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At least 199 records · Page 11

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NASA NTRS1978

On the optimization of discrete structures with aeroelastic constraints

The paper deals with the problem of dynamic structural optimization where constraints relating to flutter of a wing (or other dynamic aeroelastic performance) are imposed along with conditions of a more conventional nature such as those relating to stress under load, deflection, minimum dimensions of structural elements, etc. The discussion is limited to a flutter problem for a linear system with a finite number of degrees of freedom and a single constraint involving aeroelastic stability, and the structure motion is assumed to be a simple harmonic time function. Three search schemes are applied to the minimum-weight redesign of a particular wing: the first scheme relies on the method of feasible directions, while the other two are derived from necessary conditions for a local optimum so that they can be referred to as optimality-criteria schemes. The results suggest that a heuristic redesign algorithm involving an optimality criterion may be best suited for treating multiple constraints with large numbers of design variables.

Mcintosh, S. C., Jr.↗

NASA NTRS1979

Investigation and characterization of constraint effects on flaw growth during fatigue loading of composite materials

An investigative program is presented in an attempt to add to the current understanding of constraint effects on the response of composite materials under cyclic loading. The objectives were: (1) to use existing data and to develop additional data in order to establish an understanding and quantitative description of flaw growth in unidirectional lamina under cyclic loading at different load direction to fiber direction angles; (2) to establish a similar understanding and description of flaw growth in lamina which are embedded in laminates between other unflawed lamina; (3) to determine the nature of the influence of constraint on flaw growth by quantitatively comparing the results of the tests; and (4) to develop a model and philosophy of constraints effects based on our investigative results.

Stinchcomb, W. W.↗

NASA NTRS1984

The effect of parking orbit constraints on the optimization of ballistic planetary trajectories

The optimization of ballistic planetary trajectories is developed which includes constraints on departure parking orbit inclination and node. This problem is formulated to result in a minimum total Delta V where the entire constrained injection Delta V is included in the optimization. An additional Delta V is also defined to allow for possible optimization of parking orbit inclination when the launch vehicle orbit capability varies as a function of parking orbit inclination. The optimization problem is formulated using primer vector theory to derive partial derivatives of total Delta V with respect to possible free parameters. Minimization of total Delta V is accomplished using a quasi-Newton gradient search routine. The analysis is applied to an Eros rendezvous mission whose transfer trajectories are characterized by high values of launch asymptote declination during particular launch opportunities. Comparisons in performance are made between trajectories where parking orbit constraints are included in the optimization and trajectories where the constraints are not included.

Sauer, C. G., Jr.↗

NASA NTRS1985

Geomagnetic main field modeling using magnetohydrodynamic constraints

The influence of physical constraints are investigated which may be approximately satisfied by the Earth's liquid core on models of the geomagnetic main field and its secular variation. A previous report describes the methodology used to incorporate nonlinear equations of constraint into the main field model. The application of that methodology to the GSFC 12/83 field model to test the frozen-flux hypothesis and the usefulness of incorporating magnetohydrodynamic constraints for obtaining improved geomagnetic field models is described.

Estes, R. H.↗

NASA NTRS1986

An analytical treatment of discretely varying constraints and inertial properties in multi-body dynamics

A computerized procedure is presented for calculating instantaneous velocity changes due to discrete imposition of constraints, impact or discrete inertial changes. Such discrete changes are impulsive in nature, involve wave propagation and therefore usually require detailed modeling for accurate prediction. It is demonstrated that when modeling is coarse, physically unrealistic results can be produced. The present procedure, which concentrates on finite element analysis, reduces the need for detailed modeling by assuming that only those velocities in the neighborhood of grid points involved in the constraint or those rigidly connected to grid points involved in the constraint need be considered in calculating velocity changes. The procedure utilizes Newton's second law and as such conserves momentum where valid. Sample problems are provided.

Housner, J. M.↗

NASA NTRS1986

Geophysical constraints on the lunar interior

Geophysical constraints relating to the structure, composition, and thermal state of the moon are reviewed in an attempt to evaluate lunar origin models. Particular attention is given to constraints on the bulk composition of the crust and mantle, on the existence and size of a possible metallic core, and on the depth of initial melting and differentiation. It is noted that models such as the binary accretion hypothesis would be capable of producing a bulk moon in accordance with available geophysical constraints.

