The block layout shortest loop design problem

The purpose of this article is to formulate and solve a shortest loop problem associated with the design of material flow handling systems in factories. The problem is formulated as an Integer Linear Program (ILP) initially containing a very large number of constraints. Several simplifications are proposed in order to reduce the problem to a size amenable by standard ILP solvers. Computational results confirm the efficiency and practicality of the proposed approach.     

A Highly Efficient Multicore Floating-Point FFT Architecture Based on Hybrid Linear Algebra/FFT Cores

FFT algorithms have memory access patterns that prevent many architectures from achieving high computational utilization, particularly when parallel processing is required to achieve the desired levels of performance. Starting with a highly efficient hybrid linear algebra/FFT core, we co-design the on-chip memory hierarchy, on-chip interconnect, and FFT algorithms for a multicore FFT processor. We show that it is possible to to achieve excellent parallel scaling while maintaining power and area efficiency comparable to that of the single-core solution. The result is an architecture that can effectively use up to 16 hybrid cores for transform sizes that can be contained in on-chip SRAM. When configured with 12MiB of on-chip SRAM, our technology evaluation shows that the proposed 16-core FFT accelerator should sustain 388 GFLOPS of nominal double-precision performance, with power and area efficiencies of 30 GFLOPS/W and 2.66 GFLOPS/mm², respectively.     

Fundamental role of the retarded potential in the electrodynamics of superluminal sources

We calculate the gradient of the radiation field generated by a polarization current with a superluminally rotating distribution pattern and show that the absolute value of this gradient increases as R(7/2) with distance R, within the sharply focused subbeams that constitute the overall radiation beam from such a source. In addition to supporting the earlier finding that the azimuthal and polar widths of these subbeams become narrower (as R(-3) and R(-1), respectively) with distance from the source, this result implies that the boundary contribution to the solution of the wave equation governing the radiation field does not always vanish in the limit where the boundary tends to infinity (as is commonly assumed in textbooks and the published literature). While the boundary contribution to the retarded solution for the potential can always be rendered equal to zero by means of a gauge transformation that preserves the Lorenz condition, the boundary contribution to the retarded solution of the wave equation for the field may be neglected only if it diminishes with distance faster than the contribution of the source density. In the case of a rotating superluminal source, however, the boundary term in the retarded solution for the field is by a factor of the order of R(1/2)larger than the source term of this solution, in the limit where the boundary tends to infinity. This result explains why an argument based on the solution of the wave equation governing the field in which the boundary term is neglected [such as that presented by Hannay, J. Opt. Soc. A 23, 1530 (2006)] misses the nonspherical decay of the field that is generated by a rotating superluminal source. The only way one can calculate the free-space radiation field of an accelerated superluminal source is via the retarded solution for the potential. Our findings have implications also for the observations of the pulsar emission: The more distant a pulsar, the narrower and brighter its giant pulses should be.     

Control of epoxy creep using graphene

The creep behavior of epoxy-graphene platelet (GPL) nanocomposites with different weight fractions of filler is investigated by macroscopic testing and nanoindentation. No difference is observed at low stress and ambient temperature between neat epoxy and nanocomposites. At elevated stress and temperature the nanocomposite with the optimal weight fraction, 0.1 wt% GPLs, creeps significantly less than the unfilled polymer. This indicates that thermally activated processes controlling the creep rate are in part inhibited by the presence of GPLs. The phenomenon is qualitatively similar at the macroscale and in nanoindentation tests. The results are compared with the creep of epoxy-single-walled (SWNT) and multi-walled carbon nanotube (MWNT) composites and it is observed that creep in both these systems is similar to that in pure epoxy, that is, faster than creep in the epoxy-GPL system considered in this work.     

A review of principle and sun-tracking methods for maximizing solar systems output

Finding energy sources to satisfy the world's growing demand is one of society's foremost challenges for the next half-century. The challenge in converting sunlight to electricity via photovoltaic solar cells is dramatically reducing $/watt of delivered solar electricity. In this context the sun trackers are such devices for efficiency improvement.The diurnal and seasonal movement of earth affects the radiation intensity on the solar systems. Sun-trackers move the solar systems to compensate for these motions, keeping the best orientation relative to the sun. Although using sun-tracker is not essential, its use can boost the collected energy 10–100% in different periods of time and geographical conditions. However, it is not recommended to use tracking system for small solar panels because of high energy losses in the driving systems. It is found that the power consumption by tracking device is 2–3% of the increased energy.In this paper different types of sun-tracking systems are reviewed and their cons and pros are discussed. The most efficient and popular sun-tracking device was found to be in the form of polar-axis and azimuth/elevation types.     

