Search NASA⌕ Search

SEARCH · Search NASA

Results for “general oscillator”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

A Theory of Oscillating Edge Flames

It has been known for some years that when a near-limit flame spreads over a liquid pool of fuel, the edge of the flame can oscillate relative to a frame moving with the mean speed. Each period of oscillation is characterized by long intervals of modest motion during which the edge gases radiate like those of a diffusion flame, punctuated by bursts of rapid advance during which the edge gases radiate like those in a deflagration. Substantial resources have been brought to bear on this issue within the microgravity program, both experimental and numerical. It is also known that when a near-asphyxiated candle-flame burns at zero gravity, the edge of the (hemispherical) flame can oscillate violently prior to extinction. Thus a web-surfer, turning to the NASA web-site at http://microgravity.msfc.nasa.gov, and following the trail combustion science/experiments/experimental results/candle flame, will find photographs and a description of candle burning experiments carried out on board both the Space-shuttle and the Russian space station Mir. A brief report can also be found in the proceedings of the Fourth Workshop. And recently, in a third microgravity program, the leading edge of the flame supported by injection of ethane through the porous surface of a plate over which air is blown has been found to oscillate when conditions are close to blow-off. A number of important points can be made with respect to these observations: It is the edge itself which oscillates, advancing and retreating, not the diffusion flame that trails behind the edge; oscillations only occur under near limit conditions; in each case the Lewis number of the fuel is significantly larger than 1; and because of the edge curvature, the heat losses from the reacting edge structure are larger than those from the trailing diffusion flame. We propose a general theory for these oscillations, invoking Occam's 'Law of Parsimony' in an expanded form, to wit: The same mechanism is responsible for the oscillations in all three experiments; and no new mechanism is invoked (Occam's original 'Razor'). Such a strategy eliminates Marangoni effects as the source, for these are absent in the second and third experiments. And it eliminates arguments that point to numerically predicted gas eddies as the source, a new mechanism, unelucidated. Indeed, we hypothesize that the essential driving mechanism for the instability is a combination of large Lewis number and heat losses from the reacting structure near the flame edge. Instabilities driven by these mechanisms are commonplace in 1D configurations. Chemical reactor theory, for example, leads to system responses which mimic the response of the candle flame - steady flame, oscillations, extinction. In a combustion context, oscillating instabilities were first reported for diffusion flames in a theoretical study by Kirkby and Schmitz, and here also the instabilities are associated with near-extinction conditions, large Lewis numbers, and heat losses. And deflagrations will oscillate if the Lewis number is large enough, oscillations that are exacerbated when heat losses are present, whether global or to a surface.

Buckmaster, J.↗
NASA NTRS2015

Evaluation of Start Transient Oscillations with the J-2X Engine Gas Generator Assembly

During development of the gas generator for the liquid oxygen/liquid hydrogen propellant J-2X rocket engine, distinctive and oftentimes high-amplitude pressure oscillations and hardware vibrations occurred during the start transient of nearly every workhorse gas generator assembly test, as well as during many tests of engine system hardware. These oscillations appeared whether the steady-state conditions exhibited stable behavior or not. They occurred similarly with three different injector types, and with every combustion chamber configuration tested, including chamber lengths ranging over a 5:1 range, several different nozzle types, and with or without a side branch line simulating a turbine spin start gas supply line. Generally, two sets of oscillations occurred, one earlier in the start transient and at higher frequencies, and the other almost immediately following and at lower frequencies. Multiple dynamic pressure measurements in the workhorse combustion chambers indicated that the oscillations were associated with longitudinal acoustic modes of the combustion chambers, with the earlier and higher frequency oscillation usually related to the second longitudinal acoustic mode and the later and lower frequency oscillation usually related to the first longitudinal acoustic mode. Given that several early development gas generator assemblies exhibited unstable behavior at frequencies near the first longitudinal acoustic modes of longer combustion chambers, the start transient oscillations are presumed to provide additional insight into the nature of the combustion instability mechanisms. Aspects of the steadystate oscillations and combustion instabilities from development and engine system test programs have been reported extensively in the three previous JANNAF Liquid Propulsion Subcommittee meetings (see references below). This paper describes the hardware configurations, start transient sequence operations, and transient and dynamic test data during the start transient. The implications of these results on previous analyses and understanding of the combustion instability observed during steady-state conditions, especially the effects of injector influences, is discussed.

Hulka, J. R.↗
NASA NTRS1986

Compton-heated winds and coronae above accretion disks. II - Instability and oscillations

The stability and evolution of windy accretion disks is investigated in detail. The basic disk evolution equations are briefly recapitulated, and an idealized analytic treatment of the wind and viscosity is used to show that steady disk flow is indeed unstable for sufficiently large ratio of the mass loss rate in the wind to the central accretion rate. Numerical solutions for a more realistic and appropriate expression for the mass loss rate and the standard ad hoc alpha model prescription for the viscosity are presented. The application of these results to real systems with Compton-heated winds is discussed, and a general formula for the oscillation period is given. The prediction is compared with observed periodic behavior of Galactic X-ray sources and AGNs.

