Showing papers by "Ames Research Center published in 1993"

ZONAL TWO EQUATION k-w TURBULENCE MODELS FOR AERODYNAMIC FLOWS

Abstract: Two new versions of the k - w two-equation turbulence model will be presented. The new Baseline (BSL) model is designed to give results similar to those of the original k - w model of Wilcox. but without its strong dependency on arbitrary freestream values. The BSL model is identical to the Wilcox model in the inner SOC7£; of the boundary-layer but changes gradually to the standard k - f. model (in a k - w fonnulation) towards the boundary-layer edge. The new model is also virtually identical to the k - f. model for free shear layers. The second version of the model is called Shear-Stress Transport (SSn model. It is a variation of the BSL model with the additional ability to account for the transport of the principal turbulent shear stress in adverse pressure gradient boundary-layers. The model is based on Bradshaw's assumption that the principal shear-stress is pro­ portional to the turbulent kinetic energy, which is introduced into the definition of the eddy-viscosity. Both models are tested for a large number of different fiowfields. The results of the BSL model are similar to those of the original k - w model, but without the undesirable free stream dependency. The predictions of the SST model are also independent of the freestrearn values but show better agreement with exper­ imental data for adverse pressure gradient boundary-layer flows.

Review of chemical-kinetic problems of future NASA missions, II: Mars entries

Abstract: A number of chemical-kinetic problems related to phenomena occurring behind a shock wave surrounding an object flying in the earth atmosphere are discussed, including the nonequilibrium thermochemical relaxation phenomena occurring behind a shock wave surrounding the flying object, problems related to aerobraking maneuver, the radiation phenomena for shock velocities of up to 12 km/sec, and the determination of rate coefficients for ionization reactions and associated electron-impact ionization reactions. Results of experiments are presented in form of graphs and tables, giving data on the reaction rate coefficients for air, the ionization distances, thermodynamic properties behind a shock wave, radiative heat flux calculations, Damkoehler numbers for the ablation-product layer, together with conclusions.

Localization using nonindividualized head‐related transfer functions

Abstract: A recent development in human-computer interfaces is the virtual acoustic display, a device that synthesizes three-dimensional, spatial auditory information over headphones using digital filters constructed from head-related transfer functions (HRTFs). The utility of such a display depends on the accuracy with which listeners can localize virtual sound sources. A previous study [F. L. Wightman and D. J. Kistler, J. Acoust. Soc. Am. 85, 868-878 (1989)] observed accurate localization by listeners for free-field sources and for virtual sources generated from the subjects' own HRTFs. In practice, measurement of the HRTFs of each potential user of a spatial auditory display may not be feasible. Thus, a critical research question is whether listeners can obtain adequate localization cues from stimuli based on nonindividualized transforms. Here, inexperienced listeners judged the apparent direction (azimuth and elevation) of wideband noisebursts presented in the free-field or over headphones; headphone stimuli were synthesized using HRTFs from a representative subject of Wightman and Kistler. When confusions were resolved, localization of virtual sources was quite accurate and comparable to the free-field sources for 12 of the 16 subjects. Of the remaining subjects, 2 showed poor elevation accuracy in both stimulus conditions, and 2 showed degraded elevation accuracy with virtual sources. Many of the listeners also showed high rates of front-back and up-down confusions that increased significantly for virtual sources compared to the free-field stimuli. These data suggest that while the interaural cues to horizontal location are robust, the spectral cues considered important for resolving location along a particular cone-of-confusion are distorted by a synthesis process that uses nonindividualized HRTFs.

AutoClass: a Bayesian classification system

Abstract: This paper describes AutoClass II, a program for automatically discovering (inducing) classes from a database, based on a Bayesian statistical technique which automatically determines the most probable number of classes, their probabilistic descriptions, and the probability that each object is a member of each class. AutoClass has been tested on several large, real databases and has discovered previously unsuspected classes. There is no doubt that these classes represent new phenomena.

