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  • Other Sources  (4,406)
  • AERODYNAMICS  (2,823)
  • COMMUNICATIONS AND RADAR  (1,583)
  • Life and Medical Sciences
  • 1990-1994  (4,406)
  • 1910-1914
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  • 1
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    Recalibration of the Viyager PRA antenna for polarization sense measurement (1994)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The Voyager Planetary Radio Astronomy (PRA) antenna and receiver system provides an indication of the sense of elliptical or circular polarization of radiation that is not correct for all directions of incidence. The true sense could be determined for all directions if accurate calibration data were available. It was not feasible to make the calibration before the Voyagers were launched. Lecacheux & Ortega-Molina (1987), however, were able to derive such calibration data from planetary radio observations made in flight. They expressed their results in terms of the tilt of a plane (the E-plane) that divides the incident ray directions for which the indicated polarization sense is correct from those directions for which the indicated sense is reversed. We demonstrate that there are certain directions for which this calibration is itself in error, and that the surface dividing the two sets of incident rays is more complex than a tilted plane. We are able to make a crude approximation to the true surface from the limited data available.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 281; 3; p. 945-954
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  • 2
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    Overview of supersonic laminar flow control research on the F-16XL ships 1 and 2 (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: NASA is directing research to develop technology for a high-speed civil transport. Supersonic laminar flow control has been identified as a program element, since it offers significant drag-reduction benefits and is one of the more promising technologies for producing an economically viable aircraft design. NASA is using two prototype F-16XL aircraft to research supersonic laminar flow control. The F-16XL planform is similar to design planforms of high-speed civil transports. The planform makes the aircraft ideally suited for developing technology pertinent to high-speed transports. The supersonic laminar flow control research programs for both aircraft are described. Some general results of the ship-1 program demonstrate that significant laminar flow was obtained using laminar flow control on a highly swept wing at supersonic speeds.
    Keywords: AERODYNAMICS
    Type: SAE PAPER 921994 , ; 14 p.|SAE, Aerotech ''92 Conference; Oct 05, 1992 - Oct 08, 1992; Anaheim, CA; United States
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  • 3
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    Perspectives on hypersonic viscous flow research (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: Issues and advances in current hypersonic flow research perceived to be of interest in theoretical fluid/gas dynamics are reviewed. Particular attention is given to the hypersonic aircraft as waverider, computational methods and theoretical development in the study of viscous interaction, and boundary-layer instability and transition studies. In the present framework the study of viscous hypersonic flow faces transition problems of two kinds which represent the two major areas of current research: the turbulence transition in the high Re range and the transition to the free-molecule limit.
    Keywords: AERODYNAMICS
    Type: In: Annual review of fluid mechanics. Vol. 25 (A94-10885 01-34); p. 455-484.
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  • 4
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    Identification of aerodynamic coefficients using computational neural networks (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Keywords: AERODYNAMICS
    Type: Journal of Guidance, Control, and Dynamics (ISSN 0731-5090); 16; 6; p. 1018-1025.
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  • 5
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    Development of the wake behind a circular cylinder impulsively started into rotatory and rectilinear motion (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The temporal development of a 2D viscous incompressible flow generated by a circular cylinder started impulsively into steady rotatory and rectilinear motion is studied by integration of a velocity/vorticity formulation of the governing equations, using an explicit finite-difference/pseudo-spectral technique and an implementation of the Biot-Savart law. Results are presented for a Reynolds number of 200 (based on the cylinder diameter 2a and the magnitude U of the rectilinear velocity) for several values of the angular/rectilinear speed ratio alpha = omega(a)/U (where omega is the angular speed) up to 3.25. Several aspects of the kinematics and dynamics of the flow not considered earlier are discussed. For higher values of alpha, the results indicate that for Re = 200, vortex shedding does indeed occur for alpha = 3.25. However, consecutive vortices shed by the body can be shed from the same side and be of the same sense, in contrast to the nonrotating case, in which mirror-image vortices of opposite sense are shed alternately on opposite sides of the body. The implications of the results are discussed in relation to the possibility of suppressing vortex shedding by open or closed-loop control of the rotation rate.