Shields, G. A.↗
NASA NTRS1987

Magnetic monopole plasma oscillations and the survival of Galactic magnetic fields

This paper explores the general nature of magnetic-monopole plasma oscillations as a theoretical possibility for the observed Galactic magnetic field in the presence of a high abundance of magnetic monopoles. The modification of the hydromagnetic induction equation by the monopole oscillations produces the half-velocity effect, in which the magnetic field is transported bodily with a velocity midway between the motion of the conducting fluid and the monopole plasma. Observational studies of the magnetic field in the Galaxy, and in other galaxies, exclude the half-velocity effect, indicating that the magnetic fields is not associated with monopole oscillations. In any case the phase mixing would destroy the oscillations in less than 100 Myr. The conclusion is that magnetic monopole oscillations do not play a significant role in the galactic magnetic fields. Hence the existence of galactic magnetic fields places a low limit on the monopole flux, so that their detection - if they exist at all - requires a collecting area at least as large as a football field.

Parker, E. N.↗
NASA NTRS1989

A review of atomic clock technology, the performance capability of present spaceborne and terrestrial atomic clocks, and a look toward the future

Clocks have played a strong role in the development of general relativity. The concept of the proper clock is presently best realized by atomic clocks, whose development as precision instruments has evolved very rapidly in the last decades. To put a historical prospective on this progress since the year AD 1000, the time stability of various clocks expressed in terms of seconds of time error over one day of operation is shown. This stability of operation must not be confused with accuracy. Stability refers to the constancy of a clock operation as compared to that of some other clocks that serve as time references. Accuracy, on the other hand, is the ability to reproduce a previously defined frequency. The issues are outlined that must be considered when accuracy and stability of clocks and oscillators are studied. In general, the most widely used resonances result from the hyperfine interaction of the nuclear magnetic dipole moment and that of the outermost electron, which is characteristic of hydrogen and the alkali atoms. During the past decade hyperfine resonances of ions have also been used. The principal reason for both the accuracy and the stability of atomic clocks is the ability of obtaining very narrow hyperfine transition resonances by isolating the atom in some way so that only the applied stimulating microwave magnetic field is a significant source of perturbation. It is also important to make resonance transitions among hyperfine magnetic sublevels where separation is independent, at least to first order, of the magnetic field. In the case of ions stored in traps operating at high magnetic fields, one selects the trapping field to be consistent with a field-independent transition of the trapped atoms.

Vessot, Robert F. C.↗
NASA NTRS1987

Low-frequency atmospheric oscillations over the southeastern Pacific

A 40-50 day atmospheric oscillation has recently been reported in the southeastern Pacific based on analyses of satellite-derived microwave brightness temperature data. Prior to this, such oscillations have not been generally recognized in this region. The purpose of this note is to provide corroboration of the microwave analyses from an independent data set, four years of rawinsonde data from Easter Island. Significant spectral peaks with period 45-53 days were found in the zonal wind near the jet core (300-200 mb) and in the temperature on the underside of the jet. The signal is also present in the 100 mb temperature as shown by strong coherence between 100 and 400 mb temperature perturbations. Direct comparison of the 100 mb temperature perturbations and the microwave data shows good agreement. There is also an indication of a lower frequency signal, about 65-84 day period, in the meridional wind at 1000 mb. This signal appears to be vertically evanescent with maximum amplitude at the surface.

Graves, Charles E.↗
NASA NTRS1989

CDAW 8 observations of plasmoid signatures in the geomagnetic tail - An assessment

Magnetotail observations from the ISEE 3 distant (1983) tail mission taken during the Coordinated Data Analysis Workshop 8 (CDAW 8) A and G events are investigated. The ISEE 3 magnetic field, plasma, and energetic particle measurements taken in these two plasmoids have been analyzed and compared with various equilibrium structures and propagating waves/tail oscillation modes. Results indicate general agreement with either the closed-loop (Hones, 1977) or very small pitch angle flux rope (Hughes and Sibeck, 1987; Birn et al., 1989) models of plasmoid structure and poorer agreement with other hypotheses. Calculations based upon typical plasmoid and tail parameters are presented, indicating that the J and B force associated with the disconnected lobe field lines may be sufficient to accelerate plasmoids up to the speeds observed by ISEE 3. Overall, the energy expended in accelerating the plasmoids down the tail appears comparable to that dissipated in the inner magnetosphere and ionosphere. The study produces strong evidence in favor of the plasmoid model of substorm tail dynamics.

