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At least 433 records · Page 24

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NASA NTRS1975

Pure rotational excitation of H2 at electron impact energies of 3 to 100 eV

Cross sections for pure rotational excitation in H2 are obtained using two different crossed-beam type electron-impact spectrometers with three sets of conditions: at electron energies of 15-100 eV and scattering angles of 115 deg; 3-40 eV at 20 deg; 40 eV at 10-135 deg. For intermediate-energy electrons the pure rotational excitation cross section at large scattering angles exceeds the elastic scattering cross section at electron energies greater than about 30 eV. Rotational cross sections are found to decrease slowly with decreasing electron energy, with a magnitude at their peak (at 4 eV) about 20 times that at 100 eV. It is suggested that these results may account in part for the large population of excited rotational states observed in interstellar H2.

Srivastava, S. K.↗

NASA NTRS1975

Calculation of rotational transition probabilities in molecular collisions - Application to N2 + N2

A computational method is proposed to obtain rotational transition probabilities in collisions between two diatomic molecules. The potential method of Rabitz and an exponential approximation are used to solve the semiclassical coupled equations without invoking any perturbational technique. The collision trajectory is determined in the classical modified-wave-number approximation. The method can treat systems involving strong interactions and provide probabilities for transitions even with a multiquantum jump. A simultaneous transition in the rotational states of both molecules, i.e., the rotational-rotational energy transfer, is taken into account. An application to the system N2 + N2 is presented.

Itikawa, Y.↗

NASA NTRS1976

Timing of solar cycles by rigid internal rotations

The so-called 11-year cycle of solar activity is really more complex and contains many periods of greatly different lengths. Periods as long as 178 years and as short as 3.1 years are predicted by a theory based on beats between rigidly rotating, inertially oscillating g-modes inside the sun. Most of the beat periods are then confirmed to about 1 percent accuracy in sunspot observations. Since the agreement is of high statistical significance, one can conclude that approximate alignment of major solar oscillation modes contributes to high solar activity. The theory receives further support when tested against an independent class of observations - the large-scale magnetic sector structure. Predicted rotation rates of at least four solar oscillation modes are detected in the sector data with discrepancies all less than 0.3 percent. As a by-product of these successful fits to observation, the mean rotation of the entire solar mass becomes known. Its rotation frequency is 4.49 by 10 to the -7th power Hz, which is a sidereal period of 25.8 days. Magnetic fields have played no role in calculating the length of any of these solar cycles.

Wolff, C. L.↗

NASA NTRS1977

Effects of reset stators and a rotating, grooved stator hub on performance of a 1.92-pressure-ratio compressor stage

The overall performance and blade-element performance of a transonic fan stage are presented for two modified test configurations and are compared with the unmodified stage. Tests were conducted with reset stators 2 deg open and reset stators with a rotating grooved stator hub. Detailed radial and circumferential (behind stator) surveys of the flow conditions were made over the stable operating range at rotative speeds of 70, 90, and 100 percent of design speed. Reset stator blade tests indicated a small increase in stage efficiency, pressure ratio, and maximum weight flow at each speed. Performance with reset stators and a rotating, grooved stator hub resulted in an additional increase in stage efficiency and pressure ratio at all speeds. The rotating grooved stator hub reduced hub losses considerably.

Lewis, G. W., Jr.↗

NASA NTRS1977

Rotational joint assembly for the prosthetic leg

A rotational joint assembly for a prosthetic leg has been devised, which enables an artificial foot to rotate slightly when a person is walking, running or turning. The prosthetic leg includes upper and lower tubular members with the rotational joint assembly interposed between them. The assembly includes a restrainer mechanism which consists of a pivotably mounted paddle element. This device applies limiting force to control the rotation of the foot and also restores torque to return the foot back to its initial position.

Owens, L. J.↗

NASA NTRS1976

Solar rotation during the Maunder Minimum

Solar surface rotation has been measured from sunspot drawings made in A.D. 1642-1644, probable differences from present-day rates are found. The 17th-century sunspots rotated faster near the equator by 3 or 4%, and the differential rotation between zero and + or - 20 deg latitude was enhanced by about a factor 3. These differences are consistent features in both spots and groups of spots and in both hemispheres. It is assumed that this apparent change in surface rotation was related to the ensuing dearth of solar activity (the Maunder Minimum) which persisted until about 1715.

