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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Pitot-Pressure Measurements in Flow Fields Behind a Rectangular Nozzle with Exhaust Jet for Free-Stream Mach Numbers of 0.00, 0.60, and 1.20

An investigation has been conducted in the Langley 16-Foot Transonic Tunnel to measure the flow field in and around the jet exhaust from a nonaxisymmetric nozzle configuration. The nozzle had a rectangular exit with a width-to-height ratio of 2.38. Pitot-pressure measurements were made at five longitudinal locations downstream of the nozzle exit. The maximum distance downstream of the exit was about 5 nozzle heights. These measurements were made at free-stream Mach numbers of 0.00, 0.60, and 1.20 with the nozzle operating at a ratio of nozzle total pressure to free-stream static pressure of 4.0. The jet exhaust was simulated with high-pressure air that had an exit total temperature essentially equal to the free-stream total temperature.

Putnam, L. E.↗
NASA NTRS1985

Evolution and interaction of large interplanetary streams

A computer simulation for the evolution and interaction of large interplanetary streams based on multi-spacecraft observations and an unsteady, one-dimensional MHD model is presented. Two events, each observed by two or more spacecraft separated by a distance of the order of 10 AU, were studied. The first simulation is based on the plasma and magnetic field observations made by two radially-aligned spacecraft. The second simulation is based on an event observed first by Helios-1 in May 1980 near 0.6 AU and later by Voyager-1 in June 1980 at 8.1 AU. These examples show that the dynamical evolution of large-scale solar wind structures is dominated by the shock process, including the formation, collision, and merging of shocks. The interaction of shocks with stream structures also causes a drastic decrease in the amplitude of the solar wind speed variation with increasing heliocentric distance, and as a result of interactions there is a large variation of shock-strengths and shock-speeds. The simulation results shed light on the interpretation for the interaction and evolution of large interplanetary streams. Observations were made along a few limited trajectories, but simulation results can supplement these by providing the detailed evolution process for large-scale solar wind structures in the vast region not directly observed. The use of a quantitative nonlinear simulation model including shock merging process is crucial in the interpretation of data obtained in the outer heliosphere.

Whang, Y. C.↗
NASA NTRS1987

A comparative study of plasma heating by ion acoustic and modified two-stream instabilities at subcritical quasi-perpendicular shocks

Plasma heating due to the ion acoustic instability and the modified two-stream instability is examined for quasi-perpendicular subcritical shocks. Electron and ion heating is investigated as a function of upstream electron to ion temperature ratio and plasma beta using second-order heating rates. A simple shock model is employed in which the cross-field electron-ion drift speed is adjusted until the total (adiabatic plus anomalous) heating matches that required by the Rankine-Hugoniot relations. Quantities such as the width of the shock and the maximum electric field fluctuations are also calculated, and the results are compared with the ISEE data set of subcritical bow shock crossings. The observed width of the shock, the amount of plasma heating, and the low-frequency electric field intensity are in reasonably good agreement with the calculations for the modified two-stream instability. On the other hand, the wave intensities at higher frequency are about 4 orders of magnitude smaller than those predicted for the ion acoustic instability at saturation, consistent with the fact that the measured shock widths imply cross-field drift speeds that are below threshold for this instability. It is therefore concluded that the dissipation at these shocks is most likely due to the lowest frequency, modified two-stream instability.

Winske, D.↗
NASA NTRS1987

The design and performance of a multi-stream droplet generator for the liquid droplet radiator

Results are presented for the performance capabilities of a multistream droplet generator suitable for use in a spacecraft liquid droplet radiator heat-rejection system. The nozzle-motion mode of stream perturbation initiation was tested with a single droplet stream and found to produce data similar to those generated with the resonant cavity mode of perturbation. Tests then proceeded to a 26-orifice array; the streams of the array responded to the perturbation satisfactorily, forming uniformly separated drops.

Orme, Melissa↗
NASA NTRS1987

Measurements of the free stream fluctuations above a turbulent boundary layer

This paper investigates the velocity fluctuations in the free stream above an incompressible turbulent boundary layer developing at constant pressure. It is assumed that the fluctuations receive contributions from three statistically independent sources: (1) one-dimensional unsteadiness, (2) free stream turbulence, and (3) the potential motion induced by the turbulent boundary layer. Measurements were made in a wind tunnel with a root-mean-square level of the axial velocity fluctuations of about 0.2 percent. All three velocity components were measured using an X-wire probe. The unsteadiness was determined from the spanwise covariance of the axial velocity, measured using two single wire probes. The results show that it is possible to separate the contributions to the r.m.s. level of the velocity fluctuations, without resorting to the dubious technique of high-pass filtering. The separation could be extended to the spectral densities of the contributions, if measurements of sufficient accuracy were available. The Appendix provides a general guide for the measurement of small free stream fluctuation levels.

