Rheological model for short duration response of semi-solid metals

A thorough understanding of the rheological behavior of the semi-solid metal (SSM) alloys is of utmost importance for modeling and simulation of rheocasting and thixoforming processes. Since the duration of these processes is very short-fraction of a second-the primary focus should be on fast transient flows. We present here a relatively simple engineering model for unsteady state shear stress of semi-solid metal suspensions. The time dependent character of thixotropic semi-solids is introduced through scaling by a structural parameter, which represents the degree of connectivity or aggregation of particles in the fluid. The kinetic equation for change in structural parameter as a function of time incorporates both breakage and agglomeration of suspended entities. The proposed model simulates the fast transient, tracks the evolution or healing of structure with holding time in absence of applied shear, and computes the steady state structure for a given shearing rate after long shearing time. Moreover, it explicitly incorporates the shear yield stress as well as is in agreement with important rheological characteristics of semi-solid materials, namely, shear thickening behavior under isostructural conditions and power law relationship between steady state viscosity and shear rate. The model is validated with published data for short time measurements of Sn-15%Pb thixotropic systems with near step change in shear rate and steady state shear viscosity. It could have practical utility in simulation of flow of semi-solid materials in different die cavities

Balling and granulation kinetics revisited

Balling of finely comminuted solids by random coalescence and granulation of iron ore fines and other minerals by autolayering are two major size enlargement processes. The existing kinetic model for random coalescence does not take into account the strong dependence of coordination number on the size distribution of agglomerating entities. We present a coordination number based coalescence model, which mimics the underlying physical process more realistically. Simulations show that in spite of highly diverse model structures, random and coordination coalescence models give remarkably similar results. Only static models of autolayering are available presently. These map the input size distribution of feed solids into steady state or terminal size distribution of granules, with little or no information on the path traversed by the process. We propose a continuous-time dynamic model of autolayering within the population balance framework. The model, which is based on the proportionate growth postulate of autolayering, agrees reasonably well with experimental data

A population balance model for flocculation of colloidal suspensions by polymer bridging

A detailed population balance model for flocculation of colloidal suspensions by polymer bridging under quiescent flow conditions is presented. The collision efficiency factor is estimated as a function of interaction forces between polymer coated particles. The total interaction energy is computed as a sum of van der Waals attraction, electrical double layer repulsion and bridging attraction or steric repulsion due to adsorbed polymer. The scaling theory is used to compute the forces due to adsorbed polymer and the van der Waals attraction is modified to account for presence of polymer layer around a particle. The irregular structure of flocs is taken into account by incorporating the mass fractal dimension of flocs. When tested with experimental floc size distribution data published in the literature, the model predicts the experimental behavior adequately. This is the first attempt towards incorporating theories of polymer-induced surface forces into a flocculation model, and as such the model presented here is more general than those proposed previously

Computer aided Design and Optimization of Mineral Processing Plants by a State of the Art Simulator

Tata Research Development and Design Centre (TRDDC) has developed a state of the art mineral processing simulator called SimL8. It performs modelling, simulation and optimisation functions and provides viable strategies for enhancement of the performance of mineral processing plants. A number of case studies on plant diagnostics, grinding,classification, flotation and pressure filtration are taken up to demonstrate the utility of modelling and simulation on SimL8 platform

Self-Similar Distributions of Fine Particles Produced in Non-Linear Batch Grinding

A parametric approach is described for tracking the evolut-ion of particle size distribution with time in batch grinding in micron and sub-micron range. The approach is based on an asymptotic self-similarity solution to populat-ion balance equation of grinding. The possible presence of non-linearity in the fine grinding regime is captured by a modification to the model formulation scheme. It is assu-med that the breakage rate is a function of the grind time and therefore, indirectly of the time-dependent mean, median or a percentile size. It is shown that the self-similar character is preserved under a wide class of functions that can in principle describe the non-linear characteristics of the comminution process. The resulting parametric model describes the evolution of particle spectra. Several sets of published data are employed to validate the model

Simulation of Locked-Cycle Grinding of Multicomponent Feeds and its Implications for Stability and Control of Industrial Comminution Circuits

