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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 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Two-phase measurements of a spray in the wake of a bluff body

The dynamics of spray drop interaction with the turbulent, recirculating wake of a flat disk bluff body were investigated using a phase Doppler particle analyzer to determine drop size and velocity and the gas-phase velocity. Detailed measurements obtained included spray drop size, axial and radial velocity, angle of trajectory, and size-velocity correlations. The gas-phase velocity was determined from seeding of the two-phase flow. Results showed dramatic differences in drop behavior for various size classes when interacting with the turbulent flow field. Small drops were quickly entrained and recirculated, while initially, the larger drops continued in the general direction of the spray cone. Further downstream, significant numbers of large drops recirculated, generating a bifurcated size-velocity correlation. These lateral convections and streamwise accelerations and decelerations strongly influenced the number density along with size and velocity distributions. The complex interaction of the spray with the turbulent air-flow points out the need for spatially-resolved measurements that determine drop behavior for individual size classes, rather than characterizing a spray only via simple integral quantities such as the Sauter mean diameter.

Rudoff, R. C.↗
NASA NTRS1986

One-dimensional analysis of filamentary composite beam columns with thin-walled open sections

Vlasov's one-dimensional structural theory for thin-walled open section bars was originally developed and used for metallic elements. The theory was recently extended to laminated bars fabricated from advanced composite materials. The purpose of this research is to provide a study and assessment of the extended theory. The focus is on flexural and torsional-flexural buckling of thin-walled, open section, laminated composite columns. Buckling loads are computed from the theory using a linear bifurcation analysis and a geometrically nonlinear beam column analysis by the finite element method. Results from the analyses are compared to available test data.

Lo, Patrick K.-L.↗
NASA NTRS1984

Cellular interface morphologies in directional solidification. II - The effect of grain boundaries

A singular perturbation analysis valid for small grain-boundary slopes is used with the one-sided model for solidification to show that grain boundaries introduce imperfections into the symmetry of the developing cellular interfaces which rupture the junction between the family of planar shapes and the bifurcating cellular families. Undulating interfaces are shown to develop first near grain boundaries, and to evolve with decreasing temperature gradient either by a smooth transition from the almost planar family or by a sudden jump to moderate-amplitude cellular forms, depending on the growth rate.

Ungar, Lyle H.↗
NASA NTRS1985

Cellular interface morphologies in directional solidification. III - The effects of heat transfer and solid diffusivity

The shape and stability of two-dimensional finite-amplitude cellular interfaces arising during directional solidification are compared for several solidification models that account differently for latent heat released at the interface, unequal thermal conductivities of melt and solid, and solute diffusivity in the solid. Finite-element analysis and computer-implemented perturbation methods are used to analyze the families of steadily growing cellular forms that evolve from the planar state. In all models a secondary bifurcation between different families of finite-amplitude cells exists that halves the spatial wavelength of the stable interface. The quantitative location of this transition is very dependent on the details of the model. Large amounts of solute diffusion in the solid retard the growth of large-amplitude cells.

Ungar, Lyle H.↗
NASA NTRS1987

The delta upsilon = 1 fundamental bands of carbon monoxide in late-type stars

A program was begun to develop the fundamental vibration-rotation bands of CO near 4.7 micron, as a tool to investigate the properties of late type stellar atmospheres. The strong delta v = 1 lines are sensitive probes of the thermal structure of the outer layers, which can be strongly affected by the CO, itself: cooling in the fundamental lines can depress the surface temperature to low values that are in sharp contrast with those derived from chromospheric diagnostics like Ca II K and Mg II k. Cool atmospheric models are being constructed to reproduce observed high resolution CO spectra of several late-type stars. The sensitivity is illustrated of CO LTE spectra to variations in T sub eff, log g, (Fe/H), and isotopic abundances of C and O for a grid of such models. In parallel, the assumptions of LTE line formation are tested using newly available cross sections for translational-to-vibrational energy exchange in collisions of atomic H on CO. In addition, spectra of CO and Ca II K calculated from semi-empirical UV based hot chromospheric models are used to estimate geometrical filling factors for the CO dominated and chromospheric regions on the stellar surface, in the spirit of Ayres' thermal bifurcation scenario.

Wiedemann, G.↗
NASA NTRS1987

Numerical and analytical study of fluid dynamic forces in seals and bearings

A numerical model based on a transformed, conservative form of the three dimensional Navier-Stokes equation and an analytical model based on lumped fluid parameters are presented and compared with studies of modeled rotor bearing seal systems. The rotor destabilizing factors are related to the rotative character of the flow field. It is shown that these destabilizing factors can be reduced through a decrease in the fluid average circumferential velocity. However, the rotative character of the flow field is a complex three dimensional system with bifurcated secondary flow patterns that significantly alter the fluid circumferential velocity. By transforming the Navier-Stokes equations to those for a rotating observer and using the numerical code PHOENICS-84 with a nonorthogonal body fitted grid, several numerical experiments were carried out to demonstrate the character of this complex flow field. In general, fluid injection and/or preswirl of the flow field opposing the shaft rotation significantly intensified these secondary recirculation zones and thus reduced the average circumferential velocity; injection or preswirl in the direction of rotation significantly weakened these zones.

