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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Role of long-wavelength degrees of freedom in the rod-to-coil transition in polymers

An analytical model is fitted to experimental data to show that rod-to-coil (RTC) transformations in single strand polymers (SSPs) are close to true phase transitions. The RTC are known to occur when the temperature or pH of a solvent in which the SSP is immersed is varied abruptly. Long wavelength degrees of freedom in the SSPs induce long range interactions in the molecule which lead to a first order phase transition. The Landau formalism suggests that a long-range ordered state can appear at a low temperature. A microscopic model of the phonon degrees of freedom identifies the nonzero temperature threshold at which the phase transition will occur. The phenomenology described is similar to a previous model for transitions in double-strand polymers.

Goldenfeld, N.↗
NASA NTRS1986

Continuous, Multielement, Hot-Film Transition Gage

Accurate measurement of location where laminar boundary layer undergoes transition to turbulent one serves many purposes in basic aero-dynamic research and developmental testing. Individual gages must be staggered to prevent formation of turbulence at gages downstream. This arrangement precludes accurate measurements of laminar/turbulent transition regions along streamline. Complete understanding of performance, stability, and control of laminar-flow airplane requires knowledge of transition locations on wing surface, empennage surfaces, fuselage, and nacelles. Visual, acoustic, and electronic methods capable of providing this transition information.

Holmes, B.↗
NASA NTRS1986

Effects of compressibility and free-stream turbulence on boundary layer transition in high-subsonic and transonic flows

Based on the existing boundary layer transition data, the effects of compressibility, pressure fluctuations, and free-stream turbulence have been reexamined for subsonic and transonic flow speeds. It is confirmed that the compressibility effects may be adequately expressed in terms of a simple correlation with free-stream Mach number. Pressure fluctuations, especially at low levels, do not seem to significantly affect the transition phenomenon. Effects of free-stream turbulence in high-subsonic and transonic flows are similar to the trends observed for low-speed flows and the transition process is hastened. The trends, as seen from slender cone flow data, seem to suggest power law correlations between transition Reynolds number and free-stream turbulence.

Murthy, S. V.↗
NASA NTRS1986

The comet/solar wind transition region at Giacobini-Zinner

An account is given of the electron density, temperature and flow speed measurements made during the encounter with Comet Giacobini-Zinner by the Los Alamos plasma electron experiment on the ICE spacecraft. Between about 70,000 and 120,000 km from the nucleus of the comet, ICE found a region in which the solar wind flow speed decreased and the temperature increased by factors of about two. This transition region was characterized by large fluctuations in the plasma parameters and by highly variable electron velocity distributions. Electron temperature and density variations through the transition region reveal that ICE never crossed a short-scalelength bow shock during the encounter, although the scalelength for the gross transition in plasma properties is not incompatible with a shock dominated by the length scales of cometary ions. However, many of the electron distributions in the transition region and sheath are similar to those seen behind weak collisionless shocks elsewhere in the heliosphere. A model of the comet/solar wind interaction is suggested in which a standing shock exists sunward of the ICE trajectory, but, due to large variations in the upstream conditions, it is highly variable and perhaps only intermittent along the flanks.

Thomsen, M. F.↗
NASA NTRS1986

An interstellar line coincident with the P(2,1) transition of hydronium (H3O+)

Double-sideband observations of the Orion KL nebular region with the NRAO 12 m radiotelescope in March 1986 detected a weak emission spectral line that coincided with the P(2,1) rotation-inversion transition of H3O+. Spectral maps are provided for the observed line at 307,192 MHz with 2 and 1 MHz resolution. The line parameters, along with a calculated transition dipole moment of 1.44 D, are used to estimate column densities of 7.7 and 8.7 x 10 to the 13/sq cm for excitation temperatures of 50 and 90 K, respectively. A minimum H2 density of 1 million/cu cm is estimated for the region if the thermalization of the transition to the excitation temperatures is to occur. Finally, fractional abundances of H2O in the Orion cloud are projected to be 1/100,000 and 1/1 million for the nonthermal and thermal transition cases, respectively.

Hollis, J. M.↗
NASA NTRS1986

On the magnetic structure of the quiet transition region

Existing models of the quiet chromosphere-corona transition region predict a distribution of emission measure over temperature that agrees with observation for a T greater than about 100,000 K. These 'network' models assume that all magnetic field lines that emerge from the photosphere extend into and are in thermal contact with the corona. It is shown that the observed fine-scale structure of the photospheric magnetic network instead suggests a two-component picture in which magnetic funnels that open into the corona emerge from only a fraction of the network. The gas that makes up the hotter transition region is mostly contained within these funnels, as in standard models, but, because the funnels are more constricted in our picture, the heat flowing into the cooler transition region from the corona is reduced by up to an order of magnitude. The remainder of the network is occupied by a population of low-lying loops with lengths less than about 10,000 km. It is proposed that the cooler transition region is mainly located within such loops, which are magnetically insulated from the corona and must, therefore, be heated internally. The fine-scale structure of ultraviolet spectroheliograms is consistent with this proposal, and theoretical models of internally heated loops can explain the behavior of the emission measure below a T of aobut 100,000 K.

