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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Modeling the double-trough structure observed in broad absorption line QSOs using radiative acceleration

We present a model explaining the double trough, separated by delta v approximately = 5900 km/s, observed in the C IV lambda-1549 broad absorption line (BAL) in a number of BALQSOs. The model is based on radiative acceleration of the BAL outflow, and the troughs result from modulations in the radiative force. Specifically, where the strong flux from the Lyman-alpha lambda-1215 broad emission line is redshifted to the frequency of the N V lambda-1240 resonance line, in the rest frame of the accelerating N V ions, the acceleration increases and the absorption is reduced. At higher velocities the Lyman-alpha emission is redshifted out of the resonance and the N V ions experience a declining flux which causes the second absorption trough. A strongly nonlinear relationship between changes in the flux and the optical depth in the lines is shown to amplify the expected effect. This model produces double troughs for which the shallowest absorption between the two troughs occurs at v approximately = 5900 km/s. Indeed, we find that a substantial number of the observed objects show this feature. A prediction of the model is that all BALQSOs that show a double-trough signature will be found to have an intrinsic sharp drop in their spectra shortward of approximately 1200 A.

Arav, Nahum↗
NASA NTRS1999

Towards Understanding the Mechanism of Receptivity and Bypass Dynamics in Laminar Boundary Layers

Three problems concerning laminar-turbulent transition are addressed by solving a series of initial value problems. The first problem is the calculation of resonance within the continuous spectrum of the Blasius boundary layer. The second is calculation of the growth of Tollmien-Schlichting waves that are a direct result of disturbances that only lie outside of the boundary layer. And, the third problem is the calculation of non-parallel effects. Together, these problems represent a unified approach to the study of freestream disturbance effects that could lead to transition. Solutions to the temporal, initial-value problem with an inhomogeneous forcing term imposed upon the flow is sought. By solving a series of problems, it is shown that: A transient disturbance lying completely outside of the boundary layer can lead to the growth of an unstable Tollmien-Schlichting wave. A resonance with the continuous spectrum leads to strong amplification that may provide a mechanism for bypass transition once nonlinear effects are considered. A disturbance with a very weak unstable Tollmien-Schlichting wave can lead to a much stronger Tollmien-Schlichting wave downstream, if the original disturbance has a significant portion of its energy in the continuum modes.

Lasseigne, D. G.↗
NASA NTRS1999

Near-Infrared Laser Pumped Intersubband THz Laser Gain in InGaAs-AlAsSb-InP Quantum Wells

We investigate the possibility of using InGaAs-AlAsSb-InP coupled quantum wells to generate THz radiation by means of intersubband optical pumping. We show that large conduction band offsets of these quantum wells make it possible to use conventional near-infrared diode lasers around 1.55 micron as pump sources. Taking into account the pump-probe coherent interaction and the optical nonlinearity for the pump field, we calculate the THz gain of the quantum well structure. We show that resonant Raman scattering enhances the THz gain at low and moderate optical pumping levels. When the pump intensity is strong, the THz gain is reduced by pump-induced population redistribution and pump-probe coherent interactions.

Liu, An-Sheng↗
NASA NTRS2007

Raman-Suppressing Coupling for Optical Parametric Oscillator

A Raman-scattering-suppressing input/ output coupling scheme has been devised for a whispering-gallery-mode optical resonator that is used as a four-wave-mixing device to effect an all-optical parametric oscillator. Raman scattering is undesired in such a device because (1) it is a nonlinear process that competes with the desired nonlinear four-wave conversion process involved in optical parametric oscillation and (2) as such, it reduces the power of the desired oscillation and contributes to output noise. The essence of the present input/output coupling scheme is to reduce output loading of the desired resonator modes while increasing output loading of the undesired ones.

