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At least 1,531 records · Page 85
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Computational aspects of sensitivity calculations in transient structural analysis

A key step in the application of formal automated design techniques to structures under transient loading is the calculation of sensitivities of response quantities to the design parameters. This paper considers response quantities to the design parameters. This paper considers structures with general forms of damping acted on by general transient loading and addresses issues of computational errors and computational efficiency. The equations of motion are reduced using the traditional basis of vibration modes and then integrated using a highly accurate, explicit integration technique. A critical point constraint formulation is used to place constraints on the magnitude of each response quantity as a function of time. Three different techniques for calculating sensitivities of the critical point constraints are presented. The first two are based on the straightforward application of the forward and central difference operators, respectively. The third is based on explicit differentiation of the equations of motion. Condition errors, finite difference truncation errors, and modal convergence errors for the three techniques are compared by applying them to a simple five-span-beam problem. Sensitivity results are presented for two different transient loading conditions and for both damped and undamped cases.

Greene, William H.↗
NASA NTRS1989

Impact of small-crack effects on design-life calculations

The potential impact of the observed small-crack-effect in 2024-T3 aluminum alloy on calculated crack-growth lives is described. The significance of the small-crack effect is assessed by comparing lives computed using only large-crack data to those computed using a combined small- and large-crack data base for constant-amplitude (R = -1 and 0) and fighter-wing spectrum loading (FALSTAFF) conditions. Based on the life calculations, the small-crack effect would have no impact on life analyses that assume an initial crack larger than about 0.3 mm; only a small impact on life analyses that assume an initial crack of 0.1 mm; but a large impact on life analyses that assume an initial crack of about 0.01 mm. When small-crack effects are taken into account, fatigue life analyses based solely on crack growth may provide a viable alternative to traditional two-part initiation-plus-crack-growth life analyses.

Phillips, E. P.↗
NASA NTRS1990

Simplified Calculation Of Solar Fluxes In Solar Receivers

Simplified Calculation of Solar Flux Distribution on Side Wall of Cylindrical Cavity Solar Receivers computer program employs simple solar-flux-calculation algorithm for cylindrical-cavity-type solar receiver. Results compare favorably with those of more complicated programs. Applications include study of solar energy and transfer of heat, and space power/solar-dynamics engineering. Written in FORTRAN 77.

Bhandari, Pradeep↗
NASA NTRS1987

Program VSAERO theory document: A computer program for calculating nonlinear aerodynamic characteristics of arbitrary configurations

The VSAERO low order panel method formulation is described for the calculation of subsonic aerodynamic characteristics of general configurations. The method is based on piecewise constant doublet and source singularities. Two forms of the internal Dirichlet boundary condition are discussed and the source distribution is determined by the external Neumann boundary condition. A number of basic test cases are examined. Calculations are compared with higher order solutions for a number of cases. It is demonstrated that for comparable density of control points where the boundary conditions are satisfied, the low order method gives comparable accuracy to the higher order solutions. It is also shown that problems associated with some earlier low order panel methods, e.g., leakage in internal flows and junctions and also poor trailing edge solutions, do not appear for the present method. Further, the application of the Kutta conditions is extremely simple; no extra equation or trailing edge velocity point is required. The method has very low computing costs and this has made it practical for application to nonlinear problems requiring iterative solutions for wake shape and surface boundary layer effects.

Maskew, Brian↗
NASA NTRS1990

The calculation of the contributions to low energy e+H2 scattering from sigma u+ and Pion u symmetries using the Kohn variational method

Above incident energies of about 2 eV, the contribution to the total cross section in positron+H2 scattering from the sigma g+ symmetry is insufficient to account for the experimental value. Calculations carried out of the lowest partial waves of sigma u+ symmetry and Pion u symmetry using the Kohn variational method are described. The contributions to the total cross section from the two equivalent partial waves of Pion u symmetry significantly reduce the discrepancy with experiment up to incident energies of 4 to 5 eV. Comparisons are made with recent R-matrix calculations performed by Danby and Tennyson.

Armour, E. A. G.↗
NASA NTRS1990

En route noise: NASA propfan test aircraft (calculated source noise

The second phase of a joint National Aeronautics and Space Administration (NASA) and Federal Aviation Administration (FAA) program to study the high-altitude, low-frequency acoustic noise propagation characteristics of the Advanced Turboprop (propfan) Aircraft was conducted on April 3-13, 1989 at the White Sands Missile Range (WSMR), New Mexico. The first phase was conducted on October 26-31, 1987 in Huntsville, Alabama. NASA (Lewis) measured the source noise of the test aircraft during both phases while NASA (Langley) measured surface noise only during the second phase. FAA/NASA designed a program to obtain noise level data from the propfan test bed aircraft, both in the near field and at ground level, during simulated en route flights (35,000 and 20,000 feet ASL), and to test low frequency atmospheric absorption algorithms and prediction technology to provide insight into the necessity for regulatory measures. The curves of calculated source noise versus emission angle are based on a second order best-fit curve of the peak envelope of the adjusted ground data. Centerline and sideline derived source noise levels are shown to be in good agreement. A comparison of the Alabama chase plane source data and the calculated source noise at centerline for both the Alabama and New Mexico data shows good agreement for the 35,000 and the 20,000 feet (ASL) overflights. With the availability of the New Mexico in-flight data, further in depth comparisons will be made.

