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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Pulsed-echo interface wave characterization of bolted plates

Ultrasonic waves which propagate along the plane boundary separating two solid substrates were used to characterize aluminum and titanium plates adhesively bonded and bolted together. Wave pulses were used to produce interface waves between the aluminum and a Ti-6Al-4V titanium alloy bar by normal mode conversion. Interface wave reflections from bolt holes drilled through the interface were reconverted to surface wave pulses and detected by the same wedge by standard pulse-echo techniques. Time of flight measurements indicate that the reflected waves are a supersonic of longitudinal modes and circularly symmetric waves that propagate around the bolt hole.

Turner, T. M.↗
NASA NTRS1981

Sensor failure detection system

Advanced concepts for detecting, isolating, and accommodating sensor failures were studied to determine their applicability to the gas turbine control problem. Five concepts were formulated based upon such techniques as Kalman filters and a screening process led to the selection of one advanced concept for further evaluation. The selected advanced concept uses a Kalman filter to generate residuals, a weighted sum square residuals technique to detect soft failures, likelihood ratio testing of a bank of Kalman filters for isolation, and reconfiguring of the normal mode Kalman filter by eliminating the failed input to accommodate the failure. The advanced concept was compared to a baseline parameter synthesis technique. The advanced concept was shown to be a viable concept for detecting, isolating, and accommodating sensor failures for the gas turbine applications.

Beattie, E. C.↗
NASA NTRS1982

Aeroelastic Stability of Rotor Blades Using Finite Element Analysis

The flutter stability of flap bending, lead-lag bending, and torsion of helicopter rotor blades in hover is investigated using a finite element formulation based on Hamilton's principle. The blade is divided into a number of finite elements. Quasi-steady strip theory is used to evaluate the aerodynamic loads. The nonlinear equations of motion are solved for steady-state blade deflections through an iterative procedure. The equations of motion are linearized assuming blade motion to be a small perturbation about the steady deflected shape. The normal mode method based on the coupled rotating natural modes is used to reduce the number of equations in the flutter analysis. First the formulation is applied to single-load-path blades (articulated and hingeless blades). Numerical results show very good agreement with existing results obtained using the modal approach. The second part of the application concerns multiple-load-path blades, i.e. bearingless blades. Numerical results are presented for several analytical models of the bearingless blade. Results are also obtained using an equivalent beam approach wherein a bearingless blade is modelled as a single beam with equivalent properties. Results show the equivalent beam model.

Chopra, I.↗
NASA NTRS1980

Issues in the development of a general design algorithm for reliable failure detection

The design of residual-generation processes is briefly discussed, the goal being to develop a methodology for designing robust processes of this type. It is noted that analytical redundancy forms the basis for residual-generation, representing the relationships between the outputs of sensors and inputs of actuators via the dynamics of the system. It is because of this relationship that sensor outputs (even those of dissimilar sensors and at different times) can, in effect, be compared to ascertain whether they are consistent with normal system behavior. The residuals can be seen as constituting the discrepancy between the output resulting from such comparisons; they should display noise-like characteristics only in the normal mode. Failures in the system would lead to a discrepancy between the observed and expected behavior of the sensor outputs and hence to abnormal characteristics (failure signatures) in the residual.

Chow, E. Y.↗
NASA NTRS1981

The Lagrangian-mean motions forced by steady, dissipating equatorial waves. I

Waves are treated with a normal mode structure in order to determine the steady mean motion of the atmosphere that can be induced by dissipating equatorial waves. A model is developed which comprises a continuously stratified atmosphere at rest on the equatorial beta-plane. It is assumed that waves are excited by the corrugated bottom and are in a steady state, that dissipation is due to Newtonian cooling and Rayleigh friction, steadiness in wave magnitude is up to the second order, the waves have a long wave length, wave induced mean flows do not affect the waves, mean flows are steady, and dissipation mechanisms for the mean flows are the same as for the waves. Disturbance equations are formulated, along with Eulerian- and Lagrangian-mean flows, and the nonexistence of cross equatorial mean flows is demonstrated. Kelvin waves are shown to possess a Lagrangian-mean meridional circulation which is the same as the Eulerian-mean circulation. In the Boussinesq limit, however, neither the Eulerian- nor the Lagrangian-mean meridional circulations are caused by Kelvin waves. Further examination is made of Rossby-gravity waves and n = 1 westward propagating inertio-gravity waves.