Eddy, J. A.↗

NASA NTRS1976

The evolution of rotating stars. I - Method and exploratory calculations for a 7-solar-mass star

A method is developed which allows the evolution of rotating stars to be studied well beyond the main-sequence stage. Four different cases of redistribution of angular momentum in an evolving star are considered. Evolutionary sequences for a 7-solar-mass star, rotating according to these different cases, were computed from the zero-age main-sequence to the double-shell-source stage. Each sequence was begun with a (typical) equatorial rotational velocity of 210 km/s. On the main sequence, the effects of rotation are of minor importance. However, as the core contracts during later stages, important effects arise in all physically plausible cases. The outer regions of the cores approach critical velocities and develop unstable angular-velocity distributions. The effects of these instabilities should significantly alter the subsequent evolution.

Endal, A. S.↗

NASA NTRS1976

Absolute intensity measurement of the 4-0 vibration-rotation band of carbon monoxide

The absolute intensity of the 4-0 vibration band of CO is measured in spectra obtained using a 25-m base-path multiple-traversal absorption cell and a 5-m scanning spectrometer. The intensities of individual vibration-rotation lines in this band are determined from measurements of their equivalent widths, and absolute values for the rotationless transition moment and the vibration-rotation interaction factor are derived from the measured line strengths. The experimentally obtained vibration-rotation function is compared with a theoretical curve; agreement between theory and experiment is found to be good for the P-branch but poor for the R-branch. It is noted that numerical solutions to the radial Schroedinger equation lead to vibration-rotation function values that are in good agreement with the experiment.

Chackerian, C., Jr.↗

NASA NTRS1976

Differential rotation of photospheric magnetic fields associated with coronal holes

An interesting aspect of solar rotation is the fact that coronal holes seem to exhibit little or no differential rotation. The question is investigated of whether or not the photospheric magnetic fields underlying coronal holes also exhibit reduced differential rotation. In order to accomplish this, the daily positions of filaments and plages surrounding a large coronal hole that lasted for several disk passages were measured. The resulting differential-rotation curve was considerably flatter than the standard curve for long-lived filaments and was in remarkably good agreement with the curve found for the overlying coronal hole itself.

Adams, W. M.↗

NASA NTRS1977

Accretion of rotating fluids by barytropes - Numerical results for white-dwarf models

Numerical sequences of rotating axisymmetric nonmagnetic equilibrium models are constructed which represent the evolution of a barytropic star as it accretes material from a rotating medium. Two accretion geometries are considered - one approximating accretion from a rotating cloud and the other, accretion from a Keplerian disk. It is assumed that some process, such as Ekman spin-up or nonequilibrium oscillations, maintains nearly constant angular velocity along cylinders about the rotation axis. Transport of angular momentum in the cylindrically radial direction by viscosity is included. Fluid instabilities and other physical processes leading to enhancement of this transport are discussed. Particular application is made to zero-temperature white-dwarf models, using the degenerate electron equation of state. An initially nonrotating 0.566-solar-mass white dwarf is followed during the accretion of more than one solar mass of material. Applications to degenerate stellar cores, to mass-transfer binary systems containing white dwarfs, such as novae and dwarf novae, to Type I supernovae, and to galactic X-ray sources are considered.

Durisen, R. H.↗

NASA NTRS1977

Rotational and vibrational transitions for Li + H2 collisions

Close coupling calculations for integral and differential cross sections have been carried out for Li + H2 collisions with an ab initio Hartree-Fock potential energy surface. Rotational, vibrational, and vib-rotational excitation cross sections are reported at 0.4336 eV, 0.7 eV, and 0.8673 eV in the center of mass system. For pure rotational excitations, which dominate the inelastic scattering, coupling with vibrational states is not very important. For vibrational transitions, the influence of large multiquantum rotational transitions is far less than that found for Li(+) + H2 collisions.

Choi, B. H.↗

NASA NTRS1977

Design of experiment for earth rotation and baseline parameter determination from very long baseline interferometry

The possibility of recovering earth rotation and network geometry (baseline) parameters are emphasized. The numerical simulated experiments performed are set up in an environment where station coordinates vary with respect to inertial space according to a simulated earth rotation model similar to the actual but unknown rotation of the earth. The basic technique of VLBI and its mathematical model are presented. The parametrization of earth rotation chosen is described and the resulting model is linearized. A simple analysis of the geometry of the observations leads to some useful hints on achieving maximum sensitivity of the observations with respect to the parameters considered. The basic philosophy for the simulation of data and their analysis through standard least squares adjustment techniques is presented. A number of characteristic network designs based on present and candidate station locations are chosen. The results of the simulations for each design are presented together with a summary of the conclusions.

