Kinetic Simulation of Collisional Magnetized Plasmas with Semi-implicit Time Integration

Plasmas with varying collisionalities occur in many applications, such as tokamak edge regions, where the flows are characterized by significant variations in density and temperature. While a kinetic model is necessary for weakly-collisional high-temperature plasmas, high collisionality in colder regions render the equations numerically stiff due to disparate time scales. In this paper, we propose an implicit–explicit algorithm for such cases, where the collisional term is integrated implicitly in time, while the advective term is integrated explicitly in time, thus allowing time step sizes that are comparable to the advective time scales. This partitioning results in a more efficient algorithm than those using explicit time integrators, where the time step sizes are constrained by the stiff collisional time scales. We implement semi-implicit additive Runge–Kutta methods in COGENT, a high-order finite-volume gyrokinetic code and test the accuracy, convergence, and computational cost of these semi-implicit methods for test cases with highly-collisional plasmas.     

Impurity controlled excitations in doped quantum dot exposed to periodic external perturbation

We investigate the excitation behavior of a repulsive impurity doped quantum dot under the influence of an oscillatory external electric field. We have considered Gaussian impurity centers doped at on-center position. The investigation reveals that a variation in the strength and spatial stretch of the dopant causes maximization and minimization in the average excitation rate, respectively. The findings are expected to have important applications involving quantum dot nanodevices.     

Morphological and rheological footprints corroborating optimum foam processability of PP/ethylene acrylic elastomer blend

The focus of the present work is optimization of microcellular foam processing of Polypropylene (PP)/ethylene acrylic elastomer blends, through design of experiment approach using Taguchi technique and establishing correlation among foamability, morphological, and rheological parameters of blends. PP and elastomer are melt blended in a twin screw extruder and thereafter foaming parameter optimization is carried out using Taguchi method followed by analysis of variance (ANOVA) for batch foaming setup. Scanning electron microscope reveals development of submicron elastomer domains, which increase with the elastomer content. Further, complex viscosity and van Grup–Palmen plots are found to have a vital role in attaining cellular morphology development in blends. From Taguchi and ANOVA analysis it is established that foaming temperature has a great influence and major contribution on foamability. Blends with low elastomer content are optimized for better foamability using statistical approach and proved to be consistent with the morphological and rheological outcomes. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46322.     

Orientation Dependence of Deformation-Induced Martensite Transformation During Uniaxial Tensile Deformation of Carbide-Free Bainitic Steel

Metallurgical and Materials Transactions A - The present study investigates the orientation dependence of deformation-induced martensite (DIM) transformation in carbide-free bainitic steel using...     

Plastic collapse moment of 90° long radius elbows with internal circumferential surface crack at intrados under in-plane bending

Piping elbows under in-plane bending moment are vulnerable to cracking. The crack initiates at the surface and eventually reaches through the thickness and may lead to failure. The structural integrity assessment requires knowledge of the limit load. Limit load solutions for elbows with through-wall crack configurations are available in the open literature. But solutions for surface crack are not available. This paper presents a closed form expression for the plastic collapse moment (PCM) of 90°, long radius elbows with circumferential surface cracks at the intrados, under in-plane bending moment. The expression is derived, based on the results of non-linear (geometric and material) FE analyses covering a wide range of geometries and crack sizes. These plastic collapse moments evaluated herein will help in structural integrity assessment.     

The Influence of Surface Modification on Friction and Lubrication Mechanism Under a Bovine Serum–Lubricated Condition

Diamond-like carbon (DLC) and microdimples are two potential surface modification techniques that are extensively studied to be utilized in biotribological interfaces in order to reduce the friction coefficient and wear rate. However, in situ observation of bovine serum–lubricated DLC and microdimpled surface contacts are not well understood. In this study, a DLC-coated and a microdimpled steel ball rubbing against a Cr-coated glass disk, where 25% bovine serum was used as a lubricant and the temperature was maintained at 37°C, were investigated. The behaviors of ithe nterface were ca`ptured using optical interferometry and the friction coefficients were simultaneously measured using a torque sensor. The experimental results reveal that DLC/glass sliding is scuffing-free, with a lower friction coefficient; however, the formation of a lubricating film is insignificant. On the other hand, the dimples retained lubrication and, as a result, the wear of the glass disk was minimized; however, the friction coefficient was not reduced. Therefore, DLC and microdimples individually have few improved tribological features, but their combination should be considered to maximize performance.     

An artificial neural network model to characterize porosity defects during solidification of A356 aluminum alloy

In this study, a data-driven multilayer perceptron-based neural network model has been developed to predict the percentage of total porosity and mechanical properties, namely yield strength, ultimate tensile strength and percentage of elongation during the solidification of A356 aluminum alloy. Some of the important processing parameters such as cooling rate, solidus velocity, thermal gradient and initial hydrogen content have been considered as inputs to this model. The network training architecture has been optimized using the gradient-based Broyden–Fletcher–Goldfarb–Shanno training algorithm to minimize the network training error within few training cycles. Parametric sensitivity analysis is carried out to characterize the influence of processing parameters (inputs) on the behavior of porosity formation and simultaneously, the tensile properties of A356 alloy castings. It has been observed that initial hydrogen content in the melt and cooling rate has significant influence on the porosity formation and eventually on the tensile properties of the cast product. There has been an excellent agreement between artificial neural network predictions and the target (measured) values of porosity and the tensile properties as depicted by the regression fit between these values.     

Effect of Polar Organic Solvents on Self‐Aggregation of Some Cationic Monomeric and Dimeric Surfactants

Surface and micellization behavior of some cationic monomeric surfactants, viz., cetyldiethylethanolammonium bromide (CDEEAB), cetyldimethylethanolammonium bromide (CDMEAB), tetradecyldiethylethanolammonium bromide (TDEEAB) and dimeric surfactants, i.e., alkanediyl‐α, ω‐bis(dimethylhexadecylammonium bromide) (C₁₆‐s‐C₁₆, 2Br^? where s = 4, 12), butanediyl‐1,4‐bis(dimethyldodecylammonium bromide (C₁₂‐4‐C₁₂, 2Br^?) and 2‐butanol‐1,4‐bis(dimethyldodecylammonium bromide) (C₁₂‐4(OH)‐C₁₂, 2Br^?), was studied in water‐organic solvents [10 and 20 % v/v ethylene glycol (EG) and diethylene glycol (DEG)] by conductivity, surface tension and steady‐state fluorescence methods at 300 K. The main focus of the present work is on the study of the effect of organic solvents on the critical micelle concentration (CMC), Gibbs free energy of micellization (ΔG°_m), Gibbs free energy of transfer (ΔG°_trans), Gibbs adsorption energy (ΔG°_ads) and some interfacial parameters such as the surface excess concentration (Γ_max), minimum area per surfactant molecule (A_min) and surface pressure (π_CMC). The aggregation number (N_agg) and Stern‐Volmer quenching constant (K_SV) were also determined by the steady‐state fluorescence method. It was observed that N_agg decreased with increasing volume percent of organic solvent. The results exhibited an increase in CMC in water‐organic solvents as compared to the respective surfactants in pure water. The negative values of ΔG°_m and ΔG°_ads indicate a spontaneous micellization process. The thermodynamics of micellization revealed that the micellization‐reducing efficiency of glycols increases with the concentration and the number of ethereal oxygens in the glycol.