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NASA NTRS2008

Two-Stage Passive Vibration Isolator

The design and testing of a structural system were implemented to hold the optics of the planned Space Interferometry Mission (SIM) at positions and orientations characterized by vibrational translation and rotation errors of no more than a few nanometers or a few milliarcseconds, respectively. Much of the effort was devoted to a test bed for verifying the predicted behavior of a vibration- isolation structural subsystem working together with an active control system for positioning and orienting the SIM optics. There was considerable emphasis on the vibration-isolation subsystem, which was passive and comprised two stages. The main sources of vibration were six reaction wheels in an assembly denoted the "backpack." The first vibration-isolation stage consisted of hexapod isolator mounts - one for each reaction wheel - characterized by a natural vibration frequency of 10 Hz. The second stage was a set of three beams, disposed between the backpack and the structure that held the SIM optics, that were flexured such that they transmitted only bending loads, with a natural vibrational frequency and damping of about 5 Hz and 4 percent, respectively. Preliminary test results were presented and characterized as demonstrating the effectiveness of the two-stage vibration-isolation design.

Goullioud, Renaud↗

NASA NTRS1993

Vibration analysis of heated anisotropic plates with free edge conditions

Tabular and plotted results are presented (as a function of lamination angle and temperature) for the first two vibration frequencies and mode shapes of a heated, single-layer Boron/epoxy laminate. A Ritz formulation is used to determine the thermally induced in-plane forces and the out-of-plane vibration frequencies. This approach is formulated in terms of the out-of-plane displacement and the in-plane stress function; both the displacement and the stress function are expressed as simple algebraic polynomials modified to satisfy the essential boundary conditions. Heating is applied as a steady-state temperature distribution; material properties are considered to be temperature-dependent. To establish the correctness and effectiveness of the method, the in-plane forces are compared with Fourier series analytical results.

Locke, James E.↗

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 NTRS1991

Buckling and vibration analysis of a simply supported column with a piecewise constant cross section

An analysis and sample results for the lateral buckling and vibration of a compressively loaded column is presented whose cross section is piecewise constant along its length. The column is symmetric about its mid-span and consists of three sections, the center section having a stiffer cross section than the two identical end sections. Buckling and vibration characteristics of the column are determined from numerical solution of the exact eigenvalue problems. Parametric structural efficiency analyses are performed using a nondimensionalized set of governing equations to determine the optimum ratio between the lengths of the center section and the outer sections based on both buckling load and vibration frequency requirements. In these analyses, two relationships exist. One is between cross-sectional mass and the cross section, and the other is a high-efficiency scheme. The effect of axial load on vibration frequency is also examined and compared with that of a uniform column.

Lake, Mark S.↗

NASA NTRS1996

Chemistry by Way of Density Functional Theory

In this work we demonstrate that density functional theory (DFT) methods make an important contribution to understanding chemical systems and are an important additional method for the computational chemist. We report calibration calculations obtained with different functionals for the 55 G2 molecules to justify our selection of the B3LYP functional. We show that accurate geometries and vibrational frequencies obtained at the B3LYP level can be combined with traditional methods to simplify the calculation of accurate heats of formation. We illustrate the application of the B3LYP approach to a variety of chemical problems from the vibrational frequencies of polycyclic aromatic hydrocarbons to transition metal systems. We show that the B3LYP method typically performs better than the MP2 method at a significantly lower computational cost. Thus the B3LYP method allows us to extend our studies to much larger systems while maintaining a high degree of accuracy. We show that for transition metal systems, the B3LYP bond energies are typically of sufficient accuracy that they can be used to explain experimental trends and even differentiate between different experimental values. We show that for boron clusters the B3LYP energetics are not as good as for many of the other systems presented, but even in this case the B3LYP approach is able to help understand the experimental trends.

