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At least 919 records · Page 51

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NASA NTRS1959

Experimental Verification of Boundary-Layer Corrections in Hypersonic Nozzles

The problem of accurately predicting boundary-layer growth for supersonic nozzle design assumes increased importance as the design Mach Number of the nozzles is pushed into the hypersonic regime. Several methods of calculating this growth have been advanced which vary in ease of computation and accuracy of results. Results of the calibrations performed on two hypersonic helium nozzles recently put into operation in the 11-in. Hypersonic Tunnel Section at the NASA Langley Research Center indicate that the method employed for boundary-layer calculations in these nozzles is adequate. The flow in the test region of both nozzles has been found to be of good quality and the Mach Numbers obtained agree very satisfactorily with the desired design Mach Number.

Boundary Layer Equations↗

NASA NTRS1991

A rapid method of estimating the collision frequencies between the earth and the earth-crossing bodies

The present study examines a very fast method of calculating the collision frequency between two low-eccentricity orbiting bodies for evaluating the evolution of earth-orbiting objects such as space debris. The results are very accurate and the required computer time is negligible. The method is now applied without modification to calculate the collision frequencies for moderately and highly eccentric orbits.

Su, Shin-Yi↗

NASA NTRS1974

A preliminary theoretical line-blanketed model solar photosphere

In the theoretical approach to model-atmosphere construction, all opacities are computed theoretically and the temperature-pressure structure is determined by conservation of energy. Until recently, this has not been a very useful method for later type stars, because the line opacity was both poorly known and difficult to calculate. However, methods have now been developed that are capable of representing the line opacity well enough for construction of realistic models. A preliminary theoretical solar model is presented that produces closer agreement with observation than has been heretofore possible. The qualitative advantages and shortcomings of this model are discussued and projected improvements are outlined.

Kurucz, R. L.↗

NASA NTRS1977

Determination of pole sensitivities by Danilevskii's method

In control theory, a synonymous term for pole sensitivity is eigenvalue sensitivity. Existing methods of calculating eigenvalues are cumbersome, and cannot be trusted for systems roughly greater than tenth order. The method proposed in the present paper is applicable to high-order system. (It has been routinely used to generate eigenvalue sensitivities for systems up to 26th order, using a UNIVAC 1106.) Danilevskii's method is shown to be suitable for performing the necessary evaluations. The result is a rational function that can be used to evaluate the sensitivities for all distinct poles.

Nail, J. B.↗

NASA NTRS1982

RAXJET: A computer program for predicting transonic, axisymmetric flow over nozzle afterbodies with supersonic jet exhausts

A viscous-inviscid interaction method to calculate the subsonic and transonic flow over nozzle afterbodies with supersonic jet exhausts was developed. The method iteratively combines a relaxation solution of the full potential equation for the inviscid external flow, a shock capturing-shock fitting inviscid jet solution, an integral boundary layer solution, a control volume method for treating separated flows, and an overlaid mixing layer solution. A computer program called RAXJET which incorporates the method, illustrates the predictive capabilities of the method by comparison with experimental data is described, a user's guide to the computer program is provided. The method accurately predicts afterbody pressures, drag, and flow field properties for attached and separated flows for which no shock induced separation occurs.

Wilmoth, R. G.↗

NASA NTRS1985

Evaluation of fault-tolerant system performance by approximate techniques

An approximate method for calculating the statistics of the performance of a fault-tolerant system is developed. An approximate method is necessary because the statistical model of the system behavior is large-scale and the time horizon of interest encompasses many cycles of the Redundancy Management logic. In the development, a compact representation of the necessary information called the v-transform is introduced and discussed. Based upon this representation, an approximation that leads to a very efficient computational procedure is suggested and numerically analyzed. A very brief discussion of other related work is also presented.

Walker, B. K.↗

NASA NTRS1992

Computational techniques in tribology and material science at the atomic level

Computations in tribology and material science at the atomic level present considerable difficulties. Computational techniques ranging from first-principles to semi-empirical and their limitations are discussed. Example calculations of metallic surface energies using semi-empirical techniques are presented. Finally, application of the methods to calculation of adhesion and friction are presented.

