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At least 289 records · Page 16

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NASA NTRS1987

Problems and possible solutions involved in hard target calibration of coherent Doppler lidar

The field of radiometry is surveyed with reference to coherent light, since the surfaces to be considered here are irradiated with coherent, polarized light. Definitions of some concepts are modified. In light of these modifications calibration problems are analyzed and solutions suggested. The most important task was to develop hard targets exhibiting minimal specular reflection (mirror-like and retroreflection) and following closely a Lambertian scattering curve. Bistable reflectometer experiments and integrating sphere measurements are used to physically characterize the targets. Optical and electron microscopy are used to physically characterize the targets. Since the BRDF matrix is not measured, this capability must be developed preferably for both bistatic and monostatic reflectometer measurements. The equipment is expensive and not yet developed. Calibration can be simplified if it can be proved that the BRDF matrix is diagonal for a diffuse scatterer.

Anderson, Richard↗

NASA NTRS1992

Liquid lubricants for advanced aircraft engines

An overview of liquid lubricants for use in current and projected high performance turbojet engines is discussed. Chemical and physical properties are reviewed with special emphasis placed on the oxidation and thermal stability requirements imposed upon the lubrication system. A brief history is given of the development of turbine engine lubricants which led to the present day synthetic oils with their inherent modification advantages. The status and state of development of some eleven candidate classes of fluids for use in advanced turbine engines are discussed. Published examples of fundamental studies to obtain a better understanding of the chemistry involved in fluid degradation are reviewed. Alternatives to high temperature fluid development are described. The importance of continuing work on improving current high temperature lubricant candidates and encouraging development of new and improved fluid base stocks are discussed.

Loomis, William R.↗

NASA NTRS1993

Liquid lubricants for advanced aircraft engines

Loomis, William R.↗

NASA NTRS1995

Development of a two-equation turbulence model for hypersonic flows: Evaluation of a low Reynolds number correction to the Kappa - epsilon two equation compressible turbulence model - Volume 1

The objective of the current research is the development of an improved k-epsilon two-equation compressible turbulence model for turbulent boundary layer flows experiencing strong viscous-inviscid interactions. The development of an improved model is important in the design of hypersonic vehicles such as the National Aerospace Plane (NASP) and the High Speed Civil Transport (HSCT). Improvements have been made to the low Reynolds number functions in the eddy viscosity and dissipation of solenoidal dissipation of the k-epsilon turbulence mode. These corrections offer easily applicable modifications that may be utilized for more complex geometries. The low Reynolds number corrections are functions of the turbulent Reynolds number and are therefore independent of the coordinate system. The proposed model offers advantages over some current models which are based upon the physical distance from the wall, that modify the constants of the standard model, or that make more corrections than are necessary to the governing equations. The code has been developed to solve the Favre averaged, boundary layer equations for mass, momentum, energy, turbulence kinetic energy, and dissipation of solenoidal dissipation using Keller's box scheme and the Newton spatial marching method. The code has been validated by removing the turbulent terms and comparing the solution with the Blasius solution, and by comparing the turbulent solution with an existing k-epsilon model code using wall function boundary conditions. Excellent agreement is seen between the computed solution and the Blasius solution, and between the two codes. The model has been tested for both subsonic and supersonic flat-plate turbulent boundary layer flow by comparing the computed skin friction with the Van Driest II theory and the experimental data of Weighardt; by comparing the transformed velocity profile with the data of Weighardt, and the Law of the Wall and the Law of the Wake; and by comparing the computed results of an adverse pressure gradient with the experimental data of Fernando and Smits. Good agreement is obtained with the experimental correlations for all flow conditions.

Knight, Doyle D.↗

NASA NTRS1976

Six-beam models in radiative transfer theory

A critical analysis is given of the applicability of six-beam models to radiative transfer in particulate materials. The method of introducing transverse scattering in these models is shown to cause fundamental difficulties in the case of physically plausible phase functions; in particular, the effective absorptivity is abnormally large and thus results in incorrect reflectances and transmittances. Six-beam calculations for several media are compared with accurate solutions, with Chu-Churchill two-beam results, and with a simple modification to the Eddington approximation, the last being generally superior over a wide range of conditions.

