Approximations and Optimal Control for State-Dependent Limited Processor Sharing Queues

The paper studies approximations and control of a processor sharing (PS) server where the service rate depends on the number of jobs occupying the server. The control of such a system is implemented by imposing a limit on the number of jobs that can share the server concurrently, with the rest of the jobs waiting in a first-in-first-out (FIFO) buffer. A desirable control scheme should strike the right balance between efficiency (operating at a high service rate) and parallelism (preventing small jobs from getting stuck behind large ones). We use the framework of heavy-traffic diffusion analysis to devise near optimal control heuristics for such a queueing system. However, although the literature on diffusion control of state-dependent queueing systems begins with a sequence of systems and an exogenously defined drift function, we begin with a finite discrete PS server and propose an axiomatic recipe to explicitly construct a sequence of state-dependent PS servers that then yields a drift function. We establish diffusion approximations and use them to obtain insightful and closed-form approximations for the original system under a static concurrency limit control policy. We extend our study to control policies that dynamically adjust the concurrency limit. We provide two novel numerical algorithms to solve the associated diffusion control problem. Our algorithms can be viewed as “average cost” iteration: The first algorithm uses binary-search on the average cost, while the second faster algorithm uses Newton-Raphson method for root finding. Numerical experiments demonstrate the accuracy of our approximation for choosing optimal or near-optimal static and dynamic concurrency control heuristics.

Corrosion Behavior and Susceptibility to Stress Corrosion Cracking of Leaded and Lead-Free Brasses in Simulated Drinking Water

Duplex α + β’ brasses are widely used in drinking water distribution systems for tube fittings, valves, and ancillaries because they are low cost, easy to fabricate, and exhibit high mechanical strength. However, depending on application conditions and alloy composition, they may undergo dealloying and stress corrosion cracking. In this research, three different brass types, two leaded (CW617N and CW602N) alloys and one lead-free brass (CW724R), were investigated to assess their corrosion behavior and susceptibility to stress corrosion cracking (SCC) in simulated drinking water (SDW) solutions containing different chloride concentrations, compatible with drinking water composition requirements according to Moroccan standard NM 03.7.001. The corrosion behavior was assessed by electrochemical tests such as polarization curve recording and electrochemical impedance spectroscopy (EIS) monitoring, coupled to SEM-EDS surface observations. The susceptibility to SCC was investigated by slow strain rate tests (SSRT). The tests showed that corrosion was mainly under diffusion control and chlorides slightly accelerated corrosion rates. All alloys, and particularly CW617N, were affected by SCC under the testing conditions adopted and in general the SCC susceptibility increased at increasing chloride concentration.

Flying with Covid: The visual presence of the pandemic in airports

Since the beginning of 2020, with the eclosion of the Covid-19 pandemic, airports have been included among the main hotspots for the diffusion of the disease. Several limitations affected the possibility for people to travel, with diverse approaches between the countries, and with differences among who was authorized to travel and who was not. This caused a contraction on the number of passengers transiting in the airports in all the countries. However the commercial international aviation has never stopped, and despite the reduction of passengers the airports managed to implement health security protocols for the Covid-19 diffusion control. Before the pandemic, other challenges already affected airports’ security protocols, such as the “terrorist threat”, making of these places “nervous systems” (as defined by Maguire and Pétercsak). After one year and half from the beginning of the pandemic, with the vaccination campaigns accelerating in various countries (with the clear differences due to governments’ political choices and countries’ access to vaccines) the air travels have returned to a condition similar to previous one. An increasing number of planes flying and an increasing number of passengers can be registered everywhere. Meanwhile, the sanitary attention to the Covid-19 diffusion contention continues to be a concern in the space organization of airports.This ethnographic photoessay aims at describing the visual presence of the Covid in the airports. The work focuses on four airports in three countries the author passed through in June 2021. They are the airports of Salvador da Bahia (Brazil), Lisbon (Portugal), Rome and Venice (Italy). Despite the differences between the countries in the approached adopted to contain the diffusion of the pandemic, airports are subjected to standardized international protocols. These are intended to (re)produce similar safety measures in the diverse airports. Meanwhile, airports are designed not to be identitarian, historical and relational, but yes to be experienced as “non places” (as Augé defined these places). However, each airport introduces several dimensions of its specific location, of its specific local health politics, of its specific passengers’ flow, and so on, making of them a peculiar place to observe the space design for Covid diffusion control. Despite the definition of the Covid as an “invisible enemy”, used in general media in diverse countries, the thesis is that the presence of the virus is highly visible to everyone passing in some airport, independently from the specific country. Meanwhile, the diverse airports introduce their own local and specific visual modalities to achieve passengers. Pictures included in this ethnographic photoessay focus on some of these modalities, such as the hand gel dispensers, instructions and prohibitions for preventing Covid dissemination, among other. Covid’s aesthetics in airports highlights how the pandemic affected people visual and sensorial experiences of these places and of their designs.

