Strong Radial Gradients Research Articles

Local Axisymmetric Diffusive Stability of Weakly Magnetized, Differentially Rotating, Stratified Fluids

We study the local stability of stratified, differentially rotating fluids to axisymmetric perturbations in the presence of a weak magnetic field and of finite resistivity, viscosity, and heat conductivity. This is a generalization of the Goldreich-Schubert-Fricke (GSF) double-diffusive analysis to the magnetized and resistive, triple-diffusive case. Our fifth-order dispersion relation admits a novel branch that describes a magnetized version of multidiffusive modes. We derive necessary conditions for axisymmetric stability in the inviscid and perfect-conductor (double-diffusive) limits. In each case, rotation must be constant on cylinders and angular velocity must not decrease with distance from the rotation axis for stability, irrespective of the relative strength of viscous, resistive, and heat diffusion. Therefore, in both double-diffusive limits, solid-body rotation marginally satisfies our stability criteria. The role of weak magnetic fields is essential to reach these conclusions. The triple-diffusive situation is more complex, and its stability criteria are not easily stated. Numerical analysis of our general dispersion relation confirms our analytic double-diffusive criteria but also shows that an unstable double-diffusive situation can be significantly stabilized by the addition of a third, ostensibly weaker, diffusion process. We describe a numerical application to the Sun's upper radiative zone and establish that it would be subject to unstable multidiffusive modes if moderate or strong radial gradients of angular velocity were present.

Development of the Radially Stratified Flame Core Low NOx Burner: From Fundamentals to Industrial Applications

Research and development of the low NOx radially stratified flame core (RSFC) burner is followed from its fundamental concept through prototype burner design, pilot scale experiments at M.I.T. and scale-up and commercial design by ABB-CE (now ALSTOM Power) to applications in industrial and utility plant boilers. The principle that turbulence can be significantly damped in a rotating flow field with a strong positive radial density gradient was used to increase the fuel rich zone residence time in internally staged low NOx burners. The continuous interaction of ideas from laboratory experimental and computational studies with those from the commercial design and industrial scale tests played a pivotal role in the development of the final product, the commercial RSFC burner. Examples of application in gas, oil, and coal fired industrial and utility boilers are discussed.

Observations of PKiKP/PcP amplitude ratios and implications for Earth structure at the boundaries of the liquid core

Mining of the database produced by seismic array stations of the International Monitoring System has resulted in hundreds of new observations of precritical reflections from Earth's inner core. Here we present short‐period amplitude ratios between these PKiKP phases and corresponding reflections (PcP) from the core‐mantle boundary (CMB). PKiKP/PcP amplitude ratios are one of the few seismic observations that directly constrain the change in S velocity and density across the inner core‐outer core boundary. We measure the amplitude ratios from optimally tuned array beams and use a bootstrap technique to estimate observational uncertainties. We also use a model resampling technique to estimate uncertainties associated with the background reference model. The optimal models have S velocities at the top of the inner core less than about 2.5 km/s and density jumps across the inner core‐outer core boundary of less than about 0.45 g/cm3. These values can be reconciled with the higher estimates from normal mode constraints if strong radial gradients are present in the top of the inner core or if the outermost inner core has material properties distinct from the bulk of the inner core. We also find evidence for an ultralow‐velocity zone in the lowermost mantle beneath the eastern coast of Australia. A unique, best fitting model cannot be determined, but an example that is consistent with the data has a reduction in P velocity of 10%, a reduction in S velocity of 35%, and an increase in density of 20%. This type of structure could be explained by the presence of partial melt and increased iron content just above the CMB.