Mid- and far-infrared spectroscopy of ices - Optical constants and integrated absorbances

Abstract: Laboratory spectra through the mid-infrared (4000 to 500/cm (2.5-20 microns) have been used to calculate the optical constants (n and k) and integrated absorption coefficients (A) for a variety of pure and mixed molecular ices of relevance to astrophysics. The ices studied were H2O, CH3OH, CO2, OCS, CH4, CO2 + CH4, CO2 + OCS, CO + CH4, CO + OCS, O2 + CH4, O2 + OCS, N2 + CH4, N2 + OCS, H2O + CH4, H2O + OCS, and H2O + CH3OH + CO + NH3. In addition, the measurements have been extended through the far-infrared (500 to 50/cm (20-200 microns) for the H2O, CH3OH, and H2O + CH3OH + CO + NH3 ices.

The 1908 Tunguska explosion - Atmospheric disruption of a stony asteroid

Abstract: The explosion over Tunguska, Central Siberia, in 1908 released 10 to 20 megatons (high explosive equivalent) of energy at an altitude of about 10 km. This event represents a typical fate for stony asteroids tens of meters in radius entering the Earth's atmosphere at common hypersonic velocities. Comets and carbonaceous asteroids of the appropriate energy disrupt too high, whereas typical iron objects reach and crater the terrestrial surface.

Experimental and numerical study of a turbulent boundary layer with pressure gradients

Abstract: Results are presented of an experimental and numerical study of a turbulent boundary layer with pressure gradients conducted using the recent 'fringe method' with its numerical advantages and good inflow quality. After an inflow transient good agreement is observed; the differences, of up to 13 percent, are discussed. Moderate deviations from the law of the wall are found in the velocity profiles of the simulation. They are fully correlated with the pressure gradient, are in fair quantitative agreement with the experimental results of Nagano et al. (1992), and are roughly the opposite of uncorrected mixing-length-model predictions. Large deviations from the wall scaling are observed for other quantities, notably for the turbulence dissipation rate. The a(1) structure parameter drops mildly in the upper layer with adverse pressure gradient.

Mars atmospheric dynamics as simulated by the NASA Ames General Circulation Model: 1. The zonal-mean circulation

Abstract: The characteristics of the zonal-mean circulation and how it responds to seasonal variations and dust loading are described. This circulation is the main momentum-containing component of the general circulation, and it plays a dominant role in the budgets of heat and momentum. It is shown that in many ways the zonal-mean circulation on Mars, at least as simulated by the model, is similar to that on earth, having Hadley and Ferrel cells and high-altitude jet streams. However, the Martian systems tend to be deeper, more intense, and much more variable with season. Furthermore, the radiative effects of suspended dust particles, even in small amounts, have a major influence on the general circulation.

Digital images and human vision

Abstract: Processing of digital images destined for visual consumption raises many interesting questions regarding human visual sensitivity. This talk will survey some of these questions, including some that have been answered and some that have not. There will be an emphasis upon visual masking, and a distinction will be drawn between masking due to contrast gain control processes, and due to processes such as hypothesis testing, pattern recognition, and visual search.

Organic matter in meteorites: molecular and isotopic analyses of the Murchison meteorite.

Abstract: Carbonaceous chondrites comprise a unique subset of meteorites. Two classes of carbonaceous chondrites, the so-called CI1 and CM2 chondrites, are particularly interesting, in part because of their relatively high carbon content and the fact that most of this carbon is present as organic matter. This material is largely macromolecular but also contains a complex mixture of organic compounds that include carboxylic acids, dicarboxylic acids, amino acids, hydroxy acids, sulfonic acids, phosphonic acids, amines, amides, nitrogen heterocycles including purines and a pyrimidine, alcohols, carbonyl compounds, and aliphatic, aromatic, and polar hydrocarbons. The organic-rich CI1 and CM2 chondrites also contain an extensive clay mineralogy and other minerals that are believed to be indicative of an early episode of hydrous activity in the meteorite parent body. Recent stable isotope measurements have shown the organic matter in general, to be substantially enriched in deuterium and the discrete organic compounds to be enriched in 15N and somewhat enriched in 13C relative to terrestrial matter. These findings suggest that the organic matter is comprised of, or is closely related to, interstellar organic compounds. The organic chemistry of these meteorites is consistent with a formation scheme in which (1) a parent body was formed from volatile-rich icy planetesimals containing interstellar organic matter, (2) warming of the parent body led to an extensive aqueous phase in which the interstellar organics underwent various reactions, and (3) residual volatiles were largely lost leaving behind the suite of nonvolatile compounds that now characterize these meteorites.

