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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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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Selective extinction of marine plankton at the end of the Mesozoic era: The fossil and stable isotope record

Floral, faunal and stable isotope evidence in a continuous sequence of latest Cretaceous and earliest Tertiary shallow water marine deposits in the Mangyshlak Peninsula, USSR suggest severe environmental changes at the Cretaceous/Tertiary (K/T) boundary. Time frame is provided by nanno, micro and macrofossils as well as by magnetic stratigraphy and an iridium spike. Oxygen isotopic analyses of the bulk sediments, composed of nanno and microplankton skeletal remains, show a sharp positive spike at the K/T boundary. This shift is primarily attributed to severe cooling possibly accompanied by increased salinities of the surface mixed layer. Floral and faunal extinctions were selective, affecting approximately 90 percent of the warm water calcareous phyto and zooplankton genera in the Tethyan-Paratethyan regions. These highly diverse taxa with many endemic representatives were at the peak of their evolutionary development. Geologic evidence indicates that the terminal Cretaceous temperature decline was coeval with widespread and intense volcanic activity which reached a peak at the close of the Mesozoic Era. Increased acidity temporarily prohibited calcite nucleation of the surface dwelling warm-water plankton. Superimposed upon decreased alkalinity, severe and rapid climatic changes caused the extinction of calcareous phyto and zooplankton.

Herman, Y.↗
NASA NTRS1989

Asymptotic behavior of a flat plate wake

An experimental study has been conducted to investigate the far-field, self-similar properties of a flat plate wake. A plane turbulent wake was generated at the trailing edge of a smooth splitter plate separating two legs of a Mixing Layer Wind Tunnel, with both initial boundary layers tripped. For the present study, both legs were operated at a free-steam velocity in the test section of 15 m/s, giving a Reynolds number based on wake momentum thickness of about 1750. Single profile measurements were obtained at five streamwise locations using a Pitot probe for the mean velocity measurements and a single cross-wire probe for the turbulence data, which included statistics up to third order. The mean flow data indicated a self-similar behavior beyond a streamwise distance equivalent to about 350 wake momentum thicknesses. However, the turbulence data show better collapse beyond a distance equivalent to about 500 momentum thicknesses, with all the measured peak Reynolds stresses achieving constant, asymptotic levels. The asymptotic mean flow behavior and peak primary stress levels agree well with theoretical predictions based on a constant eddy viscosity model. The present data also agree reasonably well with previous measurements, of which only one set extends into the self-similar region. Detailed comparisons with previous data are presented and discussed in this report.

Weygandt, James H.↗
NASA NTRS1988

Advances in turbulent transport modeling based on direct simulations of turbulence

Recent work using numerical simulations to study the modeling of scalar transport is outlined. A model developed by reference to simulations for homogeneous shear flow is presented. This model predicts cross-gradient transports that can give rise to the illusion of counter-gradient transport. The model is found to work remarkably well when compared to simulations for channel flow, but fails to predict important features of the scalar fluxes in mixing layers. Reasons for this failure are discussed and suggestions are made for future work.

Reynolds, W. C.↗
NASA NTRS1989

On the three families of instability waves of high-speed jets

An analytical and computational study of the normal-mode small-amplitude waves of high-speed jets is presented. Three families of instability waves have been identified: (1) the familiar Kelvin-Helmholtz instability waves; (2) supersonic instability waves; and (3) subsonic waves. It is demonstrated that the computed wave patterns and propagation characteristics of these three wave types are consistent with the findings of Oetel (1979, 1980, 1982). The subsonic waves are shown to be unstable only for jets with mixing layers of finite thickness.