Ferrante, J.↗

NASA NTRS1993

Venus' center of mass - center of figure displacement and implications

Earth, Moon, Mars, and Venus all have centers of mass (C.M.) that are displaced from their centers of figure (C.F.) by amounts which range from 340 meters (Venus) to 2.5 km (Mars). These offsets have all been calculated from the first degree terms in spherical harmonic expansions of topography. We describe an alternate method for calculating C.M. - C.F. offsets directly from a global topographic data set and apply it to Venus. Using Magellan altimetry, we find that Venus' C.F. is displaced approximately 280 meters from its C.M. in the direction of Western Aphrodite Terra (4.4 deg S, 135.8 deg E). We investigate several simple models for this offset and find that it is most consistent with thickened crust in Ovda and Thetis Regiones (which constitute most of W. Aphrodite). The location of the C.F. offset also places constraints on the degree of crustal thickening in Western Ishtar Terra and/or this highland's mode of origin. We favor a model in which offset due to thick crust in Western Ishtar Terra is balanced by an opposing offset due to cold, downwelling mantle material beneath the highland.

Bindschadler, D. L.↗

NASA NTRS1998

Recent Advances in the Method of Forces: Integrated Force Method of Structural Analysis

Stress that can be induced in an elastic continuum can be determined directly through the simultaneous application of the equilibrium equations and the compatibility conditions. In the literature, this direct stress formulation is referred to as the integrated force method. This method, which uses forces as the primary unknowns, complements the popular equilibrium-based stiffness method, which considers displacements as the unknowns. The integrated force method produces accurate stress, displacement, and frequency results even for modest finite element models. This version of the force method should be developed as an alternative to the stiffness method because the latter method, which has been researched for the past several decades, may have entered its developmental plateau. Stress plays a primary role in the development of aerospace and other products, and its analysis is difficult. Therefore, it is advisable to use both methods to calculate stress and eliminate errors through comparison. This paper examines the role of the integrated force method in analysis, animation and design.

Patnaik, Surya N.↗

NASA NTRS1998

Recent Advances in the Method of Forces: Integrated Force Method of Structural Analysis

Patnaik, Surya N.↗

NASA NTRS1973

Calculation of separated flows at subsonic and transonic speeds

A boundary-layer integral approach is combined with a finite-difference relaxation method to calculate viscous interactions between separated flows at subsonic and transonic velocities. Results are obtained for separated laminar flows on circular-arc airfoils at zero angle of attack and are compared with data of Collins (1972). Inviscid and viscous flows are covered.

Klineberg, J. M.↗

NASA NTRS2008

Channel Temperature Determination for AlGaN/GaN HEMTs on SiC and Sapphire

Numerical simulation results (with emphasis on channel temperature) for a single gate AlGaN/GaN High Electron Mobility Transistor (HEMT) with either a sapphire or SiC substrate are presented. The static I-V characteristics, with concomitant channel temperatures (T(sub ch)) are calculated using the software package ATLAS, from Silvaco, Inc. An in-depth study of analytical (and previous numerical) methods for the determination of T(sub ch) in both single and multiple gate devices is also included. We develop a method for calculating T(sub ch) for the single gate device with the temperature dependence of the thermal conductivity of all material layers included. We also present a new method for determining the temperature on each gate in a multi-gate array. These models are compared with experimental results, and show good agreement. We demonstrate that one may obtain the channel temperature within an accuracy of +/-10 C in some cases. Comparisons between different approaches are given to show the limits, sensitivities, and needed approximations, for reasonable agreement with measurements.

Freeman, Jon C.↗

NASA NTRS1974

Addendum to 'Autoionization states in He and H/-/ by the stabilization method'

The positions of the resonances calculated by the stabilization method can be improved by inclusion of the shift as given by Hazi and Fels. It is also indicated that the phase shifts calculated by the Harris method at incident energies corresponding to the stabilized roots give the positions and the widths of the resonances reasonably well.