NGC 1052: a LINER with strong stellar Mg/Fe gradient

We analyze long slit spectroscopy of NGC 1052, the third brightest member of a group of eleven galaxies. This elliptical is a typical LINER with a stellar rotating disk. We have found a strong negative radial gradient of the abundance ratio Mg/Fe, which suggests an enhancement of α-elements particulary towards the nuclear region, and higher metallicity and age at the galactic nucleus too. We have measured the Lick line-strength indices along the major axis of the galaxy. This information, together with single-aged stellar population models of Thomas et al. (2003), was used to determine the star formation history of NGC 1052.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

CFD Optimization of Cooling Air Offtake Passages Within Rotor Cavities

In modern gas turbine engines, up to 20% of the core airflow is bled off from various compressor stages to facilitate internal cooling, bearing chamber, and rim sealing, as well as axial load management. As this secondary airflow makes no direct contribution to engine thrust, there are strong economic incentives to reduce the quantity and quality of offtake air and maximize its effective use. Secondary airflows are commonly bled off via circular drillings in the compressor rotor, thereby augmenting their swirl velocity. This results in the creation of vortices within the rotor cavity and strong radial pressure gradients opposing inflow. In the present work the relative performance of a series of noncircular offtake passages has been assessed using CFD techniques. The results of this work demonstrate the degree of control that may be exercized over swirl uptake, which can be used to suppress the creation of vortices in rotor cavities.

Use of a heterogeneous two-dimensional model to improve the primary steam reformer performance

The reforming units are basically furnaces containing burners (which provide a large amount of heat by fuel combustion) and tubes packed with supported nickel catalyst. Due to the high heat input through the reformer tube wall and the endothermic reforming reactions, the catalyst tubes are exposed to significant axial and radial temperature gradients. For this reason, a two-dimensional mathematical model that takes into account the diffusion reaction phenomena inside the particles rigorously has been used to represent the reactor. Strong radial temperature gradients in the reformer tube have been found, particularly close to the reactor entrance. These temperature differences cause significant variations in the methane reaction rate along the radial position, being the catalyst close to the reforming tube center poorly used. For this reason, the reforming tube diameter and the catalyst activity distribution were modified to use the catalyst more efficiently. The tube diameter has an important influence on the reformer performance, considerable higher conversions and reactor capacities per tube (i.e. closer equilibrium approaches) have been observed for the tubes with smaller diameters. The catalyst activity distribution also strongly impacts the reactor operation. The use of two catalysts of different activity, adequately distributed along the axial and radial directions, allowed to significantly decrease the maximum tube wall temperature and simultaneously minimize the reactor volume fraction packed with the catalyst of higher activity.

FEASIBILITY STUDIES FOR THE IMPLEMENTATION OF NUCLEAR MAGNETIC RESONANCE IN A 25T HYBRID MAGNET

As an exploratory study for NMR experiments in the future 20 MW Nijmegen high field magnet laboratory, the possibilities of field stabilization and field gradient compensation in a 25 T hybrid magnet in the present installation were evaluated. High frequency field fluctuations from the power supply can be compensated to better than 10-3 ppm in the 10 Hz-10 kHz range using a computer controlled feedback system. Field mapping by 2H magnetic resonance using a homebuilt device not only showed that there are substantial axial but also strong radial field gradients. It can be shown that for any cylindrical multicoil Bitter magnet the main components of these gradients can be compensated with simple ferromagnetic inserts. In this way we achieved a linewidth under 5 ppm in 1 mm3 without further shimming or optimization. The low frequency drift of the field due to instabilities of the present power supply and the effects of temperature fluctuations of the coil are determined by simultaneous acquisition of an in-situ deuterium reference signal together with the signal of interest. This allows for a full compensation of the field fluctuations by deconvolution techniques. We will report preliminary NMR results on solid27 Al samples in fields up to 25 T.