Meador, W. E.↗

NASA NTRS1986

Pulsar timing and general relativity

Techniques are described for accounting for relativistic effects in the analysis of pulsar signals. Design features of instrumentation used to achieve millisecond accuracy in the signal measurements are discussed. The accuracy of the data permits modeling the pulsar physical characteristics from the natural glitches in the emissions. Relativistic corrections are defined for adjusting for differences between the pulsar motion in its spacetime coordinate system relative to the terrestrial coordinate system, the earth's motion, and the gravitational potentials of solar system bodies. Modifications of the model to allow for a binary pulsar system are outlined, including treatment of the system as a point mass. Finally, a quadrupole model is presented for gravitational radiation and techniques are defined for using pulsars in the search for gravitational waves.

Backer, D. C.↗

NASA NTRS1989

Interactions between long and synoptic-scale waves. I - Instability of a nonzonal flow. II - Growth rate of long waves

The interactions between synoptic-scale and long waves were investigated analytically. First, the influence of the long wave on the synoptic-scale wave is examined. The structure of a synoptic-scale wave growing on a wavy basic scale was analyzed under an assumption that the synoptic-scale waves have the structure of the most unstable normal modes. The derived analytical solution, which is simple and is amenable to physical interpretation, can be understood in terms of eddies and their local growth rate. The analytical solution is then used to examine growth of a long wave in the presence of parameterized synoptic-scale waves. Two possibly unstable solutions were found; one is a modification of the linear long wave, and the other a strongly nonlinear solution. In both cases, the synoptic-scale wave increases the growth rate of the long wave.

Ebisuzaki, Wesley↗

NASA NTRS1991

A system for mapping sources of VHF and electric field pulses from in-cloud lightning at KSC

The literature concerning VHF radiation and wideband electric fields from in-cloud lightning is reviewed. VHF location systems give impressive radio images of lightning in clouds with high spatial and temporal resolution. Using systems based on long and short baseline time-or-arrival and interferometry, workers have detected VHF sources that move at speeds of 10(exp 5) to 10(exp 8) m/s. The more slowly moving sources appear to be associated with channel formation but the physical basis for the higher speeds is not clear. In contrast, wideband electric fields are directly related to physical parameters such as current and tortuosity. A long baseline system is described to measure simultaneously VHF radiation and wideband electric fields at five stations at Kennedy Space Center. All signals are detected over remote, isolated ground planes with fiber optics for data transmission. The modification of this system to map rapidly varying dE/dt pulses is discussed.

Thomson, Ewen M.↗

NASA NTRS2012

The Role of Forcing and Internal Dynamics in explaining the 'Medieval Climate Anomaly'

Proxy reconstructions suggest that peak global temperature during the past warm interval known as the Medieval Climate Anomaly (MCA, roughly 950-1250 AD) has been exceeded only during the most recent decades. To better understand the origin of this warm period, we use model simulations constrained by data assimilation establishing the spatial pattern of temperature changes that is most consistent with forcing estimates, model physics and the empirical information contained in paleoclimate proxy records. These numerical experiments demonstrate that the reconstructed spatial temperature pattern of the MCA can be explained by a simple thermodynamical response of the climate system to relatively weak changes in radiative forcing combined with a modification of the atmospheric circulation, displaying some similarities with the positive phase of the so-called Arctic Oscillation, and with northward shifts in the position of the Gulf Stream and Kuroshio currents. The mechanisms underlying the MCA are thus quite different from anthropogenic mechanisms responsible for modern global warming.