Use of chamber systems to measure trace gas fluxes

Abstract: Trace gas exchange between soil-plant systems and the atmosphere is a complex phenomenon driven by a different set of physical, chemical, and biological processes for each chemical species and each environment (Fig. 4-1). For example, the exchange rate of a relatively inert gas like N20 represents simply the difference between its integrated rates of production and consumption by soil biochemical processes. More reactive gases are subject to additional emission and deposition processes, such as foliar exchange (NH3) or photochemical oxidation followed by deposition of the reaction products (NO). Further confounding the measurement and understanding of trace gas exchange across the surface-atmosphere boundary, the relative importances of the source and sink processes shown in Fig. 4-1 also vary with the rate of gas transport by diffusive, advective, and plant-mediated processes, and with the time and space scales over which the exchange is considered. Because of this complexity, great care must be exercised in drawing inferences about regional to global and seasonal to annual trace gas exchange rates from fluxes measured over small areas and short times. Denmead and Raupach (1993) introduced the two most common approaches to field measurement of trace gas exchange-chamber methods and micrometeorology. Because tower-based micrometeorological techniques integrate the flux over a larger area (typically 102_103 m2), they offer a potential advantage where the exchange rate is highly variable on the local scale measured by chamber methods (typically < 1 m2). Chambers offer other advantages, including low cost and ease of use, and they represent the measurement technique of choice in process-level studies and other research requiring replicate measurements coincident in space or time. Our purpose is to provide sufficient insight into the use of chamber systems to enable poten-

Boundary conditions for direct computation of aerodynamic sound generation

Abstract: A numerical scheme suitable for the computation of both the near field acoustic sources and the far field sound produced by turbulent free shear flows utilizing the Navier-Stokes equations is presented. To produce stable numerical schemes in the presence of shear, damping terms must be added to the boundary conditions. The numerical technique and boundary conditions are found to give stable results for computations of spatially evolving mixing layers.

Surface Ices and the Atmospheric Composition of Pluto

Abstract: Observations of the 1.4- to 2.4-micrometer spectrum of Pluto reveal absorptions of carbon monoxide and nitrogen ices and confirm the presence of solid methane. Frozen nitrogen is more abundant than the other two ices by a factor of about 50; gaseous nitrogen must therefore be the major atmospheric constituent. The absence of carbon dioxide absorptions is one of several differences between the spectra of Pluto and Triton in this region. Both worlds carry information about the composition of the solar nebula and the processes by which icy planetesimals formed.

Benchmark radar targets for the validation of computational electromagnetics programs

Abstract: Results are presented of a set of computational electromagnetics validation measurements referring to three-dimensional perfectly conducting smooth targets, performed for the Electromagnetic Code Consortium. Plots are presented for both the low- and high-frequency measurements of the NASA almond, an ogive, a double ogive, a cone-sphere, and a cone-sphere with a gap.

Visual optimization of dct quantization matrices for individual images

Abstract: Many image compression standards (JPEG, MPEG, H.261) are based on the Discrete Cosine Transform (DCT). However, these standards do not specify the actual DCT quantization matrix. We have previously provided mathematical formulae to compute a perceptually lossless quantization matrix. Here I show how to compute a matrix that is optimized for a particular image. The method treats each DCT coefficient as an approximation to the local response of a visual 'channel'. For a given quantization matrix, the DCT quantization errors are adjusted by contrast sensitivity, light adaptation, and contrast masking, and are pooled non-linearly over the blocks of the image. This yields an 8x8 'perceptual error matrix'. A second non-linear pooling over the perceptual error matrix yields total perceptual error. With this model we may estimate the quantization matrix for a particular image that yields minimum bit rate for a given total perceptual error, or minimum perceptual error for a given bit rate. Custom matrices for a number of images show clear improvement over image-independent matrices. Custom matrices are compatible with the JPEG standard, which requires transmission of the quantization matrix.

