Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Instabilities”

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 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Instability of equatorial protons in Jupiter's mid-magnetosphere

Two different models for the distribution function are fit to the Jovian protons seen by Pioneer 10 inbound. The models reproduce the observed energy and angular distributions. These models are then used to assess the collisionless mirror instability. Because of the pancake proton angular distributions in the equatorial ring current region, the ring current particle population appears to be mirror unstable at times, with instability growth rates of about 10 min. Such a time is consistent with observed proton flux autocorrelation times. An instability such as this (there are other candidates) may be responsible for the previously established proton flux flowing parallel to the magnetic field away from the equatorial region.

Northrop, T. G.↗
NASA NTRS1980

The radiation of sound by the instability waves of a compressible plane turbulent shear layer

The problem of acoustic radiation generated by instability waves of a compressible plane turbulent shear layer is solved. The solution provided is valid up to the acoustic far-field region. It represents a significant improvement over the solution obtained by classical hydrodynamic-stability theory which is essentially a local solution with the acoustic radiation suppressed. The basic instability-wave solution which is valid in the shear layer and the near-field region is constructed in terms of an asymptotic expansion using the method of multiple scales. This solution accounts for the effects of the slightly divergent mean flow. It is shown that the multiple-scales asymptotic expansion is not uniformly valid far from the shear layer. Continuation of this solution into the entire upper half-plane is described. The extended solution enables the near- and far-field pressure fluctuations associated with the instability wave to be determined. Numerical results show that the directivity pattern of acoustic radiation into the stationary medium peaks at 20 degrees to the axis of the shear layer in the downstream direction for supersonic flows. This agrees qualitatively with the observed noise-directivity patterns of supersonic jets.

Tam, C. K. W.↗
NASA NTRS1980

A simple approximate result for the maximum growth rate of baroclinic instabilities

The Charney problem for baroclinic instability involves the quasi-geostrophic instability of a zonal flow on a beta plane where the zonal flow is characterized by a constant vertical shear. The atmosphere is non-Boussinesq and continuous. The solution of this problem involves confluent hypergeometric functions, and the mathematical difficulty of the problem has precluded extracting simple results of generality. It is shown that there exists very simple, powerful approximate result for the growth rate of the most rapidly growing instability, viz., that this growth rate is linearly proportional to the surface meridional temperature gradient. The coefficient of proportionality is also easily determined. The result extends to substantially more general profiles than those in the Charney problem.

Lindzen, R. S.↗
NASA NTRS1980

Sunspots and the physics of magnetic flux tubes. X - On the hydrodynamic instability of buoyant fields

The hydrodynamics of cylindrical buoyant bubbles (magnetic flux tubes) in an atmosphere is examined. Their dynamical stability is considered, with particular attention to the Rayleigh-Taylor and Kelvin-Helmholtz instabilities. The instabilities are sufficient to cause fragmentation in the absence of some strong stabilizing effect such as surface tension or an internal azimuthal magnetic field. The results of the stability analysis are first applied to rising gas bubbles in liquids. Then, since agreement with the experimental data is found, the results are applied to the similar problem of the rising magnetic flux in the sun. Magnetic flux tubes in a fluid velocity field are intrinsically unstable, unless twisted. With the known dynamical limits on the degree of twisting, an upper limit on the diameter of the order of a few hundred kilometers is found for the stability of individual flux tubes in the photosphere. Observations show that tubes, except when clustered to form sunspots, are generally not larger than this amount. Therefore, it is suggested that the hydrodynamic instability of larger tubes is a prime cause of their absence from the solar photosphere.

Tsinganos, K. C.↗
NASA NTRS1980

An explanation of the instability of the free vortex cores occurring over delta winds with raised edges

By rolling up the surfaces of discontinuity originating from the leading edge of delta wings, free vortex cores are formed above the wing. In case of greater angles of incidence, the flow in these vortex cores shows an instability which abruptly produces strong turbulence. In the present paper an explanation is given of this instability being a "frictionless instability" of the vortex core flow by increasing helical interference vortices. The occurring vortex core flows are calculated and investigated for stability by means of a stability criterion concerning flows with helical streamlines given by H. Ludwieg.

Ludwieg, H.↗
NASA NTRS1981

Existence of a natural instability not predicted by theory and connected to a wall deformation in a laminar boundary layer

Natural instability related to negative wall deformation was studied. It was shown that natural instabilities which propagate in a laminar boundary layer of a flat plate are in agreement with stability theory. It is found that if a wall has a deformation, a second frequency does exist, which is not predicted and is twice the first frequency. This second instable frequency only appears if there is a negative velocity gradient. The phenomenon is located very closely to the wall and drops off rapidly when moved away from it.

