Channel Segregation Research Articles

Numerical simulation of macrosegregation during solidification of WE43 (Mg–4.3 wt-% Y–2.0 wt-% Nd–0.6 wt-% Gd) magnesium alloy

The formation of macrosegregation driven by multicomponent thermosolutal convection during solidification of WE43 magnesium alloy is numerically studied. A two-dimensional solidification model that describes the conservation of mass, momentum, energy and solute is presented. The experiments are performed in the resin sand mould with a graphite chill placed on one side of the cavity. The validation of computer codes is carried out by comparisons with the experimentally measured cooling curves and concentrations. The influences of mould material and characteristic grain size on macrosegregation are numerically investigated. For a total sand mould casting, the magnitude of macrosegregation is reduced, although the solidification rate is slower than that cooled with a graphite chill. Increasing the characteristic grain size not only causes a serious positive segregation but also leads to the formation of channel segregates. The channel segregation occurs thanks to the weak instabilities of the growing front triggered by the fluid flow.

Comparison of Channel Segregation Formation in Model Alloys and Steels via Numerical Simulations

In the current study, the evolutions of channel segregations in several alloy systems, such as the typically used model alloys (e.g., Ga-In, Sn-Pb, Sn-Bi, Al-Cu, and Ni-based superalloy) and some special steels, are numerically simulated in a cavity solidified unidirectionally. The simulations are based on a modified continuum macrosegregation model with an extension to the multicomponent systems. The results of model alloys and steels indicate that when the thermosolutal convection is strong enough, flow instability occurs, which in turn destabilizes the mushy zone. Subsequently, the channel segregation forms with the continuous interaction between solidification and flow. The formation behavior and severity of channel segregations in various systems are different owing to their distinct melt convection strengths and solidification natures. In the current simulations, channels are apparent for model alloys with high content of solutes, whereas they are slight in some special steels, such as 27SiMn steel, and totally disappear in carbon steels. These occurrence features of channel segregation in simulations of steels are consistent with the analyses by a modified Rayleigh number associated with alloying elements, and both outcomes are well supported by the fully sectioned steel ingots in experiments.

Numerical study of influence of inclusion movement on channel segregation in Fe- 0.21 wt% C- 0.1 wt% S alloy

A two-dimensional continuum model on solute and heat transport, and fluid flow is developed to numerically investigate the influence of inclusion movement on the development of channel segregation. A trajectory model is used to track the moving path of inclusion particles. Inclusion movement affects the flow field in simulation by means of interfacial friction coefficient. Simulations are performed on the Hebditch-Hunt casting. A parametric study is carried out to study the effects of with and without inclusions, and diameter (5 × 10-6 m, 10 × 10-6 m, 20 × 10-6 m and 40 × 10-6 m) of inclusions on channel segregation. It is found that the channel segregation is strengthened with the consideration of inclusion movement. Compared to other diameters, inclusions with diameter 20 × 10-6 m are found to enhance the channel segregation. This is because the larger inclusions (40 × 10-6 m) present a faster floating velocity that reduces the interaction time between inclusion upward movement and the development of solidification front, and then lessens the disturbance to solidification front that is important to the initialization of channel segregation. The upward movement of smaller inclusions (5 × 10-6 m and 10 × 10-6 m) cannot greatly increase the upward velocity of fluid flow. Therefore, the formation of channel segregation is less affected.

Formation mechanism of channel segregation in carbon steels by inclusion flotation: X-ray microtomography characterization and multi-phase flow modeling

Recent experimental dissections of steel ingots and multi-scale simulations have led to the discovery of a potential driving force for channel segregation: the flotation of oxide-based inclusion (D. Li et al., Nat. Commun. 5:5572 (2014)). Further experimental analysis and numerical modeling are necessary to clarify this mechanism in detail. In this work, the inclusions in a carbon steel ingot that exhibits severe channel segregations were characterized by the 3D X-ray microtomography, which revealed a significant enrichment and growth of inclusions in the channels. Based on above microtomography characterization, a 2D macrosegregation model encompassing the inclusion flotation was established. In the model, the motions of solid inclusions and liquid were described using the multi-phase flow scheme within the Euler-Lagrange framework. The benchmark simulations showed that sufficient inclusion populations with appropriate sizes are capable of altering the local flow patterns and destabilize the mushy zone, initiating the subsequent channel segregation. The continuous interplay between melt convection, inclusion flotation and solidification eventually causes the formation of macroscale channel. The predicted sizes and volume fraction of inclusions that are able to trigger the channel segregation effectively are consistent with the data obtained via microtomography characterization. The macrosegregation model was then applied to predict the channel segregations in an industrial carbon steel ingot. A rather good agreement of A-segregates was achieved between the simulation and the dissected ingot.