A Spin Resonance Investigation of Magnetism and Dynamics in the Charge-transfer Salts β"-(BEDT-TTF)4[(H3O)M(C2O4)3]S

We report a spin resonance study of the family of quasi-two-dimensional organic (super)conductors β”-(BEDT-TTF)₄[(H₃O)M(C₂O₄)₃]S, where M is a 3d transition metal ion and S is a host solvent molecule. The spin systems for M = Cr³⁺ (S = 3/2) and M = Fe³⁺ (S = 5/2) are investigated by means of both resonant and field modulation techniques in the frequency range between 50 and 313 GHz. The role of the different solvent molecules in determining the degree of spin-orbit coupling and the local symmetry at the metal ion site is established. The low temperature behaviour of intensities, positions and widths of the resonant lines shows significant modifications of the spin-orbit coupling, and of the inter-and intra-ionic spin-spin inter actions. Despite the onset of a weak antiferromagnetic internal field at low temperature, the ultimate narrowing of the lines suggests spin-lattice interactions may still be the dominant relaxation process. Diamagnetic screening in the mixed state of the superconducting samples for fields parallel to the quasi-two-dimensional layers induces additional lineshifts only below B = 2.5T and T = 4K, determining the threshold of full field penetration within the anion layers.     

Fleet sizing of automated guided vehicles: a linear programming approach based on closed queuing networks

We use multi-class closed queuing networks to model operations of automated guided vehicles in a manufacturing or distribution environment. We approximate the dynamics of the system using the first moment balance equations of the embedded stochastic chain representing the network under the steady-state conditions. These moments account for loaded and empty-travel times, as well as times when vehicles are being loaded/unloaded or waiting to be dispatched. We model the steady-state behaviour of the closed queuing network by a linear program whose optimal value is the estimate of the required fleet-size. The result of the analytical model is compared with those of the simulation studies for a set of numerical examples. The comparison shows that the analytical model provides a good estimate for the required number of vehicles.     

Liquid phase surface alloying of CP-titanium with aluminum in an atmosphere of argon and nitrogen

In the present work, surface alloying of CP-titanium with different mixtures of titanium and aluminum powders in a gas mixture of 20% argon and 80% nitrogen was carried out using tungsten inert gas (TIG) process. The microstructures of the alloyed layers were investigated by optical microscope (OM), scanning electron microscope (SEM), and the phases formed were studied by X-ray diffraction analysis. The hardness of these layers was also evaluated using a microhardness machine. The results showed that the surface hardness was significantly enhanced from 175 HV_0.1 for the untreated substrate to more than 1000 HV_0.1 for the alloyed layers, due to the formation of Ti₃Al₂N₂ as well as Ti₃Al and TiN in the modified layers. It was also noticed that the alloyed layers exhibited better wear resistance as compared with the untreated substrate.     

On the interaction of Class C fly ash with Portland cement–calcium sulfoaluminate cement binder

Shrinkage cracking in concrete is a widespread problem, especially in concrete structures with high surface-to-volume ratio such as bridge decks. Expansive cements based on calcium sulfoaluminate phase were developed to mitigate the shrinkage cracking of concrete. The compressive stress induced due to restrained expansion of concrete has been shown to counteract the tensile stress generated during drying shrinkage. This research attempts to address the differential behavior of fly ash type (i.e., Class C vs. Class F) on early-age expansion and hydration characteristics of ordinary Portland cement (OPC)–calcium sulfoaluminate (CSA) cement blend. It was observed earlier that the presence of Class C fly ash (CFA), unlike Class F fly ash, shortened the expansion duration of OPC–CSA cement blend, which was hypothesized to be correlated to early depletion of gypsum. This paper presents a detailed verification of the hypothesis. Addition of external gypsum to OPC–CSA–CFA blend led to simultaneous increase in expansion and disappearance of a shoulder peak in the calorimetric curve. Thermodynamic calculations using a geochemical modeling program (GEMS-PSI) revealed higher saturation levels of ettringite in presence of external gypsum, which led to higher crystallization stress, and thereby increased expansion.     

Tuning Electronic Ground States by Using Chemical Pressure on Quasi-Two Dimensional β′′-(BEDT-TTF)4[(H3O)M(C2O4)3]·Y

We report high-field magnetotransport studies on quasi-two dimensional β′′-(BEDT-TTF)₄[(H₃O)M(C₂O₄)₃]· Y where Y is a solvent in the anionic layer. By changing the size of the solvent the low temperatures electronic behaviour varies from superconducting (for larger solvents, Y = C₆H₅NO₂ and C₆H₅CN) to metallic (for smaller solvents, Y = C₅H₅N and CH₂Cl₂). These changes in the ground state are connected with modications of the Fermi surface, which varies from having one or two pockets for the superconducting charge- transfer salts to at least four pockets in the case of metallic ones. When superconducting, the materials have very large in-plane critical fields (up to 32 T) and enhanced effective masses compared with the metallic compounds. The role of the charge-order fluctuations in stabilizing the superconducting ground state and the effects of intrinsic local disorder is discussed.