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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 757 records · Page 42
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NASA NTRS1991

A comparison of numerical methods for the Rayleigh equation in unbounded domains

A second-order finite difference and two spectral methods, including a Chebyshev tau and a Chebyshev collocation method were implemented to determine the linear hydrodynamic stability of an unbounded shear flow. The velocity profile of the basic flow in the stability analysis mimicks that of a one-stream free mixing layer. Local and global eigenvalue solution methods are used to determine individual eigenvalues and the eigenvalue spectrum, respectively. The calculated eigenvalue spectrum includes a discrete mode, a continuous spectrum associated with the equation singularity and a continuous spectrum associated with the domain unboundedness. The efficiency and the accuracy of these discretization methods in the prediction of the eigensolutions of the discrete mode were evaluated by comparison with a conventional shooting procedure. Their capabilities in mapping out the continuous eigenvalue spectra are also discussed.

Liou, W. W.↗
NASA NTRS1990

A linear shock cell model for non-circular jets using conformal mapping with a pseudo-spectral hybrid scheme

The shock structure in non-circular supersonic jets is predicted using a linear model. This model includes the effects of the finite thickness of the mixing layer and the turbulence in the jet shear layer. A numerical solution is obtained using a conformal mapping grid generation scheme with a hybrid pseudo-spectral discretization method. The uniform pressure perturbation at the jet exit is approximated by a Fourier-Mathieu series. The pressure at downstream locations is obtained from an eigenfunction expansion that is matched to the pressure perturbation at the jet exit. Results are presented for a circular jet and for an elliptic jet of aspect ratio 2.0. Comparisons are made with experimental data.

Bhat, Thonse R. S.↗
NASA NTRS1990

Physical and biological processes in the North Atlantic during the First GARP Global Experiment

The seasonal cycle of the phytoplankton productivity in the North Atlantic is investigated using monthly mean pigment composites generated by the Nimbus 7 CZCS for the period of December 1978 through December 1979. Data on the temporal and spatial distributions of surface pigments are compared with quantities derived from the First GARP Global Experiment wind data set, Climate Analysis Center SST fields, concurrent hydrogarphic station data, and climatological mixed-layer depths and heat fluxes. Time series at 11 selected sites throughout the North Atlantic basin are used to illustrate the relation of phytoplankton biomass to local physical forcing.

Mcclain, Charles R.↗
NASA NTRS1990

Instability waves in twin supersonic jets

Calculations are presented for the characteristics of instability waves in the initial mixing region of twin circular supersonic jets. Two models for the basic jet flow are used. In the first, the jets are modeled as two circular vortex sheets. In the second, realistic velocity and density profiles are used. It is shown that the unsteady flow fields of the two jets interact before the time-averaged jets flows have merged. The normal modes or instability waves are classified by their symmetry properties in the twin-jet case and their asymptotic behavior for large jet separations. Calculations of the growth rates and phase velocities are made for these modes as a function of jet separation and mixing-layer thickness. The associated pressure distributions are also presented. In the realistic jet profile calculations the effect of jet separation is found to be relatively weak. For modes that are even about the symmetry plane between the two jets the pressure levels are found to increase near this plane as the jet separation decreases.

Morris, Philip J.↗
NASA NTRS1991

Second-order closure models for supersonic turbulent flows

Recent work by the authors on the development of a second-order closure model for high-speed compressible flows is reviewed. This turbulence 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 equation 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 NTRS1991

Biological production models as elements of coupled, atmosphere-ocean models for climate research

Process models of phytoplankton production are discussed with respect to their suitability for incorporation into global-scale numerical ocean circulation models. Exact solutions are given for integrals over the mixed layer and the day of analytic, wavelength-independent models of primary production. Within this class of model, the bias incurred by using a triangular approximation (rather than a sinusoidal one) to the variation of surface irradiance through the day is computed. Efficient computation algorithms are given for the nonspectral models. More exact calculations require a spectrally sensitive treatment. Such models exist but must be integrated numerically over depth and time. For these integrations, resolution in wavelength, depth, and time are considered and recommendations made for efficient computation. The extrapolation of the one-(spatial)-dimension treatment to large horizontal scale is discussed.

Platt, Trevor↗
NASA NTRS1991

UV spectroscopy of Titan's atmosphere, planetary organic chemistry and prebiological synthesis. II - Interpretation of new IUE observations in the 220-335 nm range

The theoretical model developed by McKay et al. (1989) to characterize the size distribution, thermal structure, and chemical composition of the stratospheric haze of Titan is applied to new 220-335-nm albedo measurements obtained with the long-wavelength prime camera of the IUE during August 1987. Data and model predictions are presented in extensive graphs and discussed in detail. It is shown that a simple model with particles of one size at a given altitude does not accurately reproduce the observed features in all spectral regions, but that good general agreement is obtained using a model with a uniformly mixed layer at 150-600 km and a bimodal distribution of small 'polymer' haze particles (radius less than 20 nm) and larger haze particles (radius 100-500 nm). The number densities implied by this model require, however, a mechanism such as electrostatic charging or reaction kinetics to inhibit coagulation of the smaller particles.

