Quasinormal modes for the SdS black hole : an analytical approximation scheme

Quasinormal modes for scalar field perturbations of a Schwarzschild-de Sitter (SdS) black hole are investigated. An analytical approximation is proposed for the problem. The quasinormal modes are evaluated for this approximate model in the limit when black hole mass is much smaller than the radius of curvature of the spacetime. The model mirrors some striking features observed in numerical studies of time behaviour of scalar perturbations of the SdS black hole. In particular, it shows the presence of two sets of modes relevant at two different time scales, proportional to the surface gravities of the black hole and cosmological horizons respectively. These quasinormal modes are not complete - another feature observed in numerical studies. Refinements of this model to yield more accurate quantitative agreement with numerical studies are discussed. Further investigations of this model are outlined, which would provide a valuable insight into time behaviour of perturbations in the SdS spacetime.Comment: 12 pages, revtex, refs added and discussion expanded, version to appear in Phys. Rev.

Cell nuclei detection using globally optimal active contours with shape prior

Cell nuclei detection in fluorescent microscopic images is an important and time consuming task for a wide range of biological applications. Blur, clutter, bleed through and partial occlusion of nuclei make this a challenging task for automated detection of individual nuclei using image analysis. This paper proposes a novel and robust detection method based on the active contour framework. The method exploits prior knowledge of the nucleus shape in order to better detect individual nuclei. The method is formulated as the optimization of a convex energy function. The proposed method shows accurate detection results even for clusters of nuclei where state of the art methods fail

Interior Structure of a Charged Spinning Black Hole in (2+1)(2+1)-Dimensions

The phenomenon of mass inflation is shown to occur for a rotating black hole. We demonstrate this feature in $(2+1)$ dimensions by extending the charged spinning BTZ black hole to Vaidya form. We find that the mass function diverges in a manner quantitatively similar to its static counterparts in $(3+1)$, $(2+1)$ and $(1+1)$ dimensions.Comment: 5 pages, 2 figures (appended as postscript files), WATPHYS-TH94/0

Radiative falloff in Einstein-Straus spacetime

The Einstein-Straus spacetime describes a nonrotating black hole immersed in a matter-dominated cosmology. It is constructed by scooping out a spherical ball of the dust and replacing it with a vacuum region containing a black hole of the same mass. The metric is smooth at the boundary, which is comoving with the rest of the universe. We study the evolution of a massless scalar field in the Einstein-Straus spacetime, with a special emphasis on its late-time behavior. This is done by numerically integrating the scalar wave equation in a double-null coordinate system that covers both portions (vacuum and dust) of the spacetime. We show that the field's evolution is governed mostly by the strong concentration of curvature near the black hole, and the discontinuity in the dust's mass density at the boundary; these give rise to a rather complex behavior at late times. Contrary to what it would do in an asymptotically-flat spacetime, the field does not decay in time according to an inverse power-law.Comment: ReVTeX, 12 pages, 14 figure

Scalar wave propagation in topological black hole backgrounds

We consider the evolution of a scalar field coupled to curvature in topological black hole spacetimes. We solve numerically the scalar wave equation with different curvature-coupling constant $\xi$ and show that a rich spectrum of wave propagation is revealed when $\xi$ is introduced. Relations between quasinormal modes and the size of different topological black holes have also been investigated.Comment: 26 pages, 18 figure

Characteristics of turbulent flames in a confined and pressurised jet-in-hot-coflow combustor

Available online 16 August 2022Combustion in hot and low oxygen environments—such as those encountered in practical devices including inter-turbine burners and sequential gas turbines—is not yet fully understood at a fundamental level, particularly in terms of the effects of pressure. To meet this gap in understanding, a confined-and-pressurised jet-in-hotcoflow (CP-JHC) combustor has been developed to facilitate optical diagnostics of turbulent flames in hot and vitiated coflows for the studies of flame stabilisation, structure and soot formation at elevated pressures. The CPJHC burner has been designed for steady operation at 10 bar with internal temperatures of up to 1975 K, with a water-cooled central jet issuing into a hot oxidant stream of combustion products from a non-premixed natural gas/H2 burner. This work describes the key features and operational capabilities of the CP-JHC burner and presents a selection of experimental results showing characteristics not previously available. Specifically, temperature measurements of the hot coflow are used to estimate the enthalpy deficit of the stream, revealing an increase in thermal efficiency with increasing heat input, and a decrease with increasing pressure. Chemiluminescence imaging of OH* and CH* is performed for turbulent jet flames to study the flame structure under various operating conditions, and true-colour imaging results are also included to highlight the change in soot formation under elevated pressures. The mean images indicate a change in stabilisation behaviour with changes in pressure and jet Reynolds number (Rejet), which is further investigated by a statistical analysis of the shortexposure CH* images. This analysis reveals that an increase in Rejet from 10,000 to 15,000 leads to an increase in the mean lift-off height (from the jet exit plane) from approximately 1.5 to 6 jet diameters at atmospheric pressure, while the flames at elevated pressures show significantly less variation and tend to stabilise at the jet exit for P > 3.5 bar(a). The experimental findings are complemented by numerical simulations of laminar opposed flow flames, providing additional insights into the fundamental chemical kinetics effects which influence these flames. In particular, a monotonic reduction in both the maximum and integrated OH* and CH* mass fractions is observed with increasing pressure. This reduction is particularly pronounced at lower pressures, with a reduction to 10% of the atmospheric-pressure value at 3 bar(a) for the integrated OH* mass fraction. Additionally, this behaviour is shown to be related to the combined effects of a shift in the formation pathways and the increased impact of collisional quenching.D.B. Proud, M.J. Evans, Q.N. Chan, P.R. Medwel