Gougat, P.↗
NASA NTRS1980

The onset and development of Kelvin-Helmholtz instability at the Venus ionopause

The paper investigates the Kelvin-Helmholtz instability at the Venus ionopause resulting from the flow of the (shocked) solar wind tangential to the ionopause for the case where the interplanetary field is oriented normal to the direction of flow. It is found that gravity stabilizes the long wavelength perturbations, and the finite thickness of the boundary layer stabilizes short wavelength modes. The magnetic 'gyroviscosity' due to finite Larmor radius effects either destabilizes the boundary or stabilizes it according to whether the solar wind electric field points away from or toward the ionosphere. For solar wind and ionosphere plasma parameters consistent with Pioneer Venus observations, it is found that the instabilities with the greatest growth rates (shortest growth times) have wavelengths of 50-150 km and growth times of 0.5 to several seconds. In addition, it is found how distortion of the ionopause by Kelvin-Helmholtz instability might lead to the formation of magnetic 'flux ropes' inside the ionosphere as well as ionospheric 'bubbles' embedded in the solar wind.

Wolff, R. S.↗
NASA NTRS1981

The effects of cold plasma on the Kelvin-Helmholtz instability

The results are presented of a detailed study that examines the effects of cold plasma on the Kelvin-Helmholtz instability. Both electrostatic and electromagnetic modes are considered. The interest in the Kelvin-Helmholtz instability stems from observations of low frequency waves in the polar cusp, magnetopause, in fast moving auroral forms, the plasma pause, and other regions of magnetospheric flow boundaries. It is pointed out that the Kelvin-Helmholtz instability is driven by a velocity shear and can excite a broad spectrum of wave frequencies below the ion gyrofrequency. A model is obtained of a flow boundary that includes a density gradient perpendicular to the velocity shear. The analysis shows a compressional component in Kelvin-Helmholtz waves.

Melander, B. G.↗
NASA NTRS1981

On the role of the lower hybrid drift instability in substorm dynamics

Recent studies of the lower hybrid drift instability have shed new light on the role of this mode in field-reversed plasmas. For substorm magnetotail conditions it is found that the lower hybrid drift instability can penetrate to the neutral line and can dissipate magnetic energy at a rate of approximately 4 x 10 to the 17th erg/s. Thus this instability is capable of playing a major role in the onset of substorms and providing resistivity for reconnection processes in the context of the neutral line substorm model.

Huba, J. D.↗
NASA NTRS1981

The electromagnetic ion beam instability upstream of the earth's bow shock

The linear theory of the electromagnetic ion beam instability for arbitrary angles of propagation has been studied. The parameters considered in the theory are typical of the solar wind upstream of the earth's bow shock when a 'reflected' proton beam is present. Maximum growth occurs for propagation parallel to the ambient magnetic field B, but this instability also displays significant growth at wave-vectors oblique to B. Oblique, unstable modes seem to be the likely source of the compressive magnetic fluctuations recently observed in conjunction with the 'diffuse' ion population. An energetic ion beam does not directly give rise to linear growth of either ion acoustic or whistler mode instabilities.

Gary, S. P.↗
NASA NTRS1981

Temperature anisotropy instabilities driven by Whang's ion model

The behavior of four temperature anisotropy instabilities in a single-component space plasma is investigated theoretically using the solar wind model (i.e., heat flux modified non-Maxwellian velocity distribution functions) proposed by Whang (1972). The linear dispersion properties are studied in an infinite Vlasov plasma where the proton distribution is assumed to consist of one warm slow-speed component and where the electron background is isotropic. It is shown that the fire hose, mirror, and Harris instability do not occur under these conditions, and that only the ion cyclotron instability is of importance in Whang's ion model. Growth rates are shown to have values between 0.021 and 0.00011. Increasing temperature anisotropy causes the range frequencies where wave particle interaction occurs to be shifted toward the ion gyrofrequency.

Leubner, M. P.↗
NASA NTRS1982

Generation of instability waves at a leading edge

Two cases are considered. The first is concerned with mean flows of the Blasius type wherein the instabilities are represented by Tollmien-Schlichting waves. It is shown that the latter are generated fairly far downstream of the edge and are the result of a wave length reduction process that tunes the free stream disturbances to the Tollmien-Schlichting wave length. The other case is concerned with inflectional, uni-directional, transversely sheared mean flows. Such idealized flows provide a fairly good local representation to the nearly parallel flows in jets. They can support inviscid instabilities of the Kelvin-Helmholtz type. The various mathematically permissible mechanisms that can couple these instabilities to the upstream disturbances are discussed.

Goldstein, M. E.↗
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.↗