411 research outputs found
Towards a quantitative phase-field model of two-phase solidification
We construct a diffuse-interface model of two-phase solidification that
quantitatively reproduces the classic free boundary problem on solid-liquid
interfaces in the thin-interface limit. Convergence tests and comparisons with
boundary integral simulations of eutectic growth show good accuracy for
steady-state lamellae, but the results for limit cycles depend on the interface
thickness through the trijunction behavior. This raises the fundamental issue
of diffuse multiple-junction dynamics.Comment: 4 pages, 2 figures. Better final discussion. 1 reference adde
The Effect of Bamboo Charcoal on pH and Hardware in Dailed Well Water
The problem that is often encountered in clean water is that the quality of the water, both groundwater and river water used by the community, does not meet the requirements of healthy clean water. This study aims to determine the effectiveness of activated charcoal from bamboo on pH and decrease the hardness level of dug well water in the village of Lampoh Keude, Aceh Besar. This study used a Quasi-experimental method, water obtained from residents' wells was processed using the filtration and adsorption method, the filtration media was made using media with a volume of 20 liters which had been modified using sand with a thickness of 10 cm, 10 cm of palm fiber, and 10 gravel cm. The highest pH increase test results occurred in TH III AB media where the resulting pH was 7.7 ± 0.01 compared to 4.5 before processing. The same thing also happened to a decrease in the hardness level of residents' well water, where the thickness of the charcoal and the length of contact time greatly affected the test results, from the results of the 3 existing media tests, the best results were obtained by TH III AB media with a contact time of 30 minutes where the level The hardness obtained was 279 ± 3.46 mg/L or a decrease of 47.1% from the initial hardness level of well waste. There is an increase in the pH value of well water after processing, and that the use of bamboo charcoal can also reduce the hardness level of well wate
Unsteady Crack Motion and Branching in a Phase-Field Model of Brittle Fracture
Crack propagation is studied numerically using a continuum phase-field
approach to mode III brittle fracture. The results shed light on the physics
that controls the speed of accelerating cracks and the characteristic branching
instability at a fraction of the wave speed.Comment: 11 pages, 4 figure
Phase-Field Formulation for Quantitative Modeling of Alloy Solidification
A phase-field formulation is introduced to simulate quantitatively
microstructural pattern formation in alloys. The thin-interface limit of this
formulation yields a much less stringent restriction on the choice of interface
thickness than previous formulations and permits to eliminate non-equilibrium
effects at the interface. Dendrite growth simulations with vanishing solid
diffusivity show that both the interface evolution and the solute profile in
the solid are well resolved
Phase-field modeling of microstructural pattern formation during directional solidification of peritectic alloys without morphological instability
During the directional solidification of peritectic alloys, two stable solid
phases (parent and peritectic) grow competitively into a metastable liquid
phase of larger impurity content than either solid phase. When the parent or
both solid phases are morphologically unstable, i.e., for a small temperature
gradient/growth rate ratio (), one solid phase usually outgrows and
covers the other phase, leading to a cellular-dendritic array structure closely
analogous to the one formed during monophase solidification of a dilute binary
alloy. In contrast, when is large enough for both phases to be
morphologically stable, the formation of the microstructurebecomes controlled
by a subtle interplay between the nucleation and growth of the two solid
phases. The structures that have been observed in this regime (in small samples
where convection effect are suppressed) include alternate layers (bands) of the
parent and peritectic phases perpendicular to the growth direction, which are
formed by alternate nucleation and lateral spreading of one phase onto the
other as proposed in a recent model [R. Trivedi, Metall. Mater. Trans. A 26, 1
(1995)], as well as partially filled bands (islands), where the peritectic
phase does not fully cover the parent phase which grows continuously. We
develop a phase-field model of peritectic solidification that incorporates
nucleation processes in order to explore the formation of these structures.
Simulations of this model shed light on the morphology transition from islands
to bands, the dynamics of spreading of the peritectic phase on the parent phase
following nucleation, which turns out to be characterized by a remarkably
constant acceleration, and the types of growth morphology that one might expect
to observe in large samples under purely diffusive growth conditions.Comment: Final version, minor revisions, 16 pages, 14 EPS figures, RevTe
Phase-Field Approach for Faceted Solidification
We extend the phase-field approach to model the solidification of faceted
materials. Our approach consists of using an approximate gamma-plot with
rounded cusps that can approach arbitrarily closely the true gamma-plot with
sharp cusps that correspond to faceted orientations. The phase-field equations
are solved in the thin-interface limit with local equilibrium at the
solid-liquid interface [A. Karma and W.-J. Rappel, Phys. Rev. E53, R3017
(1996)]. The convergence of our approach is first demonstrated for equilibrium
shapes. The growth of faceted needle crystals in an undercooled melt is then
studied as a function of undercooling and the cusp amplitude delta for a
gamma-plot of the form 1+delta(|sin(theta)|+|cos(theta)|). The phase-field
results are consistent with the scaling law "Lambda inversely proportional to
the square root of V" observed experimentally, where Lambda is the facet length
and V is the growth rate. In addition, the variation of V and Lambda with delta
is found to be reasonably well predicted by an approximate sharp-interface
analytical theory that includes capillary effects and assumes circular and
parabolic forms for the front and trailing rough parts of the needle crystal,
respectively.Comment: 1O pages, 2 tables, 17 figure
Feedback control of unstable cellular solidification fronts
We present a numerical and experimental study of feedback control of unstable
cellular patterns in directional solidification (DS). The sample, a dilute
binary alloy, solidifies in a 2D geometry under a control scheme which applies
local heating close to the cell tips which protrude ahead of the other. For the
experiments, we use a real-time image processing algorithm to track cell tips,
coupled with a movable laser spot array device, to heat locally. We show,
numerically and experimentally, that spacings well below the threshold for a
period-doubling instability can be stabilized. As predicted by the numerical
calculations, cellular arrays become stable, and the spacing becomes uniform
through feedback control which is maintained with minimal heating.Comment: 4 pages, 4 figures, 1 tabl
Eutectic Colony Formation: A Stability Analysis
Experiments have widely shown that a steady-state lamellar eutectic
solidification front is destabilized on a scale much larger than the lamellar
spacing by the rejection of a dilute ternary impurity and forms two-phase cells
commonly referred to as `eutectic colonies'. We extend the stability analysis
of Datye and Langer for a binary eutectic to include the effect of a ternary
impurity. We find that the expressions for the critical onset velocity and
morphological instability wavelength are analogous to those for the classic
Mullins-Sekerka instability of a monophase planar interface, albeit with an
effective surface tension that depends on the geometry of the lamellar
interface and, non-trivially, on interlamellar diffusion. A qualitatively new
aspect of this instability is the occurence of oscillatory modes due to the
interplay between the destabilizing effect of the ternary impurity and the
dynamical feedback of the local change in lamellar spacing on the front motion.
In a transient regime, these modes lead to the formation of large scale
oscillatory microstructures for which there is recent experimental evidence in
a transparent organic system. Moreover, it is shown that the eutectic front
dynamics on a scale larger than the lamellar spacing can be formulated as an
effective monophase interface free boundary problem with a modified
Gibbs-Thomson condition that is coupled to a slow evolution equation for the
lamellar spacing. This formulation provides additional physical insights into
the nature of the instability and a simple means to calculate an approximate
stability spectrum. Finally, we investigate the influence of the ternary
impurity on a short wavelength oscillatory instability that is already present
at off-eutectic compositions in binary eutectics.Comment: 26 pages RevTex, 14 figures (28 EPS files); some minor changes;
references adde
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