Effect of electron density on shock wave propagation in optically pumped plasmas

This article discusses experimental studies of spark-generated shock wave propagation in CO-laser sustained optically pumped CO–Ar–O2 plasmas. The rotational-translational temperature of the plasma is measured by Fourier transform infrared emission spectroscopy. The electron density in the plasma is determined by microwave attenuation. The line-of-sight averaged density distribution across the propagating shock is detected by photoacoustic deflection (PAD). The measurements show that adding small amounts of oxygen (up to 0.1%) to the baseline optically pumped CO–Ar plasma increases the electron density and the ionization fraction by more than an order of magnitude (up to ne=0.9×1010 cm−3 and ne/N=0.8×10−8, respectively), while the gas temperature remains nearly constant, within 3%–5%. Therefore this approach allows varying the electron density in the plasma nearly independently of the gas temperature. The PAD measurements show considerable apparent weakening and dispersion of a shock wave propagating in the optically pumped plasma with a strong radial temperature gradient, compared to the shock propagating in a cold nonionized gas. However, varying the electron density independently of the gas temperature does not produce any detectable effect on the measured gas density distribution across the propagating shock. It is therefore concluded that the observed shock weakening is entirely due to the radial temperature gradient sustained by resonance absorption of the CO laser radiation near the centerline of the shock tube and is not affected by the presence of the charged species in the plasma.

Colors and Spectra of Kuiper Belt Objects

We present new measurements of the optical colors of Kuiper belt objects, principally from the Keck 10 m telescope. The measurements confirm the existence of a wide spread in the B-V, V-R, and R-I color indices as we found previously. Relative to the sun, the Kuiper belt objects exhibit reflected colors from nearly neutral to very red. The optical and optical-infrared (V-J) color indices are mutually correlated, showing the presence of a single reddening agent from 0.45 to 1.2 μm. On the other hand, we find no evidence for linear correlations between the color and absolute magnitude (a proxy for size), instantaneous heliocentric distance, semimajor axis, or with any other orbital property. In this regard, the Kuiper belt objects differ from the main-belt asteroids, in which strong radial color gradients exist. We find no statistically significant evidence for bimodal or other nonuniform color distributions, either in our data or in data previously reported to show such evidence. The impact resurfacing hypothesis is reexamined in the light of the new color data and is rejected as the primary cause of the observed color dispersion. We also present new near-infrared reflection spectra of 1993 SC, 1996 TS66, 1999 DE9 and 2000 EB173, taken at the Keck and Subaru telescopes. These spectra, combined with others from the published literature, provide independent evidence for compositional diversity in the Kuiper belt. Objects 2000 EB173, 1993 SC, and 1996 TS66 are spectrally bland, while 1999 DE9 shows solid-state absorption bands.

Line Forces in Keplerian Circumstellar Disks and Precession of Nearly Circular Orbits

We examine the effects of optically thick line forces on orbiting circumstellar disks, such as occur around Be stars. For radially streaming radiation (e.g., as from a point source), line forces are effective only if there is a strong radial velocity gradient, as occurs, for example, in a line-driven stellar wind. However, we emphasize here that, within an orbiting disk, the radial shear of the azimuthal velocity leads to strong line-of-sight velocity gradients along nonradial directions. As such, we show that, in the proximity of a stellar surface extending over a substantial cone angle, the nonradial components of stellar radiation can impart a significant line force to such a disk, even in the case of purely circular orbits with no radial velocity. Given the highly supersonic nature of orbital velocity variations, we use the Sobolev approximation for the line transfer, extending to the disk case the standard CAK formalism developed for line-driven winds. We delineate the parameter regimes for which radiative forces might alter disk properties; but even when radiative forces are small, we analytically quantify higher-order effects in the linear limit, including the precession of weakly elliptical orbits. We find that optically thick line forces, both radial and azimuthal, can have observable implications for the dynamics of disks around Be stars, including the generation of either prograde or retrograde precession in slightly eccentric orbits. However, our analysis here suggests a net retrograde effect, in apparent contradiction with observed long-term variations of violet/red line profile asymmetries from Be stars, which are generally thought to result from prograde propagation of a one-arm, disk-oscillation mode. We also conclude that radiative forces may alter the dynamical properties at the surface of the disk where disk winds originate, and in the outer regions far from the star, and may even make low-density disks vulnerable to being blown off completely.

