependence of IPA accuracies on single-scattering albedo is completely different. As a result, cloud optical properties retrieved from high resolution satellite images and ground-based measurements using IPA at absorbing channels will have different accuracies.

Marchak, Alexander↗
NASA NTRS1958

The Turbulent Boundary Layer on a Rough Curvilinear Surface

A number of semiempirical approximate methods exist for determining the characteristics of the turbulent boundary layer on a curvilinear surface. At present, among these methods, the one proposed by L. G. Loitsianskii is given frequent practical application. This method is sufficiently effective and permits, in the case of wing profiles with technically smooth surfaces, calculating the basic characteristics of the boundary layer and the values of the overall drag with an accuracy which suffices for practical purposes. The idea of making use of the basic integral momentum equation ((d delta(sup xx))/dx) + ((V' delta(sup xx))/V) (2 + H) = (tau(sub 0))/(rho V(exp 2)) proves to be fruitful also for the solution of the problems in the determination of the characteristics of the turbulent boundary layer on a rough surface.

Droblenkov, V. F.↗
NASA NTRS2012

Nanostructure Secondary-Mirror Apodizing Mask for Transmitter Signal Suppression in a Duplex Telescope

A document discusses a nanostructure apodizing mask, made of multi-walled carbon nanotubes, that is applied to the centers (or in and around the holes) of the secondary mirrors of telescopes that are used to interferometrically measure the strain of space-time in response to gravitational waves. The shape of this ultra-black mask can be adjusted to provide a smooth transition to the clear aperture of the secondary mirror to minimize diffracted light. Carbon nanotubes grown on silicon are a viable telescope mirror substrate, and can absorb significantly more light than other black treatments. The hemispherical reflectance of multi-walled carbon nanotubes grown at GSFC is approximately 3 to 10 times better than a standard aerospace paint used for stray light control. At the LISA (Laser Interferometer Space Antenna) wavelength of 1 micron, the advantage over paint is a factor of 10. Primarily, in the center of the secondary mirror (in the region of central obscuration, where no received light is lost) a black mask is applied to absorb transmitted light that could be reflected back into the receiver. In the LISA telescope, this is in the center couple of millimeters. The shape of this absorber is critical to suppress diffraction at the edge. By using the correct shape, the stray light can be reduced by approximately 10 to the 9 orders of magnitude versus no center mask. The effect of the nanotubes has been simulated in a stray-light model. The effect of the apodizing mask has been simulated in a near-field diffraction model. Specifications are geometry-dependent, but the baseline design for the LISA telescope has been modeled as well. The coatings are somewhat fragile, but work is continuing to enhance adhesion.

Hagopian, John↗
NASA NTRS1995

Blue lobes in the Hydra A cluster central galaxy

We present new U- and I-band images of the centrally dominant galaxy in the Hydra A cluster, obtained with the 2.5 m Isaac Newton Telescope at La Palma. The galaxy is centered in a poor, X-ray-luminous cluster whose gaseous intracluster medium is apparently cooling at a rate of m-dot(sub CF) approximately 3000 solar masses/yr. The galaxy's structure is that of a normal giant elliptical galaxy, apart from the central approximately 8 x 6 arcsec (approximately 12 x 9 kpc) region which contains an unusually blue, lobelike structure that is spatially coincident with a luminous emission-line nebula in rotation about the nucleus. Based on near spatial coincidence of the central continuum structure and the emission-line nebula, we suggest that the blue continuum is due to a warm stellar population in a central disk. In order to isolate and study the structure of the disk, we have subtracted a smooth galactic background model from the U-band image. The disk's surface brightness profiles along its major and minor axes decline roughly exponentially with radius. The disk's axial ratio is consistent with a nearly edge-on thick disk or a thin disk that is inclined with respect to the line of sight. The bluest regions, located a few arcsec on either side of the nucleus (giving the lobelike appearance), may be due to locally enhanced star formation or a seeing-blurred ring of young stars embedded in the disk observed nearly edge-on. If star-formation is occurring with the local initial mass function, the central color, surface brightness, and dynamical mass would be consistent with models for star formation at a rate of less than and approximately 1 solar masses/yr which has persisted for the past approximately 10(exp 9) yr, a short burst (10(exp 7) yr) of star formation at a rate of approximately 30 solar masses/yr which occurred less than and approximately 10(exp 8) yr ago, or an instantaneous burst of star formation which occurred approximately 5 x 10(exp 7) yr ago. While the young population contributes approximately 30%-40% of the central U-band luminosity, its mass would be less than and approximately 1% to less than and approximately 10% (10(exp 8) solar masses - 2 x 10(exp 9) solar masses of the galaxy's central dynamical mass. We consider a number of possible origins for the disk material.