Wood, David H.↗
NASA NTRS1987

On the spatial structure of the Perseids meteor stream

The analysis of radar observations of the Perseid meteor stream conducted in an ionospherical laboratory in the period from 1964 to 1981 is presented. The Perseids meteor rates were determined by the fluctuation method. Analysis of their hourly distributions showed that the stream maximum position is different for different years, i.e., the stream nodal position is constantly changing. The results of the analysis are presented and discussed.

Andreev, G. V.↗
NASA NTRS1988

Application of acoustic-Doppler current profiler and expendable bathythermograph measurements to the study of the velocity structure and transport of the Gulf Stream

The degree to which Acoustic-Doppler Current Profiler (ADCP) and expendable bathythermograph (XBT) data can provide quantitative measurements of the velocity structure and transport of the Gulf Stream is addressed. An algorithm is used to generate salinity from temperature and depth using an historical Temperature/Salinity relation for the NW Atlantic. Results have been simulated using CTD data and comparing real and pseudo salinity files. Errors are typically less than 2 dynamic cm for the upper 800 m out of a total signal of 80 cm (across the Gulf Stream). When combined with ADCP data for a near-surface reference velocity, transport errors in isopycnal layers are less than about 1 Sv (10 to the 6th power cu m/s), as is the difference in total transport for the upper 800 m between real and pseudo data. The method is capable of measuring the real variability of the Gulf Stream, and when combined with altimeter data, can provide estimates of the geoid slope with oceanic errors of a few parts in 10 to the 8th power over horizontal scales of 500 km.

Joyce, T. M.↗
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

Streaming sausage, kink and tearing instabilities in a current sheet with applications to the earth's magnetotail

This paper investigates the growth rates and eigenmode structures of the streaming sausage, kink, and tearing instabilities in a current sheet with a super-Alfvenic flow. The growth rates and eigenmode structures are first considered in the ideal incompressible limit by using a four-layer model, as well as a more realistic case in which all plasma parameters and the magnetic field vary continuously along the direction perpendicular to the magnetic field and plasma flow. An initial-value method is applied to obtain the growth rate and eigenmode profiles of the fastest growing mode, which is either the sausage mode or kink mode. It is shown that, in the earth's magnetotail, where super-Alfvenic plasma flows are observed in the plasma sheet and the ratio between the plasma and magnetic pressures far away from the current layer is about 0.1-0.3 in the lobes, the streaming sausage and streaming tearing instabilities, but not kink modes, are likely to occur.

Lee, L. C.↗
NASA NTRS1988

Observed velocity fluctuations on a major Antarctic ice stream

Recent ice velocity measurements on the major Antarctic Ice Stream B are compared with those collected 10 years ago. The results show that the ice flow in the mouth of Ice Stream B has decelerated by about 20 percent. The possible causes of this deceleration are discussed on the basis of present knowledge of current regional dynamics, and the possible effects of this deceleration on future ice stream behavior are examined.

Stephenson, S. N.↗
NASA NTRS1989

On the interpretation of measured ion streams in the wake of the Shuttle Orbiter in terms of plasma expansion processes

Measurements of the flow vector and current density of ion streams between 11 and 18 m downstream in the near wake of the Shuttle Orbiter during the Spacelab 2 mission are compared with a one-dimensional, time-dependent plasma expansion model for wake filling. The model is based on the self-consistent plasma model of Singh et al. (1987) modified to simulate the one-dimensional expansion of counter-streaming plasmas. The results show good agreement between the model and the experimental results, suggesting that the measured streams can be interpreted in terms of collisionless plasma expansion.

Singh, N.↗
NASA NTRS1990

Generation rates and chemical compositions of waste streams in a typical crewed space habitat

A judicious compilation of generation rates and chemical compositions of potential waste feed streams in a typical crewed space habitat was made in connection with the waste-management aspect of NASA's Physical/Chemical Closed-Loop Life Support Program. Waste composition definitions are needed for the design of waste-processing technologies involved in closing major life support functions in future long-duration human space missions. Tables of data for the constituents and chemical formulas of the following waste streams are presented and discussed: human urine, feces, hygiene (laundry and shower) water, cleansing agents, trash, humidity condensate, dried sweat, and trace contaminants. Tables of data on dust generation and pH values of the different waste streams are also presented and discussed.

Wydeven, Theodore↗
NASA NTRS1990

Incompressible flow computations based on the vorticity-stream function and velocity-pressure formulations

Finite element procedures and computations based on the velocity-pressure and vorticity-stream function formulations of incompressible flows are presented. Two new multistep velocity-pressure formulations are proposed and compared with the vorticity-stream function and one-step formulations. The example problems chosen are the standing vortex problem and flow past a circular cylinder. Benchmark quality computations are performed for the cylinder problem. The numerical results indicate that the vorticity-stream function formulation and one of the two new multistep formulations involve much less numerical dissipation than the one-step formulation.