Tam, L. T.↗
NASA NTRS1988

Problems in fluid dynamics

A scheme was developed for the parametric differentiation and integration of gas dynamics equations. A numerical integration of the gas dynamics equations is necessarily performed for a specific set of parameter values. The linear variational equations are obtained by differentiating the exact equations with respect to each of the relevant parameters. The resulting matrix of flow quantities is referred to as the Jacobi matrix. The subsequent procedure is then straightforward. The method was tested for two dimensional supersonic flow past an airfoil, with airfoil thickness, camber, and angle of attack varied. This approach has great potential value for rapidly assessing the effect of design changes. The other focus of the work was on problems in fluid stability, bifurcations, and turbulence.

Sirovich, Lawrence↗
NASA NTRS1987

What is the essential physics of mass loss from late-type stars?

A review is given of the data concerning mass loss from late-type stars. The major classes of mass-loss mechanisms (thermally-driven winds, radiatively-driven winds, and wave-driven winds) are discussed, and the empirical mass-loss rates and other data are compared with theoretical predictions for any of these mechanisms acting alone. It is likely that several mechanisms act together to produce the large mass-loss rates in the Mira and nonpulsating M supergiants. Studies of the solar atmosphere suggest that thermal bifurcation driven by molecular condensation instabilities may play a critical role in cooling the atmospheres of luminous cool stars and forming siicate dust. It is possible that several metastable modes of atmospheric structure may exist for a given set of stellar parameters.

Linsky, Jeffrey L.↗
NASA NTRS1988

Developing fluid flow in a curved duct of square cross-section and its fully developed dual solutions

Developing fluid flow in a curved duct of square cross-section is studied numerically by a factored ADI finite-difference method on a staggered grid. A central-difference scheme with primitive variables is used inside the computational domain to reduce numerical diffusion. Two Reynolds numbers, 574 and 790, based upon a bulk velocity and hydraulic diameter are chosen for curvature ratios of 1/6.45 and 1/2.3, respectively. It is found that the secondary flow is far more complicated than expected, with the appearance of at least two pairs of vortices. Main-flow separation is also observed for the higher curvature ratio. Furthermore, it is observed that the flow develops into two quite different states downstream, depending upon the inlet conditions. Solutions of the fully developed Navier-Stokes equations is shown to be not unique beyond a certain critical Reynolds number. Developing flow seems to evolve into the fully developed state along a particular branch into which the fully developed solution bifurcates.

Soh, W. Y.↗
NASA NTRS1988

Comparison results for ignition in conjugate systems

The exothermic reaction of an arbitrarily shaped solid in a well-mixed continuous flow of constant density and velocity is investigated analytically, with a focus on a zeroth-order reaction in a pellet. Comparison theorems for the basic problem and the eigenvalue problem of ignition are derived, and the domain dependence of the bifurcation point is explored using the method of Joseph (1976). Some typical analytical results are presented in tables and compared with published values obtained using numerical methods.

Pushpavanam, S.↗
NASA NTRS1988

Solutal convection during directional solidification

During directional solidification of a binary alloy at constant velocity, buoyancy-driven fluid flow may occur due to the solute gradients generated by the solidification process. Numerical calculations of the solute and fluid flow fields in the melt have been carried out using finite differences in a two-dimensional, time-dependent model that assumes a planar crystal-melt interface and allows time-dependent gravitational accelerations. The container walls are rigid and perfectly insulating for solute. For constant vertical gravitational accelerations, as the solutal Rayleigh number is varied, multiple steady states and time-dependent states may occur. The bifurcation from the quiescent state may be subcritical or transcritical, depending on the aspect ratio of the container. Calculations have also been performed for a gravitational acceleration that is assumed to be uniform in magnitude with its direction rotating uniformly. Numerical results have been obtained for a Schmidt number of 10 and a gravitational acceleration of 0.0001 G. The maximum variation in the solute concentration at the crystal-melt interface is calculated for various values of the rotation rate of the gravitational acceleration.

Mcfadden, G. B.↗
NASA NTRS1988

On the nonlinear interaction of Goertler vortices and Tollmien-Schlichting waves in curved channel flows at finite Reynolds numbers

The flow in a two-dimensional curved channel driven by an azimuthal pressure gradient can become linearly unstable due to axisymmetric perturbations and/or nonaxisymmetric perturbations depending on the curvature of the channel and the Reynolds number. For a particular small value of curvature, the critical neighborhood of this curvature value and critical Reynolds number, nonlinear interactions occur between these perturbations. The Stuart-Watson approach is used to derive two coupled Landau equations for the amplitudes of these perturbations. The stability of the various possible states of these perturbations is shown through bifurcation diagrams. Emphasis is given to those cases which have relevance to external flows.

