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At least 289 records · Page 16

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NASA NTRS1982

Comments on the long-term stability of the earth's obliquity

Continued tidal evolution of the earth-moon system will lengthen the day and expand the lunar orbit. Both of these changes increase the present 26,000-year equinoctial precession period. In less than two billion years, this period will become comparable to the approximately 49,000- and 69,000-year periods of important terms describing the precession of the earth's orbit plane due to planetary perturbations. These events occur when the lunar orbital semimajor axis drifts past approximately 66.5 and 68.0 earth radii, respectively, and will be accompanied by large oscillations of the obliquity and severe climatic alterations. The current rate of lunar recession implies that these resonances should have been encountered already, had present conditions been the norm throughout geologic time.

Ward, W. R.↗

NASA NTRS1983

Numerical solution to the glancing sidewall oblique shock wave/turbulent boundary layer interaction in three dimension

A supersonic three-dimensional viscous forward-marching computer design code called PEPSIS is used to obtain a numerical solution of the three-dimensional problem of the interaction of a glancing sidewall oblique shock wave and a turbulent boundary layer. Very good results are obtained for a test case that was run to investigate the use of the wall-function boundary-condition approximation for a highly complex three-dimensional shock-boundary layer interaction. Two additional test cases (coarse mesh and medium mesh) are run to examine the question of near-wall resolution when no-slip boundary conditions are applied. A comparison with experimental data shows that the PEPSIS code gives excellent results in general and is practical for three-dimensional supersonic inlet calculations.

Anderson, B. H.↗

NASA NTRS1983

Numerical solution to the glancing sidewall oblique shock wave/turbulent boundary layer interaction in three-dimension

Anderson, B. H.↗

NASA NTRS1983

Do oblique impacts produce Martian meteorites

It is pointed out that several achondritic meteorites, classified as shergottites, nakhlites, and chassignites, have a number of unusual characteristics. Following the suggestion of Wood and Ashwal (1981) these meteorites are collectively referred to as SNC meteorites. The major element compositions of the SNC meteorites are, in general, distinct from those of other meteorites and lunar samples, and similar to certain terrestrial rocks. The geochemical and geochronological characteristics of the SNC meteorites strongly imply that their parent body was on the order of lunar size or larger and geologically active. Serious attention must be given to the hypothesis of a Martian origin of the SNC meteorites and to dynamic processes capable of delivering Martian meteorites to earth. In connection with the present investigation, it is suggested that oblique impacts of large meteoroids can produce ejecta which is entrained with the ricocheting projectile and accelerated to velocities in excess of Martian escape velocity.

Nyquist, L. E.↗

NASA NTRS1983

An experimental and numerical investigation of the impingement of an oblique shock wave on a body of revolution

Attention is given to the experimental study and numerical simulation of the impingement of oblique shock wave on a cylinder, in order both to document the complex three-dimensional shock wave and boundary layer interaction occurring in practical problems (such as stores carriage interference in a supersonic tactical aircraft) and to conduct a critical comparison of experimental measurements and numerical computations for such complex flows. A thin layer approximation of the Navier-Stokes equations was solved by means of a mixed explicit-implicit scheme. Experimental measurements reveal a highly complex flow field with two distinct adjacent separation zones, regions of high cross flow, and multiply reflected shocks and expansion fans.

Brosh, A.↗

NASA NTRS1983

The obliquity of Pluto

Pluto's obliquity (the angle between its spin axis and orbit normal) varies between 102 and 126 deg over a period of about 3 million years. These oscillations are nearly sinusoidal and quite stable, leading to only modest changes in the insolation regime. Thus, Pluto's rotation has been slightly retrograde ever since its current orbit and rotation rate were established.

