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An Adaptively Inexact Method for Bilevel Learning Using Primal-Dual Style Differentiation
Get PDFWe consider a bilevel learning framework for learning linear operators. In this framework, the learnable parameters are optimized via a loss function that also depends on the minimizer of a convex optimization problem (denoted lower-level problem). We utilize an iterative algorithm called `piggyback' to compute the gradient of the loss and minimizer of the lower-level problem. Given that the lower-level problem is solved numerically, the loss function and thus its gradient can only be computed inexactly. To estimate the accuracy of the computed hypergradient, we derive an a-posteriori error bound, which provides guides for setting the tolerance for the lower-level problem, as well as the piggyback algorithm. To efficiently solve the upper-level optimization, we also propose an adaptive method for choosing a suitable step-size. To illustrate the proposed method, we consider a few learned regularizer problems, such as training an input-convex neural network
BHIS:A Bayesian-Heuristic Inference System for Recognition of Walking Gait Phases
Get PDFGait phase recognition is vital for advancing assistive robotics, enabling phase-based assistance throughout the gait cycle. This article presents a real-time method using wearable sensors and computational methods for classifying the seven gait subphases. Current methods often struggle with accuracy on unseen subjects. Furthermore, walking speed variability, hardware complexity, and response time hinder robustness, portability, and real-time performance, respectively. A Bayesian method constructs posterior belief by selecting likely phase transition candidates heuristically and combining biomechanical signal knowledge with pattern recognition techniques. The approach is validated and benchmarked against prevailing deep learning (DL) methods using two datasets, each containing data from two inertial measurement units (IMUs) attached to the midshanks of test subjects. The first dataset includes six participants, while the second dataset includes ten, all walking at their comfortable speeds. Additionally, the method is validated in real time for nine test subjects walking at varying speeds (2.2-3.5 mph). The proposed method demonstrates strong robustness, achieving average steady accuracies of 98% and 97.4% for seen and unseen subjects, respectively, with an average runtime of 1.8 ms.</p
To Change Company Culture, Focus on Systems—Not Communication
Get PDFMany leaders treat culture as a communication strategy. They believe it lives in messaging—in the articulation of purpose, the rollout of values, the tone of internal campaigns. But culture doesn’t shift because a new narrative is introduced. It shifts when systems change. When leaders take personal risks. When norms are not just declared but demonstrated. New research shows how culture doesn’t fail because it’s forgotten; it fails because it’s misunderstood. It’s treated as branding, not behavior. As output, not infrastructure. And when that happens—even the most well-meaning efforts can erode the very trust they’re meant to build
Fundamental Model for Compressor Cavity Heat Transfer: Theory and Validation
Get PDFThe buoyancy-driven flow inside compressor cavities is three-dimensional, unsteady and features a large range of time- and length-scales. The temperature and rotation of the fluid core of the cavity are influenced by an exchange (and recirculation) of enthalpy and momentum with an axial throughflow of cooling air at low radius. The complexity of the flow and conjugate nature of the heat transfer to the discs creates a challenge for the aero-engine designer when calculating thermal stresses, radial expansion, and blade-tip clearances.This paper presents a low-order model to predict the radial variation of disc and fluid-core temperatures, and the mass exchange (entrainment) to the rotating cavity. Fundamental physical principles and experimental data are used to create a single set of Rayleigh-Grashof correlations for heat transfer and radial mass flow of buoyant plumes. The model is applied to eleven test cases from experimental rigs at Bath, Dresden and Sussex, each with unique instrumentation, thermal boundary conditions, geometries and throughflow swirl. Empirical correlations for exchange and recirculation mass flow were determined for each rig using a common theoretical methodology. The model captures the heat and mass transfer characteristics with accuracy quantified relative to experimental data.New experimental data from the Bath Compressor Cavity Rig is used to validate the model under conditions of asymmetrical heating, demonstrating the effects associated with the axial gradient of temperature in the compressor are captured appropriately. The consistent agreement with experimental data and correlation methodology demonstrates a robust framework appropriate for application to thermo-mechanical design codes in the aero-engine
Scaling Sealing Performance Across Engine Operating Conditions
Get PDFThe Secondary Air System in an aero-engine uses relatively cool purge from the compressor to limit the ingress of hot annulus gases into vulnerable turbine cavities through rim seals. Superfluous use of purge is inefficient, while insufficient use leads to thermal degradation of highly-stressed turbine components. This study establishes a predictive design tool to fully characterise a rim seal across the performance envelope of the engine. Physically-informed low-order models are important in the engine design process. The Ingress Wave Model uses a single, empirically-correlated parameter to physically link the shear-driven unsteadiness in the cavity with the swirl in the annulus. In this paper, newly-collected and existing experimental data from three facilities and rim seal geometries demonstrated that this unsteadiness is a linear function of the annulus swirl, with the superposition of purge creating a weak, secondary effect. The novel introduction of the linear correlation enables predictions of sealing effectiveness across the entire engine operating range from just two data points. A larger set of data is shown to improve the accuracy and robustness. Here the method is validated by data collected at low technology readiness level (TRL). The methodology could be applied to data collected from a high TRL demonstrator engine or computational fluid dynamics. This will reduce the number of demonstrator experiments (and associated costs) during design iterations. This paper provides an original scaling methodology in the practical context of the engine design process, including the effects of density. Aero-engines operate with a significant purge-mainstream density ratio (DR), due to differences in the temperatures of the two streams. The methodology incorporates predictions of DR and is further validated against data collected at DR = 1 and 1.5. The model demonstrates that neglecting DR will provide significantly underpredicted rim seal performance.</p
The Adverse Effect of Pre-Swirl on Ingestion in a Downstream Cavity
Get PDFGas turbine designers demand accurate predictions of metal temperature to ensure acceptable operating life of components experiencing high thermal stress. Rotor-stator cavities ingest hot mainstream gas through rim seals when inadequately purged with relatively cool air bled off the compressor. Superfluous use of purge, and any associated windage increase, creates a parasitic loss in overall efficiency. Shear interaction caused by the difference in swirl between the purge and mainstream flow is a principal driver for ingestion; pre-swirled purge flow has the potential to alter the swirl gradient. This paper presents the first assessment of purge conditioning in a downstream cavity.An experimental campaign was conducted in a new aeroengine representative 1.5-stage test facility designed to facilitate expedient changeover of modular components in the downstream stator assembly. Purge flow in the downstream cavity was supplied through a series of angled injectors contained in a single component at mid-radius. Three co-swirled injection angles were tested.Measurements of CO2 gas concentration, static pressure and swirl were taken in the cavity to examine the relationship between purge-mainstream swirl gradient and ingress downstream of a rotor blade. The aero-engine designer must balance caution when employing pre-swirl to reduce disc windage; co-swirled purge increased the purge-mainstream swirl gradient and subsequently increased shear-driven ingestion
The status of development and development studies:Proceedings of the 50th anniversary symposium of the Centre for Development Studies at the University of Bath.
