A CFD assisted control system design for supercritical water cooled reactor

In this study, the methodology to construct a control system based on computational fluid dynamics (CFD) simulations is developed for supercritical water cooled reactor (SCWR). The CFD model using Reynolds Stress Model (RSM) and k-w SST model is validated with the experimental cases of steady state and vertically up flowing supercritical water in circular tubes for normal heat transfer and deteriorated heat transfer (DHT) cases. This model is extended to simulate the transient thermal-hydraulic behaviour of supercritical fluid flow and heat transfer, and the results are also compared with the 1-D numerical model, THRUST. The DHT phenomenon is investigated using the turbulence kinetic energy (TKE) and velocity distribution and its effect on heat transfer. A correlation is reported between the TKE and velocity profiles and heat transfer phenomenon at supercritical condition. The non-dimensional buoyancy and acceleration parameters are also used to predict the occurrence of DHT in the supercritical water flow in circular tubes. In the process of developing a control system for Canadian version of SCWR, system identification method is used to develop the linear dynamic models based on non-linear CFD simulations. Considering the strong cross-coupling between the inputs and outputs of the SCWR, multiple input and multiple output (MIMO) system is decoupled and is converted to several single input and single output (SISO) systems using pre-compensator. Based on the decoupled SISO systems, loop compensator is developed for the control and stability of the reactor

Segurança e privacidade em terminologia de rede

Security and Privacy are now at the forefront of modern concerns, and drive a significant part of the debate on digital society. One particular aspect that holds significant bearing in these two topics is the naming of resources in the network, because it directly impacts how networks work, but also affects how security mechanisms are implemented and what are the privacy implications of metadata disclosure. This issue is further exacerbated by interoperability mechanisms that imply this information is increasingly available regardless of the intended scope. This work focuses on the implications of naming with regards to security and privacy in namespaces used in network protocols. In particular on the imple- mentation of solutions that provide additional security through naming policies or increase privacy. To achieve this, different techniques are used to either embed security information in existing namespaces or to minimise privacy ex- posure. The former allows bootstraping secure transport protocols on top of insecure discovery protocols, while the later introduces privacy policies as part of name assignment and resolution. The main vehicle for implementation of these solutions are general purpose protocols and services, however there is a strong parallel with ongoing re- search topics that leverage name resolution systems for interoperability such as the Internet of Things (IoT) and Information Centric Networks (ICN), where these approaches are also applicable.Segurança e Privacidade são dois topicos que marcam a agenda na discus- são sobre a sociedade digital. Um aspecto particularmente subtil nesta dis- cussão é a forma como atribuímos nomes a recursos na rede, uma escolha com consequências práticas no funcionamento dos diferentes protocols de rede, na forma como se implementam diferentes mecanismos de segurança e na privacidade das várias partes envolvidas. Este problema torna-se ainda mais significativo quando se considera que, para promover a interoperabili- dade entre diferentes redes, mecanismos autónomos tornam esta informação acessível em contextos que vão para lá do que era pretendido. Esta tese foca-se nas consequências de diferentes políticas de atribuição de nomes no contexto de diferentes protocols de rede, para efeitos de segurança e privacidade. Com base no estudo deste problema, são propostas soluções que, através de diferentes políticas de atribuição de nomes, permitem introdu- zir mecanismos de segurança adicionais ou mitigar problemas de privacidade em diferentes protocolos. Isto resulta na implementação de mecanismos de segurança sobre protocolos de descoberta inseguros, assim como na intro- dução de mecanismos de atribuiçao e resolução de nomes que se focam na protecçao da privacidade. O principal veículo para a implementação destas soluções é através de ser- viços e protocolos de rede de uso geral. No entanto, a aplicabilidade destas soluções extende-se também a outros tópicos de investigação que recorrem a mecanismos de resolução de nomes para implementar soluções de intero- perabilidade, nomedamente a Internet das Coisas (IoT) e redes centradas na informação (ICN).Programa Doutoral em Informátic

A Framework for anonymous background data delivery and feedback

The current state of the industry’s methods of collecting background data reflecting diagnostic and usage information are often opaque and require users to place a lot of trust in the entity receiving the data. For vendors, having a centralized database of potentially sensitive data is a privacy protection headache and a potential liability should a breach of that database occur. Unfortunately, high profile privacy failures are not uncommon, so many individuals and companies are understandably skeptical and choose not to contribute any information. It is a shame, since the data could be used for improving reliability, or getting stronger security, or for valuable academic research into real-world usage patterns. We propose, implement and evaluate a framework for non-realtime anonymous data collection, aggregation for analysis, and feedback. Departing from the usual “trusted core” approach, we aim to maintain reporters’ anonymity even if the centralized part of the system is compromised. We design a peer-to-peer mix network and its protocol that are tuned to the properties of background diagnostic traffic. Our system delivers data to a centralized repository while maintaining (i) source anonymity, (ii) privacy in transit, and (iii) the ability to provide analysis feedback back to the source. By removing the core’s ability to identify the source of data and to track users over time, we drastically reduce its attractiveness as a potential attack target and allow vendors to make concrete and verifiable privacy and anonymity claims