A mathematical algorithm is described for cycle-wise simulation of locked-cycle grinding tests in a ball mill using multicomponent feeds. The simulation is in good agreement with extensive experimental data for quartzite and limestone mixture feeds ranging in composition from 1:3 to 3:1. The simulation model is employed for generating transients in the locked-cycle tests by imposing either relatively large step changes or random fluctuations in a narrow band width on the composition of the feed, its fineness and the grinding time. Because locked-cycle tests mimic closed-loop grinding circuits with plug flow transport through the mill, the simulation provides useful insight into the stability and control of industrial comminution circuits where ore composition and hardness and the feed rate are invariably subjected to minor fluctuations on a more or less continuous basis, and occasionally to rather abrupt large changes

MADS+: discovery of differential splicing events from Affymetrix exon junction array data

Motivation: The Affymetrix Human Exon Junction Array is a newly designed high-density exon-sensitive microarray for global analysis of alternative splicing. Contrary to the Affymetrix exon 1.0 array, which only contains four probes per exon and no probes for exon–exon junctions, this new junction array averages eight probes per probeset targeting all exons and exon–exon junctions observed in the human mRNA/EST transcripts, representing a significant increase in the probe density for alternative splicing events. Here, we present MADS+, a computational pipeline to detect differential splicing events from the Affymetrix exon junction array data. For each alternative splicing event, MADS+ evaluates the signals of probes targeting competing transcript isoforms to identify exons or splice sites with different levels of transcript inclusion between two sample groups. MADS+ is used routinely in our analysis of Affymetrix exon junction arrays and has a high accuracy in detecting differential splicing events. For example, in a study of the novel epithelial-specific splicing regulator ESRP1, MADS+ detects hundreds of exons whose inclusion levels are dependent on ESRP1, with a RT-PCR validation rate of 88.5% (153 validated out of 173 tested)

Gall Bladder And Common Bile Duct Stones – When Is Direct Cholangiography Indicated?

The medical records of 277 consecutive patients who underwent cholecystectomy for benign gall stone disease, were reviewed to determine the incidence and cause of biliary tract obstructuion

Levy flights and Levy -Schroedinger semigroups

We analyze two different confining mechanisms for L\'{e}vy flights in the presence of external potentials. One of them is due to a conservative force in the corresponding Langevin equation. Another is implemented by Levy-Schroedinger semigroups which induce so-called topological Levy processes (Levy flights with locally modified jump rates in the master equation). Given a stationary probability function (pdf) associated with the Langevin-based fractional Fokker-Planck equation, we demonstrate that generically there exists a topological L\'{e}vy process with the very same invariant pdf and in the reverse.Comment: To appear in Cent. Eur. J. Phys. (2010

White matter integrity as a predictor of response to treatment in first episode psychosis

The integrity of brain white matter connections is central to a patient's ability to respond to pharmacological interventions. This study tested this hypothesis using a specific measure of white matter integrity, and examining its relationship to treatment response using a prospective design in patients within their first episode of psychosis. Diffusion tensor imaging data were acquired in 63 patients with first episode psychosis and 52 healthy control subjects (baseline). Response was assessed after 12 weeks and patients were classified as responders or non-responders according to treatment outcome. At this second time-point, they also underwent a second diffusion tensor imaging scan. Tract-based spatial statistics were used to assess fractional anisotropy as a marker of white matter integrity. At baseline, non-responders showed lower fractional anisotropy than both responders and healthy control subjects (P < 0.05; family-wise error-corrected), mainly in the uncinate, cingulum and corpus callosum, whereas responders were indistinguishable from healthy control subjects. After 12 weeks, there was an increase in fractional anisotropy in both responders and non-responders, positively correlated with antipsychotic exposure. This represents one of the largest, controlled investigations of white matter integrity and response to antipsychotic treatment early in psychosis. These data, together with earlier findings on cortical grey matter, suggest that grey and white matter integrity at the start of treatment is an important moderator of response to antipsychotics. These findings can inform patient stratification to anticipate care needs, and raise the possibility that antipsychotics may restore white matter integrity as part of the therapeutic response