Mcnamara, Brian R.↗
NASA NTRS1975

Evaluation of a series hybrid thrust bearing at DN values to three million. 2: Fabrication and testing

Results of tests made to determine the experimental performance of a series hybrid bearing are reported. The bearing consists of a 150 mm ball bearing and a centrifugally actuated, conical, fluid film bearing fitting an envelope with an outer radius of 86.4 mm (3.4 in.) and inner radius of 71 mm (2.8 in.). Tests were conducted up to 16,500 rpm, at which speed an axial load of 15,568 N (3500 lb) was safely supported by the hybrid bearing system. Through the employment of the series hybrid bearing principle, it was possible to reduce the effective ball bearing speed to approximately one-half of the shaft speed. A reduction of this magnitude should result in a tenfold increase in the ball bearing fatigue life. A successful simulation of fluid film bearing lubricant supply failure, performed repeatedly at an operating speed of 10,000 rpm, resulted in complete and smooth change over to full scale ball bearing operation when the oil supply to the fluid film bearing was discontinued. Reactivation of the fluid film supply system produced a flawless return to the original mode of hybrid operation.

Eusepi, M.↗
NASA NTRS1982

Evaluation of LANDSAT-D Orbit Determination Using a Filter/Smoother (PREFER)

Simulated range and range rate data for five tracking stations were first generated using batch least squares orbit determination (GTDS). Then GTDS was used (in the differential correction mode) to produce a nominal trajectory which was input to PREFER. The GTDS differential correction (DC) run was made using models which differed from those used to produce the simulated data. These model differences were chosen to be fairly realistic approximations to the errors in the models actually used for operational orbit determination. Several different simulation runs were made with different types of model errors in order to determine the sensitivity to these errors. The nominal trajectory and the simulated measurement data were input to PREFER to produce a smoothed ephemeris file. Numerous runs of PREFER were made in which parameters describing the statistics of the model errors were varied. The likelihood function computed by the Kalman filter determined the ""best'' choice of input parameters. There was strong negative correlation between the likelihood function and the errors in the smoothed ephemeris.

Gibbs, B. P.↗
NASA NTRS1982

Multifrequency microwave radiometer measurements of soil moisture

Ground-based microwave radiometer experiments are carried out to investigate the effects of moisture, temperature, and roughness on microwave emission from bare soils. The measurements are made at frequencies of 0.6-0.9, 1.4, and 10.7 GHz using van-mounted radiometers to observe prepared soil sites in Kern County, CA. Brightness temperature variations of approximately 15 K at 1.4 GHz and 25 K at 10.7 GHz are observed as a result of diurnal changes in the soil temperature. Increasing the soil moisture content from 2% to 15% by volume is found to result in brightness temperature decreases of approximately 70 K at 0.775 and 1.4 GHz and 40 K at 10.7 GHz, depending, to a lesser extent, on polarization and viewing angle. The results attest the significance of soil temperature in deriving soil moisture from microwave radiometer measurements. Comparisons of the microwave measurements with theoretical predictions using smooth surface models give reasonable agreement and support previous results of this nature obtained with other soil types.

Njoku, E. G.↗
NASA NTRS1983

Numerical approximations to nonlinear conservation laws with locally varying time and space grids

Numerical approximations to the initial value problem for nonlinear systems of conservation laws are considered. The considered system is said to be hyperbolic when all eigenvalues of every real linear combination of the Jacobian matrices are real. Solutions may develop discontinuities in finite time, even when the initial data are smooth. In the investigation, explicit finite difference methods which use locally varying time grids are considered. The global CFL restriction is replaced by a local restriction. The numerical flux function is studied from a finite volume viewpoint, and a differencing technique is developed at interface points between regions of distinct time increments.