Rabin, D.↗
NASA NTRS1987

A transition detection study using a cryogenic hot film system in the Langley 0.3-meter transonic cryogenic tunnel

A transition detection study was conducted in the Langley 0.3-Meter Transonic Cryogenic Tunnel (0.3-m TCT) using a specialized hot film system designed specifically for use in cryogenic wind tunnels. The quantitative transition location data obtained at near cryogenic conditions, 360 deg R (200K) represents the first definitive transition Reynolds numbers obtained in a cryogenic wind tunnel. The model was tested at both adiabatic and nonadiabatic wall conditions with a wall-to-total temperature ratio as low as 0.47. The test results indicated an improved technique for hot-film installation and a modified data acquisition system would allow the on-line determination of the location of boundary layer transition in cryogenic wind tunnels, such as the U.S. National Transonic Facility.

Johnson, C. B.↗
NASA NTRS1987

Optical boundary-layer transition detection in a transonic wind tunnel

A high-sensitivity interferometer has been developed and used to detect boundary-layer transitions over a symmetric airfoil. The tests, which included both natural and roughness-induced transitions, were performed in a transonic wind tunnel. The measurements showed a peak amplitude rms and higher energy in the spectrum of the signal associated with transition. The tests revealed that the interferometer system can be used to locate the transition region over three-dimensional aerodynamic models.

Azzazy, M.↗
NASA NTRS1987

Statistical equilibrium in cometary C2. IV - A 10 level model including singlet-triplet transitions

Resonance fluorescence theory was used to calculate the population distribution in the energy states of the C2 molecule in comets. Ten electronic states, each with 14 vibrational states, were used in the calculations. These new calculations differ from earlier work in terms of additional electronic levels and the role of singlet-triplet transitions between the b and X levels. Since transition moments are not known, calculations are made of observable flux ratios for an array of possible values. Comparison with existing observations indicates that the a-X transition is very important, and there is marginal indication that the b-X transition is present. Swan band sequence flux ratios at large heliocentric distance are needed, as are accurate Mulliken/Swan and Phillips/Ballik-Ramsay (1963) observations.

Krishna Swamy, K. S.↗
NASA NTRS1987

Theory of transition-layer emission measures and coronae

The basic equations describing the energy equilibria, the conductive heat flux, and the temperature stratifications for stellar transition layers and coronae with 'open' field lines are summarized. The temperature dependence of the emission measures for lines originating in different temperature regions of the transition zone is determined. It is found that the stellar transition regions consist of two basically different parts: the lower part where mechnical energy input is balanced by the radiative losses, and the upper part where the mechanical energy input is balanced by the divergence of the conductive flux and radiative losses. In the lower part, the temperature stratification is determined by an equilibrium between mechanical flux input and radiative energy losses. The coronal temperatures increase with increasing mechanical flux and damping length in the upper transition zone.

Bohm-Vitense, Erika↗
NASA NTRS1987

Heat transfer and fluid mechanics measurements in transitional boundary layers on convex-curved surfaces

The test section of the present experiment to ascertain the effects of convex curvature and freestream turbulence on boundary layer momentum and heat transfer during natural transition provided a two-dimensional boundary layer flow on a uniformly heated curved surface, with bending to various curvature radii, R. Attention is given to results for the cases of R = infinity, 180 cm, and 90 cm, each with two freestream turbulence intensity levels. While the mild convex curvature of R = 180 cm delays transition, further bending to R = 90 cm leads to no signifucant further delay of transition. Cases with both curvature and higher freestream disturbance effects exhibit the latter's pronounced dominance. These data are pertinent to the development of transition prediction models for gas turbine blade design.

Wang, T.↗
NASA NTRS1987

Rough-wall turbulent boundary layers in the transition regime

An experimental investigation of turbulent boundary layers over two-dimensional spanwise groove and three-dimensional sandgrain roughnesses in the transition regime between hydraulically smooth and fully rough conditions is presented. It is found that a self-preserving state can be reached in boundary layers developing over both d-type groove and sandgrain roughnesses, and that the drag of a k-type rough wall can be reduced by lowering the spanwise aspect ratio of the roughness elements. The two roughness Reynolds numbers defining the boundaries of the transition regime of the k-type roughnesses are shown to decrease with increasing roughness-element spanwise aspect ratio, and the upper critical transition Reynolds number is shown to determine the roughness behavior in both the transition and fully rough regime.

