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NASA NTRS1983

Seismic velocity structure of the lunar mantle

The recently completed set of seismic arrival times from the Apollo lunar seismic network are inverted to estimate the average seismic velocities in three sections of the lunar mantle: two for the upper mantle and one for the middle mantle. The method used is a variation of the linearized least squares inversion where the inversion is accomplished in steps. The estimated average velocities in the upper mantle decrease from Vp = 7.74 km/sec and Vs = 4.49 km/sec in the section above 270-km depth to Vp = 7.46 km/sec and Vs = 4.25 km/sec in the section between 270- and 500-km depth, confirming the earlier finding of negative gradients based on seismic amplitude variations. The average velocities in the middle mantle between the depths of 500 km and 1000 km of Vp = 8.26 km/sec and Vs = 4.65 km/sec are significantly higher than those in the upper mantle, contradicting earlier estimates based on more limited data. The higher velocities may suggest initial melting of the moon down to at least 1000-km depth.

Nakamura, Y.↗

NASA NTRS1983

Velocity dispersions of knots in Vela X and Puppis A

Very high resolution spectroscopy of optical emission lines indicates that the velocity dispersions of knots in Vela X and Puppis A result primarily from turbulence within the emitting regions. Line-of-sight velocity dispersions (half-widths at half-maximum) for the observed knots in both remnants are about 20-30 km/s. In Vela X, these include thermal velocity dispersions of roughly 16 km/s for hydrogen and 4 km/s for nitrogen. The knots themselves move randomly relative to each other, with speeds up to 30 km/s in Vela X and between 30-80 km/s in Puppis A. Occasionally, diffuse components of the filaments are observed in Puppis A. These diffuse components have velocity dispersions between 55 and 80 km/s, but the relative contributions of turbulent and thermal motions to the dispersions are unknown. The results are compared with theory and previous observations. Direct photographs may be able to show if the forbidden O III knots are less spatially dispersed than the forbidden N II, forbidden S II, forbidden O II, and H I knots.

Shull, P., Jr.↗

NASA NTRS1983

Kinematics of velocity and vorticity correlations in turbulent flow

The kinematic problem of calculating second-order velocity moments from given values of the vorticity covariance is examined. Integral representation formulas for second-order velocity moments in terms of the two-point vorticity correlation tensor are derived. The special relationships existing between velocity moments in isotropic turbulence are expressed in terms of the integral formulas yielding several kinematic constraints on the two-point vorticity correlation tensor in isotropic turbulence. Numerical evaluation of these constraints suggests that a Gaussian curve may be the only form of the longitudinal velocity correlation coefficient which is consistent with the requirement of isotropy. It is shown that if this is the case, then a family of exact solutions to the decay of isotropic turbulence may be obtained which contains Batchelor's final period solution as a special case. In addition, the computed results suggest a method of approximating the integral representation formulas in general turbulent shear flows.

Bernard, P. S.↗

NASA NTRS1982

Correlation of flight effects on centerline velocity decay for cold-flow acoustically excited jets

Acoustic excitation can influence the large scale shear layer structure of jet flow, thereby causing the jet centerline velocity to decay more rapidly. this phenomena has numerous practical applications, such as the reduction of jet/flap impingement velocity in order to reduce flap structural loads for under-the-wing STOL aircraft concepts. Cold flow centerline velocity decay data obtained with acoustic excitation of the jet exhaust flow are correlated with and without flight effects. Initially, static data are correlated in terms of an acoustic parameter involving the threshold sound level needed to initiate acoustic excitation of the jet plume. It is shown that for a given flight speed, the same acoustic excitation parameter as that developed for the static condition is valid. Finally, an aerodynamic flight effects parameter is included to correlate the static and flight centerline velocity decay data with and without acoustic excitation.