Recent developments in high order K-exact reconstruction on unstructured meshes

Abstract: This paper presents recent improvements in high-order K-exact reconstruction on unstructured meshes. The new reconstruction procedures are incorporated into a basic upwind finite-volume scheme suitable for solving scalar advection-diffusion equations as well as the Euler and Navier-Stokes equations. Numerical calculations are performed comparing the present method with lower order accurate reconstruction procedures (piecewise constant and piecewise linear) and various competing technologies such as the fluctuation splitting method of Roe (1987) and Deconinck et al. (1992) and a system-variant of the streamline diffusion Petrov-Galerkin method developed by Hansbo (1991) and Hansbo and Johnson (1991). Five test problems are used in the numerical comparisons: scalar circular advection, transonic and supersonic Euler flow, laminar boundary-layer flow, and general compressible Navier-Stokes flow.

The three-dimensional evolution of a plane mixing layer: pairing and transition to turbulence

Abstract: The evolution of three-dimensional temporally evolving plane mixing layers through as many as three pairings has been simulated numerically. All simulations were begun from a few low-wavenumber disturbances, usually derived from linear stability theory, in addition to the mean velocity. Three-dimensional perturbations were used with amplitudes ranging from infinitesimal to large enough to trigger a rapid transition to turbulence. Pairing is found to inhibit the growth of infinitesimal three-dimensional disturbances, and to trigger the transition to turbulence in highly three-dimensional flows. The mechanisms responsible for the growth of three-dimensionality and onset of transition to turbulence are described. The transition to turbulence is accompanied by the formation of thin sheets of spanwise vorticity, which undergo secondary rollups. The post-transitional simulated flow fields exhibit many properties characteristic of turbulent flows.

Theoretical modeling of the infrared fluorescence from interstellar polycyclic aromatic hydrocarbons

Abstract: We have modeled the family of interstellar IR emission bands at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 microns by calculating the fluorescence from a size distribution of interstellar polycyclic aromatic hydrocarbons (PAHs) embedded in the radiation field of a hot star. It is found that the various emission bands are dominated by distinctly different PAHs, from molecules with much less than about 80 C atoms for the 3.3 micron feature, to molecules with 10 exp 2-10 exp 5 C atoms for the emission in the IRAS 12 and 25 micron bands. We quantitatively describe the influence on the emergent spectrum of various PAH properties such as the molecular structure, the amount of dehydrogenation, the intrinsic strength of the IR active modes, and the size distribution. Comparing our model results to the emission spectrum from the Orion Bar region, we conclude that interstellar PAHs are likely fully, or almost fully, hydrogenated. Moreover, it is found that the intrinsic strengths of the 6.2 and 7.7 micron C-C stretching modes, and the 8.6 micron C-H in-plane bending mode are 2-6 times larger than measured for neutral PAHs in the laboratory.

Generalized Smagorinsky model for anisotropic grids

Abstract: The paper revises the Smagorinsky subgrid model to properly account for grid anisotropy on the basis of energy equilibrium considerations in isotropic turbulence. For moderate resolution anisotropies, Deardorff's (1970) estimate involving an equivalent grid scale Delta(eq) = (Delta1 Delta2 Delta3) exp 1/3 is given a rigorous basis. It is argued that the results obtained should be used in conjunction with the dynamic model of Germano et al. (1991) whenever the anisotropy of the test filter differs significantly from that of the basic grid.