Towards a Framework for DHT Distributed Computing

Distributed Hash Tables (DHTs) are protocols and frameworks used by peer-to-peer (P2P) systems. They are used as the organizational backbone for many P2P file-sharing systems due to their scalability, fault-tolerance, and load-balancing properties. These same properties are highly desirable in a distributed computing environment, especially one that wants to use heterogeneous components. We show that DHTs can be used not only as the framework to build a P2P file-sharing service, but as a P2P distributed computing platform. We propose creating a P2P distributed computing framework using distributed hash tables, based on our prototype system ChordReduce. This framework would make it simple and efficient for developers to create their own distributed computing applications. Unlike Hadoop and similar MapReduce frameworks, our framework can be used both in both the context of a datacenter or as part of a P2P computing platform. This opens up new possibilities for building platforms to distributed computing problems. One advantage our system will have is an autonomous load-balancing mechanism. Nodes will be able to independently acquire work from other nodes in the network, rather than sitting idle. More powerful nodes in the network will be able use the mechanism to acquire more work, exploiting the heterogeneity of the network. By utilizing the load-balancing algorithm, a datacenter could easily leverage additional P2P resources at runtime on an as needed basis. Our framework will allow MapReduce-like or distributed machine learning platforms to be easily deployed in a greater variety of contexts

A Survey on Content Retrieval on the Decentralised Web

The control, governance, and management of the web have become increasingly centralised, resulting in security, privacy, and censorship concerns. Decentralised initiatives have emerged to address these issues, beginning with decentralised file systems. These systems have gained popularity, with major platforms serving millions of content requests daily. Complementing the file systems are decentralised search engines and name registry infrastructures, together forming the basis of a decentralised web . This survey paper analyses research trends and emerging technologies for content retrieval on the decentralised web, encompassing both academic literature and industrial projects. Several challenges hinder the realisation of a fully decentralised web. Achieving comparable performance to centralised systems without compromising decentralisation is a key challenge. Hybrid infrastructures, blending centralised components with verifiability mechanisms, show promise to improve decentralised initiatives. While decentralised file systems have seen more mature deployments, they still face challenges such as usability, performance, privacy, and content moderation. Integrating these systems with decentralised name-registries offers a potential for improved usability with human-readable and persistent names for content. Further research is needed to address security concerns in decentralised name-registries and enhance governance and crypto-economic incentive mechanisms

A Survey on Content Retrieval on the Decentralised Web

The control, governance, and management of the web have become increasingly centralised, resulting in security, privacy, and censorship concerns. Decentralised initiatives have emerged to address these issues, beginning with decentralised file systems. These systems have gained popularity, with major platforms serving millions of content requests daily. Complementing the file systems are decentralised search engines and name registry infrastructures, together forming the basis of a decentralised web. This survey paper analyses research trends and emerging technologies for content retrieval on the decentralised web, encompassing both academic literature and industrial projects. Several challenges hinder the realisation of a fully decentralised web. Achieving comparable performance to centralised systems without compromising decentralisation is a key challenge. Hybrid infrastructures, blending centralised components with verifiability mechanisms, show promise to improve decentralised initiatives. While decentralised file systems have seen more mature deployments, they still face challenges such as usability, performance, privacy, and content moderation. Integrating these systems with decentralised name-registries offers a potential for improved usability with human-readable and persistent names for content. Further research is needed to address security concerns in decentralised name-registries and enhance governance and crypto-economic incentive mechanisms

Ditto: Towards Decentralised Similarity Search for Web3 Services

The Web has become an integral part of life, and over the past decade, it has become increasingly centralised, leading to a number of challenges such as censorship and control, particularly in search engines. Recently, the paradigm of the decentralised Web (DWeb), or Web3, has emerged, which aims to provide decentralised alternatives to current systems with decentralised control, transparency, and openness. In this paper we introduce Ditto, a decentralised search mechanism for DWeb content, based on similarity search. Ditto uses locality sensitive hashing (LSH) to extract similarity signatures and records from content, which are stored on a decentralised index on top of a distributed hash table (DHT). Ditto uniquely supports numerous underlying content networks and types, and supports various use-cases, including keyword-search. Our evaluation shows that our system is feasible and that our search quality, delay, and overhead are comparable to those currently accepted by users of DWeb and search systems

