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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.

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NASA NTRS1993

Local isotropy in high Reynolds number turbulent shear flows

This is a report on the continuation of experiments, which Dr. Srinivas Veeravalli and the present author started in 1991, to investigate the hypothesis of local isotropy in shear flows. This hypothesis, which states that at sufficiently high Reynolds numbers the small-scale structures of turbulent motions are independent of large-scale structures and mean deformations, has been used in theoretical studies of turbulence and computational methods like large-eddy simulation. The importance of Kolmogorov's ideas arises from the fact that they create a foundation for turbulence theory.

Saddoughi, Seyed G.↗
NASA NTRS1992

A small-scale turbulence model

A model for the small-scale structure of turbulence is reformulated in such a way that it may be conveniently computed. The model is an ensemble of randomly oriented structured two dimensional vortices stretched by an axially symmetric strain flow. The energy spectrum of the resulting flow may be expressed as a time integral involving only the enstrophy spectrum of the time evolving two-dimensional cross section flow, which may be obtained numerically. Examples are given in which a k(exp -5/3) spectrum is obtained by this method without using large wave number asymptotic analysis. The k(exp -5/3) inertial range spectrum is shown to be related to the existence of a self-similar enstrophy preserving range in the two-dimensional enstrophy spectrum. The results are insensitive to time dependence of the strain-rate, including even intermittent on-or-off strains.

Lundgren, T. S.↗
NASA NTRS1993

Nonlinear aerodynamics and the design of wing tips

This contract began in April 1990 with studies on the effect of wingtip shape on induced drag. That work considered problems of determining induced drag from computational aerodynamic methods and examined effects of wake roll up on the induced drag of elliptical and crescent wings. The research contract was continued in April 1991 and again in April 1992 with scope augmented to include considerations other than induced drag. The idea was to develop methods for comparing wing tip shapes on the basis of their effects on total drag, structural weight, and high lift performance. To accomplish this, work was done to improve methods for multidisciplinary analysis and optimization of wings. This report describes results of research conducted from April 1992 through March 1993. The general objective was to improve an existing wing optimization method and apply the method to specific problems of interest. The method, while a valuable tool for wing tip design studies, can be applied to more general problems, and has been applied to some of these other problems during its development. Specific goals that were accomplished are listed below and are explained in more detail in attached reports: analysis methods were improved in the area of structural modeling; small improvements to the high lift model were made, and ideas were collected on how clean wing c(sub l(sub max)) should vary with sweep - these ideas should provide a guide for future examination of high lift with experiment or CFD; and a calculation for drag due to the loss of leading edge suction was added to the wing analysis.

Kroo, Ilan↗
NASA NTRS1993

On the Daubechies-based wavelet differentiation matrix

The differentiation matrix for a Daubechies-based wavelet basis is constructed and superconvergence is proven. That is, it will be proven that under the assumption of periodic boundary conditions that the differentiation matrix is accurate of order 2M, even though the approximation subspace can represent exactly only polynomials up to degree M-1, where M is the number of vanishing moments of the associated wavelet. It is illustrated that Daubechies-based wavelet methods are equivalent to finite difference methods with grid refinement in regions of the domain where small-scale structure is present.

Jameson, Leland↗
NASA NTRS1993

Local isotropy in distorted turbulent boundary layers at high Reynolds number

This is a report on the continuation of our experimental investigations of the hypothesis of local isotropy in shear flows. This hypothesis, which states that at sufficiently high Reynolds numbers the small-scale structures of turbulent motions are independent of large-scale structures and mean deformations, has been used in theoretical studies of turbulence and computational methods such as large-eddy simulation. Since Kolmogorov proposed his theory, there have been many experiments, conducted in wakes, jets, mixing layers, a tidal channel, and atmospheric and laboratory boundary layers, in which attempts have been made to verify - or refute - the local-isotropy hypothesis. However, a review of the literature over the last five decades indicated that, despite all these experiments in shear flows, there was no consensus in the scientific community regarding this hypothesis, and, therefore, it seemed worthwhile to undertake a fresh experimental investigation into this question.