Goossee, Hugues↗

NASA NTRS2011

Phase-Controlled Magnetic Mirror for Wavefront Correction

Typically, light interacts with matter via the electric field and interaction with weakly bound electrons. In a magnetic mirror, a patterned nanowire is fabricated over a metallic layer with a dielectric layer in between. Oscillation of the electrons in the nanowires in response to the magnetic field of incident photons causes a re-emission of photons and operation as a "magnetic mirror." By controlling the index of refraction in the dielectric layer using a local applied voltage, the phase of the emitted radiation can be controlled. This allows electrical modification of the reflected wavefront, resulting in a deformable mirror that can be used for wavefront control. Certain applications require wavefront quality in the few-nanometer regime, which is a major challenge for optical fabrication and alignment of mirrors or lenses. The use of a deformable magnetic mirror allows for a device with no moving parts that can modify the phase of incident light over many spatial scales, potentially with higher resolution than current approaches. Current deformable mirrors modify the incident wavefront by using nano-actuation of a substrate to physically bend the mirror to a desired shape. The purpose of the innovation is to modify the incident wavefront for the purpose of correction of fabrication and alignment-induced wavefront errors at the system level. The advanced degree of precision required for some applications such as gravity wave detection (LISA - Laser Interferometer Space Antenna) or planet finding (FKSI - Fourier-Kelvin Stellar Interferometer) requires wavefront control at the limits of the current state of the art. All the steps required to fabricate a magnetic mirror have been demonstrated. The modification is to apply a bias voltage to the dielectric layer so as to change the index of refraction and modify the phase of the reflected radiation. Light is reflected off the device and collected by a phase-sensing interferometer. The interferometer determines the initial wavefront of the device and fore optics. A wavefront correction is calculated, and voltage profile for each nanowire strip is determined. The voltage is applied, modifying the local index of refraction of the dielectric under the nanowire strip. This modifies the phase of the reflected light to allow wavefront correction.

Hagopian, John↗

NASA NTRS1982

Channel and physical models of the Jovian subnebula

A semiempirical physical model of the Jovian subnebula was developed by analogy with the primitive solar nebula itself. The chemical aspects of this model are developed according to the principles developed in the study of the thermochemistry and gas kinetic behavior of the solar nebula, but with important modifications to take into account the higher pressures and densities in the Jovian subnebula. The bulk compositions and densities of the inner satellites of Jupiter are calculated. It is proposed that Europa differs from Io chiefly in that in has suffered a less severe thermal history. The general features of this model are applicable with minor modification to the systems of Saturn and Uranus.

Lewis, J. S.↗

NASA NTRS1993

Exploring the southern ocean response to climate change

The purpose of this project was to couple a regional (Southern Ocean) ocean/sea ice model to the existing Goddard Institute for Space Science (GISS) atmospheric general circulation model (GCM). This modification recognizes: the relative isolation of the Southern Ocean; the need to account, prognostically, for the significant air/sea/ice interaction through all involved components; and the advantage of translating the atmospheric lower boundary (typically the rapidly changing ocean surface) to a level that is consistent with the physical response times governing the system evolution (that is, to the base of the fast responding ocean surface layer). The deeper ocean beneath this layer varies on time scales several orders of magnitude slower than the atmosphere and surface ocean, and therefore the boundary between the upper and deep ocean represents a more reasonable fixed boundary condition.

Martinson, Douglas G.↗

NASA NTRS1984

Radiative tearing - Magnetic reconnection on a fast thermal-instability time scale

Two energy modification mechanisms which are known to occur in sheared magnetic fields are the tearing and thermal instabilities. These processes can be studied separately with formalisms incorporating just the effective driving mechanism of interest (finite resistivity for the tearing mode and unstable radiation for the thermal mode). A model which includes both effects, and a temperature-dependent resistivity, indicates that modified forms of these two instabilities may coexist for identical physical conditions. When they are isolated computationally, one can show that their limiting growth rates are approximately those of the uncoupled instabilities. The spatial structure and energy content of these two new hybrid processes are then individually examined and are found to differ considerably from those obtained from separate treatments of the driving mechanisms. The faster radiative instability, which has a hydromagnetically scaled growth rate like the condensation mode of the thermal instability, is shown to involve a substantial amount of magnetic field reconnection. This can be partially explained by a large temperature drop (or resistivity rise) at the X-point. The island width of the Coulomb-coupled radiative mode is 30 percent of that produced by a comparable level of the slower tearing instability. In addition, the perturbed magnetic energy in the radiative instability is 5 times that of the perturbed thermal energy, indicating an appreciable modification of the initial magnetic structure.