Tam, Christopher K. W.↗
NASA NTRS1989

New instability modes for bounded, free shear flows

A class of highly amplified supersonic disturbances are found for high-speed, bounded mixing layers at high values of streamwise wavenumber. Their amplification is an order of magnitude greater than the most amplified modes, which occur at 60-65 deg at low streamwise wavenumber. These disturbances are stabilized by increasing Mach number, viscosity, and sweep; however, the effect of sweep on the most amplified mode is not significant until the wave propagation angle reaches 30 deg. The maximum growth rate of the unstable disturbances decreases as the temperature of the higher Mach number stream is increased. The structure of these disturbances is such that the phase speed with respect to the mean flow is subsonic in a small region in the center of the shear layer, and supersonic on either side of this region.

Macaraeg, Michele G.↗
NASA NTRS1989

Linear and nonlinear responses to middle latitude surface temperature anomalies

Nonlinear responses to fixed and seasonally varying surface temperature anomalies in a two-level hemispheric time-dependent coupled atmosphere-surface mixed layer model are described. Linear stationary models that are equivalent to the nonlinear time-dependent model are used to analyze these responses. A model linearized around the climatological zonal state of the time dependent model and forced by anomalous surface temperatures does not provide a reasonable estimate for the anomalous reponses, which are considerably underestimated. Better responses are obtained when the anomalous stationary nonlinear eddy fluxes are included in a model linearized around the full climatology. However, this latter model is overly sensitive, and anomalous responses are a small residual balance to the forcing by the surface temperature anomalies and the anomalous transient eddy fluxes. To better understand these linear responses, an eigenanalysis of the climatological state is performed. Seasonal anomalies appear to be dominated by one characteristic pattern near resonance which can be associated with a slowly growing coupled atmosphere-ocean instability.

Roads, John O.↗
NASA NTRS1990

Opportunities for improved understanding of supersonic and hypersonic flows

This overview describes some of the diverse applications which are foreseen in the supersonic and hypersonic regime, and points out both the need and the opportunity for improved understanding of the subject. New facilities and experimental test techniques combined with computational fluid dynamics methods are described to illustrate challenging flows over supersonic wings, separated free-shear mixing layers, and in scramjet-airframe integration.

Jones, Robert A.↗
NASA NTRS1990

Viscous three-dimensional analyses for nozzles for hypersonic propulsion

A Navier-Stokes computer code was validated using a number of two- and three-dimensional configurations for both laminar and turbulent flows. The validation data covers a range of freestream Mach numbers from 3 to 14, includes wall pressures, velocity profiles, and skin friction. Nozzle flow fields computed for a generic scramjet nozzle from Mach 3 to 20, wall pressures, wall skin friction values, heat transfer values, and overall performance are presented. In addition, three-dimensional solutions obtained for two asymmetric, single expansion ramp nozzles at a pressure ratio of 10 consists of the internal expansion region in the converging/diverging sections and the external supersonic exhaust in a quiescent ambient environment. The fundamental characteristics that were captured successfully include expansion fans; Mach wave reflections; mixing layers; and nonsymmetrical, multiple inviscid cell, supersonic exhausts. Comparison with experimental data for wall pressure distributions at the center planes shows good agreement.

Harloff, G. J.↗
NASA NTRS1990

Sources of variability in satellite-derived estimates of phytoplankton production in the eastern tropical Pacific

As a proxy for satellite CZCS observations and concurrent measurements of primary production rates, data from 138 stations occupied seasonally during 1967-1968 in the offshore eastern tropical Pacific were analyzed in terms of six temporal groups and our current regimes. Multiple linear regressions on column production Pt show that simulated satellite pigment is generally weakly correlated, but sometimes not correlated with Pt, and that incident irradiance, sea surface temperature, nitrate, transparency, and depths of mixed layer or nitracline assume little or no importance. After a proxy for the light-saturated chlorophyll-specific photosynthetic rate P(max) is added, the coefficient of determination ranges from 0.55 to 0.91 (median of 0.85) for the 10 cases. In stepwise multiple linear regressions the P(max) proxy is the best predictor for Pt.