HYDRODYNAMIC SOURCE OF THE 11-YEAR SOLAR VARIATIONS

In the traditional axisymmetric models of the 11-year solar cycle, oscillations of the magnetic fields appear in the background of nonoscillating (over time scale considered) turbulent velocity fields and differential rotation. In this paper, an alternative approach is developed: The excitation of magnetic oscillations with the 22-year period is the consequence of hydrodynamic oscillations with the 11-year period. In the excitation of hydrodynamic oscillations, two processes taking place in high latitudes near the interface between the convective and radiative zones play a key role. One is forcing of the westerly zonal flow, the conditions for which are due to deformation of the interfacial surface. The other process is the excitation of a shear instability of zonal flow as a consequence of a strong radial gradient of angular velocity. The development of a shear instability at some stage brings about the disruption of the forcing of differential rotation. In the first (hydrodynamic) part of the paper, the dynamics of axisymmetric flows near the bottom of the convection zone is numerically simulated. Forcing of differential rotation having velocity shear in latitude and the existence of solutions in the form of torsional waves with the 11-year oscillation period are shown. In the second part the dynamics of the magnetic field is studied. The most pronounced peculiarities of the solutions are the existence of forced oscillations with the 22-year period and the drift of the toroidal magnetic field component from the mid latitudes to the equator. In high and low latitudes after cycle maximum, the toroidal component is of opposite sign in accordance with observations. In the third part, the transport of momentum from the bottom of the convection zone to the outer surface by virtue of diffusivity is considered. The existence of some sources of differential rotation in the convection zone is not implied. A qualitative correspondence of the differential rotation profile in the bulk of the convection zone and on its outer surface to experimental data is shown. The time correspondence between torsional and magnetic oscillations is also in accordance with observations.

Galaxy Population Properties in the Rich Clusters MS 0839.8+2938, MS 1224.7+2007, and MS 1231.3+1542

This paper discusses the galaxy populations of three rich clusters, with redshifts 0.19 (0839+29), 0.24 (1231+15), and 0.32 (1224+20), from the database of the CNOC1 consortium. The data consist of spectra of 52 cluster members for 0839+29, 30 members for 1224+15, and 82 members for 1231+15, and there are comparable numbers of field galaxy spectra. Cluster 0839+29 is compact with no strong radial gradients, and possibly dusty. Cluster 1224+20 is isolated in redshift and has low velocity dispersion around the cD galaxy and low 4000 A break. Cluster 1231+15 is asymmetric, and we discuss the possibility that it may be a recent merger of two old clusters. We find few galaxies in 0839+29 and 1231+15 with ongoing or recently truncated star formation.

Physical Causes of Solar Activity

The magnetic fields that dominate the structure of the Sun's atmosphere are controlled by processes in the solar interior, which cannot be directly observed. Magnetic activity is found in all stars with deep convective envelopes: young and rapidly rotating stars are very active but cyclic activity only appears in slow rotators. The Sun's 11-year activity cycle corresponds to a 22-year magnetic cycle, since the sunspot fields (which are antisymmetric about the equator) reverse at each minimum. The record of magnetic activity is aperiodic and is interrupted by episodes of reduced activity, such as the Maunder Minimum in the seventeenth century, when sunspots almost completely disappeared. The proxy record from cosmogenic isotopes shows that similar grand minima recur at intervals of around 200 yr. The Sun's large-scale field is generated by dynamo action rather than by an oscillator. Systematic magnetic cycles are apparently produced by a dynamo located in a region of weak convective overshoot at the base of the convection zone, where there are strong radial gradients in the angular velocity Ω. The crucial parameter (the dynamo number) increases with increasing Ω and kinematic (linear) theory shows that dynamo action can set in at an oscillatory (Hopf) bifurcation that is probably subcritical. Although it has been demonstrated that the whole process works in a self-consistent model, most calculations have relied on mean-field dynamo theory. This approach is physically plausible but can only be justified under conditions that do not apply in the Sun. Still, mean-field dynamos do reproduce the butterfly diagram and other key features of the solar cycle. An alternative approach is to study generic behaviour in low-order models, which exhibit two forms of modulation, associated with symmetry-breaking and with reduced activity. Comparison with observed behaviour suggests that modulation of the solar cycle is indeed chaotic, i.e. deterministically rather than stochastically driven.

