14,138 research outputs found
RD-flatness and RD-injectivity
It is proved that every commutative ring whose RD-injective modules are
-RD-injective is the product of a pure semi-simple ring and a finite
ring. A complete characterization of commutative rings for which each artinian
(respectively simple) module is RD-injective, is given. These results can be
obtained by using the properties of RD-flat modules and RD-coflat modules which
are respectively the RD-relativization of flat modules and fp-injective
modules. It is also shown that a commutative ring is perfect if and only if
each RD-flat module is RD-projective.Comment: A new section is added to the version published in Communications in
Algebra where a complete proof of Theorem 3.1 is give
Bridiging designs for conjoint analysis: The issue of attribute importance.
Abstract: Conjoint analysis studies involving many attributes and attribute levels often occur in practice. Because such studies can cause respondent fatigue and lack of cooperation, it is important to design data collection tasks that reduce those problems. Bridging designs, incorporating two or more task subsets with overlapping attributes, can presumably lower task difficulty in such cases. In this paper, we present results of a study examining the effects on predictive validity of bridging design decisions involving important or unimportant attributes as links (bridges) between card-sort tasks and the degree of balance and consistency in estimated attribute importance across tasks. We also propose a new symmetric procedure, Symbridge, to scale the bridged conjoint solutions.Studies; Cooperation; Data; Problems; Effects; Decisions;
Formation of Plasmoid Chains in Fusion Relevant Plasmas
The formation of plasmoid chains is explored for the first time within the
context of the Taylor problem, in which magnetic reconnection is driven by a
small amplitude boundary perturbation in a tearing-stable slab plasma
equilibrium. Numerical simulations of a magnetohydrodynamical model of the
plasma show that for very small plasma resistivity and viscosity, the linear
inertial phase is followed by a nonlinear Sweet-Parker evolution, which gives
way to a faster reconnection regime characterized by a chain of plasmoids
instead of a slower Rutherford phase
Extended theory of the Taylor problem in the plasmoid-unstable regime
A fundamental problem of forced magnetic reconnection has been solved taking
into account the plasmoid instability of thin reconnecting current sheets. In
this problem, the reconnection is driven by a small amplitude boundary
perturbation in a tearing-stable slab plasma equilibrium. It is shown that the
evolution of the magnetic reconnection process depends on the external source
perturbation and the microscopic plasma parameters. Small perturbations lead to
a slow nonlinear Rutherford evolution, whereas larger perturbations can lead to
either a stable Sweet-Parker-like phase or a plasmoid phase. An expression for
the threshold perturbation amplitude required to trigger the plasmoid phase is
derived, as well as an analytical expression for the reconnection rate in the
plasmoid-dominated regime. Visco-resistive magnetohydrodynamic simulations
complement the analytical calculations. The plasmoid formation plays a crucial
role in allowing fast reconnection in a magnetohydrodynamical plasma, and the
presented results suggest that it may occur and have profound consequences even
if the plasma is tearing-stable.Comment: Accepted for publication in Physics of Plasma
Optimal operations and resilient investments in steam networks
Steam is a key energy vector for industrial sites, most commonly used for process heating and cooling, cogeneration of heat and mechanical power as a motive fluid or for stripping. Steam networks are used to carry steam from producers to consumers and between pressure levels through letdowns and steam turbines. The steam producers (boilers, heat and power cogeneration units, heat exchangers, chemical reactors) should be sized to supply the consumers at nominal operating conditions as well as peak demand. First, this paper proposes an Mixed Integer Linear Programing formulation to optimize the operations of steam networks in normal operating conditions and exceptional demand (when operating reserves fall to zero), through the introduction of load shedding. Optimization of investments based on operational and investment costs are included in the formulation. Though rare, boiler failures can have a heavy impact on steam network operations and costs, leading to undercapacity and unit shutdowns. A method is therefore proposed to simulate steam network operations when facing boiler failures. Key performance indicators are introduced to quantify the network’s resilience. The proposed methods are applied and demonstrated in an industrial case study using industrial data. The results indicate the importance of oversizing key steam producing equipments and the value of industrial symbiosis to increase industrial site resilience
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