Heat Carbonization and ZnCl2 Functionalization of Date Stone as an Adsorbent: Optimization of Material Fabrication Parameters and Adsorption Studies

The scientific community gave a lot attention to prepare adsorbents from different natural agriculture-based materials to be used alternative to commercial activated carbon. However, less studies on the optimization of fabrication parameters to obtain activated carbon with highly surface area and adsorption capacity. Herein, we report the synthesis conditions optimization of adsorbent based on date stone and modified with ZnCl2. To obtain a highly adsorption ability of the materials, three systematic parameters were evaluated such as the activation temperature, activation time and the functionalization ratio by ZnCl2. The optimization study showed that the best factors to fabricate an adsorbent from date stone are 700°C, 120 min and 2.0 (g/g), wherein, the specific surface area was found to be 1036 m2/g. While, the iodine and phenol numbers were 928.5 mg/g and 2.1 mmol/g, respectively. To further understand the effect of synthesis parameters, the raw and the as-synthesized activated carbon were characterized via Fourier transmission infrared spectroscopy (FT-IR), X-ray diffraction (XRD), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Batch sorption tests to remove MB from water showed a maximum adsorption capacity of 384.6 mg/g using the prepared activated carbon at pH 6 and room temperature (25±2 °C). It was found also that the kinetic adsorption data obeyed the pseudo-second order and, both external diffusion and intra-particle diffusion control the adsorption. Based on the obtained results, the optimization of synthesis conditions through experimental and mobilization studies may help the transfer of technology in terms of agriculture-based materials valorisation towards the environmental remediation.

Electrochemical Preparation of Nano-Sized Silicon as a Lithium-Ion Battery Anode Material

Highly pure silicon is an important component in photovoltaic applications and has potential in battery technology. In this study, the electrochemical behavior of Si (IV) was discussed in a NaF−LiF−Na2SiO3−SiO2 electrolyte at 750 °C , and lithium-ion battery performance with electrodeposited silicon powder as anode material were investigated. The cyclic voltammograms illustrated that the reduction of Si(IV) on an Ag electrode followed an irreversible two-step, two-electron process: Si(IV) → Si(II) and Si(II) → Si(0). Both reduction steps involved diffusion control, and the diffusion coefficients were 1.18 and 1.22 × 10−6 cm2/s, respectively. Nanoscale spherical silicon was deposited between potentials of −1.0 to −1.6 V (vs. Pt) with support of X-ray diffraction patterns, Raman spectra, and scanning electron microscopy analysis. Combining the fabricated silicon with carbon, a Si@C composite anode material for lithium-ion batteries was prepared, and its specific capacity reached 1260 mAh/g. Notably, a capacity of 200 mAh/g was maintained over 100 cycles.