The Star Formation Efficiency within Galaxies

We combine H? imaging with the CO-line observations of the Five College Radio Astronomy Observatory Extragalactic CO Survey to study the relationship between molecular gas and high-mass star formation for 568 regions in 121 galaxies at 45'' resolution. Our study finds a strong correlation between these quantities when sampled locally within galaxies, consistent with recent studies of globally averaged quantities. For spiral galaxies, there are no strong radial gradients in the star formation efficiency across the star-forming disk, although star formation efficiencies measured in the outermost regions (R > 9 kpc) of midsized galaxies tend to be systematically low. Additionally, star formation efficiencies in large (D0 > 60 kpc) galaxies are uniformly low at all radii compared with smaller galaxies. As a function of morphology and environment, the behavior of the star formation efficiency within galaxies is consistent with the results of our previous investigation of the global quantities. Among spirals the star formation efficiency does not depend on Hubble type, with a similar range of efficiencies within each type and less than 25% variation in the mean from type to type. Finally, relative to an isolated galaxy sample, the star formation efficiency is found to be sensitive only to extreme variations in the galaxy environment. The star formation efficiency decreases steadily with increasing H I deficiency among Virgo cluster spirals, and it is enhanced in strong interactions and mergers.

The Structure of the Superthin Spiral Galaxy UGC7321

AbstractUGC 7321 is an edge-on low surface brightness (LSB) spiral galaxy with a number of extraordinary properties. Its vertical scale height (~70 pc) is one of the smallest ever measured for a galaxy disk. Its disk also exhibits strong vertical and radial color gradients. UGC 7321 appears to be an extremely unevolved galaxy in both a dynamical and in a star-formation sense.

Rectangular 915 MHz mode plasma cavity: characterization and diagnostics in dinitrogen

The 915 MHz band wavelength is convenient for high power flowing plasma generation, especially in for nitrogen atom production, at low power density through large sections and gas high residence times. The chosen mode cavity when unloaded theoretically fulfills such requirements. Its qualification at power up to 2.5 kW shows a good power transfer when running with flowing nitrogen in the pressure range 150-1330 Pa. Thermal measurements made on the liquid cooling the discharge tube point out a constant power ratio (70%) released by the plasma in the form of heat. A discharge length of about 2/3 times the cavity length is deduced from the optical and emission intensities. The axial profiles of the vibrational and rotational temperatures of are presented and discussed with regard to the existence of strong radial gradients and possible excitation mechanisms. The concentration generated by this plasma source is compared at a 1.5 kW power with that of a 2450 MHz cylindrical cavity in the far remote afterglow. Finally about 3.3% of the absorbed power is transformed into dissociation with a power density less than , allowing power increases within factors much larger than ten.

Three-Dimensional Blade Boundary Layer and Endwall Flow Development in the Nozzle Passage of a Single Stage Turbine

The three-dimensional viscous flow field development in the nozzle passage of an axial flow turbine stage was measured using a “x” hot-wire probe. The measurements were carried out at two axial stations on the endwall and vane surfaces and at several spanwise and pitchwise locations. Static pressure measurements and flow visualization, using a fluorescent oil technique, were also performed to obtain the location of transition and the endwall limiting streamlines. The boundary layers on the vane surface were found to be very thin and mostly laminar, except on the suction surface downstream of 70 percent axial chord. Strong radial pressure gradient, especially close to the suction surface, induces strong radial flow velocities in the trailing edge regions of the blade. On the endwalls, the boundary layers were much thicker, especially near the suction corner of the casing surface, caused by the secondary flow. The secondary flow region near the suction surface-casing corner indicated the presence of the passage vortex detached from the vane surface. The boundary layer code accurately predicts the three-dimensional boundary layers on both vane surfaces and endwall in the regions where the influence of the secondary flow is small.