Interference-aware channel segregation based dynamic channel assignment in HetNet

Interference-aware channel segregation based dynamic channel assignment in HetNet

Functional segregation of voltage-activated calcium channels in motoneurons of the dorsal motor nucleus of the vagus.

Calcium influx elevates mitochondrial oxidant stress (mOS) in dorsal motor nucleus of the vagus (DMV) neurons that are prone to Lewy body pathologies in presymptomatic Parkinson's disease (PD) patients. In experimental PD models, treatment with isradipine, the dihydropyridine with the highest affinity to Cav1.3 channels, prevents subthreshold calcium influx via Cav1.3 channels into midbrain dopamine neurons and protects them from mOS. In DMV neurons, isradipine is also effective in reducing mOS despite overwhelming evidence that subthreshold calcium influx is negligible compared with spike-triggered influx. To solve this conundrum we combined slice electrophysiology, two-photon laser scanning microscopy, mRNA profiling, and computational modeling. We find that the unusually depolarized subthreshold voltage trajectory of DMV neurons is positioned between the relatively hyperpolarized activation curve of Cav1.3 channels and that of other high-voltage activated (HVA) calcium channels, thus creating a functional segregation between Cav1.3 and HVA calcium channels. The HVA channels flux the bulk of calcium during spikes but can only influence pacemaking through their coupling to calcium-activated potassium currents. In contrast, Cav1.3 currents, which we show to be more than an order-of-magnitude smaller than the HVA calcium currents, are able to introduce sufficient inward current to speed up firing. However, Kv4 channels that are constitutively open in the subthreshold range guarantee slow pacemaking, despite the depolarizing action of Cav1.3 and other pacemaking currents. We propose that the efficacy of isradipine in preventing mOS in DMV neurons arises from its mixed effect on Cav1.3 channels and on HVA Cav1.2 channels.

Simulation of Channel Segregation During Directional Solidification of In—75 wt pct Ga. Qualitative Comparison with In Situ Observations

Freckles are common defects in industrial casting. They result from thermosolutal convection due to buoyancy forces generated from density variations in the liquid. The present paper proposes a numerical analysis for the formation of channel segregation using the three-dimensional (3D) cellular automaton (CA)—finite element (FE) model. The model integrates kinetics laws for the nucleation and growth of a microstructure with the solution of the conservation equations for the casting, while introducing an intermediate modeling scale for a direct representation of the envelope of the dendritic grains. Directional solidification of a cuboid cell is studied. Its geometry, the alloy chosen as well as the process parameters are inspired from experimental observations recently reported in the literature. Snapshots of the convective pattern, the solute distribution, and the morphology of the growth front are qualitatively compared. Similitudes are found when considering the coupled 3D CAFE simulations. Limitations of the model to reach direct simulation of the experiments are discussed.

Low oxygen content eliminates channel segregation in cast steels

Low oxygen content eliminates channel segregation in cast steels

Interference-Aware Channel Segregation Based Dynamic Channel Assignment for Wireless Networks

SUMMARY Recently, we proposed an interference-aware channel segregation based dynamic channel assignment (IACS-DCA). In IACS-DCA, each base station (BS) measures the instantaneous co-channel interference (CCI) power on each available channel, computes the moving average CCI power using past CCI measurement results, and selects the channel having the lowest moving average CCI power. In this way, the CCI-minimized channel reuse pattern can be formed. In this paper, we introduce the autocorrelation function of channel reuse pattern, the fairness of channel reuse, and the minimum co-channel BS distance to quantitatively examine the channel reuse pattern formed by the IACS-DCA. It is shown that the IACSDCA can form a CCI-minimized channel reuse pattern in a distributed manner and that it improves the signal-to-interference ratio (SIR) compared to the other channel assignment schemes.

Mode division multiplexing of two-dimensional triangular lattice photonic crystal based on magneto-optical effect

With the rapid development of all-optical communication, the wavelength division multiplexing transmission system cannot meet the requirements of high capacity in optical networks, while mode division multiplexing uses the limited stability modes as independent channels to transmit information, improving the system capacity and spectrum efficiency, which is one of the key technologies in the construction of future optical network. In this paper, a mode division multiplexer of two-dimensional triangular lattice photonic crystal in 1.55 μm band based on the magneto-optic effect of Bi-doped compound rare earth iron garnet bulk single crystal is proposed. The defects of TbYbBiIG medium are introduced as mode splitting waveguides in photonic crystal, of which the permeability changes with the applied magnetic field in different polarization modes. Therefore, mode division multiplexing in 1.55 μm band can be achieved by controlling the propagations of TE and TM mode. The band and transmission characteristics of this device can be analyzed by using the plane wave expansion and finite difference time domain method. The results show that the transmission rates of TE and TM modes both exceed 92% and channel segregation degrees reach 19.7 dB and 42.1 dB respectively. These features indicate the important application prospect in the future high-capacity optical transmission system.

