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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Analog Delta-Back-Propagation Neural-Network Circuitry

Changes in synapse weights due to circuit drifts suppressed. Proposed fully parallel analog version of electronic neural-network processor based on delta-back-propagation algorithm. Processor able to "learn" when provided with suitable combinations of inputs and enforced outputs. Includes programmable resistive memory elements (corresponding to synapses), conductances (synapse weights) adjusted during learning. Buffer amplifiers, summing circuits, and sample-and-hold circuits arranged in layers of electronic neurons in accordance with delta-back-propagation algorithm.

Eberhart, Silvio↗
NASA NTRS1990

Controlled vortical flow on delta wings through unsteady leading edge blowing

The vortical flow over a delta wing contributes an important part of the lift - the so called nonlinear lift. Controlling this vortical flow with its favorable influence would enhance aircraft maneuverability at high angle of attack. Several previous studies have shown that control of the vortical flow field is possible through the use of blowing jets. The present experimental research studies vortical flow control by applying a new blowing scheme to the rounded leading edge of a delta wing; this blowing scheme is called Tangential Leading Edge Blowing (TLEB). Vortical flow response both to steady blowing and to unsteady blowing is investigated. It is found that TLEB can redevelop stable, strong vortices even in the post-stall angle of attack regime. Analysis of the steady data shows that the effect of leading edge blowing can be interpreted as an effective change in angle of attack. The examination of the fundamental time scales for vortical flow re-organization after the application of blowing for different initial states of the flow field is studied. Different time scales for flow re-organization are shown to depend upon the effective angle of attack. A faster response time can be achieved at angles of attack beyond stall by a suitable choice of the initial blowing momentum strength. Consequently, TLEB shows the potential of controlling the vortical flow over a wide range of angles of attack; i.e., in both for pre-stall and post-stall conditions.

Lee, K. T.↗
NASA NTRS1988

Leading edge vortex dynamics on a pitching delta wing

The leading edge flow structure was investigated on a 70 deg flat plate delta wing which was pitched about its 1/2 chord position, to increase understanding of the high angle of attack aerodynamics on an unsteady delta wing. The wing was sinusoidally pitched at reduced frequencies ranging from k being identical with 2pi fc/u = 0.05 to 0.30 at chord Reynolds numbers between 90,000 and 350,000, for angle of attack ranges of alpha = 29 to 39 deg and alpha = 0 to 45 deg. The wing was also impulsively pitched at an approximate rate of 0.7 rad/s. During these dynamic motions, visualization of the leading edge vorticies was obtained by entraining titanium tetrachloride into the flow at the model apex. The location of vortex breakdown was recorded using 16mm high speed motion picture photography. When the wing was sinusoidally pitched, a hysteresis was observed in the location of breakdown position. This hysteresis increased with reduced frequency. The velocity of breakdown propagation along the wing, and the phase lag between model motion and breakdown location were also determined. When the wing was impulsively pitched, several convective times were required for the vortex flow to reach a steady state. Detailed information was also obtained on the oscillation of breakdown position in both static and dynamic cases.

Lemay, Scott P.↗
NASA NTRS1990

Conical Euler solution for a highly-swept delta wing undergoing wing-rock motion

Modifications to an unsteady conical Euler code for the free-to-roll analysis of highly-swept delta wings are described. The modifications involve the addition of the rolling rigid-body equation of motion for its simultaneous time-integration with the governing flow equations. The flow solver utilized in the Euler code includes a multistage Runge-Kutta time-stepping scheme which uses a finite-volume spatial discretization on an unstructured mesh made up of triangles. Steady and unsteady results are presented for a 75 deg swept delta wing at a freestream Mach number of 1.2 and an angle of attack of 30 deg. The unsteady results consist of forced harmonic and free-to-roll calculations. The free-to-roll case exhibits a wing rock response produced by unsteady aerodynamics consistent with the aerodynamics of the forced harmonic results. Similarities are shown with a wing-rock time history from a low-speed wind tunnel test.

Lee, Elizabeth M.↗
NASA NTRS1990

Simulation and analysis of a delta planform with multiple jets in ground effect

The three-dimensional, thin-layer Navier-Stokes equations on a body-conforming, overset, multiple-grid topology are solved in the present numerical simulations of the flow around a 60-deg delta-planform wing equipped with two thrust-reverser jets in ground-effect conditions. Such experimentally flow-visualized features as jet-deformation and ground vertex-formation are captured by these simulations; the loss of wing-borne lift due to the 'suck-down' phenomenon, which arises as the delta planform flies at low speeds in close proximity to the ground, is also well simulated.

Chawla, K.↗
NASA NTRS1990

An embedded grid formulation applied to a delta wing

Applications using a three-dimensional embedded grid scheme are made to high angle of attack viscous flow over two bodies: a slender cone using the conical approximation and a 75 deg swept delta wing. The embedded grids are used principally to improve the numerical resolution of the separated vortical flow above the body. Detailed comparisons are made with experimental measurements of the velocity field over the delta wing. The prediction of the maximum steamwise velocity is improved using two levels of embedded grid refinement but is still less than the experimental measurements available from a laser velocimeter.

