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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Experimental and theoretical investigation of the stability of stepwise pH gradients in continuous flow electrophoresis

Isoelectric focusing in the continuous flow mode can be more quickly and economically performed by admitting a stepwise pH gradient composed of simple buffers instead of uniform mixtures of synthetic carrier ampholytes. The time-consuming formation of the pH gradient by the electric field is thereby omitted. The stability of a three-step system with arginine - morpholinoethanesulfonic acid/glycylglycine - aspartic acid is analyzed theoretically by one-dimensional computer simulation as well as experimentally at various flow rates in a continuous flow apparatus. Excellent agreement between experimental and theoretical data was obtained. This metastable configuration was found to be suitable for focusing of proteins under continuous flow conditions. The influence of various combinations of electrolytes and membranes between electrophoresis chamber and electrode compartments is also discussed.

Kuhn, Reinhard↗
NASA NTRS1987

Experimental and theoretical investigations of the optogalvanic effect in a Hg-Ar discharge

The optogalvanic effect induced in a Hg-Ar discharge by light at 546.1 nm is studied experimentally and theoretically. The plasma and its response to a disturbance in the atomic-level populations are described entirely in terms of electron collision cross sections and other fundamental parameters. In a detailed description of the laser interaction with the atomic system, it was necessary to take into account velocity-changing collisions (VCC). These collisions (VCC) strongly modify the interaction between the Doppler-broadened system and the optical field. In some Doppler-free optogalvanic spectra the dramatic influence of VCC on the velocity distribution of atomic-level population is evident. Measurements of the magnitude of the optogalvanic signal confirm most of the theoretical results. Some anomalous features of the impedance change due to a weak beam were observed when a strong counterpropagating beam was introduced.

Richardson, W. H.↗
NASA NTRS1987

Theoretical and experimental analysis of the slotted-wall flow field in a transonic wind tunnel

The flow in the vicinity of a longitudinally slotted wind-tunnel wall is theoretically analyzed, and equations describing the pressure drop across the wall are derived. The ideal form of the slotted-wall boundary condition is shown to effectively model the wall pressure drop upstream of the point of maximum model thickness providing that a zero-shift correction to the reference pressure is included in the analysis. The wall-pressure drop equations can be linearized by subtracting the tunnel-empty boundary condition. Good correlation is obtained between experimental and theoretical values for variations in Mach number and angle of attack.

Everhart, Joel L.↗
NASA NTRS1988

Experimental and theoretical study of the effects of wing geometry on a supersonic multibody configuration

An experimental and theoretical investigation of planform effects on a low-fineness ratio multibody configuration was conducted in NASA-Langley Research Center's Unitary Plan Wind Tunnel at Mach number of 1.6, 1.8, 2.0 and 2.16. Experimental and theoretical values of lift, drag, and pitching moment as well as surface pressures were obtained on several configurations which varied in both outboard-wing panel and inboard-wing panel planforms. The three outboard-wing panels were a 65 -deg delta and two trapezoidal wing planforms. An unswept and a 60-deg swept inboard-wing panels were also tested. The purpose of the study was to determine the effect of wing planform on the supersonic aerodynamics. The large trapezoidal wing provided increased performance over the small trapezoidal wing primarily due to a reduction in the zero-lift drag coefficient. The swept inboard-wing panel planforms provided a slightly higher L/D than the unswept inboard-wing panel due to a minimal improvement in zero-lift drag. Linear-theory aerodynamic codes were used to analyze the effect of planform on the supersonic aerodynamics and were found to generally produce adequate results.

Bauer, Steven X. S.↗
NASA NTRS1988

Experimental and theoretical analysis of the rate of solvent equilibration in the hanging drop method of protein crystal growth

The principles of the hanging-drop method of crystal growth are discussed, and the rate of water evaporation in a water droplet (containing protein, buffer, and a precipitating agent) suspended above a well containing a double concentration of precipitating agent is investigated theoretically. It is shown that, on earth, the rate of evaporation may be determined from diffusion theory and the colligative properties of solutions. The parameters affecting the rate of evaporation include the temperature, the vapor pressure of water, the ionization constant of the salt, the volume of the drop, the contact angle between the droplet and the coverslip, the number of moles of salt in the droplet, the number of moles of water and salt in the well, the molar volumes of water and salt, the distance from the droplet to the well, and the coefficient of diffusion of water vapor through air. To test the theoretical equations, hanging-drop experiments were conducted using various reagent concentrations in 25-microliter droplets and measuring the evaporation times at 4 C and 25 C. The results showed good agreement with the theory.