Ices on the Surface of Triton

Abstract: The near-infrared spectrum of Triton reveals ices of nitrogen, methane, carbon monoxide, and carbon dioxide, of which nitrogen is the dominant component. Carbon dioxide ice may be spatially segregated from the other more volatile ices, covering about 10 percent of Triton's surface. The absence of ices of other hydrocarbons and nitriles challenges existing models of methane and nitrogen photochemistry on Triton.

Condensation and Vaporization Studies of CH3OH and NH3 Ices: Major Implications for Astrochemistry

Abstract: In an extension of previously reported work on ices containing H2O, CO, CO2, SO2, H2S, and H2, we present measurements of the physical and infrared spectral properties of ices containing CH3OH and NH3. The condensation and sublimation behavior of these ice systems is discussed and surface binding energies are presented for all of these molecules. The surface binding energies can be used to calculate the residence times of the molecules on grain surfaces as a function of temperature. It is demonstrated that many of the molecules used to generate radio maps of and probe conditions in dense clouds, for example CO and NH3, will be significantly depleted from the gas phase by condensation onto dust grains. Attempts to derive total column densities solely from radio maps that do not take condensation effects into account may vastly underestimate the true column densities of any given species. Simple CO condensation onto and vaporization off of grains appears to be capable of explaining the observed depletion of gas phase CO in cold, dense molecular cores. This is not the case for NH3, however, where thermal considerations alone predict that all of the NH3 should be condensed onto grains. The fact that some gas phase NH3 is observed indicates that additional desorption processes must be involved. The surface binding energies of CH3OH, in conjunction with this molecule's observed behavior during warm up in H2O-rich ices, is shown to provide an explanation of the large excess of CH3OH seen in many warm, dense molecular cores. The near-infrared spectrum and associated integrated band strengths of CH3OH-containing ice are given, as are middle infrared absorption band strengths for both CH3OH and NH3.

In situ evidence of rapid, vertical, irreversible transport of lower tropospheric air into the lower tropical stratosphere by convective cloud turrets and by larger-scale upwelling in tropical cyclones

Abstract: The STEP tropical objectives were successfully met during the flight experiments conducted from Darwin, Australia, January 16 to February 16, 1987. Necessary and sufficient measurements were made in, above, and downwind from very cold cirrus clouds, produced by three convective cloud types, to demonstrate irreversible mass transports into and dehydration in the lower tropical stratosphere. The three types are defined and described in terms of the physical processes that produce them and illustrated by examples derived from in situ and remote measurements. Intense solar heating is shown to produce, in addition to the usual vertical, sea breeze circulations normal to the coastline, an unusual pair of continental spanning, horizontal circulations. An upper tropospheric-lower stratospheric anticyclonic circulation, inclined upward toward the tropics, contributes to the dehydration of dissipating cirrus anvils and intensifies the upper level, tropical easterlies. The lower tropospheric cyclonic circulation with tropical westerlies and extratropical easterlies is in direct conflict with the normal tropical easterlies and extratropical westerlies. Impulsive switches between these two opposing lower-level wind systems create conditions favorable for each of these cloud types and explain the summer season's aperiodic variability.

Reactive nitrogen and its correlation with ozone in the lower stratosphere and upper troposphere

Abstract: High resolution in situ measurements of reactive nitrogen (NO(y)) and O3 were made in the upper troposphere and lower stratosphere at a variety of latitudes and seasons. In the lower stratosphere, NO(y) and O3 are very highly positively correlated at all times and spatial scales sampled. The ratio NO(y)/NO3 is much less variable than either species measured separately. The ratio has a much weaker gradient with altitude than the mixing ratios of O3 or NO(y). The ratio is smaller and decreases more rapidly with altitude in the tropics than at midlatitudes. In the upper troposphere NO(y) and O3 are only weakly correlated. Their ratio in the tropical upper troposphere is about 0.005-0.025 and the ratio in the midlatitude upper troposphere is about 0.004-0.010. The NO(y) in the upper troposhere is probably partly due to lightning.