Experimental and numerical studies on oil spilling from damaged oil tankers

It is well understood that the spilled oil from damaged oil tankers poses a severe threat to the marine environment. Although great efforts have been devoted to studying the oil spilling from damaged oil tankers, especially double hull tanks (DHTs), the majority is subjected to an ideal condition (e.g., fixed tanks in still water; simple damage conditions) and adopts hydrostatic theories or quasi-steady models with over-simplified assumptions on data analysis or analytical prediction. These conditions or assumptions may not stand in the complex dynamic spilling process in the real spilling accident. This study brings a step further on the knowledge of oil spilling from a damaged tank by combining experimental and numerical investigations, with a focus on the dynamic spilling process from damaged oil tankers which is either fixed or subjected to motion, which have not been systematically investigated. In the experimental investigation, the submerged oil spilling from DHTs under different accidental scenarios including grounding and collision is studied. Two new sets of laboratory tests are carried out, where the damaged tank is fixed in still water. In the first set, the axial offset between the internal and the external holes on two hulls of the grounded DHT is considered to widen the scope of damage conditions which the tanker may suffer from during grounding accidents. Although all cases in this set are subjected to the same hydrostatic conditions, completely different dynamic spilling processes are observed. In the second set, the initial water thickness inside the ballast tank of the collided DHT is considered. This aims to represent the real scenarios that the external hull is generally damaged prior to the internal hull and, therefore the ballast space is partially filled by the water flowing from the surrounding environment before the internal hull is damaged. These experiments do not only advance the state of the art of the experimental study in this field, but also provide a reference for validating the numerical models developed in this study. Based on the experimental data, the correlation analysis for the discharge through the internal hole by using quasi-steady Bernoulli’s equation is presented, contributing to the development of an improved analytical model for predicting the oil spilling from damaged oil tankers. The numerical study is carried out using a numerical model developed in OpenFOAM framework, where the VOF is applied to deal with the air-oil-water multiphase flow. This model enables the users: (1) to consider air, oil and water three phases of fluid and their interaction with solid tanker hull using dynamic mesh technologies; (2) to model turbulence associated with the oil spilling process using various available turbulent models; and (3) to investigate the effects of the compressibility of the fluid. The oil spilling from damaged DHTs is simulated and validated by the experimental data. Intensive investigations are carried out to clarify uncertainties in existing numerical modelling of the oil spilling from damaged DHTs. These include (1) the associated turbulence behaviours and selecting an appropriate approach to turbulence modelling; (2) the role of fluid compressibility during the oil spilling; and (3) the effect of tank motion on the oil spilling process. For the turbulence modelling, various approaches to model the turbulence, including the large eddy simulation (LES), direct numerical simulation (DNS) and the Reynolds average Navier-Stokes equation (RANS) with different turbulence models are attempted. It is concluded that the oil spilling from DHTs is more sensitive to the turbulence modelling than that from SHTs. For DHT cases, the effective Reynolds number (Re) considering both oil outflow and water inflow is suggested to classify the significance of the turbulence and to correspondingly select the appropriate turbulence model. The investigation on the role of the air compressibility in the oil spilling from damaged DHTs reveals that the air compressibility may be considerable in a small temporal-spatial scale (e.g., jet-jet and jet-structure impact pressure), but plays an insignificant role in the macroscopic process of the oil spilling (e.g., spilling discharge and volume). In order to approach the spilling phenomena in the more realistic environment, a systematic numerical study is carried out to investigate the effect of the periodic ship motion on the oil spilling from the damaged tank. Different tank designs (i.e., SHTs and DHTs), accidental scenarios (i.e., grounding and collision) and tank motion parameters (i.e., types, frequencies and amplitude) are considered. The result indicates that the tank motion does not only cause a periodic oscillation of the oil/water flow through the broken hole, but also induces a second long-duration stage of spilling after a quasi-hydrostatic-equilibrium condition occurs, resulting in the more significant amount of spilled oil. By using both the experimental data and numerical results produced in this research, an improved prediction model for oil spilling from damaged DHTs in still is formulated. This model considers the case-dependent hydrodynamic interaction between the oil and water jet flows inside the ballast tank and its effect on the spilling process. The result using the improved model is compared with the numerical result indicating its superiority over the existing model