Bandyopadhyay, Promode R.↗
NASA NTRS1987

Prediction of transition on a flat plate under the influence of free-stream turbulence using low-Reynolds-number two-equation turbulence models

The prediction characteristics and capabilities of two popular two-equation low-Reynolds-number turbulence models (Jones, Launder 1972; Lam, Bremhorst 1981) have been evaluated with respect to the prediction of transition of a flat plate under the influence of free-stream turbulence. The sensitivity of the predictions to free-stream turbulence intensity, initial starting location of the calculation, and the assumed initial starting profiles for k and epsilon has been determined and presented. Although both models predict the correct qualitative characteristics of transition, they also exhibit significant quantitative deficiencies with regard to both the predicted location and the length of transition. A modification to the production term in the turbulent kinetic energy equation is proposed which is based on a simple stability criterion and correlated to the free-stream turbulence level. The modification becomes inactive in the fully turbulent regime, but is shown to improve both the qualitative and quantitative characteristics of the transition predictions.

Schmidt, Rodney C.↗
NASA NTRS1988

Comparison of boundary-layer transition on a cone and flat plate at Mach 3.5

NASA-Langley's Pilot Low-Disturbance Tunnel has been used to obtain boundary layer transition data on a cone and flat plate at Mach 3.5. The transition Reynolds numbers measured under these low noise conditions are higher than those in conventional noisy tunnels by a factor of 3 in the case of the cone and of seven in that of the flat plate. Transition predictions based on compressible linear stability theory and the e exp N method, for N=10, are in excellent agreement with the measured locations with transition onset for both the cone and flat plate under these low noise conditions.

Chen, F.-J.↗
NASA NTRS1988

The energy input mechanism into the lower transition regions between stellar chromospheres and coronae

The ratio of the emission line fluxes for the C II and C IV lines in the lower transition regions (T = 30,000 to 100,000 K) between stellar chromospheres and transition layers is shown to depend mainly on the temperature gradient in the line emitting regions which can therefore be determined from this line ratio. From the observed constant (within the limits of observational error) ratio of the emission line fluxes of the C II (1335 A) and C IV (1550 A) lines it is concluded that the temperature gradients in the lower transition layers are similar for the large majority of stars independently of T sub eff, L, and degree of activity. This means that the temperature dependence of the damping length for the mechanical flux must be the same for all these stars. Since for different kinds of mechanical fluxes the dependence of the damping length on gas pressure and temperature is quite different, it is concluded that the same heating mechanism must be responsible for the heating of all the lower transition layers of these stars, regardless of their chromospheric activity. Only the amount of mechanical flux changes. The T Tauri stars are exceptions: their emission lines are probably mainly due to circumstellar material.

Boehm-Vitense, Erika↗
NASA NTRS1988

Modeling of transitional flows

An effort directed at developing improved transitional models was initiated. The focus of this work was concentrated on the critical assessment of a popular existing transitional model developed by McDonald and Fish in 1972. The objective of this effort was to identify the shortcomings of the McDonald-Fish model and to use the insights gained to suggest modifications or alterations of the basic model. In order to evaluate the transitional model, a compressible boundary layer code was required. Accordingly, a two-dimensional compressible boundary layer code was developed. The program was based on a three-point fully implicit finite difference algorithm where the equations were solved in an uncoupled manner with second order extrapolation used to evaluate the non-linear coefficients. Iteration was offered as an option if the extrapolation error could not be tolerated. The differencing scheme was arranged to be second order in both spatial directions on an arbitrarily stretched mesh. A variety of boundary condition options were implemented including specification of an external pressure gradient, specification of a wall temperature distribution, and specification of an external temperature distribution. Overall the results of the initial phase of this work indicate that the McDonald-Fish model does a poor job at predicting the details of the turbulent flow structure during the transition region.

Lund, Thomas S.↗
NASA NTRS1988

Atmospheric conditions during the spring and fall transitions in the coastal ocean off western United States

Atmospheric events which force the spring and fall oceanic transitions in the coastal ocean off the west coast of North America were examined by analyzing the records of adjusted sea level (ASL), coastal wind stress, sea level atmospheric pressure (SLP), and 500-mbar heights for the years 1971-1975 and 1980-1983. The records cover periods of 91 days, centered on the dates of the spring and fall transitions as determined from coastal ASL data. It was found that the dominant mode of the ASL and coastal wind stress are similar around the times of both the spring and fall transitions, and that the time series for these modes are highly correlated with one another. Principal estimator patterns show the spatial patterns of SLP which force the ASL and coastal wind stress during the transitions.

Strub, P. Ted↗
NASA NTRS1989

Measurement of wavelengths for inner-shell transitions in Ca XVII-XIX

The wavelengths of transitions in solar flare spectra in the region 3.12-3.24 A have been measured using data recorded by a crystal spectrometer on the Air Force P78-1 spacecraft. Appearing in this wavelength region are inner-shell transitions of the type n = 1-2 in Ca XVII-XIX and resonance transitions in Ar XVII, Ar XVIII, and Fe XXV. The hydrogenic Ar XVIII 1s-1p transition was used to establish an absolute wavelength scale with an estimated uncertainty of 0.3 mA, and the relative wavelengths were measured to an uncertainty that is typically less than 0.1 mA. The presently measured wavelengths are compared with previously measured and calculated wavelengths, and the observed flare spectra are compared with synthetic spectra.

Seely, J. F.↗