Steinolfson, R. S.↗

NASA NTRS1995

Intelligent automated surface grid generation

The goal of our research is to produce a flexible, general grid generator for automated use by other programs, such as numerical optimizers. The current trend in the gridding field is toward interactive gridding. Interactive gridding more readily taps into the spatial reasoning abilities of the human user through the use of a graphical interface with a mouse. However, a sometimes fruitful approach to generating new designs is to apply an optimizer with shape modification operators to improve an initial design. In order for this approach to be useful, the optimizer must be able to automatically grid and evaluate the candidate designs. This paper describes and intelligent gridder that is capable of analyzing the topology of the spatial domain and predicting approximate physical behaviors based on the geometry of the spatial domain to automatically generate grids for computational fluid dynamics simulators. Typically gridding programs are given a partitioning of the spatial domain to assist the gridder. Our gridder is capable of performing this partitioning. This enables the gridder to automatically grid spatial domains of wide range of configurations.

Yao, Ke-Thia↗

NASA NTRS1994

Practical formulation of a positively conservative scheme

Approximate Riemann solvers have been highly successful for computing the Euler/Navier-Stokes equations, but linearized Riemann solvers are known to fail occasionally by predicting non-physical states with negative density or internal energy. Positively conservative schemes, in contrast, guarantee physical solutions from realistic input. The Harten-Lax-van Leer-Einfeldt (HLLE) scheme is a typical example of a positively conservative scheme. However, the HLLE scheme is highly dissipative at contact discontinuities and shear layers and thus it is not applicable to practicle simulations. An existing modification to the HLLE scheme, known as HLLEM, enhances the resolution to that of the Roe scheme. However, this modification violates the positivity of density and internal energy. Precise derivation of the modification yields a quatratic inequality and thus requires a case-by-case treatment. This Note describes a new, modified HLLE scheme that satisfies the positively conservative condition approximately. Sample computationa are included to demonstrate the resolution and the robustness of the scheme.

Obayashi, Shigeru↗

NASA NTRS1993

Towards a generalized computational fluid dynamics technique for all Mach numbers

Currently there exists no single unified approach for efficiently and accurately solving computational fluid dynamics (CFD) problems across the Mach number regime, from truly low speed incompressible flows to hypersonic speeds. There are several CFD codes that have evolved into sophisticated prediction tools with a wide variety of features including multiblock capabilities, generalized chemistry and thermodynamics models among other features. However, as these codes evolve, the demand placed on the end user also increases simply because of the myriad of features that are incorporated into these codes. In order for a user to be able to solve a wide range of problems, several codes may be needed requiring the user to be familiar with the intricacies of each code and their rather complicated input files. Moreover, the cost of training users and maintaining several codes becomes prohibitive. The objective of the current work is to extend the compressible, characteristic-based, thermochemical nonequilibrium Navier-Stokes code GASP to very low speed flows and simultaneously improve convergence at all speeds. Before this work began, the practical speed range of GASP was Mach numbers on the order of 0.1 and higher. In addition, a number of new techniques have been developed for more accurate physical and numerical modeling. The primary focus has been on the development of optimal preconditioning techniques for the Euler and the Navier-Stokes equations with general finite-rate chemistry models and both equilibrium and nonequilibrium thermodynamics models. We began with the work of Van Leer, Lee, and Roe for inviscid, one-dimensional perfect gases and extended their approach to include three-dimensional reacting flows. The basic steps required to accomplish this task were a transformation to stream-aligned coordinates, the formulation of the preconditioning matrix, incorporation into both explicit and implicit temporal integration schemes, and modification of the numerical flux formulae. In addition, we improved the convergence rate of the implicit time integration schemes in GASP through the use of inner iteration strategies and the use of the GMRES (General Minimized Resisual) which belongs to the class of algorithms referred to as Krylov subspace iteration. Finally, we significantly improved the practical utility of GASP through the addition of mesh sequencing, a technique in which computations begin on a coarse grid and get interpolated onto successively finer grids. The fluid dynamic problems of interest to the propulsion community involve complex flow physics spanning different velocity regimes and possibly involving chemical reactions. This class of problems results in widely disparate time scales causing numerical stiffness. Even in the absence of chemical reactions, eigenvalue stiffness manifests itself at transonic and very low speed flows which can be quantified by the large condition number of the system and evidenced by slow convergence rates. This results in the need for thorough numerical analysis and subsequent implementation of sophisticated numerical techniques for these difficult yet practical problems. As a result of this work, we have been able to extend the range of applicability of compressible codes to very low speed inviscid flows (M = .001) and reacting flows.

