Increase In Core Radius Research Articles

Tunable near-infrared optical properties of three-layered gold–silica–gold nanoparticles

The absorption spectra of gold–silica–gold nanoshells have been investigated by using Mie theory with variation of the geometry. With an increase in core radius the plasmon resonance for the lower energy mode ω−− shows a distinct redshift while for the high-energy mode ω−+ shows a blueshift. It is surprising that with increasing middle layer thickness the resonance of ω−− mode blueshifts first and then redshifts. In addition, an increase of the dielectric constant of the middle layer is found to reduce the resonance energies of the particle. The tunable near-infrared optical properties are discussed in terms of plasmon hybridization theory.

Mathematical modeling of pulsatile flow of non-Newtonian fluid in stenosed arteries

The pulsatile flow of blood through mild stenosed artery is studied. The effects of pulsatility, stenosis and non-Newtonian behavior of blood, treating the blood as Herschel–Bulkley fluid, are simultaneously considered. A perturbation method is used to analyze the flow. The expressions for the shear stress, velocity, flow rate, wall shear stress, longitudinal impedance and the plug core radius have been obtained. The variations of these flow quantities with different parameters of the fluid have been analyzed. It is found that, the plug core radius, pressure drop and wall shear stress increase with the increase of yield stress or the stenosis height. The velocity and the wall shear stress increase considerably with the increase in the amplitude of the pressure drop. It is clear that for a given value of stenosis height and for the increasing values of the stenosis shape parameter from 3 to 6, there is a sharp increase in the impedance of the flow and also the plots are skewed to the right-hand side. It is observed that the estimates of the increase in the longitudinal impedance increase with the increase of the axial distance or with the increase of the stenosis height. The present study also brings out the effects of asymmetric of the stenosis on the flow quantities.

Core performance tests for the JOYO MK-III upgrade

Many changes were made in the recent upgrade of the experimental fast reactor JOYO to the MK-III design. The core changes which were made to achieve a four-fold increase in irradiation capacity includes the introduction of a second enrichment zone, an increase in core radius and a decrease in core height. Performance tests done at low power, during the rise to power, and at full power, which focus on the neutronics characteristics, are presented. These tests include the nuclear instrumentation system response, the approach to criticality and excess reactivity evaluation, control rod worth calibrations, isothermal temperature coefficient evaluation, the calibration of the nuclear instrumentation system with reactor thermal power, and the burn-up reactivity coefficient. The measurements and comparisons with calculated predictions are shown. The design predictions are consistent with the performance test results, and all technical safety specifications are satisfied. The JOYO MK-III core will provide enhanced irradiation testing capability, as well as serve as a test bed for improving fast reactor operation, performance, and safety. Through the performance test evaluation, the core characteristics of a small size sodium cooled fast reactor with a hard neutron spectrum are clarified.

Modification of a wing tip vortex by vortex generators

The vortex wake hazard associated with transport aircraft is well known. One strategy for its alleviation is to alter the near field wake topology to distribute circulation over an increased area and reduce the consequent rolling moment on a following aircraft encountering the wake. The paper describes the results of an experimental investigation of the use of supplementary vortex generators to modify the tip vortex shed from a half-span airfoil. The experiments were conducted in a small low speed wind tunnel at a chord Reynolds number of 2.2 × 10 5 and the flow was characterised by measurement of the instantaneous and mean cross-stream flow field using particle image velocimetry. The results show that when the vortex generator is configured to produce a vortex that is co-rotating with respect to the wing-tip vortex, the two merge within five chord-lengths of the trailing edge resulting in a vortex with increased size relative to that of the baseline wake. Various vortex generator geometries were tested. A semi-circular generator produced an increase in core radius by a factor of five but the best shape was deemed triangular since generators of this shape produced substantial vortex enlargement with relatively low lift penalty. Low slenderness ratio generators were also deemed preferable since for a high slenderness generator it was shown that merging failed due to a negative vorticity buffer constrained to the region between the tip and generator vortex by the proximity of the generator vortex to the wing surface. Increasing the generator angle of attack was shown to increase the effectiveness of circulation redistribution. At the highest angles of attack increases occurred despite a decrease in VG vortex strength due to stalling. Unsteadiness in the stalled VG flow appears to enhance vorticity diffusion and it is hypothesised that it may also initiate cooperative instabilities in multiple vortex systems.