Forcing of Differential Rotation and Rossby Waves at the Interface between the Convectively Stable and Unstable Layers

A model for the high-latitude region near the interface between the solar convection and radiative zones is suggested. The essential feature of the model is an allowance for the deformation of the interface between the convectively stable and unstable parts of the layer, which appears to be due to the influence of large-scale flows on this region's conditions. Two-dimensional numerical simulations for an axisymmetric flow show that one of the consequences of the interface deformation is the appearance of a latitudinal gradient in the temperature distribution, which gives rise to a strong radial gradient of the angular velocity in the vicinity of the interface. Such a differential rotation is, in fact, the thermal wind like that in the Earth's atmosphere. It is found that the penetration of the differential rotation and meridional flow into a convectively stable part of the layer depends strongly on values of the effective turbulent viscosity and thermal conductivity in this region. Two-dimensional numerical simulations of axisymmetric and nonaxisymmetric flows show that another consequence of the deformation of the interface is the forcing of differential rotation and the Rossby wave. It is found that the pumping mechanism is similar to the deformational long-wave instability mechanism that operates in a rotating fluid layer heated from below with the deformable upper stress-free surface. Nonlinear effects lead to the limitation of the differential rotation and Rossby wave amplitude and to the establishment of stationary patterns. Stationary differential rotation is in qualitative agreement with recent results obtained by helioseismology.

The Star Formation History of the Local Group Dwarf Elliptical Galaxy NGC 185. I. Stellar Content

?????We present VI CCD photometry of ~16,000 stars in a 72 ? 72 field of the Local Group dwarf elliptical galaxy NGC 185. The resulting VI color-magnitude diagram reveals a dominant red giant branch population, an important number of luminous red stars located above the tip of the red giant branch, and a number of blue and yellow stars. Besides the nucleus, our field also covers a large, less crowded area of the galaxy. We show color-magnitude diagrams at six different distances from the nucleus. The red giant branch becomes substantially narrower at larger distances from the nucleus, while the photometry gets deeper. In this paper, we concentrate on investigating the contribution of the observational effects (mainly crowding) to this observed gradient. Although we cannot rule out here the possibility that this trend partially originates in a gradient of the characteristics of the stellar populations of the galaxy with radius, we show that a strong radial gradient exists in the observational effects that can mimic a gradient in the real properties (e.g., age, metallicity) of the stellar population. A distance modulus of m - M = 23.95 ? 0.10 has been obtained from the tip of the red giant branch, in good agreement with previous estimates. The average stellar metallicity is estimated to be [Fe/H] = -1.43 ? 0.15, and decreases for increasing galactocentric distance.

A VLA Study of the Expansion of Tycho's Supernova Remnant

We have measured the expansion of Tycho's supernova remnant (SNR) over a 10 yr interval by comparing new VLA observations at 1375 MHz made in 1994 and 1995 with previous observations made in 1983 and 1984 using the same array configurations, bandwidths, calibrators, and integration times. To compute the expansion of the outer rim, we estimate the expansion rate for radial sectors of 4°. The overall mean expansion obtained is 305 but varies between 1'' and 5'' around the shell. The weighted average fractional expansion rate is 0.1126% yr-1, corresponding to a power-law index (expansion parameter) ν ≡ d ln R/d ln t = 0.471, with an error of ~6%. We also compute the expansion of interior features, and find a comparable global expansion parameter. The value of ν obtained is in excess of the pure Sedov value of 0.4, indicating that Tycho's SNR is still in transition from an earlier phase to the Sedov adiabatic phase of evolution, and supporting expansion into a medium without a strong radial density gradient, as expected for a Type Ia supernova. In addition, the significant local variations that we observe suggest that a unique average expansion rate is an oversimplified description for Tycho. Both radio flux and polarization were found to remain almost constant during the intervening 10 years.