Hood, L. L.↗

NASA NTRS1987

Order reduction in linear state estimation under performance constraints

The design and analysis of minimal-order state estimators for possibly time-varying linear systems, under constraints on the maximal allowable mean-square error, are considered. A global lower bound on the optimal error is derived, along with a lower bound on the minimal estimator order, needed for meeting the performance constraint. The ideal reduced-order estimator which satisfies the lower bound is derived, along with conditions for its realizability. When the ideal estimator is not realizable, its structure forms a suboptimal estimator, which maintains, in some sense, a local optimality property and is called the pseudoideal estimator. The mean-square error of the pseudoideal estimator defines upper bounds on the optimal error and on the estimator order needed for meeting the performance constraint. The lower and the upper bounds on the order define a reduced search set for the design problem. When the distance between the ideal and the pseudoideal estimators is sufficiently small in a certain numerical sense, the pseudoideal estimator may be considered optimal for practical purposes.

Baram, Yoram↗

NASA NTRS1988

Minimum weight design of rotorcraft blades with multiple frequency and stress constraints

A method is developed for the minimum weight design of helicopter rotor blades having multiple coupled flap-lag natural frequency constraints, in addition to a constraint on the minimum blade autorotation value that will ensure sufficient rotary inertia for autorotation in the event of engine failure. Design variables encompass blade taper ratio, box beam dimensions, and the magnitudes of the nonstructural weights. Optimum designs have been obtained for both rectangular and tapered blades; attention is given to the optimum blade weight effect and the blade mass and stiffness distribution effects of adding constraints on higher frequencies and stresses.

Chattopadhyay, Aditi↗

NASA NTRS1988

Cluster-cluster correlations and constraints on the correlation hierarchy

The hypothesis that galaxies cluster around clusters at least as strongly as they cluster around galaxies imposes constraints on the hierarchy of correlation amplitudes in hierachical clustering models. The distributions which saturate these constraints are the Rayleigh-Levy random walk fractals proposed by Mandelbrot; for these fractal distributions cluster-cluster correlations are all identically equal to galaxy-galaxy correlations. If correlation amplitudes exceed the constraints, as is observed, then cluster-cluster correlations must exceed galaxy-galaxy correlations, as is observed.

Hamilton, A. J. S.↗

NASA NTRS1988

A new constraint on Saturn's zonal gravity harmonics from Voyager observations of an eccentric ringlet

The results obtained by Porco et al. (1984) from an analysis of the kinematics of an eccentric ringlet in Saturn's inner ring are used to add an additional constraint to the gravity solution of Null et al. (1981) and thus to derive the first experimental estimate of the Saturn's J6 gravitational field. On the assumption that the eccentricity of this ringlet is forced by the satellite Titan, the location of the Titan apsidal resonance is determined to an accuracy of + or - 13 km, which in turn provides a strong constraint on Saturn's zonal gravity harmonics. Combining this constraint with the results of Null et all. and adopting a priori values for J8, J10, and J12 based on interior models, new estimates are obtained for the first three zonal gravity coefficients J2, J4, and J6.

Nicholson, Philip D.↗

NASA NTRS1988

Constraints imposed by cosmic evolution towards the development of life

The probability of terrestrial-type life emerging in any other place of the universe will depend on the constraints imposed by cosmic evolution on that particular place. A systematic examination of cosmic constraints, which must have provided the necessary and sufficient conditions for the origin and evolution of life on earth, shows that they are concerned with the nature of the central star, the planetary system, and the specific life-bearing planet, as well as with the chemical and biological evolution processes involved. These constraints or universal requirements for life are briefly described.

Oro, J.↗

NASA NTRS1989

Evidence for an extragalactic component of the far-ultraviolet background and constraints on galaxy evolution for z between 0.1 and 0.6

A sounding rocket experiment that measured the power spectrum of small-scale fluctuations in the far-UV background is described. Evidence is presented that these fluctuations are the integrated light from distant galaxies. If this result is used as an upper limit to the integrated light from galaxies, it provides a constraint on the mean UV luminosity density in the redshift range between 0.1 and 0.6 of L(gal) less than 7 x 10 to the 7th solar luminosities/Mpc. When compared to estimates of the present UV luminosity density, this places a strong constraint on the far-UV evolution of galaxies in the last one-third of a Hubble time. This constraint can be interpreted as a limit on the average past star formation rate relative to the present if dust obscuration was not significantly greater in the past. An upper limit to the average star formation rate is derived.