Walters, R. W.↗

NASA NTRS1990

Acousto-optic signature analysis for inspection of the orbiter thermal protection tile bonds

The goal of this research is to develop a viable NDE technique for the inspection of orbiter thermal protection system (TPS) tile bonds. Phase 2, discussed here, concentrated on developing an empirical understanding of the bonded and unbonded vibration signatures of acreage tiles. Controlled experiments in the laboratory have provided useful information on the dynamic response of TPS tiles. It has been shown that several signatures are common to all the pedigree tiles. This degree of consistency in the tile-SIP (strain isolation pad) dynamic response proves that an unbond can be detected for a known tile and establish the basis for extending the analysis capability to arbitrary tiles for which there are no historical data. The field tests of the noncontacting laser acoustic sensor system, conducted at the Kennedy Space Center (KSC), investigated the vibrational environment of the Orbiter Processing Facility (OPF) and its effect on the measurement and analysis techniques being developed. The data collected showed that for orbiter locations, such as the body flap and elevon, the data analysis scheme, and/or the sensor, will require modification to accommodate the ambient motion. Several methods were identified for accomplishing this, and a solution is seen as readily achievable. It was established that the tile response was similar to that observed in the laboratory. Of most importance, however, is that the field environment will not affect the physics of the dynamic response that is related to bond condition. All of this information is fundamental to any future design and development of a prototype system.

Rodriguez, Julio G.↗

NASA NTRS1998

Cardiovascular function following reduced aerobic activity

PURPOSE: The aim of this study was to test the hypothesis that a sustained reduction of physical activity (deconditioning) would alter the cardiovascular regulatory function. METHODS: Nineteen young, healthy volunteers participated in physical deconditioning for a period of 8 wk. Before (pre) and following (post) physical deconditioning, the responses of heart rate (HR), mean arterial pressure (MAP, measured by Finapres), central venous pressure (CVP), stroke volume (SV, Doppler), and forearm blood flow (FBF, plethysmography) were determined during lower body negative pressure (LBNP). The carotid baroreflex (CBR) function was assessed using a train of pulsatile neck pressure (NP) and suction, and the aortic baroreflex control of HR was assessed during steady-state phenylephrine (PE) infusion superimposed by LBNP and NP to counteract the PE increased CVP and carotid sinus pressure, respectively. RESULTS: Active physical deconditioning significantly decreased maximal oxygen uptake (-7%) and LBNP tolerance (-13%) without a change in baseline hemodynamics. Plasma volume (-3% at P = 0.135), determined by Evans Blue dilution, and blood volume (-4% at P = 0.107) were not significantly altered. During LBNP -20 to -50 torr, there was a significantly greater drop of SV per unit decrease in CVP in the post- (14.7 +/- 1.6%/mm Hg) than predeconditioning (11.2 +/- 0.7%/mm Hg) test accompanied by a greater tachycardia. Deconditioning increased the aortic baroreflex sensitivity (pre vs post: -0.61 +/- 0.12 vs -0.84 +/- 0.14 bpm.mm-1 Hg, P = 0.009) and the slope of forearm vascular resistance (calculated from [MAP-CVP]/FBF) to CVP (-2.75 +/- 0.26 vs -4.94 +/- 0.97 PRU/mm Hg, P = 0.086). However, neither the CBR-HR (-0.28 +/- 0.03 VS -0.39 +/- 0.10 bpm.mm-1 Hg) nor the CBR-MAP (-0.37 +/- 0.16 vs -0.25 +/- 0.07 mm Hg/mm Hg) gains were statistically different between pre- and postdeconditioning. CONCLUSIONS: We concluded that the functional modification of the cardiac pressure-volume relationship resulted in the reduced LBNP tolerance, despite the accentuated aortic and cardiopulmonary baroreflex function following deconditioning.

Non-NASA Center↗