Inclusion flotation-driven channel segregation in solidifying steels.

Channel segregation, which is featured by the strip-like shape with compositional variation in cast materials due to density contrast-induced flow during solidification, frequently causes the severe destruction of homogeneity and some fatal damage. An investigation of its mechanism sheds light on the understanding and control of the channel segregation formation in solidifying metals, such as steels. Until now, it still remains controversial what composes the density contrasts and, to what extent, how it affects channel segregation. Here we discover a new force of inclusion flotation that drives the occurrence of channel segregation. It originates from oxide-based inclusions (Al2O3/MnS) and their sufficient volume fraction-driven flotation becomes stronger than the traditionally recognized inter-dendritic thermosolutal buoyancy, inducing the destabilization of the mushy zone and dominating the formation of channels. This study uncovers the mystery of oxygen in steels, extends the classical macro-segregation theory and highlights a significant technological breakthrough to control macrosegregation.

The Functional Asymmetry of ON and OFF Channels in the Lateral Geniculate Nucleus (LGN) during a Perceptual Decision Task

Early behavioral studies indicated that separate psychophysical channels underlie the perception of light (i.e. contrast increments) and dark (i.e. contrast decrements) (Anstis 1967; De Valois 1977; Krauskopf 1980; Cavanaugh and Anstis 1986; Chichilnisky and Wandell 1996; Westheimer 2007), and that human observers are faster and more accurate in perceiving contrast decrements rather than increments (Blackwell 1946; Short 1966; Bowen et al. 1989, 1992; Chan and Tyler 1992; Kontsevich and Tyler 1999; but see Komban et al. 2011). In the early visual system, the segregation of ON and OFF channels starts at bipolar cells (Nomura et al. 1994; Masu et al. 1995; Vardi and Morigawa 1997; Wassle 2004) and is preserved throughout the lateral geniculate nucleus (LGN) (Casagrande and Norton 1991; Westheimer 2007; Schiller 2010). These ON and OFF pathways were initially thought of as independent, parallel systems that mirror each other in their light/dark responses (Schiller 1992, 2010; Kremers et al. 1993; Benardete and Kaplan 1997, 1999). Several physiological studies, however, have revealed significant functional asymmetries between the two systems. On the one hand, it was reported that the ON pathway could signal both contrast increments (i.e. via increase in spikes) and decrements (via decrease in spikes) at low contrast, whereas the OFF pathway could only signal contrast decrements (Chichilnisky and Kalmar 2002). On the other hand, it was reported that over a great range of luminance levels, the contrast responses of OFF neurons were stronger and more linear than those of ON neurons (Kremkow et al. 2014), and, under scotopic conditions, OFF neurons responded to much higher temporal frequencies than ON neurons (ArmstrongGold and Rieke 2003; Pandarinath et al. 2010). Additionally, projections from the OFF pathway were reported to dominate the cortical representation of central vision (Jin et al. 2008, 2011), and visual responses to dark stimuli were reported to dominate the superficial layers of V1 (Yeh et al. 2009; Xing et al. 2010). How do the ON and OFF pathways contribute to the perception of light and dark? How can the physiological asymmetry of the ON/OFF pathways be linked to the psychophysical asymmetry of light and dark perception? To answer these questions, we recorded from LGN Parvocellular (P) ON-Center and OFF-Center neurons in awake monkeys while they passively viewed (i.e. simple fixation) and actively detected (i.e. two-alternative, forced-choice detection) contrast increment and decrement changes. Neurometric Sensitivity

指向性凝固させたSn-20mass%Bi合金鋳塊のチャンネル偏析

指向性凝固させたSn-20mass%Bi合金鋳塊のチャンネル偏析

Three-dimensional modelling and numerical simulation on segregation during Fe-Pb alloy solidification in a multiphase system