Fowlis, William W.↗
NASA NTRS1988

Theoretical study of turbulent channel flow: Bulk properties, pressure fluctuations, and propagation of electromagnetic waves

In this paper, we apply two theoretical turbulence models, DIA and the recent GISS model, to study properties of a turbulent channel flow. Both models provide a turbulent kinetic energy spectral function E(k) as the solution of a non-linear equation; the two models employ the same source function but different closures. The source function is characterized by a rate n sub s (k) which is derived from the complex eigenvalues of the Orr--Sommerfeld (OS) equation in which the basic flow is taken to be of a Poiseuille type. The O--S equation is solved for a variety of Reynolds numbers corresponding to available experimental data. A physical argument is presented whereby the central line velocity characterizing the basic flow, U0 sup L, is not to be identified with the U0 appearing in the experimental Reynolds number. The theoretical results are compared with two types of experimental data: (1) turbulence bulk properties, and (2) properties that depend stongly on the structure of the turbulence spectrun at low wave numbers. The only existing analytical expression for Pi (k) cannot be used in the present case because it applies to the case of a flat plate, not a finite channel.

Canuto, V. M.↗
NASA NTRS1988

Theoretical studies on flapped delta wings

The effects of leading edge flaps on the aerodynamic characteristics of a low aspect-ratio delta wing are studied theoretically. As an extension of the classical crossflow plane analysis and in order to include separated shear layers, an analogy between three dimensional steady conical and two dimensional unsteady self-similar flows is explored. This analogy provides a simple steady-unsteady relationship. The criteria for the validity of the steady-unsteady analogy are also examined. Two different theoretical techniques are used to represent the separated shear layers based on the steady-unsteady analogy, neglecting the trailing edge effect. In the first approach, each vortex system is represented by a pair of concentrated vortices connected to the separation points by straight feeding sheets. In the second approach, the vortex cloud method is adopted for simulating the flow field in the crossflow plane. The separated shear layers are replaced with a cloud of discrete vortices and the boundary element method is employed to represent the wing trace by a vorticity distribution. A simple merging scheme is used to model the core region of the vortical flow as a single vortex by imposing a restriction on the shear layer rotation angle. The results are compared with experiments and with results from 3-D panel calculations.

Oh, S.↗
NASA NTRS1987

Theoretical horizontal-branch evolution

The general features of the theoretical evolution of canonical horizontal-branch (HB) stars are briefly reviewed with specific emphasis on the track morphology in the HR diagram and the determination of the globular cluster helium abundance. The observational evidence for the occurrence of semiconvection is discussed together with some remaining theoretical uncertainty.

Sweigart, Allen V.↗
NASA NTRS1988

Theoretical spectrum of AlN

Theoretical spectroscopic constants and radiative lifetimes are presented for the low-lying electronic states of AlN and AlN(+). The ground state of AlN is predicted to be X 3Pi, in agreement with earlier theoretical calculations, but the A 3Pi- state is extremely low-lying. The near degeneracy of these states is analogous to the situation for Al2 and implies that sigma and pi bonds are of nearly equal strength. The 3Pi - 3Pi AlN band system observed in emission is assigned as the C 3Pi - X 3Pi transition. The C 3Pi radiative lifetime is estimated as 151, 163, and 175 ns for v-prime = 0, 1, 2 respectively. These lifetimes may be significantly shortened by dissociation by the (1) 5Pi stated. The calculated D(e) of 2.35 eV for the ground state is about half that of the isovalent BN molecule. Thus the bonding in AlN is significantly weaker than for BN owing to the smaller overlaps of the valence orbitals.

Langhoff, Stephen R.↗
NASA NTRS1989

Theoretical study of the far-infrared A 3Sigma(-)g - X 3Pi(u) transition in Al2

The transition between the X 3Pi(u) ground state of Al2 and the first excited state A 3Sigma(-)g is investigated theoretically by means of CASSCF computations. The Einstein coefficients are determined on the basis of the potentials published by Bauschlicher et al. (1989) and listed in a table. The problems involved in devising an experiment to verify the theoretical results are briefly considered.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1989

Fitting theoretical photometric functions to asteroid phase curves

The ways in which variations in the parameters of Hapke's (1981, 1984, 1986) theoretical photometric function for fitting photometric phase data of asteroids can affect the shape of the theoretical curve are considered. It is noted that, between phase angles of 2 and 25 deg, the opposition effect parameters, the roughness parameter, and the single-particle phase function parameter have similar effects on the shape and the photometric curve and are difficult to separate on the basis of disk-integrated data alone. The uniqueness of asteroid surface properties deduced from phase curve observations over a limited phase-angle range is judged to remain questionable.

Domingue, Deborah↗
NASA NTRS1989

An acoustic experimental and theoretical investigation of single disc propellers

An experimental study of the acoustic field associated with two, three, and four blade propeller configurations with a blade root angle of 50 deg was performed in the Texas A&M University 5 ft. x 6 ft. acoustically-insulated subsonic wind tunnel. A waveform analysis package was utilized to obtain experimental acoustic time histories, frequency spectra, and overall sound pressure level (OASPL) and served as a basis for comparison to the theoretical acoustic compact source theory of Succi (1979). Valid for subsonic tip speeds, the acoustic analysis replaced each blade by an array of spiraling point sources which exhibited a unique force vector and volume. The computer analysis of Succi was modified to include a propeller performance strip analysis which used a NACA 4-digit series airfoil data bank to calculate lift and drag for each blade segment given the geometry and motion of the propeller. Theoretical OASPL predictions were found to moderately overpredict experimental values for all operating conditions and propeller configurations studied.

