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Hydrodynamic Analysis of Binary Immiscible Metallurgical Flow in a Novel Mixing Process: Rheomixing
This paper presents a hydrodynamic analysis of binary immiscible metallurgical flow by a numerical simulation of the rheomixing process. The concept of multi-controll is proposed for classifying complex processes and identifying individual processes in an immiscible alloy system in order to perform simulations. A brief review of fabrication methods for immiscible alloys is given, and fluid flow aspects of a novel fabrication method – rheomixing by twin-screw extruder (TSE) are analysed. Fundamental hydrodynamic micro-mechanisms in a TSE are simulated by a piecewise linear (PLIC) volume-of-fluid (VOF) method coupled with the continuum surface force (CFS) algorithm. This revealed that continuous reorientation in the TSE process could produce fine droplets and the best mixing efficiency. It is verified that TSE is a better mixing device than single screw extruder (SSE) and can achieve finer droplets. Numerical results show good qualitative agreement with experimental results. It is concluded that rheomixing by a TSE can be successfully employed for casting immiscible engineering alloys due to its unique characteristics of reorientation and surface renewal
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Numerical analysis of the hydrodynamic behaviour of immiscible metallic alloys in twin-screw rheomixing process
A numerical analysis by a VOF method is presented for studying the hydrodynamic mechanisms of the rheomixing process by a twin-screw extruder (TSE). The simplified flow field is established based on a systematic analysis of flow features of immiscible alloys in TSE rheomixing process. The studies focus on the fundamental microstructure mechanisms of rheological behaviour in shear-induced turbulent flows. It is noted that the microstructure of immiscible alloys in the mixing process is strongly influenced by the interaction between droplets, which is controlled by shearing forces, viscosity ratio, turbulence, and shearing time. The numerical results show a good qualitative agreement with the experimental results, and are useful for further optimisation design of prototypical rheomixing processes
Fabrication and room temperature operation of semiconductor nano-ring lasers using a general applicable membrane transfer method
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 110, 171105 (2017) and may be found at https://doi.org/10.1063/1.4982621.Semiconductor nanolasers are potentially important for many applications. Their design and fabrication are still in the early stage of research and face many challenges. In this paper, we demonstrate a generally applicable membrane transfer method to release and transfer a strain-balanced InGaAs quantum-well nanomembrane of 260 nm in thickness onto various substrates with a high yield. As an initial device demonstration, nano-ring lasers of 1.5 μm in outer diameter and 500 nm in radial thickness are fabricated on MgF2 substrates. Room temperature single mode operation is achieved under optical pumping with a cavity volume of only 0.43λ03 (λ0 in vacuum). Our nano-membrane based approach represents an advantageous alternative to other design and fabrication approaches and could lead to integration of nanolasers on silicon substrates or with metallic cavity
Thick fireballs and the steep decay in the early X-ray afterglow of gamma-ray bursts
We study the early afterglows of gamma-ray bursts produced by geometrically
thick fireballs, following the development of the external shock as energy is
continually supplied to the shocked material. We study the dependence of the
early afterglow slope on the luminosity history of the central engine. The
resulting light curves are modeled with power-law functions and the importance
of a correct choice of the reference time t_0 is investigated. We find that
deviations from a simple power-law are observed only if a large majority of the
energy is released at late times. The light curve in this case can be described
as a simple power-law if the reference time is set to be close to the end of
the burst. We applied our analysis to the cases of GRB 050219a and GRB 050315.
We show that the early steep decay of the afterglow cannot result from the
interaction of the fireball with the ambient medium. We conclude that the early
X-ray afterglow emission is associated with the prompt phase and we derive
limits on the radius at which the prompt radiation is produced.Comment: Minor revisions, accepted for publication in Ap
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A Comparative Study of Prenucleation on Zr and MgO Substrates by ab initio MD Simulations
The traditional wisdom for grain refiners is by selecting the most potent solid particles (i.e. having the
least misfit between the substrate and the metal) [1,2]. A good example is the Mg-Zr grain refiner for
Mg and Mg-alloys. Zr is iso-structural to Mg with a small lattice misfit (~0.7 % at room temperature)
[3]. The melting temperature of Zr is 1855°C (2128 K), notably higher than that of Mg (649°C).
