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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Non-linear resonances in the forced responses of plates. I - Symmetric responses of circular plates

The dynamic analogue of the von Karman equations is used to study the symmetric response of a circular plate to a harmonic excitation when the frequency of the excitation is near one of the natural frequencies. It is shown that, in general, when there is no internal resonance (i.e., the natural frequencies are not commensurable), only the mode having a frequency near that of the excitation is strongly excited (i.e., is needed to represent the response in the first approximation). A clamped, circular plate is used as a numerical example to show that, when there is an internal resonance, more than one of the modes involved in this resonance can be strongly excited; moreover, when more than one mode is strongly excited, the lower modes can dominate the response, even when the frequency of the excitation is near that of the highest mode. This possibility was not revealed by any of the earlier studies which were based on the same governing equations.

Sridhar, S.↗
NASA NTRS1975

Coupling of Helmholtz resonators to improve acoustic liners for turbofan engines at low frequency

An analytical and test program was conducted to evaluate means for increasing the effectiveness of low frequency sound absorbing liners for aircraft turbine engines. Three schemes for coupling low frequency absorber elements were considered. These schemes were analytically modeled and their impedance was predicted over a frequency range of 50 to 1,000 Hz. An optimum and two off-optimum designs of the most promising, a parallel coupled scheme, were fabricated and tested in a flow duct facility. Impedance measurements were in good agreement with predicted values and validated the procedure used to transform modeled parameters to hardware designs. Measurements of attenuation for panels of coupled resonators were consistent with predictions based on measured impedance. All coupled resonator panels tested showed an increase in peak attenuation of about 50% and an increase in attenuation bandwidth of one one-third octave band over that measured for an uncoupled panel. These attenuation characteristics equate to about 35% greater reduction in source perceived noise level (PNL), relative to the uncoupled panel, or a reduction in treatment length of about 24% for constant PNL reduction. The increased effectiveness of the coupled resonator concept for attenuation of low frequency broad spectrum noise is demonstrated.

Dean, L. W.↗
NASA NTRS1976

An Experimental Investigation of Fluid Flow and Heating in Various Resonance Tube Modes

Experiments have been performed to study resonance phenomena in tubes excited by underexpanded jet flows. This investigation comprised the following: Study of the various resonance tube modes under a wide range of nozzle pressure, spacing between nozzle and tube mouth, and different tube length; the effects of these modes on the endwall pressure and gas temperature; flow visualization of both jet and tube flows by spark shadowgraph technique; and measurement of wave speed inside the tube by the laser-schlieren techniques. An extensive study of the free-jet flow was undertaken to explain important aspects of various modes of operation of resonance tube flows.

Sarohia, V.↗
NASA NTRS1976

Direct observation of the optical plasma resonance of Ag by photon-assisted tunneling

Photon-assisted tunneling is proposed for the investigation of optical plasma resonances; this is demonstrated by the direct observation of the Ag resonance in appropriate Ag-Al2O3-Al structures. A spectral scan of the ratio of the signals photoinduced by the p and s polarizations shows a large enhancement corresponding to the plasma resonance of Ag.

Jain, R. K.↗
NASA NTRS1976

Secular resonance, solar spin down, and the orbit of Mercury

A mechanism is investigated which may provide an evolutionary explanation for the large mean eccentricity and inclination of Mercury. It is proposed that if the gravitational field of the rapidly rotating early sun had a larger second-degree harmonic, the decreasing value of this harmonic during the subsequent solar spindown would drive Mercury through two secular resonances with Venus, one involving a commensurability in the apsidal motion of the two planets and the other involving their nodal rates. An analysis is performed, showing that these resonances could increase both the inclination and eccentricity of Mercury at nearly the same time, that an initial solar rotational period of 5.5 hr or less would guarantee passage through the resonances, and that a spindown time of about 1 million years could have produced the observed inclination and eccentricity.