Vonglahn, U. H.↗

NASA NTRS1984

Fluid shielding of high-velocity jet noise

Experimental noise data for a nozzle exhaust system incorporating a thermal acoustic shield (TAS) are presented to show the effect of changes in geometric and flow parameters on attenuation of high-velocity jet exhaust noise in the flyover plane. The results are presented for a 10.00-cm-diameter primary conical nozzle with a TAS configuration consisting of a 2.59- or 5.07-cm-wide annular gap. Shield-stream exhaust velocity was varied from 157 to 248 m/sec to investigate the effect of velocity ratio. The results showed that increasing the annular gap width increases attenuation of high-frequency noise when comparisons are made on the same ideal thrust basis. Varying the velocity ratio had a minor effect on the noise characteristics of the nozzles investigated.

Goodykoontz, J. H.↗

NASA NTRS1983

Measurements of wind velocity and pressure with a wave follower during Marsen

Air pressure data are used in determining the rate of momentum transfer from wind to waves. On the basis of the wind velocity measurements, the wave-induced airflow and its coherence with waves are obtained for various wind velocities and phase speeds of the ocean waves. The pressure results suggest that momentum transfer to waves can be specified by a certain relation, which is given. The wind-velocity results suggest that the wave-induced airflow is much smaller than the mean wind speed. An empirical equation is proposed for the modulation of atmospheric transfer to short waves caused by the orbital velocity of long waves.

Hsiao, S. V.↗

NASA NTRS1984

Thermospheric gravity waves near the source - Comparison of variations in neutral temperature and vertical velocity at Sondre Stromfjord

Theoretical predictions of thermospheric gravity wave oscillations are compared with observed neutral temperatures and velocities. The data were taken in February 1983 using a Fabry-Perot interferometer located on Greenland, close to impulse heat sources in the auroral oval. The phenomenon was modeled in terms of linearized equations of motion of the atmosphere on a slowly rotating sphere. Legendre polynomials were used as eigenfunctions and the transfer function amplitude surface was characterized by maxima in the wavenumber frequency plane. Good agreement for predicted and observed velocities and temperatures was attained in the 250-300 km altitude. The amplitude of the vertical velocity, however, was not accurately predicted, nor was the temperature variability. The vertical velocity did exhibit maxima and minima in response to corresponding temperature changes.

Herrero, F. A.↗

NASA NTRS1985

Velocity visualization in gaseous flows

Techniques are established for visualizing velocity in gaseous flows. Two approaches are considered, both of which are capable of yielding velocity simultaneously at a large number of flowfield locations, thereby providing images of velocity. The first technique employs a laser to mark specific fluid elements and a camera to track their subsequent motion. Marking is done by laser-induced phosphorescence of biacetyl, added as a tracer species in a flow of N2, or by laser-induced formation of sulfur particulates in SF6-H2-N2 mixtures. The second technique is based on the Doppler effect, and uses an intensified photodiode array camera and a planar form of laser-induced fluorescence to detect 2-d velocities of I2 (in I2-N2 mixtures) via Doppler-shifted absorption of narrow-linewidth laser radiation at 514.5 nm.

Hanson, R. K.↗

NASA NTRS1985

Optical absorption from the high-velocity neutral hydrogen complex C in the spectrum of the RR Lyrae star BT Draconis

The detection of interstellar Na D absorption lines at a local standard of rest (LSR) velocity of -85 km/s is reported, together with tentative detections of further Na D lines at -133 km/s and -110 km/s, in the spectrum of the RR Lyrae star BT Draconis which lies toward the high-velocity neutral hydrogen complex C (LSR velocity around -115 km/s). This suggests an upper limit of 2.1 kpc on the distance to this high-velocity complex, and indicates that the complex has near-cosmic metallicity. This result strongly favors either a model in which complex C is formed by infalling material in the halo, which has originated in the Galactic disk, or one in which it is part of a large relatively local shell structure.

Songaila, A.↗

NASA NTRS1985

Impact experiments. I - Ejecta velocity distributions and related results from regolith targets

Results are reported of laboratory measurements of the velocity distributions of powdery ejecta of 14 impacts into powders simulating regolith in near vacuum. The impact velocities ranged from 5-2321 m/sec, a velocity range believed prevalent in the early solar system. The powder was mechanically similar to lunar regolith. The projectiles comprised harder materials than the regolith, i.e., pyrex spheres, basalt spheres and cylinders, and igneous rocks. High-speed photography was employed to record the events. The differences between the present results and those from hypervelocity impacts are discussed in detail. Notably, an impact velocity threshold was detected between 10-30 m/sec, below which the amount of ejecta is less than the projectile mass.