Investigation of the Role of Polymer on the Delay of Tip Vortex Cavitation

Cavitation tunnel measurements have shown tip vortex cavitation inception is delayed when polymers are injected from the hydrofoil tip. Experiments with polymers have also shown that the polymers cause the cavitation to become more violent. To better understand the role of polymer in the delay of tip vortex cavitation, a theoretical analysis of tip vortex cavitation inception in pure water and in polymer solution is presented. The analysis shows that while polymer injection causes instability in small bubbles, its main effect is an increase in tip vortex core radius, resulting in the delay of tip vortex cavitation inception.

Postcollapse dynamics of self-gravitating Brownian particles and bacterial populations.

We address the postcollapse dynamics of a self-gravitating gas of Brownian particles in D dimensions in both canonical and microcanonical ensembles. In the canonical ensemble, the postcollapse evolution is marked by the formation of a Dirac peak with increasing mass. The density profile outside the peak evolves self-similarly with decreasing central density and increasing core radius. In the microcanonical ensemble, the postcollapse regime is marked by the formation of a "binarylike" structure surrounded by an almost uniform halo with high temperature. These results are consistent with thermodynamical predictions in astrophysics. We also show that the Smoluchowski-Poisson system describing the collapse of self-gravitating Brownian particles in a strong-friction limit is isomorphic to a simplified version of the Keller-Segel equations describing the chemotactic aggregation of bacterial populations. Therefore, our study has direct applications in this biological context.

Quasiparticle Structure in Antiferromagnetism around the Vortex and Nuclear Magnetic Relaxation Time

On the basis of the Bogoliubov–de Gennes theory for the two-dimensional extended Hubbard model, the vortex structure in d -wave superconductors is investigated including the contribution of the induced incommensurate antiferromagnetism around the vortex core. As the on-site repulsive interaction U increases, the spatial structure of charge and spin changes from the antiferromagnetic state with checkerboard modulation to that with the stripe modulation. By the effect of the induced antiferromagnetic moment, the zero-energy density of states is suppressed, and the vortex core radius increases. We also study the effect of the local density of states (LDOS) change on the site-dependent nuclear relaxation rate T 1 -1 ( r ). These results are compared with a variety of experiments performed on high T c cuprates.

Nonlinear waves in a prestressed elastic tube filled with a layered fluid

In this work, we studied the propagation of weakly nonlinear waves in a prestressed thin elastic tube filled with an incompressible layered fluid, where the outer layer is assumed to be inviscid whereas the cylindrical core is considered to be viscous. Using the reductive perturbation technique, the propagation of weakly nonlinear waves in the longwave approximation is studied. The governing equation is shown to be the Korteweg-de Vries–Burgers' (KdV–B) equation. A travelling wave type of solution to this evolution equation is sought and it is observed that the formation of shock wave becomes evident with increasing core radius parameter.

Salt induced micellization and micelle structures of' PEO/PPO/PEO block copolymers in aqueous solution