Rickley, E. J.↗
NASA NTRS1990

Calculation of flight vibration levels of the AH-1G helicopter and correlation with existing flight vibration measurements

NASA-Langley is sponsoring a rotorcraft structural dynamics program with the objective to establish in the U.S. a superior capability to utilize finite element analysis models for calculations to support industrial design of helicopter airframe structures. In the initial phase of the program, teams from the major U.S. manufacturers of helicopter airframes will apply extant finite element analysis methods to calculate loads and vibrations of helicopter airframes, and perform correlations between analysis and measurements. The aforementioned rotorcraft structural dynamics program was given the acronym DAMVIBS (Design Analysis Method for Vibrations). Sikorsky's RDYNE Rotorcraft Dynamics Analysis used for the correlation study, the specifics of the application of RDYNE to the AH-1G, and comparisons of the predictions of the method with flight data for loads and vibrations on the AH-1G are described. RDYNE was able to predict trends of variations of loads and vibrations with airspeed, but in some instances magnitudes differed from measured results by factors of two or three to one. Sensitivities were studied of predictions to rotor inflow modeling, effects of torsional modes, number of blade bending modes, fuselage structural damping, and hub modal content.

Sopher, R.↗
NASA NTRS1990

An approximate method for calculating three-dimensional inviscid hypersonic flow fields

An approximate solution technique was developed for 3-D inviscid, hypersonic flows. The method employs Maslen's explicit pressure equation in addition to the assumption of approximate stream surfaces in the shock layer. This approximation represents a simplification to Maslen's asymmetric method. The present method presents a tractable procedure for computing the inviscid flow over 3-D surfaces at angle of attack. The solution procedure involves iteratively changing the shock shape in the subsonic-transonic region until the correct body shape is obtained. Beyond this region, the shock surface is determined using a marching procedure. Results are presented for a spherically blunted cone, paraboloid, and elliptic cone at angle of attack. The calculated surface pressures are compared with experimental data and finite difference solutions of the Euler equations. Shock shapes and profiles of pressure are also examined. Comparisons indicate the method adequately predicts shock layer properties on blunt bodies in hypersonic flow. The speed of the calculations makes the procedure attractive for engineering design applications.

Riley, Christopher J.↗
NASA NTRS1989

Model calculations of spectral transmission for the CLAES etalons

This paper describes models for calculating spectral transmission for the Cryogenic-Limb-Array-Etalon-Spectrometer (CLAES) etalons. These models involve a convolution of the Airy function for a given thickness with the distribution of surface thicknesses, the effect of absorption in the substrate, and the field of view broadening as a function of etalon tilt angle. A comparison of model calculations with experimental transmission data for CLAES etalons centered at 3.52, 5.72, 8.0, and 11.86 microns showed that these models are able to provide a good description of the CLAES etalons.

James, T. C.↗
NASA NTRS1989

Test particle calculations of pick-up ions in the vicinity of Comet Giacobini-Zinner

Ionization of cometary neutral molecules produces ions which are picked-up by the solar wind. The cometary ion pick-up process for Comet Giacobini-Zinner is studied using a test particle method in which trajectories are numerically calculated for several thousand ions whose initial locations were chosen randomly with probability proportional to the neutral density. The cometary ion distribution function was calculated at several locations upstream of the bow shock. Several types of magnetic fluctuations were included. Circularly and linearly polarized monochromatic waves result in less pitch-angle scattering of pick-up ions than turbulent fluctuations which have about the same amplitude. Some anisotropy remains in the distribution function just upstream of the shock, even for the turbulent fluctuations.

Cravens, T. E.↗
NASA NTRS1989

Atomic data for opacity calculations. XI - The carbon isoelectronic sequence

Close-coupling calculations are carried out for radiative processes in neutral carbon and a number of carbon-like ions; energy levels, oscillator strengths, and photoionization cross sections have been computed for all bound states of the type 2 s(j)2p(k)nl with n not above 10 and 1 not above 3. The R-matrix method is employed to solve the coupled equations with a ten-state eigenfunction expansion for the parent ion C II and an eight-state expansion for the other boron-like target ions. A number of selected results for oscillator strengths are presented and compared with earlier data, as well as for photoionization cross sections with autoionizing resonance structures. Isoelectronic trends are discussed. The present results for the oscillator strengths of C I and N II are found to differ significantly from some earlier theoretical works for a number of transitions. However, the present C I f values are in excellent agreement with recent calculations and experimental results.