Hartmann, W. K.↗

NASA NTRS1986

Estimating Average Wind Velocity Along a Trajectory

Average Wind Velocity (VWAVE) program calculates average wind velocity over time for particular vehicle trajectory. Calculation based on wind profile, which is wind magnitude at various altitudes. Average of wind profile over altitude does not correlate well with actual apparent effect of wind. Wind profiles with low average velocities more severe than some wind profiles with high average velocities. VWAVE written in FORTRAN V for interactive execution.

Bertsch, P.↗

NASA NTRS1985

Effects of velocity profile and inclination on dual-jet-induced pressures on a flat plate in a crosswind

An experimental study was conducted to determine surface pressure distributions on a flat plate with dual subsonic, circular jets exhausting from the surface into a crossflow. The jets were arranged in both side-by-side and tandem configurations and were injected at 90 deg and 60 deg angles to the plate, with jet-to-crossflow velocity ratio of 2.2 and 4. The major objective of the study was to determine the effect of a nonuniform (vs uniform) jet velocity profile, simulating the exhaust of a turbo-fan engine. Nonuniform jets with a high-velocity outer annulus and a low-velocity core induced stronger negative pressure fields than uniform jets with the same mass flow rate. However, nondimensional lift losses (lift loss/jet thrust lift) due to such nonuniform jets were lower than lift losses due to uniform jets. Changing the injection angle from 90 deg to 60 deg resulted in moderate (for tandem jets) to significant (for side-by-side jets) increases in the induced negative pressures, even though the surface area influenced by the jets tended to reduce as the angle decreased. Jets arranged in the side-by-side configuration led to significant jet-induced lift losses exceeding, in some cases, lift losses reported for single jets.

Jakubowski, A. L.↗

NASA NTRS1986

On the advantages of the vorticity-velocity formulation of the equations of fluid dynamics

The mathematical properties of the pressure-velocity and vorticity-velocity formulations of the equations of viscous flow are compared. It is shown that a vorticity-velocity formulation exists which has the interesting property that non-inertial effects only enter the problem through the implementation of initial and boundary conditions. This valuable characteristic, along with other advantages of the vorticity-velocity approach, are discussed in detail.

Speziale, C. G.↗

NASA NTRS1985

Formation of mesosiderites by low-velocity impacts as a natural consequence of planet formation

It is argued here that, during the period of planet formation, mesosiderites originated by the low-velocity collisions of large metallic core fragments with the surface of a differentiated asteroid-size body. Relative velocities of the asteroids native to this region were low because it was distant from massive protoplanets. However, a few differentiated asteroids were destroyed by high-velocity collisions with interlopers perturbed by protoplanets into high eccentric orbits. These collisions reduced mantles and crusts to small silicate fragments, but left cores in the form of large, durable metal fragments. Mesosiderite-like pyroxenite-basalt-metal mixtures were formed when large core fragments accreted at low velocities to the regolith of an intact asteroid. Most of the olivine in mesosiderite and howardite breccias are attributed here to the mantles of the disrupted parent bodies. The low olivine contents indicate that the amount of debris accreted from disrupted asteroids was small relative to the volumes of the regoliths.

Wasson, J. T.↗

NASA NTRS1986

Temperature and velocity profiles in sooting free boundary layer flames

Temperature and velocity profiles are presented for cyclohexane, n-heptane, and iso-octane free, laminar, boundary layer, sooting, diffusion flames. Temperatures are measured with 3 mil Pt/Pt-13 percent Rh thermocouples. Corrected gas temperatures are derived by performing an energy balance of convection to and radiation from the thermocouple bead incorporating the variation of air conductivity and platinum emissivity with temperature. Velocities are measured using laser doppler velocimetry techniques. Profiles are compared with previously reported analytic temperature and velocity fields. Comparison of theoretical and experimental temperature profiles suggests improvement in the analytical treatment is needed, which accounts more accurately for the local soot radiation. The velocity profiles are in good agreement, with the departure of the theory from observation partially due to the small fluctuations inherent in these free flows.