Slavin, J. A.↗
NASA NTRS1993

Quantum harmonic oscillator in a thermal bath

The influence functional path-integral treatment of quantum Brownian motion is briefly reviewed. A newly derived exact master equation of a quantum harmonic oscillator coupled to a general environment at arbitrary temperature is discussed. It is applied to the problem of loss of quantum coherence.

Zhang, Yuhong↗
NASA NTRS1996

Collision Strengths for Electron Impact Excitation of Inelastic Transitions in Ar II

We have calculated collision strengths for electron impact excitation of inelastic transitions in Ar II using the R-matrix method in two independent nine- and 19-state close-coupling approximations. In the nine-state calculation the 3s(sup 2)3p(sup 5)2p(sup 0), 3S(sup 3)p(sup 6)S-2, 3p(sup 4)(P-3)4s(sup 2)P, 3p(sup 4)(P-3)3d(sup 2)P, 3p(sup 4)(D-1)4s(sup 2)D, 3p(sup 4)(P-3)3d(sup 2)D, 3p(sup 4)(S-1)4s(sup 2)S, 3p(sup 4)(D-1)3d(sup 2)S and 3p(sup 4)(D-1)4d(sup 2)S states are included, while in the 19-state calculation these states plus an additional ten states 3p(sup 4)(P-3)3d(sup 2)F, 3p(sup 4)(P-3)4p(sup 2)D(sup 0), 2p(sup 0), 2S(sup 0), 3p(sup 4)(D-1)4p(sup 2)P(sup 0), 2D(sup O), 2F(sup 0), 3p(sup 4)(D-1)3d(sup 2)D, p-2 and 3p(sup 4)(S-1)4p(sup 2)P(sup 0) are considered. These target states are represented by fairly extensive configuration-interaction wavefunctions which yield excitation energies and oscillator strengths that are generally in good agreement with the available most accurate calculations and the experimental values. Rydberg series of resonances converging to the excited state thresholds are included in the calculation. The effective collision strengths are obtained assuming a Maxwellian distribution of electron energies which are tabulated over the temperature range (0.5-20) x 10(exp 4) K.

Tayal, S. S.↗
NASA NTRS2004

Evaluation and Analysis of F-16XL Wind Tunnel Data From Static and Dynamic Tests

A series of wind tunnel tests were conducted in the NASA Langley Research Center as part of an ongoing effort to develop and test mathematical models for aircraft rigid-body aerodynamics in nonlinear unsteady flight regimes. Analysis of measurement accuracy, especially for nonlinear dynamic systems that may exhibit complicated behaviors, is an essential component of this ongoing effort. In this report, tools for harmonic analysis of dynamic data and assessing measurement accuracy are presented. A linear aerodynamic model is assumed that is appropriate for conventional forced-oscillation experiments, although more general models can be used with these tools. Application of the tools to experimental data is demonstrated and results indicate the levels of uncertainty in output measurements that can arise from experimental setup, calibration procedures, mechanical limitations, and input errors.

Kim, Sungwan↗
NASA NTRS2003

Evaluation and Analysis of F-16XL Wind Tunnel Data from Dynamic Tests

A series of wind tunnel tests were conducted in the NASA Langley Research Center as part of an ongoing effort to develop and test mathematical models for aircraft rigid-body aerodynamics in nonlinear unsteady flight regimes. Analysis of measurement accuracy, especially for nonlinear dynamic systems that may exhibit complicated behaviors, is an essential component of this ongoing effort. In this paper, tools for harmonic analysis of dynamic data and assessing measurement accuracy are presented. A linear aerodynamic model is assumed that is appropriate for conventional forced-oscillation experiments, although more general models can be used with these tools. Application of the tools to experimental data is demonstrated and results indicate the levels of uncertainty in output measurements that can arise from experimental setup, calibration procedures, mechanical limitations, and input errors.

Kim, Sungwan↗
NASA NTRS1997

Aeroelastic Analysis of Counter Rotation Fans

Aeroelastic problems in turbomachinery and propfans can be static or dynamic in nature. The analysis of static aeroelastic problems is involved primarily with determination: (a) of the shape of the blades and the steady aerodynamic loads on the blades (which are inter-dependent), (b) of the resultant steady stresses and (c) of the static instability (divergence) margin, if applicable. In this project, we were concerned exclusively with dynamic aeroelastic behavior. The analysis of dynamic aeroelastic problems is involved with the determination: (a) of the unsteady aerodynamic loads on blades and the dynamic motion of the blades (which are again inter-dependent), (b) of the resultant dynamic stresses and their effect on fatigue life and (c) of the dynamic instability (flutter), if applicable. There are two primary dynamic aeroelastic phenomena of interest to designers of turbomachinery and propfans: flutter and forced response. Flutter generally refers to the occurrence of rapidly growing self-excited oscillations leading to catastrophic failure of the blade. When certain nonlinear phenomena are present, flutter response may lead to a potentially dangerous limit cycle oscillation rather than an immediate catastrophic failure. Forced response generally refers to the steady-state oscillations that occur as a consequence of excitations external to the rotor in question. These excitations typically result from the presence of upstream obstructions, inflow distortions, downstream obstructions, or mechanical sources such as tip-casing contact or shaft and gear meshing. Significant forced response leads to blade fatigue, and at design conditions, generally contributes to a degradation of blade life. At other operating conditions, forced response may lead to catastrophic failure due to severe blade fatigue in a short duration of time.