Courtin, Regis↗
NASA NTRS1990

Differential absorption lidar detection of ozone in the troposphere and lower stratosphere

The DIAL technique for deriving O3 profiles from lidar measurements is discussed. Examples of a variety of O3 and aerosol measurements are presented from studies of (1) photochemically produced O3 in the summertime over the eastern United States and in biomass burning plumes during the dry season over the Amazon Basin of Brazil; (2) vertical O3 transport from the mixed layer into the free troposphere via cloud dynamics and from the stratosphere into the troposphere via tropopause fold events; and (3) O3 depletion in the O3 hole over Antarctica. The NASA airborne DIAL system is considered to be an advanced field system.

Browell, E. V.↗
NASA NTRS1991

Three-dimensional Navier-Stokes analysis of turbine passage heat transfer

The three-dimensional Reynolds-averaged Navier-Stokes equations are numerically solved to obtain the pressure distribution and heat transfer rates on the endwalls and the blades of two linear turbine cascades. The TRAF3D code which has recently been developed in a joint project between researchers from the University of Florence and NASA Lewis Research Center is used. The effect of turbulence is taken into account by using the eddy viscosity hypothesis and the two-layer mixing length model of Baldwin and Lomax. Predictions of surface heat transfer are made for Langston's cascade and compared with the data obtained for that cascade by Graziani. The comparison was found to be favorable. The code is also applied to a linear transonic rotor cascade to predict the pressure distributions and heat transfer rates.

Ameri, Ali A.↗
NASA NTRS1991

A hybrid numerical method for three-dimensional spatially-developing free-shear flows

The present novel algorithm for 3D incompressible Navier-Stokes equations treats a domain which is (1) infinite in the vertical direction, using a mapped spectral method; (2) finite in the streamwise direction, using a classical Fourier method; and (3) homogeneous in the spanwise direction, using high-order compact finite differencing. A projection method is employed which ensures the exact conservation of mass, as well as the satisfaction of the boundary conditions at infinity. The novel aspects of these methods are noted, and the code they constitute is validated in light of several test cases. Results are presented for two- and three-dimensional mixing layers.

Buell, Jeffrey C.↗
NASA NTRS1991

On the seasonal phytoplankton concentration and sea surface temperature cycles of the Gulf of Mexico as determined by satellites

Multiyear series of coastal zone color scanner (CZCS) and AVHRR observations are presently used to derive monthly climatologies of near-surface phytoplankton pigment concentration and SST for the Gulf of Mexico; these, in combination with 1946-1987 SST data and NOAA hydrographic profile data covering 1914-1985, show that the most important single factor controlling seasonal cycle surface-pigment concentration is the depth of the mixed layer. The CZCS images indicate that seasonal variation seaward of the continental shelf is synchronous throughout the Gulf. The combination of ocean color and IR images allows year-round observation of surface circulation spatial structure in the Gulf, as well as of the dispersal pattern of the Mississippi River's plume.

Mueller-Karger, Frank E.↗
NASA NTRS1991

Surface heating and patchiness in the coastal ocean off central California during a wind relaxation event

The difference between the temperature of the ocean at 4-cm and 2-m depth was continuously monitored during a cruise to the coastal transition zone off Point Arena, California, during June 1987. The two temperatures were coincident most of the time but diverged during one nearshore leg of the cruise where large temperature differences of up to 4.7 C were observed between the 4-cm and 2-m sensors, in areas which were separated by regions where the two temperatures were coincident as usual. The spatial scale of this 'patchy' thermal structure was about 5-10 km. A mixed layer model (Garwood, 1977) was used to simulate the near surface stratification when forced by the observed wind stress, surface heating, and optical clarity of the water. The model produced a thin strongly stratified surface layer at stations where exceptionally high turbidity was observed but did not produce such features otherwise. This simple model could not explain the horizontal patchiness in the thermal structure, which was likely due to patchiness in the near-surface chlorophyll distributions or to submesoscale variability of the surface wind stress.