Get PDFThe symposium brought together past and present staff and students of development studies at the University of Bath to participate in two panel-led sessions: the first on the current state of international development practice and the second on prospects for development studies. This summary includes edited versions of what each panel members said, and of the discussions these triggered. The event was opened by Phil Taylor, the University of Bath’s Vice Chancellor, who declared a strong interest in international development issues stemming from his early research into appropriate renewable energy systems for remote areas. He noted that CDS at fifty continues to thrive and grow, delivering research funded from prestigious sources including UKRI, the European Research Council, and Leverhulme Trust. Aurelie Charles, Co-director of CDS, elaborated on current activities of CDS. Ongoing research projects cover humanitarian action and the politics of famine, sustainability of food and critical minerals value chains, China’s investment in green development, and business start-ups in South Africa’s townships. Teaching also continues to thrive, with around two thousand students now having graduated from Bath with a degree that includes ‘development’ in its title (see Appendix). She drew particular attention to the value of alumni support for student work placements and doctoral field work through the Bath Research into Development (BRID) Fund
The Recyclability of Wind Turbine Blade Material, Manufacturing Process, and Recycling Technology
Get PDFAs wind energy continues to be deployed at a significantly increasing rate, the number of decommissioned wind turbines is expected to increase accordingly. To improve material efficiency, a large amount of waste requires appropriate identification and recycling, particularly the composite materials used in wind turbine blades (WTB). This study focuses on two life cycle stages, manufacturing and the decommissioning stage, which contribute most to the waste generation of WTB. This study investigates the material efficiency factors in WTB and organises fragmental information in manufacturing waste management, focusing on the recycling factor and quantifying the recyclability of wind turbine blade material regarding the different recycling technologies. This study fills the gap in existing research by evaluating recycling methods for specified carbon fibre-reinforced polymers (CFRPs) and glass fibre-reinforced polymers (GFRPs) using a revised recyclability index. Additionally, innovative sustainable materials and recent composite recycling studies have also been incorporated into the quantification and evaluation to update the current progress. The current source of WTB post-production waste, the corresponding disposal method, and opportunities were also reviewed and identified. The findings quantified recyclability and revealed that the recyclability of WTB materials varies significantly depending on the specific composite type and the recycling method employed. Furthermore, the calculated recyclability, combined with other factors such as global warming potential (GWP), cost, and technology readiness level (TRL), is discussed, along with the potential for improving material efficiency by selecting future material recycling technology and effective manufacturing waste management
Translating the International to the Local:the Case of the Women, Peace and Security Agenda in Poland and Armenia
Get PDFThrough case studies on Poland and Armenia, this chapter examines tensions that exist between the ‘cosmopolitan’ liberal peace-building agendas of the international community, which are based upon shared moral values that transcend borders and societies, and a rejection of these global norms which are considered a threat to historical traditions and values at the national level. An example of this dynamic can be observed through the United Nations’ Women, Peace and Security (WPS) agenda which aims to address the lack of female involvement and representation in conflict resolution. The WPS agenda is shaped by gender cosmopolitanism, based on the belief in universal morals and women’s rights that extend beyond national boundaries (Aggestam and True 2020). Although originating at the international level, responsibility for implementing the WPS agenda lies with individual nation-states through the creation of their own National Action Plans (NAP s). As a result, NAP s can vary greatly between nation-states in reflection of their own domestic/foreign policy and national identity, rather than the cosmopolitan values and ethics driven at the international level. Through an analysis of the NAP s from Poland (2018) and Armenia (2019, 2021), and interviews with actors directly engaged in WPS, this chapter argues that the cases explored here demonstrate a conservative interpretation and instrumental use of WPS which undermines the transformative nature of the agenda. This limited scope of implementation restricts any agonistic potential and instead reinforces existing critiques that WPS can easily be co-opted for militarist agendas
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