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Product algebras for galerkin discretisations of boundary integral operators and their applications
No full textOperator products occur naturally in a range of regularised boundary integral equation formulations. However, while a Galerkin discretisation only depends on the domain space and the test (or dual) space of the operator, products require a notion of the range. In the boundary element software package Bempp, we have implemented a complete operator algebra that depends on knowledge of the domain, range, and test space. The aim was to develop a way of working with Galerkin operators in boundary element software that is as close to working with the strong form on paper as possible, while hiding the complexities of Galerkin discretisations. In this article, we demonstrate the implementation of this operator algebra and show, using various Laplace and Helmholtz example problems, how it significantly simplifies the definition and solution of a wide range of typical boundary integral equation problems
Shear and breathing modes of layered materials
No full textLayered materials (LMs), such as graphite, hexagonal boron nitride, and transition-metal dichalcogenides, are at the center of an ever-increasing research effort, due to their scienti c and technological relevance. Raman and infrared spectroscopies are accurate, non destructive, approaches to determine a wide range of properties, including the number of layers and the strength of the interlayer interactions. We present a general approach to predict the complete spectroscopic fan diagrams, i.e., the relations between frequencies and number of layers, N, for the optically active shear and layer-breathing modes of any multilayer comprising N>=2 identical layers. In order to achieve this, we combine a description of the normal modes in terms of a one-dimensional mechanical model, with symmetry arguments that describe the evolution of the point group as a function of N. Group theory is then used to identify which modes are Raman and/or infrared active, and to provide diagrams of the optically active modes for any stack composed of identical layers. We implement the method and algorithms in an open-source tool directly available on the Materials Cloud portal, to assist any researcher in the prediction and interpretation of such diagrams. Our work will underpin future efforts on Raman and infrared characterization of known, and yet not investigated, LMs
A graphical method for determining truss stability
No full textGraphic statics has been used for over 150 years, having been pioneered by the likes of Maxwell, Cremona, Culmann and Rankine, and has recently seen a resurgence in popularity because of its use in design. However, it is only concerned with equilibrium; as any engineer will testify, whilst equilibrium is necessary, it is not sufficient and stability must also be obtained. This paper develops a novel graphical method for determining the stability and stiffness of prestressable structures. By considering the weighted sum of the Maxwell-Minkowski diagram, the stiffness and stability of the structural mechanisms can be determined. This work extends to cover structures with multiple mechanisms and has been compared to results obtained through experimentation and the finite element method. Furthermore, it extends the work on stiffness to provide a graphical method to estimate the natural frequency of a truss. Whilst this method accurately determines the stiffness of structures, it represents a significant development in the field of graphic statics as it allows an engineer to 'eye-ball' the stability of a given truss. Engineers can also manipulate the form and force diagrams, as desired, to adjust the stiffness of their structure accordingly, whilst being able to visualise the process. Much of the previous work in this area relies heavily upon large matrices, while this method allows a more intimate and hands-on alternative
Impact of dislocations in monolithic III-V lasers on silicon: A theoretical approach
No full text© 2020 SPIE. The growth of reliable III-V quantum well (QW) lasers on silicon remains a challenge as yet unmastered due to the issue of carrier migration into dislocations. We have recently compared the functionality of quantum dots (QDs) and QWs in the presence of high dislocation densities using rate equation travelling-wave simulations, which were based on 10-μm large spatial steps, and thus only allowed the use of effective laser parameters to model the performance degradation resulting from dislocation-induced carrier loss. Here we increase the resolution to the sub-micrometer level to enable the spatially resolved simulation of individual dislocations placed along the longitudinal cavity direction in order to study the physical mechanisms behind the characteristics of monolithic 980 nm In(Ga)As/GaAs QW and 1.3 μm QD lasers on silicon. Our simulations point out the role of diffusion-assisted carrier loss, which enables carrier migration into defect states resulting in highly absorptive regions over several micrometers in QW structures, whereas QD active regions with their efficient carrier capture and hence naturally reduced diffusion length show a higher immunity to defects. An additional interesting finding not accessible in a lower-resolution approach is that areas of locally reduced gain need to be compensated for in dislocation-free regions, which may lead to increased gain compression effects in silicon-based QD lasers with limited modal gain
Uncertainty Management in Situation Awareness for Cyber-Physical Systems
No full textCyber-Physical Systems (CPS) are a result of highly cross-disciplinary processes and are evolving to perform increasingly challenging tasks in dynamically changing environments. This leads to an increasing CPS complexity and therefore the management of uncertainty to ensure the trustworthiness of these systems is needed. Our paper focuses on uncertainty management (UM) both in general and more specifically in the context of CPS situation awareness (SA). The motivation behind this is the important role of SA and its many inherent uncertainties. To this end, firstly, a literature review is conducted to acquire the state of the art of UM. Later, we present findings and observations from the literature review, with two main challenges identified - inconsistent understanding and terminology among a multitude of uncertainty perspectives, and a lack of collaboration among different communities. On this basis, lastly, two case studies are conducted to exemplify the challenges and provide brief ideas on how to deal with them. The whole investigation in the paper suggests an urgent strengthening of common understanding through enhanced collaboration and regulations
