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On the effect of boundary condition uncertainty on robust topology optimization of aerospace structures
No full textUncertainty quantification (UQ) within topology optimization (TO) is a growing trend, with designers recognizing that deterministic analysis does not reflect the natural variabilities found in real-world structural performance. Thus far the majority of works have dealt with uncertain loading and material properties; however, minimal research has considered uncertain boundary conditions (BCs), which play a vital role for static and dynamic analysis involving compliance and natural frequency objectives and/or constraints. BCs uncertainty is studied in this paper by assuming a random percentage of a cantilever can be freed to rotate and/or translate in space. This is firstly demonstrated through a case study on a flat plate wing where the frequency gap between two modes is maximized, and secondly through optimizing the wing of a sensorcraft where compliance is minimized. A robust objective is formulated via a weighted sum of the mean and standard deviation, which is approximated efficiently using a Non-Intrusive Polynomial Chaos Expansion (NIPC). Both studies demonstrate the pitfalls of a deterministic optimum in dealing with uncertain BCs, and how using Robust TO (RTO) can effectively reduce the variance and worst-case performance of the objective. Experimental validation for the flat plate wing is also undertaken to ensure practical feasibility of the RTO topologies and to provide motivation to study BCs uncertainty further
The case for reviewing laboratory-based road transport simulations for packaging optimisation
No full textToday, there exist a number of standards designed to assist packaging engineers with implementing suitable laboratory testing regimes for road transport. However, these standards generally focus on translational vibrations and do not include other motions that may affect survival rates during transport (e.g., pitch and roll). The standards also do not account for the significant variations in vibration (root mean square [rms]) levels that are clearly evident during transport. Further, the analysis and interpretation of vibration frequency spectra typically ignore the possible presence of harmonics or shocks. Most standards also advocate some form of time compression to reduce testing duration by artificially amplifying the simulated vibrations. Each of these individual approaches combines to render the simulated vibrations currently in use unrepresentative of what occurs during transport, thereby making it difficult to optimise packaging systems. This article focuses on road transport shocks and vibrations and highlights the shortcomings of proposing and making changes to test methods based on limited data obtained from specific transport scenarios. It argues that only once all the evidence, taking into account a broader set of scenarios from multiple studies, has been collected and the correct scientific analysis applied, should changes to test protocols be proposed and implemented. The paper includes specific recommendations for further evidence collection and analysis for each of the main issues associated with road transport vibrations, namely, spectral shape, rms levels and test duration, nonvibratory events such as shocks and multiaxis vibrations
Current Status in Building a Compact and Mobile HTS MRI Instrument
No full textMagnetic Resonance Imaging (MRI) is an indispensable tool in monitoring lipid-rich tissues and the musculoskeletal system. It however requires maintaining a huge homogeneous (1 to 7 T) field. That is where superconductors come into play since they are the only feasible way to create such fields. The state-of-the-art technology utilizes low critical temperature superconductors (LTS) which require cooling down to liquid helium temperatures (4.2K). The new generation superconductors, aka the high critical temperature superconductors (HTS), can operate up to relatively elevated temperatures (technically above 100K's). In addition to that, they can withstand larger magnetic field strengths which can theoretically go up to the order of hundred Teslas in magnitude. It is evident that LTS wires of current MRI magnet technology is at their limits and new pursuits such as reaching higher fields and reducing operational/maintenance costs are in the favor of HTS tapes. There is, in fact, another advantage beyond minimization of the cryogenic costs, namely the compactness of the overall system. Utilizing certain auxiliary techniques such as flux pumping and conductive cooling of the tapes, MRI magnets can be reduced in size substantially. That, in turn, enables constructing much smaller thus mobile MRI scanners which can be brought to patients' doorsteps. Here we present the current status of our project aiming to build an HTS MRI machine dedicated to use as a mobile head scanner
Environmental hotspots analysis: A systematic framework for food supply chains and implementation case in the UK poultry industry
No full textEnvironmental sustainability analyses of end-to-end food supply chain (SC) operations need to be performed regularly to accommodate reconfiguration opportunities arising in the global business landscape. This research scrutinises the pertinent literature and identifies the challenges of data availability, data obsolescence, computational complexity, and data specificity that associate to well-established environmental assessment methodologies, and proposes a stepwise approach that considers key players and processes for generating “close to real-time snapshots” of the main environmental hotspots for the focus firm. The applicability of the proposed systematic approach is demonstrated via an implementation at a resource-intensive sector of significant scale, i.e., the UK poultry industry. Overall, this research contributes to the SC environmental sustainability management domain by guiding the mapping and identification of environmental hotspots across end-to-end networks of operations in the form of a stepwise framework, and through articulating several research propositions