Interestingly, Zr is chemically repulsive to Mg with a heat of mixing +6 kJ/mol, and there forms no
Zr-Mg intermetallic compound [3,4]. Zr particles have been considered to be good for heterogeneous
nucleation of Mg through structural templating according to the epitaxial nucleation model [5].
Recently, it was also suggested that the most effective grain refinement can be achieved by the least
potent particles (i.e. having largest misfit) if no other more potent particles of significance exist in the
melt [6]. The role of MgO in heterogeneous nucleation of Mg can be a good demonstration of this
theory [6]. [...]EPSRC is gratefully acknowledged for supporting the EPSRC Centre under grant EP/H026177/
Hyperaccretion Disks around Neutron Stars
(Abridged) We here study the structure of a hyperaccretion disk around a
neutron star. We consider a steady-state hyperaccretion disk around a neutron
star, and as a reasonable approximation, divide the disk into two regions,
which are called inner and outer disks. The outer disk is similar to that of a
black hole and the inner disk has a self-similar structure. In order to study
physical properties of the entire disk clearly, we first adopt a simple model,
in which some microphysical processes in the disk are simplified, following
Popham et al. and Narayan et al. Based on these simplifications, we
analytically and numerically investigate the size of the inner disk, the
efficiency of neutrino cooling, and the radial distributions of the disk
density, temperature and pressure. We see that, compared with the black-hole
disk, the neutron star disk can cool more efficiently and produce a much higher
neutrino luminosity. Finally, we consider an elaborate model with more physical
considerations about the thermodynamics and microphysics in the neutron star
disk (as recently developed in studying the neutrino-cooled disk of a black
hole), and compare this elaborate model with our simple model. We find that
most of the results from these two models are basically consistent with each
other.Comment: 44 pages, 10 figures, improved version following the referees'
comments, main conclusions unchanged, accepted for publication in Ap
Catastrophic Photo-z Errors and the Dark Energy Parameter Estimates with Cosmic Shear
We study the impact of catastrophic errors occurring in the photometric
redshifts of galaxies on cosmological parameter estimates with cosmic shear
tomography. We consider a fiducial survey with 9-filter set and perform photo-z
measurement simulations. It is found that a fraction of 1% galaxies at
z_{spec}~0.4 is misidentified to be at z_{phot}~3.5. We then employ both chi^2
fitting method and the extension of Fisher matrix formalism to evaluate the
bias on the equation of state parameters of dark energy, w_0 and w_a, induced
by those catastrophic outliers. By comparing the results from both methods, we
verify that the estimation of w_0 and w_a from the fiducial 5-bin tomographic
analyses can be significantly biased. To minimize the impact of this bias, two
strategies can be followed: (A) the cosmic shear analysis is restricted to
0.5<z<2.5 where catastrophic redshift errors are expected to be insignificant;
(B) a spectroscopic survey is conducted for galaxies with 3<z_{phot}<4. We find
that the number of spectroscopic redshifts needed scales as N_{spec} \propto
f_{cata}\times A where f_{cata}=1% is the fraction of catastrophic redshift
errors (assuming a 9-filter photometric survey) and A is the survey area. For
A=1000 {deg}^2, we find that N_{spec}>320 and 860 respectively in order to
reduce the joint bias in (w_0,w_a) to be smaller than 2\sigma and 1\sigma. This
spectroscopic survey (option B) will improve the Figure of Merit of option A by
a factor \times 1.5 thus making such a survey strongly desirable.Comment: 25 pages, 9 figures. Revised version, as accepted for publication in
Ap
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