Ang, J. A.↗

NASA NTRS1986

Instantaneous velocity profile measurements in a turbulent boundary layer

Instantaneous wall shear stress and streamwise velocities have been measured simultaneously in a flat-plate, turbulent boundary layer at moderate Reynolds number in an effort to provide experimental support for large eddy simulations. Data were obtained using a buried-wire, wall shear gage and a hot-wire rake positioned in the log region of the flow. Fluctuations of the instantaneous U(+) versus Y(+) profiles about a mean law of the wall are shown to be significant and complex. Peak cross-correlation values between wall shear stress and the velocities are high, and reflect the passage of a large structure inclined at a small angle to the wall. Estimates of this angle are consistent with those made by other investigators. Conditional sampling techniques were used to detect the passage of various sizes and types of flow disturbances (events), and to estimate their mean frequency of occurrence. Events characterized by large aand sudden streamwise accelerations were found to be highly coherent throughout the log region and were strongly correlated with large fluctuations in wall shear stress. Phase randomness between the near-wall quantities and the outer velocities was small. The results suggest that the flow events detected by conditional sampling applied to velocities in the log region may be related to the bursting process.

Robinson, S. K.↗

NASA NTRS1986

The influence of velocity variability on the determination of wind profiles

High sensitivity radars allow the determination of velocity estimates at time resolutions down to one minute or better. Because of the variability introduced to the mean wind due to turbulence and waves, the high resolution profiles may not be too useful for forecasting applications, although they yield the most realistic estimate of the instantaneous wind profile. Profiles of wind speed and direction, vertical velocity and echo power, which were deduced in real-time on 23 August 1981 with the spaced antenna drift mode of the SOUSY-VHF-Radar are shown. Whereas these profiles were measured within 1 minute, the operating routine allowed the selection of variable (longer) measuring periods, and one has to search for the optimum duration of the data averaging period. A high time resolution wind vector diagram is given which gives an idea of the temporal variability. The data were obtained with the spaced antenna technique, which allows a good estimate of the horizontal wind without having to correct for the vertical velocity component. The wind vectors specifically indicate a quasi-periodic variation in direction. This is assumed to be due to gravity waves since the vertical velocity also shows periodical variations with the same period. The consistency of these spaced-antenna VHF radar results along with the radiosonde data convinced researchers that the method is quite suitable for wind profiling applications.

Rottger, J.↗

NASA NTRS1986

Determination of the Brunt-Vaisala frequency from vertical velocity spectra

Recent work on the spectra of vertical velocity oscillations due to gravity waves in the troposphere, stratosphere and the mesosphere has revealed a typical feature which we call the Brunt-Vaisala cutoff. Several observers noticed a spectral peak near the Brunt-Vaisala frequency. This peak often is characterized by a very steep slope at the high frequency part, but a fairly shallow slope towards lower frequencies. Some example spectra of stratosphere observations are given. This distinct spectral shape (most clear at the upper height 22.5 km) can be explained by the fact that the vertical velocity amplitudes of atmospheric gravity waves increase with frequency up to their natural cutoff at the Brunt-Vaisala frequency. The measurement of the frequency of the peak in a vertical velocity spectrum was found to yield most directly the Brunt-Vaisala-frequency profile. Knowing the Brunt-Vaisala frequency profile, one can deduce the potential temperature profile, if one has a calibration temperature at one height. However, even the uncalibrated profile will be quite useful, e.g., to determine fronts (defined by temperature inversions) and the tropopause height. This method fails for superadiabatic lapse rates when the Brunt-Viasala frequency is imaginary. The application of this method will also be difficult when the wind velocity is too high, causing the Doppler effect to smear out the total spectrum and blur the Brunt-Vaisala cutoff. A similar deficiency will also appear if the gravity-wave distribution has a maximum in wind direction.

Rottger, J.↗