Conducting Polymer-Ionic Liquid Electrode Arrays for High-Density Surface Electromyography
No full textSurface electromyography (EMG) is used as a medical diagnostic and to control prosthetic limbs. Electrode arrays that provide large-area, high density recordings have the potential to yield significant improvements in both fronts, but the need remains largely unfulfilled. Here, digital fabrication techniques are used to make scalable electrode arrays that capture EMG signals with mm spatial resolution. Using electrodes made of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) composites with the biocompatible ionic liquid (IL) cholinium lactate, the arrays enable high quality spatiotemporal recordings from the forearm of volunteers. These recordings allow to identify the motions of the index, little, and middle fingers, and to directly visualize the propagation of polarization/depolarization waves in the underlying muscles. This work paves the way for scalable fabrication of cutaneous electrophysiology arrays for personalized medicine and highly articulate prostheses
Electronic properties and tunability of the hexagonal SiGe alloys
No full textHexagonal (2H) germanium is found to be a direct bandgap semiconductor, showing the potential of efficient light emission. Based on 2H-Ge, the structure and electronic properties of 2H-SiGe alloys are studied in detail by hybrid functional calculations. By varying the Si content of the 2H-SiGe alloys, the bandgap is found to be direct for Si contents smaller than 0.35. We find that the key factor in determining the indirect-to-direct transition of the band structures for 2H-SiGe alloys originates from the variation of lattice constant. Furthermore, the Si-rich 2H-SiGe alloy can be changed from indirect to direct bandgap by strain engineering. Furthermore, we consider the effective electron masses (me), band alignments with several oxides, optical absorption properties, and vacancy formation energies of 2H-SiGe alloys, which show that the direct-gap 2H-SiGe alloys have the potential for optoelectronic applications
Structural and electronic engineering of Ir-Doped Ni-(Oxy)hydroxide nanosheets for enhanced oxygen evolution activity
No full textDiscovering highly active and stable electrocatalysts for the oxygen evolution reaction (OER) is critical to the commercial development of many next-generation energy conversion and storage devices, with Fe-doped nickel (oxy)hydroxide representing one of the most promising OER catalysts developed to date. However, the active sites and mechanism of OER on Fedoped nickel (oxy)hydroxide catalysts remain unclear. To gain deeper insights into the role of metal dopants in enhancing OER activity, we explored here the role of Ir-doping in the OER performance of nickel (oxy)hydroxide catalysts, placing particular emphasis on the nature of the active site. Density functional theory calculations with Hubbard U correction revealed that Ir-doping of a β-NiOOH(001) surface enhanced the electric conductivity while also activating an oxygen site involving three Ni atoms (Ni3 site) to realize a remarkably low OER overpotential of only ν = 0.46 V, much lower than the overpotential on the oxygen site involving Ir + two Ni atoms (IrNi2 site, ν = 0.77 V) or the oxygen site involving three Ni atoms in pristine β-NiOOH (ν = 0.66 V). Guided by the computational results, ultrathin Ir-doped Ni(OH)2 nanosheets were then fabricated through a combination of hydrothermal assembly and liquid exfoliation, with the nanosheets transforming to Ir-doped NiOOH during OER and offering superior activity relative to pristine Ni(OH)2 nanosheets or a commercial IrO2 catalyst, thereby validating the theoretical predictions. The computational and experimental results thus conclusively demonstrate that Ir-doping and nanosheet engineering are synergistic strategies for tuning the electronic and structural properties of nickel (oxy)hydroxides for improved oxygen evolution electrocatalysis
Multi-class cost-constrained random coding for correlated sources over the multiple-access channel
No full textThis paper studies a generalized version of multi-class cost-constrained random-coding ensemble with multiple auxiliary costs for the transmission of N correlated sources over an N-user multiple-access channel. For each user, the set of messages is partitioned into classes and codebooks are generated according to a distribution depending on the class index of the source message and under the constraint that the codewords satisfy a set of cost functions. Proper choices of the cost functions recover different coding schemes including message-dependent and message-independent versions of independent and identically distributed, independent conditionally distributed, constant-composition and conditional constant composition ensembles. The transmissibility region of the scheme is related to the Cover-El Gamal-Salehi region. A related family of correlated-source Gallager source exponent functions is also studied. The achievable exponents are compared for correlated and independent sources, both numerically and analytically
Balanced truncation of k-positive systems
No full textThis paper considers balanced truncation of discrete-time Hankel -positive systems, characterized by Hankel matrices whose minors up to order k are nonnegative. Our main result shows that if the truncated system has order or less, then it is Hankel totally positive (-positive), meaning that it is a sum of first order lags. This result can be understood as a bridge between two known results: the property that the first-order truncation of a positive system is positive (), and the property that balanced truncation preserves state-space symmetry. It provides a broad class of systems where balanced truncation is guaranteed to result in a minimal internally positive system