Aqueous micellar solutions of two moderately hydrophilic polyethylene oxide/polypropylene oxide/polyethylene oxide (PEO/PPO/PEO) triblock copolymers, pluronics P84 and P104 are examined by small angle neutron scattering (SANS), viscosity and cloud point measurements. Micellar structures of pluronic P84 in aqueous solution are determined as a function of its concentrations (5 and 10 wt%) and added KCl concentrations (0–2 M). 5 wt% solutions of both the block copolymers contain spherical micelles with hydrophobic core of PPO and corona of highly hydrated ethylene oxide subchains. The presence of added neutral salt favors micellization at markedly lower concentration/temperature unlike conventional surfactants. The addition of the salt causes dehydration of ethylene oxide units from hydrated PEO shell from the side of PPO core, leading to an increase in core radius ( R C) of spherical micelles. Micellar volume fraction ( φ) increases with pluronic concentrations. It however increases at lower salt concentrations (up to 0.5 M) but is found to be independent at higher salt concentrations. Aggregation number ( N ag) of P84 in aqueous salt solutions increases from 72 in water to 141 in 2 M KCl. No growth of spherical micelles is observed in unimer-to-micelle transition region and at temperatures below 20–22°C to its cloud point (CP) as hard sphere remains almost constant ( R hs∼70–74 Å) with increasing salt concentration. Micelles remain spherical throughout the KCl concentration range studied.

Collision of a Vortex Jet with Stator Vanes

We consider the unsteady behavior of the e owe eld due to the slicing of a jet and a vortex with axial e ow, that is, a vortex jet, by a pair of thin e at plates. In the case of a vortex, the geometry is an idealization of the collision between a tip vortex shed from a rotor blade and two downstream stator blades in an axial e ow turbomachine. The e ow is assumed to be inviscid, and the main result of the calculations is that the vorticity e eld near each plate is radically altered. The role of the axial e ow in the vortex is to decrease the vortex core radius and to increase the swirl on the suction side while the vortex core radius increases and the swirl decreases on the pressure side. The suction peak due to the vortex grows on the suction side and decreases on the pressure side both for the jet and the vortex.

Theoretical and numerical analysis of wake vortices

The rst part of the paper analyses the linear dynamics of a vortex pair thanks to a normal mode analysis. The basic flow is a superposition of two Lamb-Oseen vortices of radius a and distance b. The long-wave Crow instability and the short-wave Widnall instabilities are fully characterized for dierent aspect ratios a=b of the vortex pair. In particular, as a=b increases, it is shown that the antisymmetric Widnall instabilities are favoured for short-wave perturbations. This constitutes an explanation for the results obtained by Leweke & Williamson (1998). The second part of the paper deals with turbulence in the Batchelor vortex. A 3D Large Eddy Simulation (LES) shows that, starting from a linearly unstable wake-type vortex, high levels of turbulence can be obtained. But, the mean-flow then rapidly evolves towards a stable state. These results are in accordance with those of Ragab & Sreedhar (1995). This paper analyses some linear and non-linear properties of the dynamics of vortices. Two problems will be discussed. The rst concerns the linear dynamics of two Lamb-Oseen vortices. The second deals with the turbulence eects on a Batchelor-type vortex. These constitute two dierent aspects of the general airplane vortex wake hazard problem. Linear stability of two counterrotating vortices, including the (long-wave) Crow instability (Crow 1970) and the (short-wave) Widnall instability (Tsai & Widnall 1976 and Moore & Saman 1975), may contribute to an aircraft vortex wake dispersion. In the present paper, these two instabilities are fully characterised in a unied frame by means of a normal mode analysis. The development of turbulence and its impact on aircraft vortices is still a matter of question. A temporal numerical simulation of an unstable Batchelor vortex is performed in order to get an insight into this problem. The rst part of this paper is devoted to linear dynamics. We rst obtain the bending wave oscillation frequencies of a single Lamb-Oseen vortex by means of a shooting method. It is shown that a single Lamb-Oseen vortex is stable. Then, a spectral collocation and matrix eigenvalue method is used to analyse the dynamics of a vortex pair. In particular, the Crow instability in the long-wave limit and the Widnall instabilities are recovered by this general method. We also prove that increasing the core radius makes the interaction between the two vortices stronger. It is shown, for example, that the antisymmetric mode of the Widnall instability is promoted as the core radius increases. The second part deals with turbulence in the Batchelor vortex. It is shown that high levels of turbulence can be obtained in the vortex. Starting from a linearly unstable wake-type vortex, the

Bertlmann-Martin inequality for a weakly bound particle

The Bertlmann-Martin (BM) inequality for the rms radius is checked against the exact value in the case of a particle in a weak central potential having a single bound state. A number of usual potentials are considered in one and three dimensions. The BM inequality is poorly saturated but the correction factor needed to recover the exact rms radius is weakly depending on the shape of the potential, except for the Hulth?n potential. For potentials having a hard-core component, we found that , which depends on the bound-state energy, tends to a universal curve as the hard core radius increases.