Joint UAVs’ Load Balancing and UEs’ Data Rate Fairness Optimization by Diffusion UAV Deployment Algorithm in Multi-UAV Networks

Unmanned aerial vehicles (UAVs) can be deployed as base stations (BSs) for emergency communications of user equipments (UEs) in 5G/6G networks. In multi-UAV communication networks, UAVs’ load balancing and UEs’ data rate fairness are two challenging problems and can be optimized by UAV deployment strategies. In this work, we found that these two problems are related by the same performance metric, which makes it possible to optimize the two problems simultaneously. To solve this joint optimization problem, we propose a UAV diffusion deployment algorithm based on the virtual force field method. Firstly, according to the unique performance metric, we define two new virtual forces, which are the UAV-UAV force and UE-UAV force defined by FU and FV, respectively. FV is the main contributor to load balancing and UEs’ data rate fairness, and FU contributes to fine tuning the UEs’ data rate fairness performance. Secondly, we propose a diffusion control stratedy to the update UAV-UAV force, which optimizes FV in a distributed manner. In this diffusion strategy, each UAV optimizes the local parameter by exchanging information with neighbor UAVs, which achieve global load balancing in a distributed manner. Thirdly, we adopt the successive convex optimization method to update FU, which is a non-convex problem. The resultant force of FV and FU is used to control the UAVs’ motion. Simulation results show that the proposed algorithm outperforms the baseline algorithm on UAVs’ load balancing and UEs’ data rate fairness.

New Venture Creation: A Drift-Variance Diffusion Control Model

A Model for New Venture Creation New ventures go through multiple stages: In the early stage, there is a business concept and preliminary evidence supporting the concept. In later stages, there are revenues and sales. In each stage, there are usually milestones for the venture to meet in order for investors to provide additional funding. Otherwise, the venture is abandoned. The entrepreneur can engage in a set of costly activities that aim to create value and reach the appropriate milestones. In “New Venture Creation: A Drift-Variance Diffusion Control Model,” Wang and Zenios develop a framework for new venture creation. The authors provide theoretical guidance on the optimal policy, which is relatively simple to describe. Their analysis reveals a trade-off between how costly an activity is and how much upside potential the activity generates, and their result shows how a new venture creator can manage that trade-off.

Steel Corrosion Patterns in Neutral and Alkaline Iron Oxide Slurry

The study identifies and theoretically substantiates the steel corrosion patterns in aqueous slurry of iron-ore concentrate. The purpose of the study was to examine the effect that the content of dissolved oxygen(PO2), pH, the concentration of chloride ions (CCl-) and the rate of movement of the iron oxide slurry (ω) produce on the corrosion losses of steel 20. Comparative analysis of the corrosion rate values obtained by the gravimetric method and the polarization resistance method showed that the reciprocal of the polarization resistance and the corrosion rate change symbatically with increasing pH, the corrosion rate values are quantitatively well consistent with each other. To identify the corrosion patterns, the method of polarization resistance was used. The dependence of the corrosion rate (ρ) on рН revealed two characteristic areas: in the first one, there is no dependence of the rate on pH (6.5--9.0); in the second one, there is a sharp decrease in corrosion losses when the pH goes from 9.0 to 12.5. For pH = 6.5--9.0, the corrosion rate increases linearly with an increase in the partial pressure of oxygen, and corrosion losses in the slurry are higher than in the background solution. The dependence ρ(√ω) is linear over the entire pH range (6.0--9.0), which indicates the diffusion control of the corrosion process. Findings of research show that in order to protect carbon steel from corrosion in the iron oxide slurry, it is necessary to take into account the pH and О2 concentration. Optimal reduction of corrosion losses can be achieved by alkalizing the slurry and removing dissolved oxygen

Grain Growth Control of Dielectric and Magnetic Ceramics

The sintering process transported the atoms in the materials by decreasing the total interface energy. The microstructure changes as a result of grain growth and densification under the capillary driving force due to the interface curvature among grains. The grain growth rate is expressed as the product of the interface mobility and the driving force. According to grain growth theories, the mobility of the interface governed by diffusion control is constant but interface mobility is nonlinear when the movement of an interface is governed by interface reaction. As the growth rate is nonlinear for the regime of interface reaction control, the grain growth is nonstationary with annealing time. The microstructure can be controlled by changing the growth rate of an individual grain with the correlation between the maximum driving force and the critical driving force for appreciable growth. The present paper discusses applications of the principle in the fabrication of dielectric and magnetic ceramic materials.