Krist, Sherrie L.↗
NASA NTRS1989

Calculating climate attractor dimension from delta O-18 records by the Grassberger-Procaccia algorithm

The Grassberger-Procaccia method of calculating dimension from a time series is applied to 14 late Pleistocene delta O-18 records. A step-by-step sequence leading from data to the Grassberger-Procaccia dimension is outlined, and the problems encountered when dealing with observed (as opposed to theoretical) data are discussed; for the climatic proxy data these problems include situations where the time series is not very long, is noisy and/or smoothed, and is not sampled at a constant time interval. The delta O-18 records to be used are described, and the results are presented and compared with previously published dimension calculations. New dimension interpretations are assessed, and an example using a synthetic time series that illustrates the possible error due to inconsistencies in the time scale is analyzed.

Maasch, Kirk A.↗
NASA NTRS1990

A computational study of incipient leading-edge separation on a 65-deg delta wing at M = 1.60

A computational study on a 65-deg delta wing at a freestream Mach number of 1.60 has been conducted by obtaining conical Reynolds-averaged Navier-Stokes solutions on a parametric series of geometries which varied in leading-edge radius and/or circular-arc camber. The computational results showed that increasing leading-edge radius or camber can delay the onset of leading-edge separation on the leeside of a delta wing at a specific angle of attack. Reynolds number was varied from 1 x 10 to the 6th to 5 x 10 to the 6th for a turbulent boundary-layer and was shown to have a minor effect on the effectiveness of leading-edge radius and/or camber in delaying the onset of leading-edge separation. Both laminar and turbulent boundary-layer models were investigated at a Reynolds number of 1 x 10 to the 6th, and the predicted flow pattern was found to change from attached flow for the turbulent boundary-layer model to separated flow for the laminar boundary-layer model. Based upon these results, three wind-tunnel models have been designed to be tested in the Langley Unitary Plan Wind Tunnel.

Mcmillin, S. Naomi↗
NASA NTRS1989

Satellite mapping of Nile Delta coastal changes

Multitemporal, multispectral scanner (MSS) landsat data have been used to monitor erosion and sedimentation along the Rosetta Promontory of the Nile Delta. These processes have accelerated significantly since the completion of the Aswan High Dam in 1964. Digital differencing of four MSS data sets, using standard algorithms, show that changes observed over a single year period generally occur as strings of single mixed pixels along the coast. Therefore, these can only be used qualitatively to indicate areas where changes occur. Areas of change recorded over a multi-year period are generally larger and thus identified by clusters of pixels; this reduces errors introduced by mixed pixels. Satellites provide a synoptic perspective utilizing data acquired at frequent time intervals. This permits multiple year monitoring of delta evolution on a regional scale.

Blodget, H. W.↗
NASA NTRS1989

An embedded grid formulation applied to delta wings

An embedded grid algorithm for the Euler and/or Navier-Stokes equations is developed and applied to delta wings at high angles of attack in low speed flow. The Navier-Stokes code is an implicit, finite volume algorithm, using flux difference splitting for the convective and pressure terms and central differencing for the viscous and heat transfer terms. Calculations are compared with detailed experimental results over an angle of attack range up to and beyond the maximum lift coefficient, corresponding to vortex breakdown at the trailing edge, for a delta wing nominally of unit aspect ratio. The results indicate that the overall flowfield, including surface pressures, surface streamlines, and vortex trajectories, can be simulated accurately with the global grid version of the present algorithm. However, comparison of computed velocities and vorticity with experimentally measured off-body values at an angle of attack of 20.5 deg indicates the core region is substantially more diffuse in the computations than that measured with either a five-hole probe or a laser velocimeter. Embedded grids, used to improve the numerical discretization in the core region, are formulated within the framework of the implicit, upwind-biased multi-grid algorithm. Structured levels of local nested refinements are made. Three-dimensional results for both Euler and Navier-Stokes calculations are shown, with up to 3 levels of embedded refinement. The embedding procedure was effective in eliminating a crossflow secondary separation produced in the Euler solutions on coarse grids.

