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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
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NASA NTRS1982

An electrostatic parabolic density drift instability

Assuming a uniform magnetic field and using the local approximation, the Vlassov theory for an electrostatic instability driven by a parabolic density gradient is considered. It is found that the instability grows only in the case of a sufficiently large, positive second derivative of the density. Linear growth rate parametric dependences are given, in addition to weakly nonlinear calculations on wave-particle transport due to this instability.

Gary, S. P.↗
NASA NTRS1982

The effects of transmitter/receiver clock time-base instability on coherent communication system performance

The purpose of this paper is to present a model and associated analysis of the deleterious effects which both transmitter and receiver data clock time-base instability have on bit-error probability. The types of time-base instability modeled and analyzed include what is commonly referred to as bit jitter (epoch jitter), bit-jitter rate (time interval jitter), and data asymmetry. The bit-error probability is determined as a function of the statistical characteristics of the time-base instability, the data-transition density, the signal-to-noise ratio in the bit-rate bandwidth and the mechanizations of both the data-bit detector and bit synchronizer. The latter is manifested through the bit synchronizer's natural frequency and loop-damping parameters. Numerical results which indicate the sensitivity of the system bit-error rate to various parameters of the model are provided.

Chie, C. M.↗
NASA NTRS1982

Effects of heat loss, preferential diffusion, and flame stretch on flame-front instability and extinction of propane/air mixtures

Flame configurations, flame-front cellular instability, and extinction of propane/air mixtures in the stagnation-point flow are experimentally studied for their dependence on downstream heat loss, preferential diffusion, and flame stretch. Boundaries for lean- and rich-limit extinction, stabilization of corrugated flames, and local extinction caused by sharp curvatures are mapped for varying propane concentrations and freestream velocities. Flame location and temperature at extinction are determined as functions of stagnation surface temperature, extent of preheating, propane concentration, and freestream velocity. Results substantiate the theoretical predictions of the different extinction modes for lean and rich flames in the absence of downstream heat loss, and yield useful insight on the extinction characteristics when finite downstream heat loss does exist. It is further shown that flame-front instability occurs only for rich mixtures in accordance with preferential diffusion considerations, and that flame stretch has a stabilizing effect such that flame-front instability is completely inhibited before the onset of extinction.

Ishizuka, S.↗
NASA NTRS1982

The instability of sheared liquid layers

The critical values of the Reynolds number (Rc) above which unstable travelling waves exist in a steady shear flow are calculated with the inclusion of a term in the normal stress boundary condition omitted by Miles (1960). Results show a value of Rc = 34.2, compared to Miles' value of 203, for the case of a zero-surface tension interface. The critical wavenumber, critical phase speed, and Rc are calculated as functions of a nondimensional surface tension, and the mechanism of the instability is investigated by examining the disturbance energy equation. A long wave instability is found at small values of the Reynolds number when the shear flow has a parabolic velocity profile. In addition, these results are extended to larger values of the wavenumber by numerical methods, and the relation between this long wave instability and the profile curvature is examined.

Smith, M. K.↗
NASA NTRS1982

The problem of dimensional instability in airfoil models for cryogenic wind tunnels

The problem of dimensional instability in airfoil models for cryogenic wind tunnels is discussed in terms of the various mechanisms that can be responsible. The interrelationship between metallurgical structure and possible dimensional instability in cryogenic usage is discussed for those steel alloys of most interest for wind tunnel model construction at this time. Other basic mechanisms responsible for setting up residual stress systems are discussed, together with ways in which their magnitude may be reduced by various elevated or low temperature thermal cycles. A standard specimen configuration is proposed for use in experimental investigations into the effects of machining, heat treatment, and other variables that influence the dimensional stability of the materials of interest. A brief classification of various materials in terms of their metallurgical structure and susceptability to dimensional instability is presented.

Wigley, D. A.↗
NASA NTRS1982

The investigation of flow instabilities on a rotating disk with curvature in the radial direction

The major objective is to explore any visible differences of the flow field with wall curvature of the test body, including possible interaction between Taylor-Gortler instabilities present along concave walls and the inflexional instabilities investigated here. An experimental study was conducted with emphasis placed on making visual observations and recording photographically the flow instabilities present under three different rotating bodies: a flat disk, a concave paraboloid, and a convex paraboloid. The data collected for the three test bodies lead to the conclusion that the wall curvature of the concave and convex paraboloids did not alter the observed flow field significantly from that observed on the flat disk.