Saddoughi, Seyed G.↗
NASA NTRS1994

Inducer analysis/pump model development

Current design of high performance turbopumps for rocket engines requires effective and robust analytical tools to provide design information in a productive manner. The main goal of this study was to develop a robust and effective computational fluid dynamics (CFD) pump model for general turbopump design and analysis applications. A finite difference Navier-Stokes flow solver, FDNS, which includes an extended k-epsilon turbulence model and appropriate moving zonal interface boundary conditions, was developed to analyze turbulent flows in turbomachinery devices. In the present study, three key components of the turbopump, the inducer, impeller, and diffuser, were investigated by the proposed pump model, and the numerical results were benchmarked by the experimental data provided by Rocketdyne. For the numerical calculation of inducer flows with tip clearance, the turbulence model and grid spacing are very important. Meanwhile, the development of the cross-stream secondary flow, generated by curved blade passage and the flow through tip leakage, has a strong effect on the inducer flow. Hence, the prediction of the inducer performance critically depends on whether the numerical scheme of the pump model can simulate the secondary flow pattern accurately or not. The impeller and diffuser, however, are dominated by pressure-driven flows such that the effects of turbulence model and grid spacing (except near leading and trailing edges of blades) are less sensitive. The present CFD pump model has been proved to be an efficient and robust analytical tool for pump design due to its very compact numerical structure (requiring small memory), fast turnaround computing time, and versatility for different geometries.

Cheng, Gary C.↗
NASA NTRS1994

Electron density measurements during the NLC-91 campaign

A Super Arcas rocket, MISTI B, containing DC and RF probes, was launched as a part of the PMSE (Polar Mesosphere Summer Echoes) Salvo during the NLC-91 (Noctilucent Cloud) campaign to measure electron density irregularities with high spatial resolution. Measurements of large and small scale structures in the electron density were made on rocket ascent and descent at the altitudes of 86.5 and 88.5 +/- 0.5 km corresponding to the two altitudes of strongest backscatter recorded by the nearby CUPRI (Cornell University Portable Radar Interferometer) radar. Power spectra of the fluctuations shows two different structuring and scattering mechanisms exist at altitudes only 1 km apart. Since the rocket apogee was 89 km, the rocket was in the height range 88.5 +/- 0.5 km for 30 seconds giving an unusual measurement of horizontal structure over a distance of 5.5 km. Using the simultaneous DC and RF probe measurements of electron depletions and sharp gradient in the lower layer, the role of aerosols in creating these depletions and gradients is speculated upon.

Ulwick, J. C.↗
NASA NTRS1994

On the wavelet optimized finite difference method

When one considers the effect in the physical space, Daubechies-based wavelet methods are equivalent to finite difference methods with grid refinement in regions of the domain where small scale structure exists. Adding a wavelet basis function at a given scale and location where one has a correspondingly large wavelet coefficient is, essentially, equivalent to adding a grid point, or two, at the same location and at a grid density which corresponds to the wavelet scale. This paper introduces a wavelet optimized finite difference method which is equivalent to a wavelet method in its multiresolution approach but which does not suffer from difficulties with nonlinear terms and boundary conditions, since all calculations are done in the physical space. With this method one can obtain an arbitrarily good approximation to a conservative difference method for solving nonlinear conservation laws.