Savchenkov, Anatoliy↗
NASA NTRS1993

Observations of nonlinear interactions in directionally spread shoaling surface gravity waves

Shoaling wave fields generated in laboratory experiments were analyzed to determine the sensitivity of nonlinear interactions to the directional distributions of incident waves. Peaks in the directional spectra observed in shallow water were consistent with near resonating, quadratic interactions between two primary waves transferring energy to a third wave with the sum frequency and vector sum wavenumber of the primary waves. Directionally colinear waves forced a higher-frequency wave propagating in the same direction as the primary waves, while directionally spread (i.e., noncolinear) primary waves forced a higher-frequency wave that propagated in a direction between those of the interacting primary waves. Deepwater wave fields with similar frequency spectra but different directional spectra evolved to different shallow-water directional spectra, yet their shallow-water frequency spectra were remarkably similar. This result suggests that the shape of the directional spectrum of the incident wave field has only a small effect on the magnitudes of nonlinear energy transfer during shoaling. The principal effect of directionality in the incident wave field is on the directions, not the amplitudes, of the nonlinearly generated waves. The laboratory data demonstrate clearly the importance of triad interactions between noncolinear and colinear shoaling waves.

Elgar, Steve↗
NASA NTRS1983

Terrestrial versus Jovian VLF chorus - A comparative study

The relevant parameters of the magnetospheres of Jupiter and earth are investigated based on the wave-particle resonant interactions that are believed to be responsible for the generation of VLF chorus emissions observed on Voyager 1. Expressions are derived for the wave-particle interaction length and the nonlinearity parameter, and the values of these parameters are compared with those calculated for the earth's magnetosphere. It is determined that the typical interaction lengths are at least 2-5 times larger in the Jovian than in the terrestrial magnetosphere, and that the wave intensity necessary to reach the threshold of nonlinearity in the Jovian magnetosphere is 5-100 times lower. Measurements by Voyager 1 show that the inferred wave magnetic field intensities of the Jovian chorus are in the range of reported intensities for terrestrial chorus, probably due to the fact that the fluxes of few keV resonant particles found in the Jovian magnetosphere were typically two orders of magnitude higher. Growth rate measurements on Voyager 1 broadband wave data are employed to confirm that the temporal growth rates of Jovian chorus bursts are higher than for the earth.

Inan, U. S.↗
NASA NTRS1987

Nonlinear absorption in AlGaAs/GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition

This paper reports the study of growth conditions for achieving the sharp exciton resonances and low-intensity saturation of these resonances in AlGaAs-GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition. Low growth temperature is necessary to observe this sharp resonance feature at room temperature. The optimal growth conditions are a tradeoff between the high temperatures required for high quality AlGaAs and low temperatures required for high-purity GaAs. A strong optical saturation of the excitonic absorption has been observed. A saturation density as low as 250 W/sq cm is reported.

Lee, H. C.↗
NASA NTRS1974

Effect of nonsymmetrical flow resistance upon orifice impedance

Previous laboratory work has indicated that an orifice in a thin sheet behaves in a quasisteady manner under acoustical excitation. Also, it has been found that the steady flow resistance of an orifice may be dependent upon the direction of flow, especially in the presence of a crossflow on one side of the hole. An analytical study is presented which assumes a nonreactive orifice in an infinite baffle. The pressure difference across the orifice varies sinusoidally with amplitude 1.0 and average value -P. The orifice resistance is discontinuous at zero velocity and exhibits the constant values when the velocity is nonzero. The resultant velocity has power in all harmonics of the excitation frequency, providing an explanation of the even harmonic excitation observed by other investigators, but not predicted by symmetric nonlinearity. A quasilinear resistance is defined and found to be relatively insensitive to the presence or absence of a resonant backing cavity.

Posey, J. W.↗
NASA NTRS1977

Revenge of tiny Miranda

An investigation is conducted concerning the resonance model for the rings of Uranus proposed by Dermott and Gold (1977). Dermott and Gold had dismissed resonances involving Miranda as insignificant. The reported investigation shows, however, that the strongest resonances are all associated with Miranda. It is also found that the hypothesis that the rings are made up of librating particles is incorrect. If the ring positions are determined by resonances, the control is more subtle than previously suggested. One possibility is that the rings are the crests of nonlinear density waves in an optically thin disk of particles.