Daudpota, Q. Isa↗
NASA NTRS1988

Numerical results for axial flow compressor instability

Using Cornell's supercomputing facilities, an extensive study of the Moore-Greitzer model was carried out, which gives accurate and reliable information about compressor instability. The bifurcation analysis in the companion paper shows the dependence of the mode of compressor response on the shape of the rotating stall characteristic. The numerical results verify and extend this with a more accurate representation of the characteristic. The effect of the parameters on the shape of the rotating stall characteristic is investigated, and it is found that the parameters with the strongest effects are the inlet length, and the shape of the compressor pressure rise vs. mass flow diagram (i.e. tall diagrams vs. shallow diagrams). The effects of inlet guide vane loss on the characteristic are discussed.

Mccaughan, F. E.↗
NASA NTRS1988

The influence of and the identification of nonlinearity in flexible structures

Several models were built at NASA Langley and used to demonstrate the following nonlinear behavior: internal resonance in a free response, principal parametric resonance and subcritical instability in a cantilever beam-lumped mass structure, combination resonance in a parametrically excited flexible beam, autoparametric interaction in a two-degree-of-freedom system, instability of the linear solution, saturation of the excited mode, subharmonic bifurcation, and chaotic responses. A video tape documenting these phenomena was made. An attempt to identify a simple structure consisting of two light-weight beams and two lumped masses using the Eigensystem Realization Algorithm showed the inherent difficulty of using a linear based theory to identify a particular nonlinearity. Preliminary results show the technique requires novel interpretation, and hence may not be useful for structural modes that are coupled by a guadratic nonlinearity. A literature survey was also completed on recent work in parametrically excited nonlinear system. In summary, nonlinear systems may possess unique behaviors that require nonlinear identification techniques based on an understanding of how nonlinearity affects the dynamic response of structures. In this was, the unique behaviors of nonlinear systems may be properly identified. Moreover, more accutate quantifiable estimates can be made once the qualitative model has been determined.

Zavodney, Lawrence D.↗
NASA NTRS1988

Radar investigation of asteroids and planetary satellites

The aim is to make radar reconnaissance of near-Earth asteroids, mainbelt ateroids, the Galilean satellites, the Martian satellites, and the largest Saturnian satellites, using the Arecibo 13-cm and the Goldstone 3.5-cm systems. Measurements of echo strength, polarization, and delay/Doppler distribution of echo power provide information about dimensions, spin vector, large-scale topography, cm-to-m-scale morphology, and surface bulk density. The observations also yield refined estimates of target orbital elements. Radar signatures were measured for 31 mainbelt asteroids and 16 near-Earth asteroids since this task began eight years ago. The dispersion in asteroid radar albedoes and circular polarization ratios is extreme, revealing huge differences in surface morphologies, bulk densities, and metal concentration. For the most part, correction between radar signature and VIS/IR class is not high. Many near-Earth asteroids have extremely irregular, nonconvex shapes, but some have polar silhouettes that appear only slightly noncircular. The signatures of 1627 Ivar, 1986 DA, and the approximately 180-km mainbelt asteroid 216 Kleopatra suggest bifurcated shapes. Observational milestones during 1987 and 1988 are noted.

Ostro, Steven J.↗
NASA NTRS1989

Low Reynolds number numerical solutions of chaotic flow

Numerical computations of two-dimensional flow past an airfoil at low Mach number, large angle of attack, and low Reynolds number are reported which show a sequence of flow states leading from single-period vortex shedding to chaos via the period-doubling mechanism. Analysis of the flow in terms of phase diagrams, Poincare sections, and flowfield variables are used to substantiate these results. The critical Reynolds number for the period-doubling bifurcations is shown to be sensitive to mesh refinement and the influence of large amounts of numerical dissipation. In extreme cases, large amounts of added dissipation can delay or completely eliminate the chaotic response. The effect of artificial dissipation at these low Reynolds numbers is to produce a new effective Reynolds number for the computations.

Pulliam, Thomas H.↗
NASA NTRS1989

Numerical simulation of axisymmetric turbulent flow in combustors and diffusers

Numerical studies of turbulent flow in an axisymmetric 45-deg-expansion combustor and bifurcated diffuser are presented. The Navier-Stokes equations incorporating a k-epsilon model were solved in a nonorthogonal curvilinear coordinate system. A zonal-grid method, where the flow field was divided into several subsections, was developed. This approach permitted different computational schemes to be used in the various zones. In addition, grid generation was made a more simple task. Boundary overlap and interpolating techniques were used, and an adjustment of the flow variables was required to assure conservation of mass flux. Three finite-differencing methods (hybrid, quadratic upwind, and skew upwind) were used to represent the convection terms. Results were compared with existing experimental data. In general, good agreement between predicted and measured values was obtained.

Yung, Chain-Nan↗