Perpendicular blade vortex interaction

The interaction between a streamwise vortex and a spanwise blade was studied in incompressible flow for blade-vortex separations between ±1/8 chord. Three-component velocity and turbulence measurements were made from 4 chord lengths upstream to 15 chord lengths downstream of the blade using miniature four-sensor hot-wire probes. The interaction of the vortex with the blade causes an almost immediate loss in vortex core circulation. Downstream of the blade the core becomes embedded in the blade wake and begins to grow rapidly. Core radius increases and peak tangential velocity decreases but circulation remains roughly constant. True turbulence levels within the core are much larger downstream than upstream of the blade. Outside of the core the interaction produces a substantial region of turbulent flow associated with the blade and vortex generator wakes. Overall, perpendicular blade vortex interaction substantially alters the flow and produces a much larger and more intense region of turbulent flow than that presented by the undisturbed vortex. These results have significant implications for the prediction of both impulsive and broadband helicopter noise.

Stellar surface density distributions in ten open clusters

Abstract A method based on the numerical differentiation of the best mean-square fit polynomial for N(r) (the number of stars in the circle of radius r in projection) is used for the determination of the stellar surface density in star clusters, F(r), free from the “interval error”. Structural features seen in NGC 1245, NGC 1502 and IC 1848 as steps on the F(r) curve are found to be significantly stronger than fluctuations of background stellar density. Such features may be the manifestations of non-stationary dynamical processes in these clusters. Star counts in clusters NGC 1502, NGC 2420 and IC 1848 were carried out on the plates taken with different exposures. It was found that the different structural features are due to the stars of different brightness. The cluster core radius increases and the concentration parameter of the cluster King model decreases in average with the increase of the limiting magnitude. The cluster radius slightly increases with the increase of the limiting magnitude only in IC 1848. Thus, the structure of the clusters varies unhomologically with the limiting magnitude.

Unsteady flow phenomena over delta wings at high angle of attack

Experiments show that coherent pressure fluctuations observed on delta wings are due to the helical mode instability of the vortex breakdown flowfield. No dominant frequency in the spectra of pressure fluctuations on the wing surface was observed after the breakdown reached the apex of the wing, although the vortex shedding could be detected in the wake. Measurements of pressure fluctuations at different streamwise locations on the wing suggest that the dimensionless frequency fx/U^ is nearly constant for a given geometry, which implies increasing wavelength in the streamwise direction. For different wings, this nondimensional frequency is shown to be a function of nondimensional circulation T/U^x only. Both the wavelength of the disturbances and the core radius increase with the nondimensional circulation at a fixed streamwise location. The wavelength normalized by the core radius is around 3-4, which is much smaller than the predictions for the Q vortex.

Buckling of a columnar vortex

The instability of a columnar vortex under axial compression is studied analytically. Prediction of the change with time of an initial perturbation in the form of a small-amplitude helical wave on the vortex core is reduced to solution of a system of two linear, second-order ordinary differential equations. For small rates of compression, a closed-form asymptotic solution of these equations is obtained using a two-variable expansion in time. The results indicate that a vortex under compression may bend (or ‘‘buckle’’) or it may remain columnar with an increase in core radius. The choice of which of these possible outcomes will occur is found to depend on the value of a single dimensionless parameter given by the product of the wave number k of the helical perturbation and the radius σ of the vortex core. When kσ is greater than a critical value, found to be approximately 1.77, the perturbation amplitude will decrease with axial compression and no ‘‘buckling’’ will occur. When kσ is less than this critical value, the perturbation amplitude will increase as the vortex axis is compressed and ‘‘buckling’’ will be observed.

