Viscous nozzle flow as a Sturm-Liouville problem

In a slender nozzle the mean stationary flow is one dimensional. I analyse this one dimensional flow introducing a small artificial viscosity into the conservation equations, and considering the coefficient of viscosity and the size of the constriction of first and second order smallness respectively. The so resulting approximation scheme leads to a Sturm-Liouville problem with the quadratically not integrable solutions playing a significant role; they describe subsonic flows. The summary at the end of this note enumerates one by one the different results obtained.     

Predictions and Analyses on the Growth Behavior of Oxide Scales Formed on Ferritic–Martensitic in Supercritical Water

Oxidation of Metals - For 9–12Cr ferritic–martensitic steels in supercritical water, the dependencies of the thicknesses of three oxide layers (diffusion, inner, and outer layers) on...     

Probing and repairing damaged surfaces with nanoparticle-containing microcapsules

Nanoparticles have useful properties, but it is often important that they only start working after they are placed in a desired location. The encapsulation of nanoparticles allows their function to be preserved until they are released at a specific time or location, and this has been exploited in the development of self-healing materials and in applications such as drug delivery. Encapsulation has also been used to stabilize and control the release of substances, including flavours, fragrances and pesticides. We recently proposed a new technique for the repair of surfaces called 'repair-and-go'. In this approach, a flexible microcapsule filled with a solution of nanoparticles rolls across a surface that has been damaged, stopping to repair any defects it encounters by releasing nanoparticles into them, then moving on to the next defect. Here, we experimentally demonstrate the repair-and-go approach using droplets of oil that are stabilized with a polymer surfactant and contain CdSe nanoparticles. We show that these microcapsules can find the cracks on a surface and selectively deliver the nanoparticle contents into the crack, before moving on to find the next crack. Although the microcapsules are too large to enter the cracks, their flexible walls allow them to probe and adhere temporarily to the interior of the cracks. The release of nanoparticles is made possible by the thin microcapsule wall (comparable to the diameter of the nanoparticles) and by the favourable (hydrophobic-hydrophobic) interactions between the nanoparticle and the cracked surface.     

Engineering design optimization using species-conserving genetic algorithms

The species conservation technique described here, in which the population of a genetic algorithm is divided into several groups according to their similarity, is inspired by ecology. Each group with similar characteristics is called a species and is centred on a dominating individual, called the species seed. A genetic algorithm based on this species conservation technique, called the species-conserving genetic algorithm (SCGA), was established and has been proved to be effective in finding multiple solutions of multimodal optimization problems. In this article, the SCGA is used to solve engineering design optimization problems. Different distance measures (measures of similarity) are investigated to analyse the performance of the SCGA. It is shown that the Euclidean distance is not the only possible basis for defining a species and sometimes may not make sense in engineering applications. Two structural design problems are used to demonstrate how the choice of a meaningful measure of similarity will help the exploration for significant designs.     

Model order selection of nonlinear input–output models––a clustering based approach

Selecting the order of an input–output model of a dynamical system is a key step toward the goal of system identification. The false nearest neighbors algorithm (FNN) is a useful tool for the estimation of the order of linear and nonlinear systems. While advanced FNN uses nonlinear input–output data-based models for the model-based selection of the threshold constant that is used to compute the percentage of false neighbors, the computational effort of the method increases along with the number of data and the dimension of the model. To increase the efficiency of this method, in this paper we propose a clustering-based algorithm. Clustering is applied to the product space of the input and output variables. The model structure is then estimated on the basis of the cluster covariance matrix eigenvalues. The main advantage of the proposed solution is that it is model-free. This means that no particular model needs to be constructed in order to select the order of the model, while most other techniques are ‘wrapped' around a particular model construction method. This saves the computational effort and avoids a possible bias due to the particular construction method used. Three simulation examples are given to illustrate the proposed technique: estimation of the model structure for a linear system, a polymerization reactor and the van der Vusse reactor.     

Exciton Recombination in Formamidinium Lead Triiodide: Nanocrystals versus Thin Films

The optical properties of the newly developed near‐infrared emitting formamidinium lead triiodide (FAPbI₃) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady‐state and time‐resolved photoluminescence. The excitonic emission is dominant in NC ensemble because of the localization of electron–hole pairs. A promisingly high quantum yield above 70%, and a large absorption cross‐section (5.2 × 10^?13 cm^?2) are measured. At high pump fluence, biexcitonic recombination is observed, featuring a slow recombination lifetime of 0.4 ns. In polycrystalline thin films, the quantum efficiency is limited by nonradiative trap‐assisted recombination that turns to bimolecular at high pump fluences. From the temperature‐dependent photoluminescence (PL) spectra, a phase transition is clearly observed in both NC ensemble and polycrystalline thin film. It is interesting to note that NC ensemble shows PL temperature antiquenching, in contrast to the strong PL quenching displayed by polycrystalline thin films. This difference is explained in terms of thermal activation of trapped carriers at the nanocrystal's surface, as opposed to the exciton thermal dissociation and trap‐mediated recombination, which occur in thin films at higher temperatures.