Thomas, James L.↗
NASA NTRS1990

Detailed flow-field measurements over a 75 deg swept delta wing

Results from an experimental investigation documenting the flowfield over a 75 deg swept delta wing at an angle-of-attack of 20.5 deg are presented. Results obtained include surface flow visualization, off-body flow visualization, and detailed flowfield surveys for various Reynolds numbers. Flowfield surveys at Reynolds numbers of 0.5, 1.0, and 1.5 million based on the root chord were conducted with both a Pitot pressure probe and a 5-hole pressure probe; and 3-component laser velocimeter surveys were conducted at a Reynolds number of 1.0 million. The Pitot pressure surveys were obtained at 5 chordwise stations, the 5-hole probe surveys were obtained at 3 chordwise stations and the laser velocimeter surveys were obtained at one station. The results confirm the classical roll up of the flow into a pair of primary vortices over the delta wing. The velocity measurements indicate that Reynolds number has little effect on the global structure of the flowfield for the Reynolds number range investigated. Measurements of the non-dimensional axial velocity in the core of the vortex indicate a jet like flow with values greater than twice freestream. Comparisons between velocity measurements from the 5-hole pressure probe and the laser velocimeter indicate that the pressure probe does a reasonable job of measuring the flowfield quantities where the velocity gradients in the flowfield are low.

Kjelgaard, Scott O.↗
NASA NTRS1991

Delta VLBI data performance in the Galileo spacecraft Earth flyby of December 1990

The performance of the delta very long baseline interferometry (Delta VLBI) data type during the recent Earth flyby of the Galileo spacecraft is presented. First, the data are given significance by showing why the mission design requires an accurate flyby. Then, the data's performance is analyzed, including its operational features, as well as its accuracy. Finally, the effectiveness of these data in improving the accuracy of flyby is shown. Comparisons are made between the expected and actual performances of the data. The processing time, reliability, and accuracy of these data were very good, and a solid gain was made in the encounter accuracy.

Gray, D. L.↗
NASA NTRS1991

Conical Euler methodology for unsteady vortical flows about rolling delta wings

A conical Euler methodology was developed to study unsteady vortex-dominated flows about rolling highly swept delta wings undergoing either forced or free-to-roll motions. The flow solver of the code involves a multistage Runge-Kutta time-stepping scheme which uses a finite-volume spatial discretization of the Euler equations on an unstructured grid of triangles. Results are presented for a 75-deg swept sharp-leading-edge delta wing at a freestream Mach number of 1.2 and at alpha = 10, 20, and 30 deg. At the 10 and 20 deg, forced harmonic analyses indicate that the rolling moment coefficients provide a positive damping which is verified by free-to-roll calculations. In contrast, at 30 deg, a forced harmonic analysis indicates that the rolling moment coefficient provides a negative damping at the small roll amplitudes. A free-to-roll calculation for this case produces an initially divergent response, but as the amplitude of motion grows with time, the response transitions to a wing-rock type of limit cycle oscillation.

Lee, Elizabeth M.↗
NASA NTRS1991

Unsteady supersonic flow around delta wings with symmetric and asymmetric flaps oscillation

A parametric study is presented to investigate the effect of reduced frequency of the leading-edge flaps on the locally-conical, unsteady, supersonic flow around a delta wing. This study covers symmetric and antisymmetric forced oscillation of the leading-edge flaps. The effects of the freestream Mach number and angle of attack are also presented. The problem is solved using time-accurate integration of the unsteady, compressible, thin-layer Navier-Stokes equations and the unsteady, linearized, Navier-displacement equations. The delta wing is of aspect ratio of 1.5 and its leading-edge flaps are hinged at 65 percent of the local-half span length. The reduced frequency is varied between 2 pi and pi/2. Two supersonic flow conditions have been investigated; the first is for a freestream Mach number of 2.4 and an angle of attack of 19 deg and the second is for a freestream Mach number of 1.5 and an angle attack of 15 deg.

Kandil, Osama A.↗
NASA NTRS1991

Conical Euler simulation of wing rock for a delta wing planform

Unsteady, vortex-dominated flowfields are presently studied by using the conical Euler equations as an efficient first step toward investigation of the full three-dimensional problem, under the assumption that the supersonic flow about a delta wing is conical and therefore allows the three-dimensional problem to be reduced to a two-dimensional one. Attention is given to the case of a delta wing undergoing wing-rock motion. The code developed has also been modified to allow treatment of the 'free-to-roll' case.

Lee, Elizabeth M.↗
NASA NTRS1991

Optically induced absorption modulation in a periodically delta-doped InGaAs/GaAs multiple quantum well structure

Strong optically induced absorption modulation has been achieved in a periodically delta-doped InGaAs/GaAs multiple quantum well structure. The use of delta-doping has enabled efficient modulation in short-period structures using a low-power semiconductor laser. With an excitation intensity of 100 mW/sq cm an absolute quantum well absorption change of more than 9000 cm has been measured corresponding to a differential absorption change as high as 58 percent at the excitonic resonance.

Larsson, A.↗
NASA NTRS1991

Four new Delta Scuti stars

Four new Delta Scuti stars are reported. Power, modified into amplitude, spectra, and light curves are used to determine periodicities. A complete frequency analysis is not performed due to the lack of a sufficient time base in the data. These new variables help verify the many predictions that Delta Scuti stars probably exist in prolific numbers as small amplitude variables. Two of these stars, HR 4344 and HD 107513, are possibly Am stars. If so, they are among the minority of variable stars which are also Am stars.

Schutt, R. L.↗