Dermanis, A.↗

NASA NTRS1978

Development of a cryogenic rotating heat pipe joint

The performance of two critical technology components required for a continuously rotatable heat pipe: (1) a low-leakage rotatable coupling for the heat pipe pressure vessel, and (2) a rotatable internal wick, is reported. Performance and leakage requirements were established based on 12 months operation of a cryogenic rotatable heat pipe on a satellite in earth orbit.

Source record↗

NASA NTRS1977

The rotation of Uranus

The rotation rate of Uranus is investigated by measuring the tilts of reflected Fraunhofer lines observed through a slit spectrograph. The data obtained are corrected for the effect of astronomical seeing, and the data-reduction procedures are outlined in detail. Observations are discussed which indicate that the planet's pole of rotation is parallel to the pole of its satellites' orbits and that Uranus may not rotate as a solid body. A rotational period of 15.57 + or - 0.80 hr is derived for northern midlatitudes on Uranus by adopting a planetary diameter of 51,800 + or - 600 km. Possible sources of small systematic errors are briefly considered.

Brown, R. A.↗

NASA NTRS1977

White-dwarf variability and the rotation of g-modes

The multiperiodic behavior clearly evident after Fourier analysis of many DA white-dwarf light curves is interpreted as arising from outbursts whose timing is regulated by the rotation and oscillation of the star. A model is proposed on the basis of three main assumptions: (1) a broad array of g-mode oscillations is taking place all the time in DA white-dwarf variables; (2) slow rotation permits most of the g-modes to couple and form a small group of nonlinear modes, each characterized by one of the indices of the spherical harmonic functions involved; and (3) the observed brightenings of the star are produced by upwelling of convection on a large horizontal scale when the antinodes of the nonlinear oscillation patterns periodically rotate into alignment and cause a temporary local enhancement of energy per unit volume. This model is used to match precisely almost every strong periodicity observed in the complex light curves of the DA white dwarfs G207-9, G38-29, G29-38, and HL Tau 76. If the model has been applied correctly, the results indicate that all four of these variables are rotating slowly with periods in the range from 250 to 500 sec.

Wolff, C. L.↗

NASA NTRS1977

Anomalous solar rotation in the early 17th century

The character of solar rotation has been examined for two periods in the early 17th century for which detailed sunspot drawings are available: A.D. 1625 through 1626 and 1642 through 1644. The first period occurred 20 years before the start of the Maunder sunspot minimum, 1645 through 1715; the second occurred just at its commencement. Solar rotation in the earlier period was much like that of today. In the later period, the equatorial velocity of the sun was faster by 3 to 5 percent and the differential rotation was enhanced by a factor of 3. The equatorial acceleration with declining solar activity is in the same sense as that found in recent Doppler data. It seems likely that the change in rotation of the solar surface between 1625 and 1645 was associated with the onset of the Maunder Minimum.

Eddy, J. A.↗

NASA NTRS1978

Formation of a rotational accretion column

We consider the problem of the flow produced by the accretion of gas from a rotating cloud onto a point gravitational source. It is shown that, for a cloud initially in uniform rotation, the effect of accretion is to produce a Taylor column in the cloud parallel to the rotation axis. Only gas within the column is accreted; the gas outside the column undergoes oscillatory flow, which is probably subject to shear instability. The column forms in approximately one-half rotation period from the time at which accretion begins. Its initial radius is determined by the accretion rate and the angular velocity of the cloud. Application of the results to the formation of Jupiter by accretion is discussed.

Cassen, P.↗

NASA NTRS1978

Nonlinear flap-lag-axial equations of a rotating beam with arbitrary precone angle

In an attempt both to unify and extend the analytical basis of several aspects of the dynamic behavior of flexible rotating beams, the second-degree nonlinear equations of motion for the coupled flapwise bending, lagwise bending, and axial extension of an untwisted, torsionally rigid, nonuniform, rotating beam having an arbitrary angle of precone with the plane perpendicular to the axis of rotation are derived using Hamilton's principle. The derivation of the equations is based on the geometric nonlinear theory of elasticity and the resulting equations are consistent with the assumption that the strains are negligible compared to unity. No restrictions are imposed on the relative displacements or angular rotations of the cross sections of the beam other than those implied by the assumption of small strains. Illustrative numerical results, obtained by using an integrating matrix as the basis for the method of solution, are presented both for the purpose of validating the present method of solution and indicating the range of applicability of the equations of motion and the method of solution.

Kvaternik, R. G.↗