Intemann, P. A.↗
NASA NTRS1982

Origin and control of instability in SCR/triac three-phase motor controllers

The energy savings and reactive power reduction functions initiated by the power factor controller (PFC) are discussed. A three-phase PFC with soft start is examined analytically and experimentally to determine how well it controls the open loop instability and other possible modes of instability. The detailed mechanism of the open loop instability is determined and shown to impose design constraints on the closed loop system. The design is shown to meet those constraints.

Dearth, J. J.↗
NASA NTRS1982

Stall induced instability of a teetered rotor

Recent tests on the 38m Mod-0 horizontal experimental wind turbine yielded quantitative information on stall induced instability of a teetered rotor. Tests were conducted on rotor blades with NACA 230 series and NACA 643-618 airfoils at low rotor speeds to produce high angles of attack at relatively low wind speeds and power levels. The behavior of the rotor shows good agreement with predicted rotor response based on blade angle of attack calculations and airfoil section properties. The untwisted blades with the 64 series airfoil sections had a slower rate of onset of rotor instability when compared with the twisted 230 series blades, but high teeter angles and teeter stop impacts were experienced with both rotors as wind speeds increased to produce high angles of attack on the outboard portion of the blade. The relative importance of blade twist and airfoil section stall characteristics on the rate of onset of rotor unstability with increasing wind speed was not established however. Blade pitch was shown to be effective in eliminating rotor instability at the expense of some loss in rotor performance near rated wind speed.

Glasgow, J. C.↗
NASA NTRS1982

The baroclinic instability of highly structured one-dimensional basic states

A frictionless, adiabatic, quasi-geostrophic model on a beta-plane was employed to examine the baroclinic instability of highly structured one-dimensional basic states. Definitions of the model eigenvalue equation, the boundary conditions, and the fluxes and energetics are provided, and highly structured average wind speed and temperature profiles were selected to represent a wide range of latitudes. Attention was given to the instability source, wavelength, growth rate, phase velocity, steering levels, and the vertical structure of their amplitude, phase, meridional entropy transport, and conversion of potential-to-kinetic energy. Green modes, Charney-Eady modes, and the modes defined in the paper are examined in connection with the highly structured profiles. The eigenvalue approach is shown to be superior to the numerical time integration approach and necessary conditions for instabilities are defined.

Fullmer, J. W. A.↗
NASA NTRS1981

The effects of transmitter/receiver clock time base instability on coherent communication system performance

The purpose of this paper is to present a model and associated analysis of the deleterious effects which both transmitter and receiver data clock time-base instability have on bit error probability. The types of time-base instability modeled and analyzed include bit jitter (epoch jitter), bit jitter rate (time interval jitter) and data asymmetry. The bit error probability is determined as a function of the statistical characteristics of the time-base instability, the data transition density, the signal-to-noise ratio in the bit rate bandwidth and the mechanizations of both the data bit detector and bit synchronizer. Numerical results are provided.

Chie, C. M.↗
NASA NTRS1983

A kinetic cross-field streaming instability

In a high-beta plasma the so-called modified-two-stream instability, which results from strongly magnetized electrons drifting relative to unmagnetized ions across a homogeneous magnetic field, is misnamed because the mode is highly kinetic, particularly when the relative streaming velocity exceeds the Alfven speed of the plasma. This kinetic cross-field streaming instability is investigated in detail, examining the effect of the electromagnetic terms and the stability boundaries in both low- and high-beta plasmas. An approximate dispersion relation showing the relation of this mode to the whistler is derived and solutions of it are compared with those obtained from the exact dispersion relation. The kinetic mode, unlike the usual modified-two-stream instability, is not stabilized by electromagnetic effects when the relative electron-ion drift speed exceeds the Alfven speed.

Wu, C. S.↗
NASA NTRS1983

Energy dynamics in stressed magnetic fields - The filamentation and flare instabilities

The thermal and tearing instabilities are believed to be the two primary temperature modification mechanisms in sheared astrophysical magnetic fields. The former gives rise to the formation of cool filaments and the latter to the release of magnetic energy. It has long been known that these processes are interrelated, most conspicuously in the case of the solar corona where prominences often precede flares within the same magnetic structure. It is also clear, from first principles, that the energy transport underlying the thermal instability should have a strong effect on the resistivity which facilitates magnetic tearing, and that the energy release of the latter should affect the temperature drop of the former. This paper describes some results of the first calculations which attempt to unify the dynamic treatment of these two coexisting instabilities. Growth rates as a function of resistivity, and examples of the primary mode structures are provided, along with a discussion of some critical aspects of the interaction of these two astrophysical energy flux mechanisms.