Osher, S.↗
NASA NTRS1990

On the maximum rate of change in sunspot number growth and the size of the sunspot cycle

Statistically significant correlations exist between the size (maximum amplitude) of the sunspot cycle and, especially, the maximum value of the rate of rise during the ascending portion of the sunspot cycle, where the rate of rise is computed either as the difference in the month-to-month smoothed sunspot number values or as the 'average rate of growth' in smoothed sunspot number from sunspot minimum. Based on the observed values of these quantities (equal to 10.6 and 4.63, respectively) as of early 1989, it is inferred that cycle 22's maximum amplitude will be about 175 + or - 30 or 185 + or - 10, respectively, where the error bars represent approximately twice the average error found during cycles 10-21 from the two fits.

Wilson, Robert M.↗
NASA NTRS1994

A method for correcting aspect solution errors in ROSAT HRI observations of compact sources

X-ray point sources observed with the ROSAT High Resolution Imager (HRI) often appear elongated over scales of approximately 5 sec-10 sec from the image core. This elongation has been attributed to errors in the attitude correction as the satellite is wobbled during the observations, and affects sources with both soft and hard X-ray spectra. In this paper, I report the results of an attempt to rid a high signal-to-noise observation of the soft X-ray point source HZ 43 of its characteristic elongation. I divided the observation into 181 separate images, each containing photons from only a small region on the detector through which the source passed during the satellite's wobble. By measuring the positions of the individual image centroids, I found clear evidence for systematic offsets from a common mean by up to approximately +/- 3 sec in both right ascension and declination as a function of phase in the satellite wobble. Shifting the subimages to a common center and then restacking them into a single image measurably improved the symmetry of the point-spread function. HRI observations are wobbled primarily to smooth out variations in the pixel-to-pixel sensitivity of the detector and also to extend the lifetime of the microchannel plates in the detector since these decay at a given location as a function of the number of photons detected. However, the elongations introduced by the aspect errors inhibit the identification of possible extended X-ray emission associated with sources such as pulsars and active galactic nuclei. In light of these results, I suggest that until the aspect errors are understood, observations of compact sources, where this effect may be important, should not be wobbled.

Morse, Jon A.↗
NASA NTRS1999

The gravity field of Mars: results from Mars Global Surveyor

Observations of the gravity field of Mars reveal a planet that has responded differently in its northern and southern hemispheres to major impacts and volcanic processes. The rough, elevated southern hemisphere has a relatively featureless gravitational signature indicating a state of near-isostatic compensation, whereas the smooth, low northern plains display a wider range of gravitational anomalies that indicates a thinner but stronger surface layer than in the south. The northern hemisphere shows evidence for buried impact basins, although none large enough to explain the hemispheric elevation difference. The gravitational potential signature of Tharsis is approximately axisymmetric and contains the Tharsis Montes but not the Olympus Mons or Alba Patera volcanoes. The gravity signature of Valles Marineris extends into Chryse and provides an estimate of material removed by early fluvial activity.

long duration↗
NASA NTRS1993

The reionization of the universe: The feedback of galaxy formation on the intergalactic medium

The thermal and ionization evolution of a uniform intergalactic medium (IGM) composed of H and He, undergoing reionization, including the mean effect of gas clumps embedded in a smoothly distributed ambient gas were calculated. The rate equations for ionization and recombination were solved together with the equations of energy conservation, including the effects of cosmological expansion, radiative and Compton cooling, and the diffuse flux emitted by the gas, and radiative transfer. The contribution to the continuum opacity of the universe due to the observed quasar absorption line clouds (QALC'S) were included. A variety of sources of photoionization, including quasars and primeval galaxies, as well as the possibility that hydrodynamical processes deposit thermal energy in the IGM were considered. Applications of these calculations including the evolution of the Ly-alpha forest clouds are described. A self-consistent treatment of the thermal and ionization history of the intergalactic medium (IGM) must take account of the growth of structure in the universe, since the mean density of the IGM corresponds primarily to the time-varying uncollapsed fraction of the baryon-electron component of the matter, and the collapsed fraction, in turn, can have a feedback effect on this uncollapsed fraction by releasing ionizing radiation and thermal energy and by contributing to the opacity of the universe. The coupled evolution of the IGM and the emerging structure with a special focus on the reionization of the IGM, which is believed to have been completed by some redshift z is approximately greater than 4, as inferred from the absence of the Gunn-Peterson effect in the spectra of high z quasars, are studied. The results and implications of detailed, numerical calculations of the thermal and ionization balance and radiative transfer in a uniform IGM of H and He, including the mean effect of an evolving distribution of gas clumps embedded in a smoothly distributed ambient gas is described.