Bhatia, A. K.↗

NASA NTRS1961

Application of Monte Carlo Technique for Determining Maneuvering Loads from Statistical Information on Airplane Motions

Results of a statistical analysis of horizontal-tail loads on a fighter airplane are presented. The data were obtained from a number of operational training missions with flight at altitudes up to about 50,000 feet and at Mach numbers up to 1.22. The analysis was performed to determine the feasibility of calculating horizontal-tail load from data on the flight conditions and airplane motions. In the analysis the calculated loads are compared with the measured loads for the different types of missions performed. The loads were calculated by two methods: a direct approach and a Monte Carlo technique. The procedures used and some of the problems associated with the data analysis are discussed. frequencies of occurrence of tail loads of given magnitudes are derived from statistical information on the flight quantities. In the direct method, a time history of tail load is calculated from time-history measurements of the flight quantities. The Monte Carlo method could be useful for extending loads information for design of prospective airplanes . For the Monte Carlo method, the The results indicate that the accuracy of loads, regardless of the method used for calculation, is largely dependent on the knowledge of the pertinent airplane aerodynamic characteristics and center-of-gravity location. In addition, reliable Monte Carlo results require an adequate sample of statistical data and a knowledge of the more important statistical dependencies between the various flight conditions and airplane motions.

Hamer, Harold A.↗

NASA NTRS2018

Mars Optimal Aerobrake Maneuver Estimation

Mars science satellites often perform orbit changes to obtain different measurements, ground tracks or relay operations. Large reductions in semi-major axis and eccentricity can be done efficiently using the atmospheric drag, a.k.a aerobrake. Aerobraking is one of the most challenging planetary orbit maneuvers in terms of planning and operations. The most important consideration for aerobraking is maintaining the spacecraft's periapsis within an allocated atmospheric density corridor, which is accomplished by raising or lowering periapsis through one or a series of very small and short maneuvers. These maneuvers must be performed as efficiently as possible due to propellant constraints. Work herein details a fast and accurate method to calculate the required impulsive velocity changes in the orbit to guarantee that the spacecraft remains in a prescribed density corridor. The method makes use of the orbit's state transition matrix to map the solution space around the reference orbit. It evaluates the most efficient maneuver epochs to target a given periapsis change with a linear optimal control for single or multiple maneuvers. A fast calculation of the maneuver allows for a more comprehensive evaluation of the trade space, and the selected maneuver may be re-targeted later with a higher- fidelity model. Comparisons against fully propagated models and direct method optimizations demonstrate the new method's performance.

Sarli, Bruno V.↗

NASA NTRS1966

Use of the method of dynamic rigidities for calculating the frequencies of natural vibrations of built-up shells

Frequencies of natural vibrations in built-up shells determined by dynamic rigidities method

RIGIDITY↗

NASA NTRS1992

Comparison of two-dimensional and three-dimensional droplet trajectory calculations in the vicinity of finite wings

Computational predictions of ice accretion on flying aircraft most commonly rely on modeling in 2D. These 2D methods treat an aircraft geometry either as wing-like with infinite span, or as an axisymmetric body. Recently, fully 3D methods have been introduced that model an aircraft's true 3D shape. Because 3D methods are more computationally expensive than 2D methods, 2D methods continue to be widely used. However, a 3D method allows investigation of whether it is valid to continue applying 2D methods to a finite wing. The extent of disagreement between LEWICE, a 2D method, and LEWICE3D, a 3D method, in calculating local collection efficiencies at the leading edge of finite wings is investigated.

Mohler, Stanley R., Jr.↗

NASA NTRS1992

Integrated aerodynamic-structural-control wing design

The aerodynamic-structural-control design of a forward-swept composite wing for a high subsonic transport aircraft is considered. The structural analysis is based on a finite-element method. The aerodynamic calculations are based on a vortex-lattice method, and the control calculations are based on an output feedback control. The wing is designed for minimum weight subject to structural, performance/aerodynamic and control constraints. Efficient methods are used to calculate the control-deflection and control-effectiveness sensitivities which appear as second-order derivatives in the control constraint equations. To suppress the aeroelastic divergence of the forward-swept wing, and to reduce the gross weight of the design aircraft, two separate cases are studied: (1) combined application of aeroelastic tailoring and active controls; and (2) aeroelastic tailoring alone. The results of this study indicated that, for this particular example, aeroelastic tailoring is sufficient for suppressing the aeroelastic divergence, and the use of active controls was not necessary.

Rais-Rohani, M.↗