Ward, W. R.↗
NASA NTRS1976

Dynamical study of the Hilda asteroids. I - Resonant orbital motion of the PLS objects from the Palomar-Leiden Survey

Schubart's (1968) model of a planar elliptic restricted three-body problem is used to study the orbital motion of the Hilda asteroids from the Palomar-Leiden Survey. The 3:2 resonant coupling to Jupiter of some of these small asteroids is found to be stable. However, some of the small asteroids with absolute magnitude greater than 15 have large amplitude of variation in their orbital elements in one libration period. Since the lifetime scales against catastrophic collision of the Hilda asteroids are estimated to be several times larger than those of the main-belt objects, a significant portion of these resonant asteroids could be the original members of the Hilda group. From this point of view, it is suggested that such 'size dependence' of resonant orbital motions might be the result of cosmogonic effects of jet-stream accretion.

Ip, W.-H.↗
NASA NTRS1977

Collisionless dissociation of SF6 using two resonant frequency CO2 laser fields

The collisionless dissociation of SF6 has been studied using simultaneous irradiation by two frequencies from a CO2 laser which are both nearly resonant with the SF6nu3 absorption band. It was found that the dissociation was enhanced, and occurred over a wider frequency range, than for single frequency dissociation. No threshold effect was observed for a weak resonant and a much higher energy field pumping slightly off-resonance. For such two frequency irradiation, the peak in the dissociation curve was found to be shifted to lower frequencies with respect to that for single frequency dissociation.

Gower, M. C.↗
NASA NTRS1979

Calculation of resonant effects in electron-impact excitation of positive ions Application to oxygen VII

The general reaction theory of Feshbach is applied to the calculation of resonant effects in near-threshold electron-positive-ion excitation. The theory divides configuration space into open- and closed-channel parts, resonance effects being described by the closed-channel part. The open-channel part is handled in a distorted-wave approximation to the set of open-channel coupled equations. Various methods are suggested for handling the closed-channel part. However, an 'attached-excited-target approximation' is used explicitly, which is further approximated by a set of uncoupled closed-channel equations. As an example, the 1 1S-2 1P excitation cross section of O VII below the 3 3S threshold is calculated. Various distorted-wave approximations are investigated and results from most of them are quite similar. Resonant effects arising from the attachment of the colliding electron with the 3 3S state are found to be small, but other close-lying n = 3 states have not yet been included.

Pindzola, M. S.↗
NASA NTRS1979

Intense electrostatic waves near the upper hybrid resonance frequency

Plasma wave measurements using instruments on the Imp 6 and Hawkeye satellites are utilized in a study of very intense electrostatic waves near the upper hybrid resonance frequency in the region just outside the plasmapause. Studies of these electrostatic disturbances show that the events occur at local times and at magnetic latitudes varying from the equator to 50 deg, and the polarization of these waves is such that the wave electric field vector is oriented perpendicular to the geomagnetic field. In most cases the center frequency of the intense waves corresponds to an (n + 1/2) fg(-) harmonic near the upper hybrid resonance frequency. The hot distribution on function is described for a few events showing temperature anisotropy and a loss cone distribution. A possible mechanism for producing intense waves near the upper hybrid resonance frequency is suggested, and evidence which indicates that the intense electrostatic waves may be a source of nonthermal continuum radiation is given.

Kurth, W. S.↗
NASA NTRS1980

The influence of wall resonances on the levitation of objects in a single-axis acoustic processing chamber

Instabilities were observed in high temperature, single axis acoustic processing chambers. At certain temperatures, strong wall resonances were generated within the processing chamber itself and these transverse resonances were thought sufficient to disrupt the levitation well. These wall resonances are apparently not strong enough to cause instabilities in the levitation well.

Ross, B. B.↗
NASA NTRS1981

A theory for predicting boundary impedance and resonance frequencies of slotted-wall wind tunnels, including plenum effects

Wave-induced resonance associated with the geometry of wind-tunnel test sections can occur. A theory that uses acoustic impedance concepts to predict resonance modes in a two dimensional, slotted wall wind tunnel with a plenum chamber is described. The equation derived is consistent with known results for limiting conditions. The computed resonance modes compare well with appropriate experimental data. When the theory is applied to perforated wall test sections, it predicts the experimentally observed closely spaced modes that occur when the wavelength is not long compared with he plenum depth.