Jameson, Leland↗
NASA NTRS1994

Small-scale behavior in distorted turbulent boundary layers at low Reynolds number

During the last three years we have conducted high- and low-Reynolds-number experiments, including hot-wire measurements of the velocity fluctuations, in the test-section-ceiling boundary layer of the 80- by 120-foot Full-Scale Aerodynamics Facility at NASA Ames Research Center, to test the local-isotropy predictions of Kolmogorov's universal equilibrium theory. This hypothesis, which states that at sufficiently high Reynolds numbers the small-scale structures of turbulent motions are independent of large-scale structures and mean deformations, has been used in theoretical studies of turbulence and computational methods such as large-eddy simulation; however, its range of validity in shear flows has been a subject of controversy. The present experiments were planned to enhance our understanding of the local-isotropy hypothesis. Our experiments were divided into two sets. First, measurements were taken at different Reynolds numbers in a plane boundary layer, which is a 'simple' shear flow. Second, experiments were designed to address this question: will our criteria for the existence of local isotropy hold for 'complex' nonequilibrium flows in which extra rates of mean strain are added to the basic mean shear?

Saddoughi, Seyed G.↗
NASA NTRS1994

The effects of Kelvin-Helmholtz instability on resonance absorption layers in coronal loops

One of the long-standing uncertainties in the wave-resonance theory of coronal heating is the stability of the resonance layer. The wave motions in the resonance layer produce highly localized shear flows which vary sinusoidally in time with the resonance period. This configuration is potentially susceptible to the Kelvin-Helmholtz instability (KHI), which can enhance small-scale structure and turbulent broadening of shear layers on relatively rapid ideal timescales. We have investigated numerically the response of a characteristic velocity profile, derived from resonance absorption models, to finite fluid perturbations comparable to photospheric fluctuations. We find that the KHI primarily should affect long (approximately greater than 6 x 10(exp 4) km) loops where higher velocity flows (M approximately greater than 0.2) exist in resonance layers of order 100 km wide. There, the Kelvin-Helmholtz growth time is comparable to or less than the resonance quarter-period, and the potentially stabilizing magnetic effects are not felt until the instability is well past the linear growth stage. Not only is the resonance layer broadened by the KHI, but also the convective energy transport out of the resonance layer is increased, thus adding to the efficiency of the wave-resonance heating process. In shorter loops, e.g., those in bright points and compact flares, the stabilization due to the magnetic field and the high resonance frequency inhibit the growth of the Kelvin-Helmholtz instability beyond a minimal level.

Karpen, Judith T.↗
NASA NTRS1993

Variations in Venus cloud particle properties: A new view of Venus's cloud morphology as observed by Galileo Near-Infrared Mapping Spectrometer

Using Venus nightside data obtained by the Galileo Near-Infrared Mapping Spectrometer (NIMS), we have studied the correlation of 1.74 and 2.30 micrometer radiation which is transmitted through the clouds. Since the scattering and absorption properties of the cloud particles are different at these two wavelengths, one can distinguish between abundance variations and variations in the properties of the cloud particles themselves. The correlation of intensities shows a clustering of data into five distinct branches. Using radiative transfer calculations, we interpret these branches as regions of distinct but different mixes of Mode 2' and 3 particles. The data and calculations indicate large differences in these modal ratios, the active cloud regions varying in content from nearly pure Mode 2' particles to almost wholly Mode 3. The spatial distribution of these branches shows large scale sizes and both hemispheric symmetries and asymmetries. High-latitude concentrations of large particles are seen in both hemispheres and there is banded structure of small particles seen in both the North and South which may be related. The mean particle size in the Northern Hemisphere is greater than found in the South. If these different branch regions are due to mixing of vertically stratified source regions (e.g. photochemical and condensation source mechanisms), then the mixing must be coherent over very large spatial scales.