Martin, Christopher↗

NASA NTRS1989

Astrophysical and cosmological constraints to neutrino properties

The astrophysical and cosmological constraints on neutrino properties (masses, lifetimes, numbers of flavors, etc.) are reviewed. The freeze out of neutrinos in the early Universe are discussed and then the cosmological limits on masses for stable neutrinos are derived. The freeze out argument coupled with observational limits is then used to constrain decaying neutrinos as well. The limits to neutrino properties which follow from SN1987A are then reviewed. The constraint from the big bang nucleosynthesis on the number of neutrino flavors is also considered. Astrophysical constraints on neutrino-mixing as well as future observations of relevance to neutrino physics are briefly discussed.

Kolb, Edward W.↗

NASA NTRS1990

Resource allocation using constraint propagation

The concept of constraint propagation was discussed. Performance increases are possible with careful application of these constraint mechanisms. The degree of performance increase is related to the interdependence of the different activities resource usage. Although this method of applying constraints to activities and resources is often beneficial, it is obvious that this is no panacea cure for the computational woes that are experienced by dynamic resource allocation and scheduling problems. A combined effort for execution optimization in all areas of the system during development and the selection of the appropriate development environment is still the best method of producing an efficient system.

Rogers, John S.↗

NASA NTRS1989

Local parallel models for integration of stereo matching constraints and intrinsic image combination

Parallel relaxation computations such as those of connectionist networks offer a useful model for constraint integration and intrinsic image combination in developing a general-purpose stereo matching algorithm. This paper describes such a stereo algorithm that incorporates hierarchical, surface-structure, and edge-appearance constraints that are redefined and integrated at the level of individual candidate matches. The algorithm produces a high percentage of correct decisions on a wide variety of stereo pairs. Its few errors arise when the correlation measures defined by the constraints are either weakened or ambiguous, as in the case of periodic patterns in the images. Two additional mechanisms are discussed for overcoming the remaining errors.

Stewart, Charles V.↗

NASA NTRS1990

Stability constraints for line scanning cameras in orbit

Requirements, parameters, and constraints influencing the design of a line scanner camera system for imaging from orbit are described. The driving requirements are the area of the site to be imaged in each pass and the resolution as determined by the size of a pixel projected on the planet surface. Allowable jitter and distortion, required SNR, maximum data rate, and maximum variation of the emission angle during imaging are among the constraints. Simplified equations incorporating these requirements, constraints, and parameters are developed and presented.

Bernard, Douglas E.↗

NASA NTRS1989

Observational constraints on interstellar dust models

No single model has been able to account for all of the observed spectroscopic properties of interstellar or circumstellar dust. The reason for this is that, despite the agreement that the grains are composed of silicaceous/metal oxide and carbonaceous material, there is strong disagreement as to their exact structure and composition. This led Draine and Lee (1984) to use interstellar extinction data to define an interstellar graphitic material; new observational findings have made even that identification uncertain. But the great advantage of their approach is that they used observations at all of the wavelengths available to define the material. Here, the authors attempt a variation of that approach. They examine recent UV and IR data and attempt to put constraints on the possible types of interstellar grain composition, and to connect these constraints with grain models. A summary of some of the important constraints imposed by the observations is given.

Hecht, James H.↗

NASA NTRS1991

Two Point Exponential Approximation Method for structural optimization of problems with frequency constraints

The point exponential approximation method was introduced by Fadel et al. (Fadel, 1990), and tested on structural optimization problems with stress and displacement constraints. The reports in earlier papers were promising, and the method, which consists of correcting Taylor series approximations using previous design history, is tested in this paper on optimization problems with frequency constraints. The aim of the research is to verify the robustness and speed of convergence of the two point exponential approximation method when highly non-linear constraints are used.

Fadel, G. M.↗