Keith, Theo G., Jr.↗
NASA NTRS1982

Raindrop oscillations

A model of the change in shape of a raindrop is presented. Raindrops measured by two orthogonal cameras were classified by shape and orientation to determine the nature of the oscillation. A physical model based on potential energy was then developed to study the amplitude variation of oscillating drops. The model results show that oscillations occur about the equilibrium axis ratio, but the time average axis ratio if significantly more spherical for large amplitudes because of asymmetry in the surface potential energy. A generalization of the model to oscillations produced by turbulence yields average axis ratios that are consistent with the camera measurements. The model results for average axis ratios were applied to rainfall studies with a dual polarized radar.

Beard, K. V.↗
NASA NTRS1988

Properties of finite difference models of non-linear conservative oscillators

Finite-difference (FD) approaches to the numerical solution of the differential equations describing the motion of a nonlinear conservative oscillator are investigated analytically. A generalized formulation of the Duffing and modified Duffing equations is derived and analyzed using several FD techniques, and it is concluded that, although it is always possible to contstruct FD models of conservative oscillators which are themselves conservative, caution is required to avoid numerical solutions which do not accurately reflect the properties of the original equation.

Mickens, R. E.↗
NASA NTRS1990

An intercomparison of intraseasonal variability in general circulation models and observations

Low frequency oscillations appearing in three GCM seasonal cycle integrations are compared with the analyses of the European Center for Medium Range Weather Forecasting (ECMWF). All three models have the same resolution: 4 deg latitude by 5 deg longitude, with 9 levels. The dominant phase speeds and the differential vertical structure of the heating profiles in the GCMs are in general agreement with current theory involving the positive feedback between latent heating and moist static stability. All three GCMs fail to capture the detailed evolution in the different stages of the development and decay of the oscillation. The results suggest that an improvement in the boundary layer moisture processes may be crucial for a better simulation of the oscillation.

Park, Chung-Kyu↗
NASA NTRS1986

Hopf bifurcation with dihedral group symmetry - Coupled nonlinear oscillators

The theory of Hopf bifurcation with symmetry developed by Golubitsky and Stewart (1985) is applied to systems of ODEs having the symmetries of a regular polygon, that is, whose symmetry group is dihedral. The existence and stability of symmetry-breaking branches of periodic solutions are considered. In particular, these results are applied to a general system of n nonlinear oscillators coupled symmetrically in a ring, and the generic oscillation patterns are described. It is found that the symmetry can force some oscillators to have twice the frequency of others. The case of four oscillators has exceptional features.

Golubitsky, Martin↗
NASA NTRS2003

Resonance Effects in the NASA Transonic Flutter Cascade Facility

Investigations of unsteady pressure loadings on the blades of fans operating near the stall flutter boundary are carried out under simulated conditions in the NASA Transonic Flutter Cascade facility (TFC). It has been observed that for inlet Mach numbers of about 0.8, the cascade flowfield exhibits intense low-frequency pressure oscillations. The origins of these oscillations were not clear. It was speculated that this behavior was either caused by instabilities in the blade separated flow zone or that it was a tunnel resonance phenomenon. It has now been determined that the strong low-frequency oscillations, observed in the TFC facility, are not a cascade phenomenon contributing to blade flutter, but that they are solely caused by the tunnel resonance characteristics. Most likely, the self-induced oscillations originate in the system of exit duct resonators. For sure, the self-induced oscillations can be significantly suppressed for a narrow range of inlet Mach numbers by tuning one of the resonators. A considerable amount of flutter simulation data has been acquired in this facility to date, and therefore it is of interest to know how much this tunnel self-induced flow oscillation influences the experimental data at high subsonic Mach numbers since this facility is being used to simulate flutter in transonic fans. In short, can this body of experimental data still be used reliably to verify computer codes for blade flutter and blade life predictions? To answer this question a study on resonance effects in the NASA TFC facility was carried out. The results, based on spectral and ensemble averaging analysis of the cascade data, showed that the interaction between self-induced oscillations and forced blade motion oscillations is very weak and can generally be neglected. The forced motion data acquired with the mistuned tunnel, when strong self-induced oscillations were present, can be used as reliable forced pressure fluctuations provided that they are extracted from raw data sets by an ensemble averaging procedure.

Lepicovsky, J.↗