Bauschlicher, Charles W., Jr.↗

NASA NTRS2012

Accurate ab initio Quartic Force Fields of Cyclic and Bent HC2N Isomers

Highly correlated ab initio quartic force field (QFFs) are used to calculate the equilibrium structures and predict the spectroscopic parameters of three HC2N isomers. Specifically, the ground state quasilinear triplet and the lowest cyclic and bent singlet isomers are included in the present study. Extensive treatment of correlation effects were included using the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T). Dunning s correlation-consistent basis sets cc-pVXZ, X=3,4,5, were used, and a three-point formula for extrapolation to the one-particle basis set limit was used. Core-correlation and scalar relativistic corrections were also included to yield highly accurate QFFs. The QFFs were used together with second-order perturbation theory (with proper treatment of Fermi resonances) and variational methods to solve the nuclear Schr dinger equation. The quasilinear nature of the triplet isomer is problematic, and it is concluded that a QFF is not adequate to describe properly all of the fundamental vibrational frequencies and spectroscopic constants (though some constants not dependent on the bending motion are well reproduced by perturbation theory). On the other hand, this procedure (a QFF together with either perturbation theory or variational methods) leads to highly accurate fundamental vibrational frequencies and spectroscopic constants for the cyclic and bent singlet isomers of HC2N. All three isomers possess significant dipole moments, 3.05D, 3.06D, and 1.71D, for the quasilinear triplet, the cyclic singlet, and the bent singlet isomers, respectively. It is concluded that the spectroscopic constants determined for the cyclic and bent singlet isomers are the most accurate available, and it is hoped that these will be useful in the interpretation of high-resolution astronomical observations or laboratory experiments.

Inostroza, Natalia↗

NASA NTRSDate not supplied

Computing rovibrational, vibrational, and cascade emission spectra for comparison to astronomical observations

Over the last several years, our group has been involved in developing approaches to compute highly accurate spectroscopic constants and vibrational frequencies for small transient molecules that may be used in the interpretation and assignment of high-resolution laboratory experiments as well as high-resolution astronomical spectra. Additionally, we have used the computed spectroscopic constants to simulate purely rotational and rovibrational spectra so that these may be compared directly with high-resolution astronomical observations, and we have worked on developing approaches that can be applied to much larger molecules, such as polycyclic aromatic hydrocarbon (PAH) molecules, where we can explicitly determine anharmonic corrections to vibrational frequencies as well as take into account intensity sharing due to resonances. In addition, for PAH molecules we have used the anharmonic data to compute a library of temperature-dependent anharmonic vibrational spectra, with proper inclusion of polyad resonances, which can then be used to model the cascade emission spectra of PAH molecules – the type of spectra directly observed by astronomers. Another part of our work in spectroscopic signatures involves computing highly accurate line lists for common molecules, such as CO2, SO2, and NH3, which occur in many astrophysical environments, including the atmospheres of exoplanents, and often need to have their lines identified in high-resolution observations in order to determine which lines are due to other molecules. In order to characterize the atmospheres of hot exoplanets, these line lists need to be very accurate and extend to very high energies. I will discuss our latest work in these areas of astrochemical spectroscopy research.

Timothy J Lee↗

NASA NTRS1996

Deformation of Fluid Column by Action of Axial Vibration and Some Aspects of High-Rate Thermocapillary Convection

The deformation of the fluid column by an action of a low-frequency vibration is considered. It is shown that behavior of the free fluid surface depends on the frequency of applied vibration and its amplitude. In the area of very low frequencies when fluid has time to comment on travel of bounding solid walls limiting column, the harmonical oscillations of free surface with given frequency are observed. With increase of vibration frequency the steady-state relief on free fluid surface is formed. If the amplitude of vibration is very small and the frequency corresponding to the first peak in the vibration spectrum on the Mir orbital station, the deformation of free surface tends to zero. Fluid flow induced thermocapillary effect on deformed free surface is more unstable as in the case of smooth cylindrical surface. It was shown that width of heating zone affects very essentially the flow pattern and transition to oscillatory regime of thermocapillary convection.

Feonychev, Alexander I.↗

NASA NTRS1985

The Influence of Low Frequency Mechanical Vibrations on the Growth of Single Crystals

The optimum conditions for crystal growth are usually achieved either by suppressing convective fluid flows (e.g., by the use of a low-gravity environment) or by over-riding thermal and solutal convection by the use of a strong stirring action. A novel stirring technique has been developed which involves subjecting a vertical crucible to a circle in a horizontal plane (without rotation). Use of an amplitude of 3 mm at a frequency of approx 6 Hz produced complete mixing of a non-uniform aqueous liquid in a few seconds. The mixing action involved the downward flow of liquid in the outer annulus of the liquid, driven by surface waves. When the downward flowing liquid reaches the bottom of the crucible, it is reflected in a central, upward flowing spiral. This flow pattern should be beneficial for crystal growth by the Bridgman method since it will sweep impurities away from the walls and produce a more convex solid-liquid interface. Initial attempts to apply the new stirring technique to CdTe crystal growth did not show significant improvement in the number of crystals nucleated, but the interface shape appeared to be close to that predicted.