Bumann, Elizabeth A.↗
NASA NTRS1989

Theoretical modelling of Algol disks

A brief review of various theoretical approaches to model accretion disks is presented. Emphasis is given to models that determine self-consistently the structure of a disk together with the radiation field. It is argued that a proper treatment of the vertical structure is essential for calculating theoretical spectra to be compared with observations. In particular, it is shown that hot layers above an accretion disk (sometimes called disk 'chromospheres' or 'coronae'), whose presence is indicated by recent UV observations of strong emission lines of highly ionized species, may be explained using simple energy balance arguments.

Hubeny, I.↗
NASA NTRS1989

Laboratory and theoretical studies of baroclinic processes

An understanding is being developed for processes which may be important in the atmosphere, and the definition and analysis of baroclinic experiments utilizing the geophysical fluid flow cells (GFFC) apparatus in microgravity space flights. Included are studies using numerical codes, theoretical models, and terrestrial laboratory experiments. The numerical modeling is performed in three stages: calculation of steady axisymmetric flow, calculation of fastest-growing linear eigenmodes, and nonlinear effects (first, wave-mean flow interactions, then wave-wave interactions). The code can accommodate cylindrical, spherical, or channel geometry. It uses finite differences in the vertical and meridional directions, and is spectral in the azimuthal. The theoretical work was mostly in the area of effects of topography upon the baroclinic instability problem. The laboratory experiments are performed in a cylindrical annulus which has a temperture gradient imposed upon the lower surface and an approximately isothermal outer wall, with the upper and inner surfaces being nominally thermally insulating.

Miller, Timothy↗
NASA NTRS1990

Theoretical survey on positronium formation and ionisation in positron atom scattering

The recent theoretical studies are surveyed and reported on the formation of exotic atoms in positron-hydrogen, positron-helium and positron-lithium scattering specially at intermediate energy region. The ionizations of these targets by positron impact was also considered. Theoretical predictions for both the processes are compared with existing measured values.

Basu, Madhumita↗
NASA NTRS1990

Localization effects in radiationally disordered high-temperature superconductors: Theoretical interpretation

Theoretical interpretation of recent experiments on radiationally disordered high-temperature superconductors is presented, based on the concepts of mutual interplay of Anderson localization and superconductivity. Microscopic derivation of Ginzburg-Landau coefficients for the quasi-two-dimensional system in the vicinity of localization transition is given in the framework of the self-consistent theory of localization. The 'minimal metallic conductivity' for the quasi-two-dimensional case is enhanced due to a small overlap of electronic states on the nearest neighbor conducting planes. This leads to a stronger influence of localization effects than in ordinary (three-dimensional) superconductors. From this point of view even the initial samples of high-temperature superconductors are already very close to Anderson transition. Anomalies of H(c2) are also analyzed, explaining the upward curvature of H(c2)(T) and apparent independence of dH(c2)/dT (T = T(sub c)) on the degree of disorder as due to localization effects. Researchers discuss the possible reasons of fast T(sub c) degradation due to the enhanced Coulomb effects caused by the disorder induced decrease of localization length. The appearance and growth of localized magnetic moments is also discussed. The disorder dependence of localization length calculated from the experimental data on conductivity correlates reasonably with the theoretical criterion for suppression of superconductivity in the system with localized electronic states.

Goshchitskii, B. N.↗
NASA NTRS1990

Theoretical study of turbulent channel flow - Bulk properties, pressure fluctuations, and propagation of electromagnetic waves

In this paper, we apply two theoretical turbulence models, DIA and the recent GISS model, to study properties of a turbulent channel flow. Both models provide a turbulent kinetic energy spectral function E(k) as the solution of a non-linear equation; the two models employ the same source function but different closures. The source function is characterized by a rate n sub s (k) which is derived from the complex eigenvalues of the Orr-Sommerfeld (OS) equation in which the basic flow is taken to be of a Poiseuille type. The O-S equation is solved for a variety of Reynolds numbers corresponding to available experimental data. A physical argument is presented whereby the central line velocity characterizing the basic flow, U0 sup L, is not to be identified with the U0 appearing in the experimental Reynolds number. The theoretical results are compared with two types of experimental data: (1) turbulence bulk properties, and (2) properties that depend strongly on the structure of the turbulence spectrum at low wave numbers. The only existing analytical expression for Pi (k) cannot be used in the present case because it applies to the case of a flat plate, not a finite channel.

Canuto, V. M.↗
NASA NTRS1990

Experimental and theoretical study of Rayleigh-Lamb waves in a plate containing a surface-breaking crack

Scattering of Rayleigh-Lamb waves by a normal surface-breaking crack in a plate has been studied both theoretically and experimentally. The two-dimensionality of the far field, generated by a ball impact source, is exploited to characterize the source function using a direct integration technique. The scattering of waves generated by this impact source by the crack is subsequently solved by employing a Green's function integral expression for the scattered field coupled with a finite element representation of the near field. It is shown that theoretical results of plate response, both in frequency and time, are similar to those obtained experimentally. Additionally, implication for practical applications are discussed.

Paffenholz, Joseph↗