Open Research Exeter - University of Exeter
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Re-programmable mechanical metamaterials for tunable energy dissipation through pre-stressing: Concept, mechanics and data-driven design
No full textNonlinear structures exhibiting negative stiffness have been harnessed as fundamental building blocks for reversible energy dissipation mechanical metamaterials through the utilisation of elastic instability. Traditional metamaterials suffer from a fixed energy dissipation performance once manufactured. We propose a novel approach using elastic tailoring, where the pre-stressing level within individual building blocks is adjusted by altering their boundary conditions. This innovative method allows for re-programming the energy dissipation performance in the post-manufacturing regime, offering unprecedented design flexibility and performance adaptability. We demonstrate this concept using shallow arches as building blocks, achieving elastic tailoring by manipulating the horizontal displacement of the supports. A parametric finite element (FE) model is developed in the commercial FE package Abaqus to analyse the nonlinear response of both the single building block and overall mechanical metamaterial sheet. Based on the numerical results, prototype metamaterial sheets with re-programmable energy dissipation are fabricated and experimentally tested. Excellent agreement is observed between the experimental results and numerical simulations. We demonstrate that the energy dissipation performance of the metamaterial can be increased by a magnitude via elastic tailoring Furthermore, we train a surrogate model based on neural network to map the geometry and pre-compression magnitude of the building block arch to the key performance indicators of the nonlinear equilibrium path. We have developed a performance-based inverse design framework that combines an optimization algorithm with the surrogate model. This tool enables the customization of individual building blocks to ensure that the overall metamaterial sheet meets the target performance. This work paves the way for the development of reprogrammable metamaterials through a simple and versatile elastic tailoring approach
Old ties, new relations? Taliban’s victory and violent jihadi groups in Afghanistan
No full textThis paper analyzes the relationship between the Taliban and violent jihadi extremist (VJE) organizations present and/or operating in Afghanistan. Jihadi relations are examined within the analytical frameworks of cooperation, and competition. By conducting an in-depth analysis of the Taliban’s interactions with other violent extremist groups – such as Al-Qaeda, Islamic State-Khorasan Province, and Tehrik-e-Taliban Pakistan – we highlight the nuanced dynamics of old ties forged through years of jihad and their continuation under the Taliban’s regime. We argue that the Taliban’s relationships with these VJEs are shaped by a shared experience of jihad and sustained primarily through ideological, tactical, and strategic cooperation. However, the Taliban’s rise to power and subsequent focus on consolidating control have altered these relationships. While old ties remain resilient – due to personal connections and organizational-level cooperation – new relationships are characterized by economic links, reciprocity, and the Taliban’s efforts to maintain a delicate balance of power as the de facto government. Challenging the Taliban within the network has led to violent conflicts despite ideological commonalities with other VJEs
Defending mental fictionalism: A précis of Mind as Metaphor
No full textMind as Metaphor develops a new approach to the mind. This approach is called mental fictionalism. It claims that our concept of mind is fundamentally metaphorical: we project the “outer world” of human culture (especially spoken and written language) onto the “inner world” of the mind. This inner world is a useful fiction: it does not exist, and yet talking as if it exists allows us to make sense of people’s behavior. The result is a view that sits somewhere between Cartesianism and behaviorism. If Cartesianism takes the metaphor of the inner world metaphor too seriously, behaviorism fails to take it seriously enough, overlooking the crucial role it plays in our concept of mind
Escape by jumps and diffusion by α -stable noise across the barrier in a double well potential
No full textMany physical and chemical phenomena are governed by stochastic escape across potential barriers. The escape time depends on the structure of the noise and the shape of the potential barrier. By applying -stable noise from the = 2 Gaussian noise limit to the < 2 jump processes, we find a continuous transition of the mean escape time from the usual dependence on the height of the barrier for Gaussian noise to a dependence solely on the width of the barrier for -stable noise. We consider the exit problem of a process driven by -stable noise in a double well potential. We study individually the influences of the width and the height of the potential barrier in the escape time, and we show through scalings that the asymptotic laws are described by a universal curve independent of both parameters. When the dependence in the stability parameter is considered, we see that there are two different diffusive regimes in which diffusion is described either by Kramer’s time or by the corresponding asymptotic law for -stable noise. We determine the regions of the noise parameter space in which each regime prevails and exploit this result to construct an anomalous example in which a double well potential exhibit a different diffusion regime in each well for a wide range of parameters
Enhanced Current Limiting Strategy for Droop-Controlled Grid-Connected Inverters During LVRT
No full textRenewable energies mostly need power electronic devices to convert and adapt the energy generated to be available for the loads. Electrical engineers’ major challenge in the energy transition is keeping high energy quality and standards without compromising the stability of electric power systems. Microgrids are seen as an affordable and efficient answer to integrating them with the energy mix.