Goldreich, P.↗
NASA NTRS1989

Time evolution from linear to nonlinear stages in magnetohydrodynamic parametric instabilities

The nonlinear evolution of the magnetohydrodynamic (MHD) parametric instability of wave fluctuations propagating along an unperturbed magnetic field is investigated. Both a magnetohydrodynamic perturbation-theoretical approach and a nonlinear MHD simulation are used. It is shown that high harmonic waves are rapidly excited by wave-wave coupling, and that the wave spectrum evolves from a state containing a small number of degrees of freedom in k space to one which contains a large number of degrees of freedom. It is found that the spectral evolution prior to nonlinear saturation is well described by the prturbation theory. During this stage, the ratio of the growth rate of the nth harmonic wave to the linear growth rate of the fundamental wave is n. The nonlinear saturation stage is characterized by a frequency shift of the fundamental wave that destroys the wave-wave resonance condition which, in turn, causes the wave amplitude to cease its growth.

Hoshino, M.↗
NASA NTRS1997

An Experimental Investigation on the Static Equilibria and Dynamics of Liquid Bridges

A liquid bridge is a volume of liquid held between two or more solid supports. In the case of small disk supports with a sharp edge, the contact line between the bridge and the support disk will be anchored along the edge of the disk. For these cases the solid presents a geometrical singularity and the contact angle is indeterminate within a given range. This dissertation presents research conducted on liquid bridges with anchored contact lines. The three major topics covered are: determining the role of support geometry on static equilibria, liquid bridge dynamical behavior, and forces exerted by a liquid bridge on a support structure. The work was primarily experimental and conducted in a "Plateau tank" that allowed for the simulation of equivalent low-gravity conditions. The main thrust of the experimental work involved the use of a high resolution optical measurement system for imaging the dynamic zone shape, measurement of the static and dynamic contact angles and non-invasive analysis of excited surface modes. The liquid bridge was manipulated by computer controlled linear actuators which allowed precise control over the physical characteristics of the bridge. Experiments have been carried out to locate a bifurcation point along the maximum volume axisymmetric stability margin. Below the critical slenderness the bifurcation from an axisymmetric to a stable nonaxisymmetric configuration is supercritical. However, above this critical slenderness, the bifurcation is subcritical. A series of experiments analyzed the effect on axisymmetric bridge stability by using support disks of different radii, The shape behavior as transition points were approached, as well as the limiting case of a vanishing support radius was investigated. Experiments were performed to determine the resonant frequencies of axisymmetric bridges subject to lateral vibrations. Anomolous results led to a series of experiments to characterize nonlinearities present in the dynamic bridge shape. Finally, an attempt was made to experimentally measure the force exerted by the bridge on the lower support disk. This was done through use of a force balance apparatus. Particular attention was paid to the behavior of the bridge as the minimum volume stability limit was approached.

Resnick, Andrew Howard↗
NASA NTRS1979

Non-linear dynamic response of a wind turbine blade

The paper outlines the nonlinear dynamic analysis of an isolated three-degree flap-lag-feather wind turbine blade under a gravity field and with shear flow. Lagrangian equations are used to derive the nonlinear equations of motion of blade for arbitrarily large angular deflections. The limit cycle analysis for forced oscillations and the determination of the principal parametric resonance of the blade due to periodic forces from the gravity field and wind shear are performed using the harmonic balance method. Results are obtained first for a two-degree flap-lag blade, then the effect of the third degree of freedom (feather) is studied. The self-excited flutter solutions are obtained for a uniform wind and with gravity forces neglected. The effects of several parameters on the blade stability are examined, including coning angle, structural damping, Lock number, and feather frequency. The limit cycle flutter solution of a typical configuration shows a substantial nonlinear softening spring behavior.

Chopra, I.↗
NASA NTRS1972

Turbulence of electrostatic electron cyclotron harmonic waves observed by Ogo 5.

