Nonlinear terahertz properties of n-type quantum-wellheterostructures

In this letter, we study the effect of many body interactions on the collective response of confined electrons in doped quantum-well (QW) heterostructures to intense far-infrared radiation. Absorption lineshapes are computed both by numerically integrating the equations of motion and by using the appropriately time-averaged equations. For a two-subband double-QW system optical bistability is observed and its parameter range is given. For an asymmetric triple-QW structure driven at ω≈E₂-E₀ Hopf bifurcations occur which generate a strong response at a frequency incommensurate with the drive frequency or any natural frequency of the system     

Photocatalytic reactors: Reaction kinetics in a flat plate solar simulator

The kinetics of the photocatalytic decomposition of low concentrations of trichloroethylene (TCE) in water was modeled and the reaction parameters have been evaluated for different catalyst loadings. The employed reactor is a flat plate configuration irradiated by tubular lamps that have emission in the 300–400 nm wavelength range.

The mass conservation model is two-dimensional while the developed radiation model is two-dimensional in space and two directional in radiation propagation. The performance of the photoreactor with this reaction can be properly represented employing only two lumped kinetic constants that can be derived from a 12 steps, complete reaction sequence. The deduced kinetic model has explicit functional dependencies for the local volumetric rate of photon absorption (LVRPA) and the effect of the catalyst concentration:

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. Values of the kinetic constants are: =1.94×10⁻⁹ mol g^1/2 cm⁻² s^−1/2 einstein^−1/2 and ₃=5.52×10⁶ cm³ mol⁻¹. As derived from the reaction sequence and validated with experiments, it was observed that the reaction rate is proportional to the square root of the LVRPA. The dependence on the catalyst loading, well described by the model, is more complex due to its characteristic effect on the light distribution inside the reaction space.     

A reactive MILP approach to the multidepot heterogeneous fleet vehicle routing problem with time windows

The time‐window‐constrained vehicle routing problem (VRPTW) is a well‐known combinatorial problem. Its goal is to discover the best set of routes for a vehicle fleet in order to service a given number of customers at minimum cost. Vehicle capacity, maximum service time and time‐window constraints must be satisfied. Most proposed VRPTW optimizing approaches intend to discover the best or a near‐optimal solution at once. Improvement methods are old strategies that apply heuristics to insert customers into tours and/or rearrange nodes to obtain better routes. They are performed until no further improvement is achieved. Little research has been focused on model‐based reactive approaches seeking a better solution by exploring a small solution space around the current solution. This work presents a new model‐based improvement methodology for the multi‐depot heterogeneous‐fleet VRPTW problem to enhance an initial solution through solving a series of MILP mathematical problems that allow exchanges of nodes among tours and node reordering on every route. By restricting the range of improvement options, the problem size can be bounded and a limited number of binary variables is required for real‐world problems. The improvement formulation is based on a continuous time‐domain representation that handles assignment and sequencing decisions through different sets of binary variables and uses the notion of a generalized predecessor instead of a direct predecessor. Several types of VRPTW problems have been efficiently solved.     

Experimental and Numerical Investigation of a Reactive Absorption Column for Chlorine Gas Mitigation

Chlorine gas is extensively used in a variety of chemical processes as an end product or reactant, for instance, in the conversion of phthalocyanine dye. Reactive absorption (RA) columns can be applied for the treatment of the waste gas streams at the outlet of the reaction equipment, in order to satisfy the environmental regulations, and these should be prepared for potential accidents. The objective of this study was to evaluate an industrial-scale RA column for the treatment of chlorine gas stream in a phthalocyanine plant in the case of an accident. A coupled experimental and numerical study was performed. A factorial design was applied for the definition of a set of experimental conditions to be evaluated in a pilot-scale plant operated in the laboratory. The data collected were treated by means of the response surface methodology, and coupled numerical schemes allowed the determination of the mass transfer coefficient and the reaction rate constant. Finally, numerical studies were carried out to assess the industrial-scale column behavior. It was found that the industrial absorption column was able to treat a stream of chlorine at a flow rate of 900 kg/h with an efficiency of approximately 79%.     

Fluid Flow and Mechanisms of Momentum Transfer in a Six-Strand Tundish

Flow in a six-strand billet tundish, using turbulence inhibitors (TIs), was characterized using inputs of a pulsed tracer and mathematical simulations. It was found that to control turbulence attaining high fluid fractions under plug flow patterns, the key parameter for designing TIs is the dissipation rate of kinetic energy. TI designs that induce steep dissipation gradients are less efficient as flow controllers than those designs that yield more prolonged dissipation gradients from the inhibitor bottom to the bulk flow. A direct relationship between the dissipation of kinetic energy and the linear acceleration of the smallest turbulent eddies in the flow was established through dimensional analysis. The inhibitor with the highest linear accelerations of eddies in the viscous sublayer at the Kolmogorov scale, for a given liquid flow rate, yields the better flow control.     

Sensor‐Based Long Baseline Navigation: Observability Analysis and Filter Design

This paper presents a novel long baseline (LBL) position and velocity navigation filter for underwater vehicles based directly on the sensor measurements. The solution departs from previous approaches as the range measurements are explicitly embedded in the filter design, therefore avoiding inversion algorithms and allowing also the consideration of the cases of reduced numbers of readings, in particular when there are only two or three distance measurements. The nonlinear system dynamics are considered to their full extent and no linearizations are carried out whatsoever. The filter error dynamics are globally exponentially stable (GES) and it is shown, in a realistic simulation environment, that the filter achieves similar performance to the extended Kalman filter (EKF) and outperforms linear position and velocity filters based on algebraic estimates of the position obtained from the range measurements.     

Globally exponentially stable filters for source localization and navigation aided by direction measurements

This paper presents a set of filters with globally exponentially stable error dynamics for source localization and navigation, in 3-D, based on direction measurements from the agent (or vehicle) to the source, in addition to relative velocity readings of the agent. Both the source and the agent are allowed to have constant unknown drift velocities and the relative drift velocity is also explicitly estimated. The observability of the system is studied and realistic simulation results are presented, in the presence of measurement noise, that illustrate the performance of the achieved solutions. Comparison results with the Extended Kalman Filter are also provided and similar performances are achieved.