International grid codes and standards are evolving to regulate the connection and disconnection of inverter-based resources; this thesis proposes a microgrid design to mitigate power cuts and extend the energy supply in developed countries. A hybrid 100 kW commercial inverter droop control is studied and upgraded to allow the inverter to limit the current to ride through voltage drops. This way,
the UPS inverter can seamlessly supply energy to the local loads when the grid collapses.
Grid-connected inverters with LCL filters require active damping to mitigate system resonance, typically achieved via inductor or capacitor current feedback loops. While inductor current feedback is favoured for its inherent current-limiting capability, capacitor current feedback provides a more inductive output impedance, enabling droop control with smaller inductors, improved transient response,
and reduced total harmonic distortion (THD). Limiting the current in grid-connected inverters during voltage drops using the capacitor current as active damping is the main topic of the thesis. A dynamic voltage feedforward using a high-pass filter in
a current control mode helps to reduce the current overshoot in front of a voltage sag. Simulation results show a reduction in the inverter side current from 1.89 p.u. to 1.09 p.u.
A similar strategy has been explored in a hybrid droop-controlled inverter that
can work grid-connected and standalone, only by changing some controller parameters. The strategy introduces double voltage feedforward, ensuring a rapid response to grid voltage sags and effectively limiting overshoot. A small signal model is developed to analyse stability, and the controller is validated through detailed simulations and experimental testing. Results demonstrate significant improvements in transient response, from 99 ms to 21.7 ms in the active power rising time, which is 78.62% faster, and current limitation preventing premature disconnection when the voltage grid drops from 230 V to 30 V; from a overshoot
of 3.8 p.u. to 1.35 p.u. on the inductor current, approaching well-suited for next-generation grid-forming and grid-following inverters.
The inverter and a static switch were manufactured in collaboration with HiT Power. Experimental tests have been conducted in the power lab of the University of Exeter- Penryn campus to verify and validate the theoretical and simulation results. The equipment has been sent and commissioned in Johannesburg, South Africa. Field data from another microgrid project installed in Nepal using the same
100 kW commercial inverter was available before and after the software was updated with the current limitation strategy developed in this research, showing a significant impact on the number of trips in front of power outages and low voltage
situations, from 475 trips on February to 8 in August
Conceptualising research environments using biological niche concepts
No full textSeveral philosophers of science have taken inspiration from biological research on niches to conceptualise scientific practice. We systematise and extend three niche-based theories of scientific practice: conceptual ecology, cognitive niche construction, and scientific niche construction. We argue that research niches are a promising conceptual tool for understanding complex and dynamic research environments, which helps to investigate relevant forms of agency and material and social interdependencies, while also highlighting their historical and dynamic nature. To illustrate this, we develop a six-point framework for conceptualising research niches. Within this framework, research niches incorporate multiple and heterogenous material, social and conceptual factors (multi-dimensionality); research outputs arise, persist and differentiate through interactions between researchers and research niches (processes); researchers actively respond to and construct research niches (agency); research niches enable certain interactions and processes and not others (capability); and research niches are defined in relation to particular entities, such as individual researchers, disciplines, or concepts (relationality), and in relation to goals, such as understanding, solving problems, intervention, or the persistence of concepts or instruments (normativity)
Replicating community dynamics reveals how initial composition shapes the functional outcomes of bacterial communities
No full textBacterial communities play key roles in global biogeochemical cycles, industry, agriculture, human health, and animal husbandry. There is therefore great interest in understanding bacterial community dynamics so that they can be controlled and engineered to optimise ecosystem services. We assess the reproducibility and predictability of bacterial community dynamics by creating a frozen archive of hundreds of naturally-occurring bacterial communities that we repeatedly revive and track in a standardised, complex resource environment. Replicate communities follow reproducible trajectories and the community dynamics closely map to ecosystem functioning. However, even under standardised conditions, the communities exhibit tipping-points, where small differences in initial community composition create divergent compositional and functional outcomes. The predictability of community trajectories therefore requires detailed knowledge of rugged compositional landscapes where ecosystem properties are not the inevitable result of prevailing environmental conditions but can be tilted toward different outcomes depending on the initial community composition. Our results shed light on the relationship between composition and function, opening new avenues to understand the feasibility and limitations of function prediction in complex microbial communities
What can a corpus tell us about school writing? Findings, challenges, and future directions