Ramp, Steven R.↗
NASA NTRS1991

Optically thin cirrus clouds over oceans and possible impact on sea surface temperature of warm pool in western Pacific

Over the convectively active tropical ocean regions, the measurement made from space in the IR and visible spectrum have revealed the presence of optically thin cirrus clouds, which are quite transparent in the visible and nearly opaque in the IR. The Nimbus-4 IR Interferometer Spectrometer (IRIS), which has a field of view (FOV) of approximately 100 km, was utilized to examine the IR optical characteristics of these cirrus clouds. From the IRIS data, it was observed that these optically thin cirrus clouds prevail extensively over the warm pool region of the equatorial western Pacific, surrounding Indonesia. It is found that the seasonal cloud cover caused by these thin cirrus clouds exceeds 50 percent near the central regions of the warm pool. For most of these clouds, the optical thickness in the IR is less than or = 2. It is deduced that the dense cold anvil clouds associated with deep convection spread extensively and are responsible for the formation of the thin cirrus clouds. This is supported by the observation that the coverage of the dense anvil clouds is an order of magnitude less than that of the thin cirrus clouds. From these observations, together with a simple radiative-convective model, it is inferred that the optically thin cirrus can provide a greenhouse effect, which can be a significant factor in maintaining the warm pool. In the absence of fluid transports, it is found that these cirrus clouds could lead to a runaway greenhouse effect. The presence of fluid transport processes, however, act to moderate this effect. Thus, if a modest 20 W/sq m energy input is considered to be available to warm the ocean, then it is found that the ocean mixed-layer of a 50-m depth will be heated by approximately 1 C in 100 days.

Prabhakara, C.↗
NASA NTRS1992

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

The basic objective of this research is to extend the capabilities of Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS) for the computational analyses of high speed reacting flows. In the efforts related to LES, we were primarily involved with assessing the performance of the various modern methods based on the Probability Density Function (PDF) methods for providing closures for treating the subgrid fluctuation correlations of scalar quantities in reacting turbulent flows. In the work on DNS, we concentrated on understanding some of the relevant physics of compressible reacting flows by means of statistical analysis of the data generated by DNS of such flows. In the research conducted in the second year of this program, our efforts focused on the modeling of homogeneous compressible turbulent flows by PDF methods, and on DNS of non-equilibrium reacting high speed mixing layers. Some preliminary work is also in progress on PDF modeling of shear flows, and also on LES of such flows.

Givi, P.↗
NASA NTRS1990

Aerosol optical thickness measurements during FIFE '89

The measurements used for correction and calibration are presented which permit the estimation of atmospheric effects on reflected and transmitted solar radiation. Four sun-photometers are calibrated and used to derive aerosol optical thicknesses that agree with expected uncertainties, and lower values and higher values are associated with cool dry northerly flows and warm humid southerly flows, respectively. The rapid increase in the vertical aerosol optical thickness after sunrise is related to the growth of the mixing layer which can be inferred from the 2D maps of the instantaneous aerosol number densities.

Halthore, Rangasayi N.↗
NASA NTRS1992

Turbulent reacting flow computations including turbulence-chemistry interactions

A two-equation (k-epsilon) turbulence model has been extended to be applicable for compressible reacting flows. A compressibility correction model based on modeling the dilatational terms in the Reynolds stress equations has been used. A turbulence-chemistry interaction model is outlined. In this model, the effects of temperature and species mass concentrations fluctuations on the species mass production rates are decoupled. The effect of temperature fluctuations is modeled via a moment model, and the effect of concentration fluctuations is included using an assumed beta-pdf model. Preliminary results obtained using this model are presented. A two-dimensional reacting mixing layer has been used as a test case. Computations are carried out using the Navier-Stokes solver SPARK using a finite rate chemistry model for hydrogen-air combustion.

Narayan, J. R.↗
NASA NTRS1992

Modeling deep convection in the Greenland Sea

The development of deep convective events in the high-latitude ocean is studied using a three-dimensional, coupled ice-ocean model. Oceanic mixing is described according to the level 2.5 turbulence closure scheme in which convection occurs in a continuous way, i.e., convective adjustment is not invoked. The model is forced by strong winds and surface cooling. Strong upwelling at the multilyear ice edge and consequent entrainment of warm Atlantic waters into the mixed layer is produced by winds parallel to the ice edge. Concomitant cooling drives deep convection and produces chimneylike structures. Inclusion of a barotropic mean flow over topography to the model provides important preconditioning and selects the location of deep convection. The most efficient preconditioning occurs at locations where the flow ascends a slope. In a stratified environment similar to the Greenland Sea with a 12 m/s wind the model simulations show that localized deep convection takes place after about 10 days to depths of 1000 m.

Hakkinen, S.↗
NASA NTRS1992

The relationship between delta C-13 of organic matter and (CO2(aq)) in ocean surface water - Data from the JGOFS site in the northeast Atlantic Ocean and a model

Consistent with the hypothesis that plankton delta C-14 and (CO2(aq)) are inversely related, increases in both sinking and suspended particulate organic matter (POM) delta C-13 detected by the Joint Global Ocean Flux Study (JGOFS) were highly negatively correlated with mixed-layer (CO2(aq)). A model of plant delta C-13 by Farquhar et al. (1982) is adapted to show that under a constant phytoplankton demand for CO2 an inverse nonlinear suspended POM delta C-13 response to ambient (CO2(aq)) is expected. Differences between predicted and observed suspended POM delta C-13 vs. (CO2(aq)) trends and among observed relationships can be reconciled if biological CO2 demand is allowed to vary.

Rau, G. H.↗