Barger, R. L.↗
NASA NTRS1981

Radiation losses in resonant tubes

The role of radiation losses in a resonant tube - the transfer of sound energy from the test gas through the confines of the tube to the ambient atmosphere - is investigated both theoretically and experimentally. It is concluded that losses of this sort make no noticeable contribution to the background losses, even at high gas pressures. Nevertheless, if an axial resonant frequency of the test gas closely matches a longitudinal resonant frequency of the tube wall, the internal sound pressure can excite tube displacements of disproportionate magnitude and thus cause excessive losses, not due to radiation but rather to frictional dissipation related to the motion of the tube structure. This mechanism is effective when the mode numbers of the gas and tube have opposite parity, and makes sound absorption measurements at the fundamental gas frequency particularly problematical. An experimental investigation confirms the existence of such additional losses when the parity condition is fulfilled.

Zuckerwar, A. J.↗
NASA NTRS1983

Using SAW Resonators in RF Oscillators

Surface-acoustic-wave (SAW) resonators used as frequency-determining elements in radio-frequency oscillators circuits. Oscillators are frequencymodulated, phase-modulated, or pulse-modulated. SAW resonators are especially applicable to low-power subminiature applications, such as biotelemetry and wind-tunnel instrumentation, where they advantageously replace crystals. Resonators are smaller than crystals and very thin--advantage where small package size is important.

Westbrook, R. M.↗
NASA NTRS1981

IUE observations of pre-main-sequence stars. I - Mg II and Ca II resonance line fluxes for T Tauri stars

IUE satellite and Lick 3 m reflector image tube scanner measurements of the Mg II and Ca II resonance lines in a sample of T Tauri stars are the basis of a discussion of the Mg II h and k line emission and the Ca II H and K line emission, within the context of stellar chromospheres. Corroborative evidence is presented for the chromospheric origin of these resonance lines, and chromospheric radiative loss rates in the Mg II and Ca II resonance lines are derived. It is found that the degree of nonradiative heating present in the outer atmospheres of T Tauri stars generally exceeds that of the RS CVn systems, as well as the dMe stars and other active chromospheric dwarfs, and it is inferred that the surfaces of such pre-main sequence stars are covered by regions similar to solar plages. The mean chromospheric electron density of T Tauri stars is determined as 10 to the 11th/cu cm.

Giampapa n, M. S.↗
NASA NTRS1982

Correlation of the effects of grazing flow on the impedance of Helmholtz resonators

Measurements of the acoustic impedance of a two-dimensional Helmholtz resonator, i.e., a cavity coupled to the environment through a high-aspect ratio slot are presented as a function of the velocity of a steady flow above the orifice of the resonator. The structure of the unsteady flow above the orifice is outlined from hot-wire measurements. A simplified model of it is proposed and analyzed, leading to a successful correlation of both the resistance and the reactance of the resonator.

Walker, B. E.↗
NASA NTRS1982

Cyclotron resonance effects on stochastic acceleration of light ionospheric ions

The production of energetic ions with conical pitch angle distributions along the auroral field lines is a subject of considerable current interest. There are several theoretical treatments showing the acceleration (heating) of the ions by ion cyclotron waves. The quasi-linear theory predicts no acceleration when the ions are nonresonant. In the present investigation, it is demonstrated that the cyclotron resonances are not crucial for the transverse acceleration of ions by ion cyclotron waves. It is found that transverse energization of ionospheric ions, such as He(+), He(++), O(++), and O(+), is possible by an Electrostatic Hydrogen Cyclotron (EHC) wave even in the absence of cyclotron resonance. The mechanism of acceleration is the nonresonant stochastic heating. However, when there are resonant ions both the total energy gain and the number of accelerated ions increase with increasing parallel wave number.

Singh, N.↗