Takahashi, M.↗
NASA NTRS1982

Examination of time series through randomly broken windows

The study of many astronomical problems involves a consideration of time series. The present investigation was prompted by a problem related to the determination of normal modes of oscillation of the sun. It is desirable to have an approach for obtaining an estimate of the accuracy with which oscillation modes may be determined when it appears possible to observe the sun over a long interval of time losing some blocks of time because of cloud cover. The present investigation has the objective to develop a model for addressing problems of this type. After presentation of a few general formulae, the problem is considerably simplified by supposing that observations are made over a large number N of equal time intervals, each of length t, so that the total time interval T is given by T = Nt. With certain additional simplifying assumptions, attention is given to the statistical properties of two functions which are representative of 'randomly broken' window functions.

Sturrock, P. A.↗
NASA NTRS1980

Rotor dynamic simulation and system identification methods for application to vacuum whirl data

Methods of using rotor vacuum whirl data to improve the ability to model helicopter rotors were developed. The work consisted of the formulation of the equations of motion of elastic blades on a hub using a Galerkin method; the development of a general computer program for simulation of these equations; the study and implementation of a procedure for determining physical parameters based on measured data; and the application of a method for computing the normal modes and natural frequencies based on test data.

Berman, A.↗
NASA NTRS1983

Effects of specimen resonances on acoustic-ultrasonic testing

The effects of specimen resonances on acoustic ultrasonic (AU) nondestructive testing were investigated. Selected resonant frequencies and the corresponding normal mode nodal patterns of the aluminum block are measured up to 75.64 kHz. Prominent peaks in the pencil lead fracture and sphere impact spectra from the two transducer locations corresponded exactly to resonant frequencies of the block. It is established that the resonant frequencies of the block dominated the spectral content of the output signal. The spectral content of the output signals is further influenced by the transducer location relative to the resonant frequency nodal lines. Implications of the results are discussed in relation to AU parameters and measurements.

Williams, J. H., Jr.↗
NASA NTRS1983

Studies with the Pinhole/Occulter Facility

The scientific justifications for high-resolution hard X-ray astronomy are reviewed, and a scheme for making such observations from the Space Shuttle payload bay is presented. High-resolution X-ray observations at photon energies above 10 keV are important for the understanding of the physics of solar flares, coronal disturbances related to flares, and corona fine structure, as well as nonsolar X-ray sources. In order to study these phenomena, concepts have been developed for the Shuttle Pinhole/Occulter Facility (P/OF), an instrument based on the principles of the pinhole camera which will have an angular resolution of 0.2 arc sec. The proposed P/OF configuration consists of four separate telescopes or position-sensitive counters mounted on a detector plane and looking toward the target through separate portions of an aperture mask (occulter plane). The two planes are separated by a self-deployable 50-m boom, which is to be made essentially rigid by direct control of its lowest normal modes.

Tandberg-Hanssen, E.↗
NASA NTRS1982

Calculation of SF6-/SF6 and Cl-/CFCl3 electron attachment cross sections in the energy range 0-100 meV

Electron attachment cross sections for the processes SF6-/SF6 and Cl-/CFCl3 are calculated in a local theory using a model in which diatomic-like potential energy curves for the normal modes are constructed from available spectroscopic data. Thermally populated vibrational and rotational levels are included. Good agreement is found with experimental cross sections in the energy range 5-100 meV for a particular choice of potential energy curve parameters.

Chutjian, A.↗
NASA NTRS1983

Testing solar models with global solar oscillations in the 5-minute band

Frequencies of solar oscillation for normal modes described by spherical harmonics with l-values between 0 and 4 are computed. The method of computation is discussed and some of the theoretical uncertainties are examined. It is shown that the standard solar model has eigenfrequencies which do not agree with the frequencies observed for the low l-modes to within the estimated accuracy of either the observed or theoretical frequencies. Four non-standard models are considered: (1) the interior Z abundance is lower than the surface abundance; (2) the interior Z abundance is higher than the surface abundance; (3) the interior Z abundance is altered by mixing; and (4) a large primordial magnetic field remains in the solar core. The effect of all these models on the solar neutrino flux is considered, with the result that the high-Z model is rejected. The conclusions of Bahcall and Ulrich (1971) that a primordial magnetic field increases the neutrino flux are disputed.

Ulrich, R. K.↗
NASA NTRS1982

An integrated regenerative air revitalization system for spacecraft

Progress towards development of an air revitalization system (ARS) for spacecraft breathable atmosphere regeneration is assessed, and a preliminary design for a one-person ARS is described. The ARS is considered a necessary component of any permanently manned orbital station, and studies have demonstrated that penalties for expendable air supplies justify an ARS for missions longer than 40 days. CO2 must be removed and O2 returned along with N2, which can be extracted from hydrazine, with the H2 component returning to the operation of the CO2 reduction subsystem. An experimental ARS (ARX-1) features a cabin humidity control unit, a CO2 concentrator, an air-cooled CO2 reduction reactor, an oxygen generator (electrolysis), the hydrazine N2 generator, and a water handling unit. A 120-day test demonstrated one-button startup and 480 hr operation in a normal mode.