No full textMuch child corpus research has focused on the language of school writing. In the first part of this paper, I discuss what this work can contribute to theory and educational practice. I then look in detail at a recent large-scale study conducted in England in order to illustrate, and critically discuss, key corpus methods and findings. In the final part of the paper, I discuss prospects for future work, focusing in particular on the issue of defining and analysing educational text types. I look at why this task is so central to valid research on school writing and why it has been so problematic, despite decades of attention from researchers. I then introduce and discuss the prospects for one promising approach to such analysis
Tree species hyperdominance and rarity in the South American Cerrado
No full textThe South American Cerrado, the largest savanna of the Americas and the world's most tree-biodiverse, is critically endangered, with just 8% protected and more than half deforested. However, the extent of its tree diversity and abundance remains poorly quantified. Using a unique biome-wide eco-floristic dataset with 222 one-hectare plots, we estimate the Cerrado has ~1605 tree species and has extreme hyperdominance, with fewer than 2% (30 species) accounting for half of all trees. A single family, Vochysiaceae, represents 17% of all trees, and the most abundant species, Qualea parviflora, accounts for 1 in 14 trees. In contrast, 63% of the species are rare, with fewer than 100 trees across all plots. Remote sensing and spatial modelling suggest the Cerrado has lost 24 billion trees since 1985, equivalent to three times the Earth's human population. We estimate up to 800 tree species may remain undetected in Cerrado ecosystems and could face extinction in a few decades due to deforestation. This hyperdominance parallels patterns in Amazonian forests and highlights risks both biomes face for species loss due to fragmentation, deforestation, and land-use change. Our findings highlight the Cerrado’s critical but undervalued role in global biodiversity, its vulnerabilities, and the urgent need for conservation to avoid irreversible species and biome loss
Social Response to Environmental Extremes: A Computational Approach
No full textExtreme weather events and environmental hazards are increasing in frequency, intensity, and coverage, posing significant global risks to human health, safety, and infrastructure. Traditional environmental monitoring methods often fail to capture the real-time social impacts of these events due to their limited spatial and temporal resolution. Social sensing, the systematic collection and analysis of social media data to observe real-world events, uniquely captures subjective, human-centred responses to environmental phenomena, offering insights into public sentiment and emotional impacts that traditional methods cannot provide. However, existing approaches often stop at detection, overlooking specific impacts on communities whilst lacking the contextual nuances necessary to understand the event properly.
This thesis explores how social sensing can be used to investigate the specific impacts of natural hazards, monitoring and analysing public sentiment nuances within the impacted areas. Three initial case studies were carried out, demonstrating the capacity for social sensing to be extended beyond simple event detection. The first study examines public responses to heatwaves in the UK, the US, and Australia, revealing cultural and regional differences in how populations perceive and react to extreme heat. Despite experiencing milder conditions, the UK showed heightened concern about individual well-being, whilst Australian and US users focused more on environmental impacts. The second case study evaluates how social media and government data can be used to investigate the 2018 Kerala floods, highlighting social sensing’s potential to provide timely and accurate information for disaster relief, especially in less economically developed regions. The third case study focuses on the 2018 Kīlauea volcanic eruption, where social media was used to track changes in community sentiment and responses to evolving volcanic activity.
Through these case studies, the limitations of general-purpose sentiment analysis models became evident, which struggle to capture context-sensitive linguistic nuances, resulting in inaccurate or unspecific conclusions. This led to the creation of CIDER (Context Informed Dictionary and sEmantic Reasoner), a Python library designed to infer the valence of sentiment-laden terms within specific discussions before providing sentiment scores for individual texts. This novel tool enhances sentiment analysis by incorporating domain-specific context, making it particularly effective in interpreting the language used in social media during environmental hazards. Beyond sentiment, CIDER also creates linguistic classifiers for additional dimensions, such as gender, temperature, and "northernness/southernness", further improving our understanding of language variation on social media. When applied to weather-related social media content in the UK, the tool outperformed existing general-purpose sentiment analysis methods, revealing complex emotional responses to various weather conditions and demonstrating the importance of context-sensitive sentiment analysis in understanding public reactions to environmental changes.
This research provides valuable insights for policymakers, disaster risk managers, and environmental communicators by addressing key limitations in social sensing methodologies. Integrating CIDER into the social sensing pipeline improves the capture of real-time, granular public sentiment data, enhancing situational awareness during environmental crises. This not only advances disaster monitoring and impact-based forecasting but also contributes to more effective emergency response and preparedness strategies. Additionally, exploring linguistic variations in social media content related to environmental phenomena offers a deeper understanding of regional and cultural differences in public perception, which is essential for effective communication and policy-making. Overall, this thesis demonstrates the value of combining social sensing with advanced linguistic analysis to provide actionable insights during environmental crises, helping build more resilient societies in the face of increasing environmental risks