Carlson, R. W.↗
NASA NTRS1994

Multifrequency studies of bright radio supernova remnants. 2: W49B

We report radio observations of the supernova remnant W49B using the Very Large Array (VLA) of the NRAO at 90, 20, and 6 cm. This work continues the study of the properties of young, bright supernova remnants (SNRs) begun with 3C 391 (Moffett & Reynolds 1994). Here we present high-resolution images of total intensity, polarization, and spectral index of W49B. In large-scale morphology it is basically a box-shaped remnant expanding into an apparently inhomogeneous medium, but we also find small-scale structures, arcs and filaments, which lie interior to the outer shell in projection. The actual spatial position of the filaments is unknown, though we suspect that they are in the remnant interior rather than on the front or back face of the blast wave. In addition, their distribution in two dimensions suggests the projection of a helical structure. The shell edge, sharply rising in brightness for at least 3/4 of its circumference, still remains unresolved at our highest resolution (4 sec). This for our assumed distance of 8 kpc, the width of the region in which the emission appears is less than 0.16 pc, indicating short mean free paths for shock-accelerated electrons and high levels of MHD turbulence presumably causing the scattering. We find no polarized flux at 90 or 20 cm, with 3 sigma upper limits in polarized intensity of 36 mJy at 90 cm (45 sec resolution) and 7.2 mJy at 20 cm (5 sec resolution), or 22 micro-Jy/sq arcsec and 370 micro-jy/sq arcsec, respectively, for any emission extended on those scales. Polarized flux is present at 6 cm, but at a very low mean polarized fraction (total polarized flux divided by total flux) of 0.44% +/- 0.06%, which, as for 3C 391, is much lower than typical for bright SNRs at this frequency. The morphology in polarized intensity is poorly correlated with that in total intensity. We see excursions in the polarized fraction up to at least 10% in a few locations, but even where polarization is seen, typical levels are a few percent. Tangled or disordered magnetic fields in the emitting region of the radio shell may be responsible for depolarizing the radio synchrotron radiation, but some form of internal Faraday depolarization may also occur. We estimate the foreground Faraday rotation measure to be about -450 rad/sq m, similar to that found for 3C 391, which is also the Galactic plane and just about as distant. Spectral index images created from the total intensity images show that the spectral index across W49B is constant to within about Delta alpha approximately 0.1 in bright regions. This result weakly supports a common origin of the radio-emitting electrons, as in the blast wave, rather than in inhomogeneous turbulent regions of differing properties due to the stochastic (second-order Fermi) acceleration process. Variations at the level of Delta alpha approximately 0.1 are seen, but their significance is doubtful. New observations at 90 cm, using experimental three-dimensional imaging technology, may improve on this limit.

Moffett, David A.↗
NASA NTRS1994

The rotation of photospheric magnetic fields: A random walk transport model

In an earlier study of solar differential rotation, we showed that the transport of magnetic flux across latitudes acts to establish quasi-stationary patterns, therby accounting for the observed rigid rotation of the large-scale photospheric field. In that paper, the effect of supergranular convection was represented by a continuum diffusion, limiting the applicability of the calculations to large spatial scales. Here we extend the model to scales comparable to that of the supergranulation itself by replacing the diffusive transport with a discrete random walk process. Rotation curves are derived by cross-correlating the simulated photospheric field maps for a variety of time lags and spatial resolutions. When the lag between maps is relatively short less than or approximately = 15 days), the midlatitude correlation functions show two distinct components: a broad feature associated with the large-scale unipolar patterns and a narrow feature originating from small magnetic structures encompossing from one to several supergranular cells. By fitting the broad component we obtain the rigid rotation profile of the patterns, whereas by fitting the narrow component, we recover the differential rate of the photospheric plasma itself. For time lags of 1 month or greater, only the broad feature associated with the long-lived patterns remains clearly identifiable in the simulations.

Wang, Y. -M.↗
NASA NTRS1996

Multipath analysis diffraction calculations

This report describes extensions of the Kirchhoff diffraction equation to higher edge terms and discusses their suitability to model diffraction multipath effects of a small satellite structure. When receiving signals, at a satellite, from the Global Positioning System (GPS), reflected signals from the satellite structure result in multipath errors in the determination of the satellite position. Multipath error can be caused by diffraction of the reflected signals and a method of calculating this diffraction is required when using a facet model of the satellite. Several aspects of the Kirchhoff equation are discussed and numerical examples, in the near and far fields, are shown. The vector form of the extended Kirchhoff equation, by adding the Larmor-Tedone and Kottler edge terms, is given as a mathematical model in an appendix. The Kirchhoff equation was investigated as being easily implemented and of good accuracy in the basic form, especially in phase determination. The basic Kirchhoff can be extended for higher accuracy if desired. A brief discussion of the method of moments and the geometric theory of diffraction is included, but seems to offer no clear advantage in implementation over the Kirchhoff for facet models.