Pinatubo and pre-Pinatubo optical-depth spectra: Mauna Loa measurements, comparisons, inferred particle size distributions, radiative effects, and relationship to lidar data

Abstract: The Ames airborne tracking sunphotometer was operated at the National Oceanic and Atmospheric Administration (NOAA) Mauna Loa Observatory (MLO) in 1991 and 1992 along with the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) automated tracking sunphotometer and lidar June 1991 measurements provided calibrations, optical-depth spectra, and intercomparisons under relatively clean conditions; later measurements provided spectra and comparisons for the Pinatubo cloud plus calibration checks June 1991 results are similar to previous MLO springtime measurements, with midvisible particle optical depth tau(sub p)(lambda = 0526 microns) at the near-background level of 0012 +/- 0006 and no significant wavelength dependence in the measured range (lambda = 038 to 106 microns) The arrival of the Pinatubo cloud in July 1991 increased midvisible particle optical depth by more than an order of magnitude and changed the spectral shape of tau(sub p)(lambda) to an approximate power law with an exponent of about -14 By early September 1991, the spectrum was broadly peaked near 05 microns, and by July 1992, it was peaked near 08 microns Our optical-depth spectra include corrections for diffuse light which increase postvolcanic midvisible tau(sub p) values by 1 to 3% (ie, 00015 to 00023) NOAA- and Ames Research Center (ARC)-measured spectra are in good agreement Columnar size distributions inverted from the spectra show that the initial (July 1991) post-Pinatubo cloud was relatively rich in small particles (r less than 025 microns), which were progressively depleted in the August-September 1991 and July 1992 periods Conversely, both of the later periods had more of the optically efficient medium-sized particles (025 less than r less than 1 micron) than did the fresh July 1991 cloud These changes are consistent with particle growth by condensation and coagulation The effective, or area-weighted, radius increased from 022 +/- 006 micron in July 1991 to 056 +/- 012 micron in August-September 1991 and to 086 +/- 029 micron in July 1992 Corresponding column mass values were 48 +/- 07, 91 +/- 27, and 55 +/- 2 micro g/sq cm, and corresponding column surface areas were 44 +/- 05, 29 +/- 02, and 11 +/- 01 sq micron/sq cm, Photometer-inferred column backscatter values agree with those measured by the CMDL lidar on nearby nights Combining lidar-measured backscatter profiles with photometer-derived backscatter-to-area ratios gives peak particle areas that could cause rapid heterogeneous loss of ozone, given sufficiently low particle acidity and suitable solar zenith angles (achieved at mid- to high latitudes) Top-of-troposphere radiative forcings for the September 1991 and July 1992 optical depths and size distributions over MLO are about -5 and -3 W 1/sq m, respectively (hence comparable in magnitude but opposite in sign to the radiative forcing caused by the increase in manmade greenhouse gases since the industrial revolution) Heating rates in Pinatubo layer over MLO are 055 +/- 013 and 041 +/- 014 K/d for September 1991 and July 1992, respectively

Propagation velocity of perturbations in turbulent channel flow

Abstract: A database obtained from direct numerical simulation of a turbulent channel flow is analyzed to extract the streamwise component of the propagation velocity V of velocity, vorticity, and pressure fluctuations from their space-time correlations. A surprising result is that V is approximately the same as the local mean velocity for most of the channel, except for the near-wall region. For y(+) less than 15, V is virtually constant, implying that perturbations of all flow variables propagate like waves near the wall. In this region V is 55 percent of the centerline velocity Uc for velocity and vorticity perturbations and 75 percent of U sub c for pressure perturbations. This is equal to U at y(+) = 15 for velocity and vorticity perturbations, and equal to U at y(+) = 20 for pressure perturbations, indicating that the dynamics of the nearwall turbulence is controlled by turbulence structures present near y(+) about 15-20. Scale dependence of V is also examined by analyzing the bandpass-filtered flow fields. This paper contains comprehensive documentation on the propagation velocities, which should prove useful in the evaluation of Taylor's hypothesis.