Luo, D.↗
NASA NTRS1989

The application of ab initio calculations to molecular spectroscopy

The state of the art in ab initio molecular structure calculations is reviewed with an emphasis on recent developments, such as full configuration-interaction benchmark calculations and atomic natural orbital basis sets. It is found that new developments in methodology, combined with improvements in computer hardware, are leading to unprecedented accuracy in solving problems in spectroscopy.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1989

Rapid calculation of radiative heating rates and photodissociation rates in inhomogeneous multiple scattering atmospheres

The solution of the generalized two-stream approximation for radiative transfer in homogeneous multiple scattering atmospheres is extended to vertically inhomogeneous atmospheres in a manner which is numerically stable and computationally efficient. It is shown that solar energy deposition rates, photolysis rates, and infrared cooling rates all may be calculated with the simple modifications of a single algorithm. The accuracy of the algorithm is generally better than 10 percent, so that other uncertainties, such as in absorption coefficients, may often dominate the error in calculation of the quantities of interest to atmospheric studies.

Toon, Owen B.↗
NASA NTRS1989

Calculated emission rates for barium releases in space

The optical emissions from barium releases in space are caused by resonance and fluorescent scattering of sunlight. Emission rates for the dominant ion and neutral lines are calculated assuming the release to be optically thin and the barium to be in radiative equilibrium with the solar radiation. The solar spectrum has deep Fraunhofer absorption lines at the primary barium ion resonances. A velocity component toward or away from the sun will Doppler shift the emission lines relative to the absorption lines and the emission rates will increase many-fold over the rest value. The Doppler brightening is important in shaped charge or satellite releases where the barium is injected at high velocities. Emission rates as a function of velocity are calculated for the 4554, 4934, 5854, 6142 and 6497 A ion emission lines and the dominant neutral line at 5535 A. Results are presented for injection parallel to the ambient magnetic field, B, and for injection at an angle to B.

Stenbaek-Nielsen, H. C.↗
NASA NTRS1989

The application of ab initio calculations to molecular spectroscopy

The state of the art in ab initio molecular structure calculations is reviewed, with an emphasis on recent developments such as full configuration-interaction benchmark calculations and atomic natural orbital basis sets. It is shown that new developments in methodology combined with improvements in computer hardware are leading to unprecedented accuracy in solving problems in spectroscopy.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1990

An approximate method for the calculation of nonequilibrium radiative heat transfer

A simplified method for the calculation of radiative heat transfer in hypersonic reentry flows where local thermal nonequilibrium (LTNE) conditions exist is presented. This method has been incorporated into the computer code RADMC (RADiation/Modified Corrections), which utilizes an approximate 'two-state' model of the electronic levels of atoms and molecules in the determination of the populations of the excited states for both LTNE and local thermal equilibrium (LTE) conditions for each of the relevant chemical species. The population data are used to calculate a correction factor to the radiative cross sections in an eight-step nongray radiative heat-transfer model. Results have been obtained for various points along the proposed trajectory of the Aeroassist Flight Experiment and the Fire II trajectory, and compared with results obtained for the same flowfields from the NEQAIR radiative heat-transfer code.

Greendyke, Robert B.↗
NASA NTRS1990

Calculations of the flow past bluff bodies, including tilt-rotor wing sections at alpha = 90 deg

An attempt was made to model in two dimensions the effects of rotor downwash on the wing of the tilt-rotor aircraft and to compute the drag force on airfoils at - 90 deg angle of attack, using a well-established Navier-Stokes code. However, neither laminar nor turbulent calculations agreed well with drag and base-pressure measurements at high Reynolds numbers. Therefore, further efforts were concentrated on bluff-body flows past various shapes at low Reynolds numbers, where a strong vortex shedding is observed. Good results were obtained for a circular cylinder, but the calculated drag of a slender ellipse at right angles to the freestream was significantly higher than experimental values reported in the literature for flat plates. Similar anomalous results were obtained on the tilt-rotor airfoils, although the qualitative effects of flap deflection agreed with the wind tunnel data. The ensemble of results suggest that there may be fundamental differences in the vortical wakes of circular cylinders and noncircular bluff bodies.

Raghavan, V.↗
NASA NTRS1990

High-voltage plasma interactions calculations using NASCAP/LEO

This paper reviews four previous simulations (two laboratory and two space-flight) of interactions of a high-voltage spacecraft with a plasma under low-earth orbit conditions, performed using a three-dimensional computer code NASCAP/LEO. Results show that NASCAP/LEO can perform meaningful simulations of high-voltage plasma interactions taking into account three-dimensional effects of geometry, spacecraft motion, and magnetic field. Two new calculations are presented: (1) for current collection by 1-mm pinholes in wires (showing that a pinhole in a wire can collect far more current than a similar pinhole in a flat plate); and (2) current collection by Charge-2 mother vehicle launched in December 1985. It is shown that the Charge-2 calculations predicted successfully ion collection at negative bias, the floating potential of a probe outside or inside the sheath under negative bias conditions, and magnetically limited electron collection under electron beam operation at high altitude.

Mandell, M. J.↗