VAEM: A deep generative model for heterogeneous mixed type data
No full textDeep generative models often perform poorly in real-world applications due to the heterogeneity of natural data sets. Heterogeneity arises from data containing different types of features (categorical, ordinal, continuous, etc.) and features of the same type having different marginal distributions. We propose an extension of variational autoencoders (VAEs) called VAEM to handle such heterogeneous data. VAEM is a deep generative model that is trained in a two stage manner such that the first stage provides a more uniform representation of the data to the second stage, thereby sidestepping the problems caused by heterogeneous data. We provide extensions of VAEM to handle partially observed data, and demonstrate its performance in data generation, missing data prediction and sequential feature selection tasks. Our results show that VAEM broadens the range of real-world applications where deep generative models can be successfully deployed
Evaluating bird collision risk of a high-speed railway for the crested ibis
No full textWe evaluate the bird-train collision risk associated with a recent high-speed railway project for the crested ibis (Nipponia nippon) and other local bird species in the Qinling Mountains, China. Using ground-walked line transect surveys, we surveyed the population abundance, distance to the railway bridge, and bridge-crossing behaviors of the birds in the study area. The results show that: (1) The crested ibis and the little egret (Egretta garzetta) were the two most abundant waterfowl species in the study area. The relative abundance index of these two species were about 43.33% and 42.75%, respectively; (2) 92.86% of the observed crested ibises were firstly detected within the range of 0 m to 25 m of the vicinity of the railway bridge; (3) The ratio between crossing over and under the railway bridge is about 7:3, which we found that crested ibises preferred flying over the bridge, but little egrets were more likely to cross the railway below the bridge. In the end, we recommend that, while the collision risk for the crested ibis could be low, protection structures, such as barrier poles or fences, should still be considered to avoid possible collisions of other co-habitants and promote conservation for multiple bird species in the area
Effect of natural graphite fineness on the performance and electrical conductivity of cement paste mixes for self-sensing structures
No full textCementitious composites are the most widely used construction materials; however, their poor durability necessitates frequent monitoring and repairs. The emergence of self-sensing composites could reduce the need for costly and time-consuming structural inspections. Natural graphite, due to its low cost and wide availability, is a promising additive to generate an electrically conductive network which could ultimately lead to a self-sensing mechanism. Despite several studies using natural graphite as a conductive additive, the effect of its fineness on the cementitious composite’s performance has not been explored. This study experimentally investigated the effect of three graphite products of varying fineness on the early age, mechanical, and electrical conductivity performance of cement pastes. The fluidity of the graphite-cement paste reduced significantly with increasing graphite fineness, and graphite did not affect the cement hydration. The finer the graphite, the lower the effect on the mechanical performance, as confirmed by compressive strength testing and micro-indentation. Electrical conductivity testing showed that the percolation threshold depended on the graphite fineness and was found at ~20 wt % for the fine and medium graphite, while it increased to 30–40 wt % for the coarse graphite. This is the first study that has investigated holistically the effect of graphite fineness on the performance of cement pastes and will pave the way for using this material as an additive for self-sensing structures
Novel WDM-PON system with shared wavelength locking and full C-band tunability
No full textIn this article the operation of a novel, prototypical WDM-PON system with bi-directional data rates of up to 10.3 Gb/s is experimentally demonstrated. In the optical line terminal (OLT) a novel photonic integrated circuit (PIC) is deployed integrating an array of tunable lasers with Mach-Zehnder modulators and a star coupler. Wavelength control is achieved by a pilot-tone based scheme with a shared wavelength-locker (etalon). In the optical network unit (ONU) a novel Aluminium-containing laser is used, which has been specially designed for operation between 70°C to 90°C without requiring a thermo-electric cooler (TEC). For reduced cost and complexity the tunable laser is not equipped with an etalon. Instead it employs a generic contol algorithm, which is augmented by a pilot tone based scheme is used for centralized wavelength locking of all channels at the OLT
Thermal Design of a 2-Phase Flow Cooled Medium-frequency 140kVA Transformer for Railway Applications
No full textIn this paper, a cooling scheme and the corresponding thermal design method is proposed for high power high frequency transformer, which is used in power electronic transformer device for railway traction applications. Particularly, to address the overheating problem resulting from dimensional restriction and electrical insulation in railway applications, the cooling scheme based on actively cooled system with R245fa refrigerants is further enhanced by 2-phase flow cooling. The mathematic model of transformer copper and iron loss is first introduced. Subsequently, the required cooling area of transformer core is quantitatively calculated using thermal network method and the total pressure loss and heat transfer in two-phase flow is determined accordingly. Finally, the proposed thermal design method and corresponding cooling scheme are verified through the experiment on a 5.5kHz 140kVA transformer