Co-combustion characteristics and kinetics of microalgae chlorella vulgaris and coal through TGA

Published online: 26 Dec 2018The combustion characteristics and kinetics of microalgae (Chlorella vulgaris) and sub-bituminous coal blends (CCBs) are studied by a thermogravimetric analyzer (TGA), and those of pure Chlorella vulgaris (C. vulgaris) and coal were also taken respectively as control groups. The microalgae to coal blending ratio (MCR) is 3/7, 5/5 and 7/3. The results showed that three stages were observed during the combustion of CCBs. And the main combustion of CCBs was occurred at the second stage ranged from 254.6 ~ 389.4°C to 698.7 ~ 741.0°C. Both of the ignition temperature (Ti) and the final temperature detected when stabilization of samples mass occurred (Tf) were decreased as the C. vulgaris content increased in the CCB. The maximum combustion rate (Rmax) of C. vulgaris was maximum. The average reaction rate (Rv) was firstly decreased, and then increased as the content of C. vulgaris in CCBs increasing. With the increasing content of C. vulgaris, both of the ignition index (Di) and the comprehensive combustion characteristic index for the blends (SM) were increased. Some deviations from their expected characteristics indicate interaction. As the heating rate (β) increases, Ti, the peak temperature (Tp), the reaction rate at the peaks (Rp), Rv and Tf were all increased significantly, while the residual mass (Mr) was first increased, and then decreased. For CCBs, the activation energy (E) was the first decreased, and then increased, and the minimum E was obtained as MCR = 5/5. Among all the samples, E of pure coal was the minimum one. Finally, kinetic triplets were determined by the Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), and master-plots method, they are respectively E = 62.90, 108.99, 85.28, 92.27, 104.98 kJ/mol, the reaction order (n) = 1.4, 4.1, 2.7, 3.2, 4 and the frequency factor (A) = 6.38 × 105, 1.05 × 106, 2.29 × 104, 8.73 × 104, 2.93 × 106 min−1 for the coal, blends with MCR of 3/7, 5/5, 7/3 and C. vulgaris combustion at β = 20°C/min.Chunxiang Chen, Qing Nian Chan, Paul R. Medwell and Guan Heng Yeo

Radiative falloff of a scalar field in a weakly curved spacetime without symmetries

We consider a massless scalar field propagating in a weakly curved spacetime whose metric is a solution to the linearized Einstein field equations. The spacetime is assumed to be stationary and asymptotically flat, but no other symmetries are imposed -- the spacetime can rotate and deviate strongly from spherical symmetry. We prove that the late-time behavior of the scalar field is identical to what it would be in a spherically-symmetric spacetime: it decays in time according to an inverse power-law, with a power determined by the angular profile of the initial wave packet (Price falloff theorem). The field's late-time dynamics is insensitive to the nonspherical aspects of the metric, and it is governed entirely by the spacetime's total gravitational mass; other multipole moments, and in particular the spacetime's total angular momentum, do not enter in the description of the field's late-time behavior. This extended formulation of Price's falloff theorem appears to be at odds with previous studies of radiative decay in the spacetime of a Kerr black hole. We show, however, that the contradiction is only apparent, and that it is largely an artifact of the Boyer-Lindquist coordinates adopted in these studies.Comment: 17 pages, RevTeX

Bose-Einstein Condensate in Weak 3d Isotropic Speckle Disorder

The effect of a weak three-dimensional (3d) isotropic laser speckle disorder on various thermodynamic properties of a dilute Bose gas is considered at zero temperature. First, we summarize the derivation of the autocorrelation function of laser speckles in 1d and 2d following the seminal work of Goodman. The goal of this discussion is to show that a Gaussian approximation of this function, proposed in some recent papers, is inconsistent with the general background of laser speckle theory. Then we propose a possible experimental realization for an isotropic 3d laser speckle potential and derive its corresponding autocorrelation function. Using a Fourier transform of that function, we calculate both condensate depletion and sound velocity of a Bose-Einstein condensate as disorder ensemble averages of such a weak laser speckle potential within a perturbative solution of the Gross-Pitaevskii equation. By doing so, we reproduce the expression of the normalfluid density obtained earlier within the treatment of Landau. This physically transparent derivation shows that condensate particles, which are scattered by disorder, form a gas of quasiparticles which is responsible for the normalfluid component