Banse, Karl↗
NASA NTRS1990

A simple parcel method for prediction of cumulus onset and area-averaged cloud amount over heterogeneous land surfaces

A set of PBL data selected from the Wangara experiment is used to examine the relationship between cumulus amount and the calculated relative humidity of rising parcels. The Wilde et al. (1985) method is compared to two explicit parcel methods. It is shown that predictions using parcel methods, in which surface air is lifted to its equilibrium level while being diluted by a defined amount of mixed layer air, are superior for cloud amount, especially during periods of rapid cloud onset when the mean boundary layer top is swiftly rising through a near-neutral layer. Pending independent verification, it appears that an rms error in instantaneous cloud amount of + or - 10 percent may be achievable.

Wetzel, Peter J.↗
NASA NTRS1990

A comparison of ship and Coastal Zone Color Scanner mapped distribution of phytoplankton in the southeastern Bering Sea

Twenty-one Coastal Zone Color Scanner (CZCS) images of the southeastern Bering Sea are examined in order to map the near-surface distribution of phytoplankton during 1979 and 1980. The information is compared with the mesoscale (100-1000 km) distribution of phytoplankton inferred from pooled ship sampling obtained during the Processes and Resources of the Bering Shelf (PROBES) intensive field study during the late 1970s and early 1980s. The imagery indicates that open-water phytoplankton blooms occur first in late April in coastal waters, peak in early May over the middle shelf, and decay rapidly afterwards, reaching concentration minima in June in both regions. These patterns show that the earlier ship observations are valid for most of the eastern Bering shelf. A very tight correlation is found between the PROBES surface chlorophyll a concentrations and mean mixed-layer chlorophyll concentrations. The significant discrepancies between CZCS and ship-based chlorophyll estimates may be due to aliasing in time by the CZCS. It is concluded that neither satellite nor ship alone can do an adequate job of characterizing the physics or biological dynamics of the ocean.

Mcclain, C. R.↗
NASA NTRS1989

A spectral multi-domain technique applied to stability and transition of incompressible and compressible flows

Techniques for the numerical simulation of compressible and incompressible viscous flows are described and demonstrated. The focus is on a multidomain spectral method based on the flux-balance domain-interface scheme of Macaraeg and Streett (1986). The fundamental principles of the method are reviewed, and it is shown that the multidomain spectral approach facilitates the treatment of instability and the transition to turbulence. Typical numerical results for the Taylor-Couette problem and for bounded compressible high-speed mixing layers are presented in graphs and briefly characterized.

Macaraeg, Michele↗
NASA NTRS1990

Amazon Basin ozone and aerosol - Wet season observations

The ozone and aerosol number density data obtained from aircraft flights over the Amazon Basin (AB) during April and May of 1987 are summarized with particular attention given to the general characteristics of ozone and aerosol for wet season (WS), compared to dry season (DS), meteorology. It is concluded that the AB provides favorable conditions for the production of hydroxyl radical and subsequent chemical and photochemical reactions which generate or consume key atmospheric trace species. It is determined that a negative ozone gradient exists within the mixed layer (ML). WSML ozone is about 7 ppbv lower than that of DS. The net sink characteristics of the WSML are seen throughout the troposphere of the AB in that the ozone at 3-4 km altitude is typically 15-25 ppbv as compared to DS values of 30-35 ppbv. In terms of aerosol source characteristics of the AB, ML aerosols are 5-10 times higher than observed in the troposphere with ML values of 100-200 aerosols/cu cm. An analysis of aerosol size distribution data is presented.