Van Hoven, G.↗
NASA NTRS1983

Evidence for helical kink instability in the Venus magnetic flux ropes

Empirical models of the magnetic field structure of flux ropes found in the Venus ionosphere are seen as suggesting that the ropes are unstable to long-wavelength (more than 100 km) helical-kink perturbations. The onset of such an instability can explain the apparent volume distribution of flux ropes with altitude, as well as their orientation as a function of altitude. In the subsolar region, the fraction of volume occupied by flux ropes increases from approximately 20 percent at high altitudes to more than 50 percent at low altitudes; this is a greater increase than would be expected if ropes convect downward as simple straight horizontal cylinders. The helical kink instability raises the fractional volume occupied by ropes by turning the originally straight, horizontal flux tubes into corkscrew-shaped structures as they convect to lower altitudes. It is noted that this instability also explains why high altitude ropes tend to be horizontal and low altitude ropes appear to have almost any orientation.

Elphic, R. C.↗
NASA NTRS1984

Laboratory studies of baroclinic instability at small Richardson number

As part of the support program for the Atmospheric General Circulation Experiment, laboratory studies of baroclinic and other convective instabilities were performed for a thin layer of fluid between thermally conducting horizontal discs. There were three types of modes identified. The first has a spiral-arm appearance, and exists for large enough horizontal thermal forcing, weak enough static stability, and large enough rotation. The source of this wave is shown to be the Eady mode of instability. The second mode is due to convective instability in the thermal boundary layers which exist due to the thermally conducting horizontal boundaries. Finally, for strong enough negative static stability, thermal convection of the Benard type appears. The most significant result is that the symmetric (Solberg) mode was not found, even though the infinite-plane theory predicts this mode under certain experimental conditions.

Miller, T. L.↗
NASA NTRS1983

Pedersen density drift instabilities

This paper describes the linear kinetic theory of electrostatic-drift instabilities driven by Pedersen and density-drift velocities. The model uses a uniform magnetic field B; a weak, uniform density gradient in the x direction; and a weak, uniform electric field in the y direction. Weak charged-neutral collisions are represented by the addition of BGK model terms to the Vlasov equation. The resulting local dispersion equation is used to study the properties of the associated instabilities at ka(i) greater than about 1 (where k is the wave number and a(i) is the ion gyroradius). Results show that the E x B gradient drift instability at ka(i) = about 1 may grow in the auroral ionosphere primarily in the vicinity of 200 km and only if the electron density is sufficiently small.

Gary, S. P.↗
NASA NTRS1984

Kinetic theory of current and density drift instabilities with weak charged-neutral collisions

This paper describes the linear kinetic theory of electrostatic instabilities driven by a density gradient drift and a magnetic-field-aligned current in a plasma with weak charged neutral collisions. The configuration is that of a uniform magnetic field B, a weak, uniform density gradient in the x direction and a weak, uniform electric field in the z direction. Collisions are represented by the BGK model. The transition from the (kinetic) universal density drift instability to the (fluidlike) current convective instability is studied in detail, and the short wavelength properties of the latter mode are investigated.

Gary, S. P.↗
NASA NTRS1984

Sound generated by instability waves of supersonic flows. I Two-dimensional mixing layers. II - Axisymmetric jets

An investigation is conducted of the phenomenon of sound generation by spatially growing instability waves in high-speed flows. It is pointed out that this process of noise generation is most effective when the flow is supersonic relative to the ambient speed of sound. The inner and outer asymptotic expansions corresponding to an excited instability wave in a two-dimensional mixing layer and its associated acoustic fields are constructed in terms of the inner and outer spatial variables. In matching the solutions, the intermediate matching principle of Van Dyke and Cole is followed. The validity of the theory is tested by applying it to an axisymmetric supersonic jet and comparing the calculated results with experimental measurements. Very favorable agreements are found both in the calculated instability-wave amplitude distribution (the inner solution) and the near pressure field level contours (the outer solution) in each case.

Tam, C. K. W.↗