Surface brightness profiles for five rich star clusters in the Large Magellanic Cloud

Surface brightness profiles are presented for five rich star clusters in the Large Magellanic Cloud (NGC 1783, 1856, 1987, 2107 and 2108), with ages between 10 8 and 10 9 yr. The profiles are well represented by King-like models, and the core radii follow the trend, of increasing core radius with age, found in a previous study. If this trend reveals expansion due to mass loss through stellar evolution, then the new data reinforce the suggestion that the slope of the initial mass function, which determines the rate of expansion, is consistent with the Salpeter value (x = 1.35), and is not in general as steep as x = 2.5

Core expansion in young star clusters in the Large Magellanic Cloud

view Abstract Citations (66) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Core Expansion in Young Clusters in the Large Magellanic Cloud Elson, Rebecca A. W. ; Freeman, Kenneth C. ; Lauer, Tod R. Abstract We have measured the core radii of 18 rich star clusters in the Large Magellanic Cloud (LMC) with ages 10^7^-10^9^ yr. Data for an additional 17 clusters with ages 10^6^-10^10^ yr are available in the literature. The combined sample shows that the core radii increase from ~0 to ~5 pc between ~ 10^6^ and 10^9^ yr, and then begin to decrease again. The expansion of the cores is probably driven by mass loss from evolving stars. Models of cluster evolution show that the rate of increase in core radius is sensitive to the slope of the initial mass function. The observed core radius-age relation for the LMC clusters favors an initial mass function with slope slightly flatter than the Salpeter value. Publication: The Astrophysical Journal Pub Date: December 1989 DOI: 10.1086/185610 Bibcode: 1989ApJ...347L..69E Keywords: Globular Clusters; Gravitational Collapse; Magellanic Clouds; Astronomical Models; Expansion; Stellar Cores; Stellar Mass Ejection; Astrophysics; CLUSTERS: DYNAMICS; CLUSTERS: GLOBULAR; GALAXIES: MAGELLANIC CLOUDS full text sources ADS | data products SIMBAD (36) NED (2)

Mode conversion in optical fibers with monotonically increasing core radius

In this paper we discuss mode conversion in a straight fiber which maintains its circular cross section but whose radius a(z) increases monotonically with the length coordinate z so that it passes from the single-mode to the multimode regime. Our principal concern is with step-index fibers which are treated in the scalar approximation. We study conversion of power from the incident dominant mode to other core and cladding modes for several functions a(z) . The results of our calculations show clearly that the dominant mode is able to adapt itself to the changing fiber radius provided the change is gradual. For comparison, we also consider a fiber with an infinitely extended radial parabolic-index profile as an approximation to graded index fibers. We show that it too exhibits mode conversion similar to that of the step-index fiber, provided that the comparison is made on the basis of corresponding mode spot sizes instead of actual core radii.

Primordial stellar evolution - The protostar phase

The structure and evolution of a protostar forming from a cloud composed of pure hydrogen and helium gas are calculated. Using an accretion rate of 0.0044 solar mass/yr, the collapse of the cloud is followed numerically as a sequence of steady state accretion flows onto the hydrostatic core, which grows from an initial mass of 0.01 solar mass to 10.5 solar masses. The core is surrounded by an optically thick radiative precursor for most of its evolution. The core radius reaches 47 solar radii when the mass is 1 solar mass. For sufficiently massive cores, the deep interior contracts strongly, driving out a 'luminosity wave' which reaches the surface when the mass is 8 solar masses. This results in a large increase in core radius, the establishment of surface convection, and the disappearance of the radiative precursor. The dependence of core radius on the mass and accretion rate is analytically derived, and a new table or Rosseland mean opacities for metal-free gas is presented.