Feigelson, R. S.↗

NASA NTRS1991

Probabilistic structural analysis of adaptive/smart/intelligent space structures

A three-bay, space, cantilever truss is probabilistically evaluated for adaptive/smart/intelligent behavior. For each behavior, the scatter (ranges) in buckling loads, vibration frequencies, and member axial forces are probabilistically determined. Sensitivities associated with uncertainties in the structure, material and load variables that describe the truss are determined for different probabilities. The relative magnitude for these sensitivities are used to identify significant truss variables that control/classify its behavior to respond as an adaptive/smart/intelligent structure. Results show that the probabilistic buckling loads and vibration frequencies increase for each truss classification, with a substantial increase for intelligent trusses. Similarly, the probabilistic member axial forces reduce for adaptive and intelligent trusses and increase for smart trusses.

Pai, Shantaram S.↗

NASA NTRS1992

Probabilistic structural analysis of adaptive/smart/intelligent space structures

Pai, Shantaram S.↗

NASA NTRS1998

On vibrational imperfection sensitivity of Augusti's model structure in the vicinity of a non-linear static state

In this paper we present a closed-form solution for vibrational imperfection sensitivity the effect of small imperfections on the vibrational frequencies of perturbed motion around the static equilibrium state of Augusti's model Structure (a rigid link, pinned at one end to a rigid foundation and supported at the other by a linear extensional spring that retains its horizontality, as the system deflects). We also treat a modified version of that model with attendant slightly different dynamics. It is demonstrated that the vibrational frequencies decreases as the initial imperfections increase.

Elishakoff, Isaac↗

NASA NTRS1993

Wavelength-resolved emission spectroscopy of the alkoxy and alkylthio radicals in a supersonic jet

Wavelength-resolved emission spectra of methoxy (CH3O) and methylthio (CH3S) radicals have been obtained in a supersonic jet environment with a resolution of 0.3 nm by dispersing the total laser-induced fluorescence with a 0.6 m monochromator. A detailed analysis of the single vibronic level dispersed fluorescence spectra yields the following vibrational frequencies for CH3O in the X(2)E state; nu(sub 1 double prime) = 2953/cm, nu(sub 2 double prime) = 1375/cm, nu(sub 3 double prime) = 1062/cm, nu(sub 4 double prime) = 2869/cm, nu(sub 5 double prime) = 1528/cm and nu(sub 6 double prime) = 688/cm. A similar analysis of the wavelength-resolved emission spectra of CH3S provides the following ground state vibrational frequencies: nu(sub 2 double prime) = 1329/cm, nu(sub 3 double prime) = 739/cm and nu(sub 6 double prime) = 601/cm. An experimental uncertainty of 20/cm is estimated for the assigned frequencies.

Misra, Prabhakar↗

NASA NTRS2006

Stability Limits of a PD Controller for a Flywheel Supported on Rigid Rotor and Magnetic Bearings

Active magnetic bearings are used to provide a long-life, low-loss suspension of a high-speed flywheel rotor. This paper describes a modeling effort used to understand the stability boundaries of the PD controller used to control the active magnetic bearings on a high speed test rig. Limits of stability are described in terms of allowable stiffness and damping values which result in stable levitation of the nonrotating rig. Small signal stability limits for the system is defined as a nongrowth in vibration amplitude of a small disturbance. A simple mass-force model was analyzed. The force resulting from the magnetic bearing was linearized to include negative displacement stiffness and a current stiffness. The current stiffness was then used in a PD controller. The phase lag of the control loop was modeled by a simple time delay. The stability limits and the associated vibration frequencies were measured and compared to the theoretical values. The results show a region on stiffness versus damping plot that have the same qualitative tendencies as experimental measurements. The resulting stability model was then extended to a flywheel system. The rotor dynamics of the flywheel was modeled using a rigid rotor supported on magnetic bearings. The equations of motion were written for the center of mass and a small angle linearization of the rotations about the center of mass. The stability limits and the associated vibration frequencies were found as a function of nondimensional magnetic bearing stiffness and damping and nondimensional parameters of flywheel speed and time delay.