Noyes, G. P.↗
NASA NTRS1983

Charney's problem for baroclinic instability applied to barotropic instability

If barotropically unstable easterly jets may be approximated by broken line profiles, it becomes possible to use the solutions of the Charney (1947) problem for baroclinic instabilithy in order to solve the barotropic problem. Results are presented for easterly jets having an arbitrary degree of symmetry, and it is noted that the convenient calculation obtained for the growth of both localized and normal mode perturbations is important in the study of monsoon depressions over India.

Lindzen, R. S.↗
NASA NTRS1983

Energetics and optimum motion of oscillating lifting surfaces

Low-frequency, unsteady, lifting-line theory is used to characterize the energetics and optimum motion of an unswept rigid wing oscillating harmonically in an inviscid, incompressible flow. The energetics calculations account for the leading edge suction force, the power absorbed in the wing oscillations, and the energy loss rate produced by vortex shedding. Optimization is achieved by minimizing the average energy loss rate in relation to a given thrust, and a unique solution is found in the three dimensional case for low, reduced frequencies. The two-dimensional solution is nonunique, a condition which is examined in terms of the normal modes of the energy loss rate matrix. An invisible mode with a hydrodynamic efficiency of 100 pct is obtained in the two-dimensional case, causing the nonuniqueness of the solution by yielding no fixed positive thrust through perfect unsteady feathering.

Ahmadi, A. R.↗
NASA NTRS1983

Absolute barotropic instability and monsoon depressions

Monsoon depressions over the Bay of Bengal develop almost entirely in July and August. After studies conducted by Lindzen et al. (1980) and Stevens and Lindzen (1978), only barotropic instability remains as a mechanism for the development of the wave disturbances associated with monsoon depressions. The present investigation has the objective to show that barotropic instability is able to explain the wave aspects of monsoon depressions, but that normal mode analysis is inadequate. It is found that the local barotropically unstable response to regional perturbations in the Bay of Bengal during July and August will be dominated by the lower troposphere. The analysis clearly identifies the features of the mean flow which lead to monsoon depressions in July. The features include the development of an easterly jet as opposed to semijet structure in the mean flow, and the development of a modest easterly flow at the jet center as opposed to westerly flow.

Lindzen, R. S.↗
NASA NTRS1983

The Riemann disks. II - Stability

The Riemann disks are the two-dimensional analogs of the Riemann S-type ellipsoids. They form a simple yet rich family of galactic bar models. A method is developed to find their normal modes to arbitrary order, and the stability analysis is carried out for all modes with order not greater than 5.

Weinberg, M. D.↗
NASA NTRS1983

Initial-value problems for Rossby waves in a shear flow with critical level

The time-dependent evolution of sheared Rossby waves starting from an initial disturbance is studied for the simple case in which the shear is uniform. The uniform-shear assumption allows explicit solutions to be obtained which are useful in addressing the issue of the long-time asymptotic approach to normal modes and in assessing the relative importance of viscosity, nonlinearity and time-dependence in the evolution of Rossby waves in the presence of critical layers.

Tung, K. K.↗
NASA NTRS1983

Waveform inversion of mantle Love waves: The born seismogram approach

Normal mode theory, extended to the slightly laterally heterogeneous Earth by the first-order Born approximation, is applied to the waveform inversion of mantle Love waves (200-500 sec) for the Earth's lateral heterogeneity at l=2 and a spherically symmetric anelasticity (Q sub mu) structure. The data are from the Global Digital Seismograph Network (GDSN). The l=2 pattern is very similar to the results of other studies that used either different methods, such as phase velocity measurements and multiplet location measurements, or a different data set, such as mantle Rayleigh waves from different instruments. The results are carefully analyzed for variance reduction and are most naturally explained by heterogeneity in the upper 420 km. Because of the poor resolution of the data set for the deep interior, however, a fairly large heterogeneity in the transition zones, of the order of up to 3.5% in shear wave velocity, is allowed. It is noteworthy that Love waves of this period range can not constrain the structure below 420 km and thus any model presented by similar studies below this depth are likely to be constrained by Rayleigh waves (spheroidal modes) only.

Tanimoto, T.↗