Clustering with the Average Silhouette Width

The Average Silhouette Width (ASW) is a popular cluster validation index to estimate the number of clusters. The question whether it also is suitable as a general objective function to be optimized for finding a clustering is addressed. Two algorithms (the standard version OSil and a fast version FOSil) are proposed, and they are compared with existing clustering methods in an extensive simulation study covering known and unknown numbers of clusters. Real data sets are analysed, partly exploring the use of the new methods with non-Euclidean distances. The ASW is shown to satisfy some axioms that have been proposed for cluster quality functions. The new methods prove useful and sensible in many cases, but some weaknesses are also highlighted. These also concern the use of the ASW for estimating the number of clusters together with other methods, which is of general interest due to the popularity of the ASW for this task

Cloaking using the anisotropic multilayer sphere

We studied a Spherically Radially Anisotropic (SRA) multilayer sphere with an arbitrary number of layers. Within each layer permittivity components are different from each other in radial and tangential directions. Under the quasi-static approximation, we developed a more generalized mathematical model that can be used to calculate polarizability of the SRA multilayer sphere with any arbitrary number of layers. Moreover, the functionality of the SRA multilayer sphere as a cloak has been investigated. It has been shown that by choosing a suitable contrast between components of the permittivity, the SRA multilayer sphere can achieve threshold required for invisibility cloaking

Multi-level supervisory emergency control for operation of remote area microgrid clusters

Remote and regional areas are usually supplied by isolated and self-sufficient electricity systems, which are called as microgrids (MGs). To reduce the overall cost of electricity production, MGs rely on non-dispatchable renewable sources. Emergencies such as overloading or excessive generation by renewable sources can result in a substantial voltage or frequency deviation in MGs. This paper presents a supervisory controller for such emergencies. The key idea is to remedy the emergencies by optimal internal or external support. A multi-level controller with soft, intermedial and hard actions is proposed. The soft actions include the adjustment of the droop parameters of the sources and the controlling of the charge/discharge of energy storages. The intermedial action is exchanging power with neighboring MGs, which is highly probable in large remote areas. As the last remedying resort, curtailing loads or renewable sources are assumed as hard actions. The proposed controller employs an optimization technique consisting of certain objectives such as reducing power loss in the tie-lines amongst MGs and the dependency of an MG to other MGs, as well as enhancing the contribution of renewable sources in electricity generation. Minimization of the fuel consumption and emissions of conventional generators, along with frequency and voltage deviation, is the other desired objectives. The performance of the proposal is evaluated by several numerical analyses in MATLAB®

Introduction to radar scattering application in remote sensing and diagnostics: Review

The manuscript reviews the current literature on scattering applications of RADAR (Radio Detecting And Ranging) in remote sensing and diagnostics. This paper gives prime features for a variety of RADAR applications ranging from forest and climate monitoring to weather forecast, sea status, planetary information, and mapping of natural disasters such as the ones caused by earthquakes. Both the fundamental parameters involved in scattering mechanisms of RADAR applications and the factors affecting RADAR performances are also discusse

Green and facile synthesis of cerium doped Ni3Fe electrocatalyst for efficient oxygen evolution reaction

Electrochemical water splitting is the most promising pathway to produce high-purity hydrogen to alleviate global energy crisis. This reaction demands inexpensive, efficient and robust electrocatalyst for its commercial use. Herein, we demonstrate an effective, facile and scalable method for the synthesis of cerium doped Ni3Fe nanostructures as an electrocatalyst for oxygen evolution reaction (OER) by following simple chemical bath deposition route. The different molar ratios (3, 6 and 12 mM) of cerium in the chemical bath were used to study its effect on the structural and the electrochemical properties of the Ni3Fe nanostructured films. Doping of cerium contents induced variations in the morphology of deposited Ni3Fe nanostructures. The optimized electrocatalyst Ni3Fe/Ce-6 yielded high surface area catalyst nanosheets uniformly deposited on three-dimensional conductive scaffold to ensure increase in the exposure of doped Ni3Fe catalytic sites with high electrical conductivity. As a result, this earth-abundant electrocatalyst affords high OER performance with a small overpotential of 310 mV versus reversible hydrogen electrode (RHE) at 10 mA cm-2 and retains good stability up to ~ 10 h in alkaline electrolyte. This scalable strategy has great potential in future advancement of efficient and low-cost electrocatalysts for their large-scale application in energy conversion systems.                     KEY WORDS: Oxygen evolution, Electrocatalyst, Ni3Fe nanostructures, Cerium, Alkaline electrolyte   Bull. Chem. Soc. Ethiop. 2020, 34(2), 353-363 DOI: https://dx.doi.org/10.4314/bcse.v34i2.1

Fistulotomy versus fistulectomy for simple fistula in ano: a retrospective cohort study

OBJECTIVE: To compare fistulotomy with fistulectomy for wound healing, duration of surgery, post-operative pain, incontinence and recurrence in patients with fistula in ano. METHODS: This retrospective cohort study was conducted at the Aga Khan University Hospital, Karachi, and comprised data of adults who had undergone fistulotomy/fistulectomy for simple fistula in ano from January 2007 to August 2012. Data collection was done in August 2013 using questionnaire and telephonic interviews. Outcome variables like duration of wound healing, recurrence, incontinence, duration of surgery and post-operative pain were compared in both the groups. SPSS 19 was used for data analysis. RESULTS: Of the 192 cases, there were 96(50%) in each group. The mean age was 40.51 years (range: 21-72 years) in the fistulotomy group and 41.14 years (range: 21-66 years) in the fistulectomy group (p=0.66). Both groups were comparable for baseline demographic variables. The median duration of wound healing was shorter in the fistulotomy group 15 days (Interquartile range: 7-20 days) compared to the fistulectomy group 30 days (Interquartile range: 15-42 days) (p\u3c0.001). The incidence of recurrence was comparable in fistulotomy vs. fistulectomy (3[3.12%] vs. 4[4.16%]; p=0.70). The incidence of incontinence was higher in fistulotomy compared to fistulectomy (5[5.3%] vs. 12[12.5%]; p=0.07). The severity of incontinence was also compared but the difference was insignificant (p=0.06). The median duration of surgery was significantly shorter in fistulotomy group 17 minutes (Interquartile range: 12-25 minutes) compared to fistulectomy group 25 minutes Interquartile range: 20-35 minutes (p\u3c0.001). The median post-operative pain in the surgical day care unit and at the first follow-up in clinic was zero for both groups. CONCLUSIONS: Fistulotomy yielded better results than fistulectomy since it significantly decreased the duration of wound healing and duration of surgery without increasing the incidence of recurrence, incontinence and post-operative pain