The three-dimensional mathematical model for a three-phase flow during its horizontai solidification is studied using fluid dynamics method based on Eulerian-Eulerian and volume of fraction methods, in which the mass, momentum, species, and enthalpy conservation equations of the Fe-Pb alloy solidification process are solved simultaneously. Effects of Pb area quadratic gradient (∇ (∇SPb)) and Pb concentration quadratic gradient (∇ (∇CPb)) on the segregation formation are investigated. Results show that the segregation mode is manifested as X-segregates in the upper and V-segregates in the lower part during flow-solidification of liquid phase and gas phase. The X-segregates result from the phase transformation driving force of gas phase and “scattering” is due to the orientation of phase transition. When t >tc the lower ∇ (∇SPb) and ∇ (∇CPb) curves cause a larger yielding rate of Pb with a larger down angle of X-segregates and a smaller up angle of X-segregates and V-segregates. All these are favorable for the formation of a well-dispersed microstructure. In addition, the gas-liquid two-phase flow interaction term has an effect on channel segregation, showing that channels occur only in the region where the flow-phase transition interaction term (ul·∇cl and ug·∇cg) is negative. With a negative flow-phase transition interaction term the increase in flow velocity due to the flow perturbation and flow-phase transition interaction becomes more negative, thus the channel continues to grow and tends to be stable. Calculated results show good agreement with experimental data.

A numerical simulation of columnar solidification: influence of inertia on channel segregation

We investigate the role of the inertia of the flow through the dendritic mushy zone in the numerical prediction of channel segregations during columnar solidification. The contribution of inertia is included in the momentum transport equation through the quadratic Forchheimer correction term. The study reveals a significant influence of the Forchheimer term in the vicinity of the liquidus front, i.e. at high liquid fractions. The natural convective flow field in this region is modified due to the additional inertial drag. This strongly influences the convective transport of solute and thereby incurs a modification of the dynamics of the advancement of the mushy zone. The most notable consequence is a significant decrease in the predicted channel segregation.

Modeling of Multiscale and Multiphase Phenomena in Materials Processing

In order to demonstrate how CFD can help scientists and engineers to better understand the fundamentals of engineering processes, a number of examples are shown and discussed. The paper covers (i) special aspects of continuous casting of steel including turbulence, motion and entrapment of non-metallic inclusions, and impact of soft reduction; (ii) multiple flow phenomena and multiscale aspects during casting of large ingots including flow-induced columnar-to-equiaxed transition and 3D formation of channel segregation; (iii) multiphase magneto-hydrodynamics during electro-slag remelting; and (iv) melt flow and solidification of thin but large centrifugal castings.

指向性凝固させたSn-20mass%Bi合金インゴット中チャンネル偏析の3次元数値解析

Channel segregation behavior of a lateral directional Sn-20mass%Bi ingots was examined by X-ray computing tomography (X-ray CT) and three-dimensional numerical analysis. The channel segregation with string shape from near the cold wall side was observed by the X-ray CT and those segregation patterns could be reproduced by the numerical analysis. Low concentration regions also appeared above the channel segregation in both X-ray CT images and numerical analysis results. These results suggest that the proposed numerical method is a reasonable model to simulate the channel segregation. Detailed investigation of the effects of convection on the generation of channel segregation showed that a solidification-accelerated region (SA-region) and solidification-retarded region (SR-region) were formed by the fluid flow. At the SA-region, solidification was accelerated by the inflow of lower Bi concentrated liquid. On the other hand, in the SR-region, solidification was retarded by the inflow of the liquid with high Bi concentration from the SA-region. It was found that the SA-region (called donor part) and SR-region (called acceptor part) group into pairs, and generate the channel segregation.

Enhancing the phase segregation and connectivity of hydrophilic channels by blending highly sulfonated graft copolymers with fluorous homopolymers

The influence of tuning the ionic content of membranes by blending, as opposed to varying the degree of sulfonation, is evaluated. Membranes of fully sulfonated poly(vinylidene fluoride-co-chlorotrifluoroethylene)-g-poly(styrene sulfonic acid) blended with PVDF were prepared and investigated for morphology, water sorption, and proton transport properties. The blend membranes exhibit conductivities superior to pure graft copolymers under fully humidified conditions despite their lower water uptake. Transmission electron microscopy images of the blends reveal that the membranes comprise a combination of macro-phase segregated regions of ion-rich and PVDF-rich domains, and, at higher PVDF contents, ion-rich nano-scale domains within fluorine-rich domains.

VEP contrast sensitivity responses reveal reduced functional segregation of visual processing channels in autism

VEP contrast sensitivity responses reveal reduced functional segregation of visual processing channels in autism

Calling signals in sympatric species of the far-eastern Aphrophora (Homoptera: Auchenorrhyncha: Aphrophoridae): regularities of communication channel segregation

Calling signals in sympatric species of the far-eastern Aphrophora (Homoptera: Auchenorrhyncha: Aphrophoridae): regularities of communication channel segregation