Shapiro, Paul R.↗
DOE OSTI2022

A Realistic Full-Scale 3D Modeling of Turning Using Coupled Smoothed Particle Hydrodynamics and Finite Element Method for Predicting Cutting Forces

Computational modelling is an effective technique for understanding the complex physics of machining. Large deformations, material separation, and high computational requirements are the key challenges faced while simulating machining. This work introduces a full-scale three-dimensional model of turning operations using a combined approach based on the Smoothed Particle Hydrodynamics (SPH) and Finite Element (FE) methods. By exploiting the advantages of each method, this approach leads to high-fidelity coupled SPH-FE machining models. Cutting forces and chip morphology are the primary results of interest. The machining models are validated with the results of turning experiments. Two-dimensional machining model underpredicts the cutting force and feed force by approximately 49% and 70%, respectively. Moreover, passive force cannot be predicted using the two-dimensional model. On the other hand, with the three-dimensional models developed in this manuscript, the difference between the total simulated force and experimentally measured force is ∼17%. The chip morphologies correlate with experiments in terms of the direction of the chip movement and the “long” continuous chips observed while turning Al 6061. This work expands the realm of machining simulations from two-dimensional orthogonal machining or sectional three-dimensional model to a full-scale realistic simulation. The encouraging simulation results show the potential to study more complex phenomena, such as machining stability and tool path modulation.

42 ENGINEERING↗
DOE OSTI2020

Beam-energy dependence of the production of light nuclei in Au + Au collisions

We study in the coalescence model the collision energy dependence of (anti-)deuteron and (anti- )triton production in the most central Au+Au collisions at $\sqrt{s_{NN}} $ = 7.7, 11.5, 19.6, 27, 39, 62.4 and 200 GeV. The needed phase-space distribution of nucleons at the kinetic freeze-out is generated from a new 3D hybrid dynamical model (iEBE-MUSIC) by using a smooth crossover equation of state (EoS) without a QCD critical point. Our model calculations predict that the coalescence parameters of (anti-)deuteron (B 2 (d) and B 2 ($\bar{d}$)) decrease monotonically as the collision energy increases, and the light nuclei yield ratio N t N p /$Nd^{2}_{d}$ remains approximately a constant with respect to the collision energy. These calculated observables fail to reproduce the non-monotonic behavior of the corresponding data from the STAR Collaboration. Without including any effects of the critical point in our model, our results serve as the baseline predictions for the yields of light nuclei in the search for the possible QCD critical points from the experimental beam energy scan of heavy ion collisions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS1976

Design and evaluation of a 3 million DN series-hybrid thrust bearing

The design and experimental evaluation of a series-hybrid thrust bearing, consisting of a 150-mm ball bearing and a centrifugally actuated, conical, fluid-film bearing, is presented. Tests were conducted up to 16,000 rpm and at this speed an axial load of 15,600 N (3500 lb) was safely supported by the hybrid bearing system. Through the series-hybrid bearing principle, the effective ball bearing speed was reduced to approximately one-half of the shaft speed. A speed reduction of this magnitude would result in a tenfold increase in the ball bearing fatigue life. A successful evaluation of fluid-film bearing lubricant supply failure was performed repeatedly at an operating speed of 10,000 rpm. A complete and smooth changeover to full-scale ball bearing operation was effected when the oil supply to the fluid-film bearing was cut off. Reactivation of the fluid-film oil supply system produced a flawless return to the original mode of hybrid operation.

Scibbe, H. W.↗