Statham, Richard B.↗
NASA NTRS1996

Stratospheric Meteorological Conditions for the 3-12 Nov. 1994 ATMOS/ATLAS-3 Measurements

During the ATLAS-3 mission (3-12 Nov 1994), United Kingdom Meteorological Office fields show that the SH vortex was still strong below about 700 K (approximately 16 hPa), with coherent vortex fragments apparent up to about 1300 K (approximately 3 hPa). The SH vortex was shifted off the pole toward 270 deg E throughout ATLAS-3, although its shape varied from day to day. SH temperatures were increasing during ATLAS-3; temperatures below 188 K were last seen 35-45 days before the mission, but were below 195 K as late as 2 Nov 1994. The NH polar vortex had developed at levels above about 550 K (approximately 35 hPa). Simulated high-resolution potential vorticity (PV) fields clearly show low latitude air being drawn up around the polar vortices in both hemispheres. These fields indicate that meteorological analyses underestimate the amount of atmospheric variability, including the strength of local PV gradients, and small-scale structure. Structure such as that in the simulated fields can result in apparent discrepancies between tracer measurements and PV fields.

Manney, G. L.↗
NASA NTRS1996

Reconstruction of the Mesoscale Velocity Shear Seaward of Coastal Upwelling Regions from the Refraction of the Surface Wave Field

The objective of this project is to study fronts that develop at the boundary between cold water recently upwelled to the surface through Ekman divergence, and warmer surrounding waters. This specific objective was suggested by studying the small scale structure of upwelling fronts (coastal, island, and equatorial) through shipboard surveys and infrared satellite images. Constraints on the shuttle equator crossing imposed by other land sites precluded a coverage of the area targeted in the initial SIR-C proposal, the California Current. The site was then relocated to the Equatorial Pacific upwelling tongue, that can be satisfactorily imaged for a wide range of longitudes of the equator crossing. Some limited data was nevertheless obtained over coastal upwelling off California in 1989, using the JPL AIRSAR in multifrequency mode, and over island upwelling off Hawaii in 1990, using the radar in along-track interferometric mode.

Flament, Pierre↗
NASA NTRS1998

High Resolution Imaging of the Sun with CORONAS-1

We applied several image restoration and enhancement techniques, to CORONAS-I images. We carried out the characterization of the Point Spread Function (PSF) using the unique capability of the Blind Iterative Deconvolution (BID) technique, which recovers the real PSF at a given location and time of observation, when limited a priori information is available on its characteristics. We also applied image enhancement technique to extract the small scale structure imbeded in bright large scale structures on the disk and on the limb. The results demonstrate the capability of the image post-processing to substantially increase the yield from the space observations by improving the resolution and reducing noise in the images.

Karovska, Margarita↗
NASA NTRS1998

A Search for Hydrodynamical Interaction Between the ISM and Radio Jets in IC4296

The ROSAT HRI Data set on IC 4296 has been reduced and analysed. A draft paper on the small-scale structure of x-ray emission and its relation to the radio emission has been written. Mackie left the Smithsonian Astrophysical Observatory in September 1997 and in January 1998 he joined the staff at Carter Observatory, New Zealand. Mackie is currently (May 1998) upgrading computer software at Carter to run IRAF-PROS and XSPEC x-ray software packages in order to reduce and analyze archival ROSAT PSPC data on IC 4296. The PSPC results are needed to investigate the hot gas temperature and abundance properties that will be used in conjunction with the radio jet properties to determine the hydrodynamical interaction status of IC 4296, and finalize the results of a research paper.

Mackie, Glen↗