Gregory, Gerald L.↗
NASA NTRS1989

Modeling of high speed chemically reacting flow-fields

The SPARK3D and SPARK3D-PNS computer programs were developed to model 3-D supersonic, chemically reacting flow-fields. The SPARK3D code is a full Navier-Stokes solver, and is suitable for use in scramjet combustors and other regions where recirculation may be present. The SPARK3D-PNS is a parabolized Navier-Stokes solver and provides an efficient means of calculating steady-state combustor far-fields and nozzles. Each code has a generalized chemistry package, making modeling of any chemically reacting flow possible. Research activities by the Langley group range from addressing fundamental theoretical issues to simulating problems of practical importance. Algorithmic development includes work on higher order and upwind spatial difference schemes. Direct numerical simulations employ these algorithms to address the fundamental issues of flow stability and transition, and the chemical reaction of supersonic mixing layers and jets. It is believed that this work will lend greater insight into phenomenological model development for simulating supersonic chemically reacting flows in practical combustors. Currently, the SPARK3D and SPARK3D-PNS codes are used to study problems of engineering interest, including various injector designs and 3-D combustor-nozzle configurations. Examples, which demonstrate the capabilities of each code are presented.

Drummond, J. P.↗
NASA NTRS1990

Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows

This research is involved with the implementation of advanced computational schemes based on large eddy simulations (LES) and direct numerical simulations (DNS) to study the phenomenon of mixing and its coupling with chemical reactions in compressible turbulent flows. In the efforts related to LES, a research program to extend the present capabilities of this method was initiated for the treatment of chemically reacting flows. In the DNS efforts, the focus is on detailed investigations of the effects of compressibility, heat release, and non-equilibrium kinetics modelings in high speed reacting flows. Emphasis was on the simulations of simple flows, namely homogeneous compressible flows, and temporally developing high speed mixing layers.

Givi, Peyman↗
NASA NTRS1991

Second-order closure models for supersonic turbulent flows

Recent work on the development of a second-order closure model for high-speed compressible flows is reviewed. This turbulent closure is based on the solution of modeled transport equations for the Favre-averaged Reynolds stress tensor and the solenoidal part of the turbulent dissipation rate. A new model for the compressible dissipation is used along with traditional gradient transport models for the Reynolds heat flux and mass flux terms. Consistent with simple asymptotic analyses, the deviatoric part of the remaining higher-order correlations in the Reynolds stress transport equations are modeled by a variable density extension of the newest incompressible models. The resulting second-order closure model is tested in a variety of compressible turbulent flows which include the decay of isotropic turbulence, homogeneous shear flow, the supersonic mixing layer, and the supersonic flat-plate turbulent boundary layer. Comparisons between the model predictions and the results of physical and numerical experiments are quite encouraging.

Speziale, Charles G.↗
NASA NTRS1990

Viscous three-dimensional analyses for nozzles for hypersonic propulsion

A Navier-Stokes computer code was validated using a number of two- and three-dimensional configurations for both laminar and turbulent flows. The validation data covers a range of freestream Mach numbers from 3 to 14, including wall pressures, velocity pressure, and skin friction. Nozzle flow fields computed for a generic scramjet nozzle from Mach 3 to 20, wall pressures, wall skin friction values, heat transfer values, and overall performance are presented. In addition, three-dimensional solutions obtained for two asymmetric, single expansion ramp nozzles at a pressure ratio of 10 consists of the internal expansion region in the converging/diverging sections and the external superonic exhaust in a quiescent ambient environment. The fundamental characteristics that were captured successfully include expansion fans; Mach wave reflections; mixing layers; and nonsymmetrical, multiple inviscid cell, supersonic exhausts. Comparison with experimental data for wall pressure distributions at the center planes shows good agreement.

Harloff, G. J.↗
NASA NTRS1991

Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows

This research is involved with the implementations of advanced computational schemes based on large eddy simulations (LES) and direct numerical simulations (DNS) to study the phenomenon of mixing and its coupling with chemical reactions in compressible turbulent flows. In the efforts related to LES, a research program was initiated to extend the present capabilities of this method for the treatment of chemically reacting flows, whereas in the DNS efforts, focus was on detailed investigations of the effects of compressibility, heat release, and nonequilibrium kinetics modeling in high speed reacting flows. The efforts to date were primarily focussed on simulations of simple flows, namely, homogeneous compressible flows and temporally developing hign speed mixing layers. A summary of the accomplishments is provided.

Givi, Peyman↗