Dobrovolskis, A. R.↗

NASA NTRS1984

The structure of oblique subcritical bow shocks - ISEE 1 and 2 observations

ISEE 1 and 2 spacecraft magnetic field data are used to determine the scale lengths of various elements of shock structure, as well as wavelengths and wave polarizations, in a study of structural elements which include shock ramps and precursor wave trains over a series of oblique, low Mach number terrestrial bow shocks. Dissipative processes are reflected in the damping of the precursors, and dissipative scale lengths are of 200-800 km, or several times greater than shock thicknesses. The source of dissipation in the shocks does not appear to be the wave-wave decay of the whistlers, for which no evidence is found. The interaction of the whistler itself with upstream electrons is suggested as a simple and self-consistent explanation for the observed wave train damping.

Mellott, M. M.↗

NASA NTRS1982

Oblique focus ICCD laboratory evaluation

An oblique focus intensified charge coupled device (ICCD) was constructed and operated in a vacuum system. Special gratings were obtained and an optical system set up to try to model a candidate UV spectrometer (Milieu Interstellaire et Intergalactique-MISIG), and to produce small enough images to test the theoretical subpixel resolution capability of the ICCD system. The efforts were only partly successful. Based on the results, a similar detector was built and flown successfully on a Princeton rocket program.

York, D. G.↗

NASA NTRS1985

Martian atmospheric photochemistry and composition during periods of low obliquity

During periods of low obliquity, previous work has shown that martian CO2 partial pressures decreased to 0.1 mbar; CO2 partial pressures decreased to 0.02 mbar prior to the formation of the Tharsis bulge. The permanent polar caps act as a cold trap and projected global average water vapor abundances drop to possibly as low as 0.00001 precipitable micron. As a result, the odd hydrogen catalytic cycle would not be effective at recombining CO and O back into CO2, and as much as 0.12 mbar of CO and 0.06 mbar of O2 could exist. These increased abundances would radically affect surface oxidation, change the lower atmospheric thermal structure, and completely alter the upper atmosphere.

Lindner, B. L.↗

NASA NTRS1984

In-flight total forces, moments and static aeroelastic characteristics of an oblique-wing research airplane

A low-speed flight investigation has provided total force and moment coefficients and aeroelastic effects for the AD-1 oblique-wing research airplane. The results were interpreted and compared with predictions that were based on wind tunnel data. An assessment has been made of the aeroelastic wing bending design criteria. Lateral-directional trim requirements caused by asymmetry were determined. At angles of attack near stall, flow visualization indicated viscous flow separation and spanwise vortex flow. These effects were also apparent in the force and moment data.

Curry, R. E.↗

NASA NTRS1984

Flight-determined aerodynamic derivatives of the AD-1 oblique-wing research airplane

The AD-1 is a variable-sweep oblique-wing research airplane that exhibits unconventional stability and control characteristics. In this report, flight-determined and predicted stability and control derivatives for the AD-1 airplane are compared. The predictions are based on both wind tunnel and computational results. A final best estimate of derivatives is presented.

Sim, A. G.↗

NASA NTRS1985

Oblique, parallel, and quasi-parallel morphology of collisionless shocks

The question as to whether nonperpendicular, oblique shocks are disguised in the bow shock system by its infinite radius-of-curvature is addressed in light of the most recent data, which indicate that the quasi-perpendicular/quasi-parallel division is not merely real, but intrinsic. An attempt is presently made to summarize observational results on nonperpendicular shocks with the express intention of understanding the shock as an interactive structure that is dependent on the geometry of shock propagation. A foundation for the reconciliation of some rather complex or apparently contradictory quasi-parallel features is suggested.

Greenstadt, Eugene W.↗

NASA NTRS1987

An integrated study of structures, aerodynamics and controls on the forward swept wing X-29A and the oblique wing research aircraft

The results of an integrated study of structures, aerodynamics, and controls using the STARS program on two advanced airplane configurations are presented. Results for the X-29A include finite element modeling, free vibration analyses, unsteady aerodynamic calculations, flutter/divergence analyses, and an aeroservoelastic controls analysis. Good correlation is shown between STARS results and various other verified results. The tasks performed on the Oblique Wing Research Aircraft include finite element modeling and free vibration analyses.