Kascak, Albert F.↗

NASA NTRS1996

Finite-Element Vibration Analysis and Modal Testing of Graphite Epoxy Tubes and Correlation Between the Data

Structural materials in the form of graphite epoxy composites with embedded rubber layers are being used to reduce vibrations in rocket motor tubes. Four filament-wound, graphite epoxy tubes were studied to evaluate the effects of the rubber layer on the modal parameters (natural vibration frequencies, damping, and mode shapes). Tube 1 contained six alternating layers of 30-degree helical wraps and 90-degree hoop wraps. Tube 2 was identical to tube 1 with the addition of an embedded 0.030-inch-thick rubber layer. Tubes 3 and 4 were identical to tubes 1 and 2, respectively, with the addition of a Textron Kelpoxy elastomer. This report compares experimental modal parameters obtained by impact testing with analytical modal parameters obtained by NASTRAN finite-element analysis. Four test modes of tube 1 and five test modes of tube 3 correlate highly with corresponding analytical predictions. Unsatisfactory correlation of test and analysis results occurred for tubes 2 and 4 and these comparisons are not shown. Work is underway to improve the analytical models of these tubes. Test results clearly show that the embedded rubber layers significantly increase structural modal damping as well as decrease natural vibration frequencies.

Taleghani, Barmac K.↗

NASA NTRS1995

Ab Initio Characterization of Triatomic Bromine Molecules of Potential Interest in Stratospheric Chemistry

The equilibrium structures, harmonic vibrational frequencies, quadratic force fields, dipole moments, and IR intensities of several triatomic bromine compounds of known or potential importance in stratospheric ozone depletion chemistry have been determined using the CCSD(T) electron correlation method in conjunction with a basis set of triple zeta double polarized (TZ2P) quality. Specifically, the molecules included in the present study are HOBr, HBrO, FOBr, FBrO, BrNO, BrON, Br2O, BrBrO, BrCN, BrNC, ClOBr, ClBrO, and BrClO. Very accurate isomeric energy differences have also been determined at the CCSD(T) level with atomic natural orbital basis sets that include through g-type functions. In most cases, the isomer with a normal neutral Lewis dot structure is the lowest energy form, with the single exception that FBRO is predicted to be 11.1 kcal/mol (0 K) lower in energy than FOBr. In all cases, however, the hypervalent isomer is more stable relative to the isomer with a normal Lewis dot structure as compared to the chlorine analogs. Consistent with this observation, the energy of the last three molecules given above increases in the order ClOBr less than ClBrO less than BrClO. The CCSD(T)/TZ2P geometries and vibrational frequencies are in good agreement with the available experimental data. Heats of formation are determined for all species using a combination of theoretical isomeric, homodesmic, and isodesmic reaction energies. The accuracy of these quantities is ultimately dependent on the reliability of the experimental heat of formation of HOBr.

Lee. Timothy J.↗

NASA NTRS1995

Ab Initio Characterization of Triatomic Bromine Molecules of Potential Interest in Stratospheric Chemistry

The equilibrium structures, harmonic vibrational frequencies, quadratic force fields, dipole moments, and IR intensities of several triatomic bromine compounds of known or potential importance in stratospheric ozone depletion chemistry have been determined using the CCSD(T) electron correlation method in conjunction with a basis set of triple zeta double polarized (TZ2P) quality. Specifically, the molecules included in the present study are HOBr, HBrO, FOBr, FBrO, BrNO, BrON, Br2O, BrBrO, BrCN, BrNC, ClOBr, ClBrO, and BrClO. Very accurate isomeric energy differences have also been determined at the CCSD(T) level with atomic natural orbital basis sets that include through g-type functions. In most cases, the isomer with a normal neutral Lewis dot structure is the lowest energy form, with the single exception that FBrO is predicted to be 11.1 kcal/mol (0 K) lower in energy than FOBr. In all cases, however, the hypervalent isomer is more stable relative to the isomer with a normal Lewis dot structure as compared to the chlorine analogs. Consistent with this observation, the energy of the last three molecules given above increases in the order ClOBr less than ClBrO less than BrClO. The CCSD(T)/TZ2P geometries and vibrational frequencies are in good agreement with the available experimental data. Heats of formation are determined for all species using a combination of theoretical isomeric, homodesmic, and isodesmic reaction energies. The accuracy of these quantities is ultimately dependent on the reliability of the experimental heat of formation of HOBr.

Lee, Timothy J.↗

NASA NTRS1974

Process to restore obliterated serial numbers on metal surfaces

Metal smeared into grooves of serial numbers by grinding or filing can be cleaned out by process called cavitation. Ultrasonic vibrator generates very high frequency vibrations in water which create millions of microscopic bubbles. Cavitation bubbles impact metal surface at thousands of pounds per square inch pressure. Metal particles filling grooves are broken away.

Young, S. G.↗