Analysis of VLF emissions that have been observed near 3/2, 5/2, and 7/2 f sub H by Ogo 5 in the magnetosphere (f sub H is the electron cyclotron frequency) in the light of the mechanism used for the diffuse plasma resonance f sub Dn observed by Alouette 2 and Isis 1. The VLF emission is considered to be generated by nonlinear coupling mechanisms in certain portions of the observation as the f sub Dn is enhanced by its association with nonlinear wave-particle interaction of the electrostatic electron cyclotron harmonic wave, including the instability due to the nonlinear inverse Landau damping mechanism in the turbulence. The difference between the two observations is in the excitation mechanism of the turbulence; the turbulence in the plasma trough detected by Ogo 5 is due to natural origins, whereas the ionospheric topside sounder makes the plasma wave turbulence artificially by submitting strong stimulation pulses. Electron density values in the plasma trough are deduced by applying the f sub Dn-f sub N/f sub H relationship obtained from the Alouette 2 experiment as well as by applying the condition for the wave-particle nonlinear interactions. The electron density values reveal good agreement with the ion density values observed simultaneously by the highly sensitive ion mass spectrometer.

Oya, H.↗
NASA NTRS2001

Design and Fabrication of a Fabry-Perot Electrooptic Modulator

The research to design a Fabry-Perot electrooptic modulator with an intracavity electrooptically active organic material is based on the initial results of Wang et. al. using poled polymer thin films. The main feature of the proposed device is the observation that in traditional electrooptic modulators like a Pockels cell, it requires few kilovolts of driving voltage to cause a 3 dB modulation even in high figure-of-merit electrooptic materials like LiNbO3. The driving voltage for the modulator can be reduced to as low as 10 volts by introducing the electrooptic material inside the resonant cavity of a Fabry-Perot modulator. This is because the transmission of the Fabry-Perot cavity varies nonlinearly with the change of refractive index or phase of light due to applied electric field. We describe in this report the progress made so far in the design and fabrication of the proposed device.

Banks, C.↗
NASA NTRS2001

Design and Fabrication of a Fabry-Perot Electrooptic Modulator

The research to design a Fabry-Perot electrooptic modulator with an intracavity electrooptically active organic material is based on the initial results of Wang et. al. [1] using poled polymer thin films. The main feature of the proposed device is the observation that in traditional electrooptic modulators like a Pockels cell, it requires few kilovolts of driving voltage to cause a 3 dB modulation even in high figure-of-merit electrooptic. materials like LiNbO3. The driving voltage for the modulator can be reduced to as low as 10 volts by introducing the electrooptic material inside the resonant cavity of a Fabry-Perot modulator. This is because the transmission of the Fabry-Perot cavity varies nonlinearly with the change of refractive index or phase of light due to applied electric field. We describe in this report the progress made so far in the design and fabrication of the proposed device.

Banks, C.↗
NASA NTRS2002

Fabry-Perot Interferometer-Based Electrooptic Modulator using LiNbO3 and Organic Thin Films

We report the study of a Fabry-Perot electro-optical modulator using thin crystalline film NPP, and Crystalline LiNbO3. We are able to observe 14, and 60 percent degree of modulation. Measurements were carried using a standard lock-in amplifier with a silicon detector. The proposal to design a Fabry-Perot electro-optic modulator with an intracavity electro-optically active organic material was based on the initial results using poled polymer thin films. The main feature of the proposed device is the observation that in traditional electrooptic modulators like a Packets cell, it requires few kilovolts of driving voltage to cause a 3 dB modulation even in high figure-of-merit electrooptic materials like LiNbO3. The driving voltage for the modulator can be reduced to as low as 10 volts by introducing the electrooptic material inside die resonant cavity of a Fabry-Perot modulator. This is because the transmission of the Fabry-Perot cavity varies nonlinearly with the change of refractive index or phase of light due to applied electric field.

Banks, C.↗
NASA NTRS1999

Semi-monolithic cavity for external resonant frequency doubling and method of performing the same

The fabrication of an optical cavity for use in a laser, in a frequency doubling external cavity, or any other type of nonlinear optical device, can be simplified by providing the nonlinear crystal in combination with a surrounding glass having an index of refraction substantially equal to that of the nonlinear crystal. The closed optical path in this cavity is formed in the surrounding glass and through the nonlinear crystal which lies in one of the optical segments of the light path. The light is transmitted through interfaces between the surrounding glass in the nonlinear crystal through interfaces which are formed at the Brewster-angle to minimize or eliminate reflection.

Hemmati, Hamid↗