Dawson, Kenneth S.↗

NASA NTRS1987

Conceptual Final Paper on the Preliminary Design of an Oblique Flying Wing SST

A conceptual Oblique Flying Wing Supersonic Transport Aircraft (OFW, or surfplane because of its shape) was first proposed in 1957. It was reintroduced in 1987 in view of the emerging technology of artificial stabilization. This paper is based on the performance and economics study of an M2 B747-100B replacement aircraft. In order to make a fair comparison of this configuration with the B747, an end-sixties structural technology level is assumed. It is shown that a modern stability and control system can balance the aircraft and smooth out gusts, and that the OFW configuration equals or outperforms the B747 in speed, economy and comfort.

Van der Velden, Alexander J. M.↗

NASA NTRS1988

Experimental and numerical investigation of the effect of distributed suction on oblique shock wave/turbulent boundary layer interaction

A combined experimental and numerical study of the interaction of an incident oblique shock wave with a turbulent boundary layer on a rough plate and on a porous plate with suction is presented. The experimental phase involved the acquisition of mean data upstream of, within, and downstream of the interaction region at Mach numbers 2.5 and 3.0. Data were taken at unit Reynolds numbers of 1.66 E7 and 1.85 E7 m respectively, and for flow deflection angles of 0, 4, 6 and 8 degs. Measured data include wall static pressure, pitot pressure profiles, and local bleed distributions on the porous plate. On the rough plate, with no suction, the boundary layer profiles were modified near the wall, but not separated for the 4 deg flow deflection angle. For the higher deflection angles of 6 and 8 degs, the boundary layer was separated. Suction increases the strength of the incident shock required to separate the turbulent boundary layer; for all shock strengths tested, separation is completely eliminated. The pitot pressure profiles are affected throughout the whole boundary layer; they are fuller than the ones obtained on the rough plate. It is also found that the combination of suction and roughness introduces spatial perturbations.

Benhachmi, Driss↗

NASA NTRS1987

Electric field measurements at subcritical, oblique bow shock crossings

ISEE-1 electric field measurements at three oblique, subcritical dispersive bow shock crossings are presented. The potential drops across the shock due to the large spatial scale normal component of the electric field were found to vary between 340 and 520 V. The measurements provide the first observations in a space plasma of the oscillations in the normal component of the electric field connected with the whistler precursor phase standing at a collisionless shock. Intense, rapidly varying electric fields with peak amplitudes ranging up to 100 mV/m were observed at the magnetic ramp of the shock in the high time resolution data.

Wygant, J. R.↗

NASA NTRS1988

Oblique-wing research airplane motion simulation with decoupling control laws

A large piloted vertical motion simulator was used to assess the performance of a preliminary decoupling control law for an early version of the F-8 oblique wing research demonstrator airplane. Evaluations were performed for five discrete flight conditions, ranging from low-altitude subsonic Mach numbers to moderate-altitude supersonic Mach numbers. Asymmetric sideforce as a function of angle of attack was found to be the primary cause of both the lateral acceleration noted in pitch and the tendency to roll into left turns and out of right turns. The flight control system was shown to be effective in generally decoupling the airplane and reducing the lateral acceleration in pitch maneuvers.

Kempel, Robert W.↗

NASA NTRS1989

Nonlinear evolution of interacting oblique waves on two-dimensional shear layers

The effects of critical layer nonlinearity are considered on spatially growing oblique instability waves on nominally two-dimensional shear layers between parallel streams. The analysis shows that three-dimensional effects cause nonlinearity to occur at much smaller amplitudes than it does in two-dimensional flows. The nonlinear instability wave amplitude is determined by an integro-differential equation with cubic type nonlinearity. The numerical solutions to this equation are worked out and discussed in some detail. The numerical solutions always end in a singularity at a finite downstream distance.

Goldstein, M. E.↗