    Keywords: AERODYNAMICS
    Type: Journal of Fluid Mechanics (ISSN 0022-1120); p. 449-484.
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  • 6
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    A time-accurate high-resolution TVD scheme for solving the Navier-Stokes equations (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: A total variation diminishing (TVD) scheme has been developed and incorporated into an existing time-accurate high-resolution Navier-Stokes code. The accuracy and the robustness of the resulting solution procedure have been assessed by performing many calculations in four different areas: shock tube flows, regular shock reflection, supersonic boundary layer, and shock boundary layer interactions. These numerical results compare well with corresponding exact solutions or experimental data.
    Keywords: AERODYNAMICS
    Type: Computers & Fluids (ISSN 0045-7930); 22; 5-Apr; p. 517-528.
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  • 7
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    Mitigation of multipath in DGPS ground reference stations (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: Multipath represents one of the most serious threats to accuracy in Differential GPS (DGPS). The theoretical foundations of multipath are presented and several multipath mitigation techniques are reviewed. Special emphasis is placed on signal diffraction methods. Reflections enter peaks and nulls in the distorted antenna pattern in a random manner. This, coupled with the oscillations in relative phase (with respect to the direct signal), results in a multipath error signature which is noise-like. Since it is noise-like, it may be significantly reduced by averaging the code phase measurements against the more stable carrier-phase measurements. Exploitation of this effect to reduce multipath error represents a significant increase in accuracy for real time DGPS. Experiments have been performed which verify these conclusions.
    Keywords: COMMUNICATIONS AND RADAR
    Type: In: Institute of Navigation, National Technical Meeting, San Diego, CA, Jan. 27-29, 1992, Proceedings (A93-21176 06-04); p. 105-114.
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  • 8
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    Viscous and inviscid instabilities of a trailing vortex (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: The linear stability of the trailing line vortex model of Batchelor (1964) is studied using a spectral collocation and matrix eigenvalue method. The entire unstable region in the swirl/axial wavenumber parameter space is mapped out for various azimuthal wavenumbers for both the inviscid and viscous stability problem. The results of the study provide a direct numerical validation of the large-azimuthal-wavenumber asymptotic analysis of Leibovich and Stewartson (1983). It is shown that accurate results are obtained up to azimuthal wavenumbers of 10,000 and greater, and the agreement with the asymptotic theory is excellent.
    Keywords: AERODYNAMICS
    Type: Journal of Fluid Mechanics (ISSN 0022-1120); p. 91-114.
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  • 9
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    Exploratory study of wall pressure fluctuations in a Mach 5, sharp fin-induced turbulent interaction (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Keywords: AERODYNAMICS
    Type: AIAA Journal (ISSN 0001-1452); 30; 9 Se; 2188-219
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  • 10
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    Data compression of stereopairs (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: This paper addresses the problem of stereopair data compression. Two fundamentally different techniques for compressing stereopairs are discussed. The first technique, called disparity-compensated transform-domain predictive coding, attempts to minimize the mean-square error between the original stereopair and the compressed stereopair. The second technique, called mixed-resolution coding, is a psychophysically justified technique that exploits known facts about human stereovision to code stereopairs in a subjectively acceptable manner. A method for assessing the quality of compressed stereopairs is also presented. It involves measuring the ability of an observer to perceive depth in coded stereopairs. It is found that observers generally perceived objects to be further away in compressed stereopairs than they did in originals. Finally, the problem of coding stereopairs is considered from a rate-distortion perspective. It is proved that the rate distortion limit for coding stereopairs cannot in general be achieved by a coder that first codes and decodes the right picture sequence independently of the left picture sequence, and then codes and decodes the left picture sequence given the decoded right picture sequence.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Communications (ISSN 0090-6778); 40; 4 Ap; 684-696
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