Tezduyar, T. E.↗
NASA NTRS1990

Spiral structure of M51: Streaming motions across the spiral arms

The atomic hydrogen (HI) and the H alpha emission line in the grand-design spiral galaxy M51 have been observed with the Westerbork Synthesis Radio Telescope and the Taurus Fabry-Perot imaging spectrometer, respectively. Across the inner spiral arms significant tangential and radial velocity gradients are detected in the H alpha emission after subtraction of the axi-symmetric component of the velocity field. The shift is positive on the inside and negative on the outside of the northern arm. Across the southern arm this situation is reversed. The direction of the shifts is such that the material is moving inward and faster compared to circular rotation in both arms, consistent with the velocity perturbations predicted by spiral density wave models for gas downstream of a spiral shock. The observed shifts amount to 20 to 30 km (s-1), corresponding to streaming motions of 60 to 90 km (s-1) in the plane of the disk (inclination angle 20 degrees). Comparable velocity gradients have also been observed by Vogel et al. in the CO emission from the inner northern arm of M51. The streaming motions in M51 are about 2 to 3 times as large as the ones found in HI by Rots in M81, and successfully modelled by Visser with a self-consistent density wave model. Researchers have not been able to detect conclusively streaming motions in the HI emission from the arms, perhaps due to the relatively poor angular resolution (approx. 15 seconds) of the HI observations.

Tilanus, R. P. J.↗
NASA NTRS1991

Free stream capturing in fluid conservation law for moving coordinates in three dimensions

The free-stream capturing technique for both the finite-volume (FV) and finite-difference (FD) framework is summarized. For an arbitrary motion of the grid, the FV analysis shows that volumes swept by all six surfaces of the cell have to be computed correctly. This means that the free-stream capturing time-metric terms should be calculated not only from a surface vector of a cell at a single time level, but also from a volume swept by the cell surface in space and time. The FV free-stream capturing formulation is applicable to the FD formulation by proper translation from an FV cell to an FD mesh.

Obayashi, Shigeru↗
NASA NTRS1990

Sea level variabilities in the Gulf Stream between Cape Hatteras and 50 deg W - A Geosat study

Sea level variabilities in the Gulf Stream between Cape Hatteras and 50 deg W were examined by studying sea level residuals, relative to a 2-yr mean sea level, obtained from Geosat altimetry data for the period between November 1986 and December 1988. An array of sea-level time series was constructed for a region bounded by 30 deg N and 45 deg N in latitude and by 80 deg W and 50 deg W longitude. It is shown that the spectral characteristics of this time series varies with geographic location along the Gulf Stream path. Concurrent NOAA IR images are used to aid in the interpretation of sea level observations in terms of the variability of the stream's path, demonstrating the synergistic value of the combination of satellite-altimeter and IR data.

Vazquez, Jorge↗
NASA NTRS1991

Numerical simulations of high-speed solar wind streams within 1 AU and their signatures at 1 AU

A parametric study of the evolution within, and signatures at, 1 AU of high-speed streams is performed with the use of a MHD two-and-a-half-dimensional time-dependent model. This study is an extension of an earlier one by Smith and Dryer (1990) who examined the ecliptic plane consequences of relatively short-duration, energetic solar disturbances. The present study examines both the erupting and corotating parts of long-duration, high-speed streams characteristic of coronal hole flows. By examining the variation of the simulated plasma velocity, density, temperature, and magnetic field at 1 AU, as well as the location of the solar coronal hole sources relative to the observer at 1 AU, it was possible to provide some insight into the identification of the solar sources of interplanetary disturbances. Two definitions for angle locating the solar source of interplanetary disturbances at 1 AU are presented and discussed. The results are applied to the suggestion by Hewish (1988) that low-latitude coronal holes are suitably positioned to be the sources of major geomagnetic storms when the holes are in the eastern half of the solar hemisphere at the time of the commencement of the storm. The results indicate that, for these cases, the streams emanating from within the hole must be very fast, greater than 1000 km/s, or very wide, greater than 60 deg, at the inner boundary of 18 solar radii.

Smith, Z.↗
NASA NTRS1990

Receptivity of boundary layers to acoustic and vortical free-stream disturbances

The receptivity mechanisms by which free-stream disturbances generate instability waves in laminar boundary layers are discussed. Free-stream disturbances have wavelengths which are generally much longer than those of instability waves. Hence, the transfer of energy from the free-stream disturbance to the instability wave requires a wavelength conversion mechanism. Recent analyses using asymptotic methods have shown that the wavelength conversion takes place in regions of the boundary layer where the mean flow adjusts on a short streamwise length scale. This paper reviews recent progress in the theoretical understanding of these phenomena and suggests directions for future research in this area.

Kerschen, Edward J.↗