Numerical Simulations of Vortex-Induced Vibrations in Marine Riser Pipes and Circular Cylinders

This thesis presents and discusses the results of two distinct investigations. The first is a Direct Numerical Simulation investigation of prescribed transverse oscillations of a two-dimensional circular cylinder in a fluid flow of Reynolds number 100. The second involves the numerical simulation of the Vortex-Induced Vibrations of long riser pipes in the sub-critical Reynolds number regime, using a strip theory code that employed a Large Eddy Simulation model. Before commencing the long riser investigation the code was thoroughly benchmarked against data from appropriate prescribed cross-stream oscillation experiments; the results of that benchmarking work are also presented in this thesis. The principal objectives of the low Reynolds number Direct Numerical Simulations were to use prescribed oscillations to explain phenomena that have been observed in free oscillation experiments, and also to investigate the different levels and types of synchronisation that exist between the cylinder and its wake in a given amplitude-frequency domain. It was found that the contour of zero hydrodynamic excitation closely matches the response envelopes reported from experimental and numerical investigations of the transverse Vortex-Induced Vibrations of lightly damped cylinders. Furthermore, the zero contour inferred that the maximum amplitude of free cross-stream vibration is 0.56 cylinder diameters in Reynolds number 100 flow, and the shape of the contour confirmed the existence of hystereses at low and high reduced velocities in free vibration. The present study also revealed two new coalesced shedding modes, here labelled C∗(2S) and C∗(P+S), that differ in their formation mechanism from the known C(2S) mode. In the benchmarking of the Large Eddy Simulation code at sub-critical Reynolds numbers a clear trend was observed in which the prediction of the flow physics was altered by changing the level of sub-grid scale turbulence dissipation in the code’s Smagorinsky turbulence dissipation model. It was found that by carefully tuning the level of turbulent dissipation the code could deliver very good predictions of the key physical quantities important in Vortex-Induced Vibrations; namely the component of the lift coefficient at the oscillation frequency and the phase angle by which this lift coefficient leads the cylinder displacement. Regarding the simulations of the Vortex-Induced Vibrations of a long model riser, it has been shown that responses in high modes of vibration at harmonics of the displacement-dominant response frequency (at 3 and 5 times the cross-stream displacement dominant frequency in the cross-stream direction and at 2 and 3 times the in-line displacement dominant frequency in the in-line direction) can be important with regard to the curvature variation along the riser, and can therefore contribute very significantly to the overall fatigue damage rate experienced by a riser undergoing VIV. Comparisons with experimental data in terms of maximum and mean displacements and modes and frequencies of vibration, were generally good for both uniform and linearly sheared flow profiles. Furthermore, it was observed that the majority of the responses involved travelling waves, even when the flow profile was uniform

Shared Arrangements: practical inter-query sharing for streaming dataflows

Current systems for data-parallel, incremental processing and view maintenance over high-rate streams isolate the execution of independent queries. This creates unwanted redundancy and overhead in the presence of concurrent incrementally maintained queries: each query must independently maintain the same indexed state over the same input streams, and new queries must build this state from scratch before they can begin to emit their first results. This paper introduces shared arrangements: indexed views of maintained state that allow concurrent queries to reuse the same in-memory state without compromising data-parallel performance and scaling. We implement shared arrangements in a modern stream processor and show order-of-magnitude improvements in query response time and resource consumption for interactive queries against high-throughput streams, while also significantly improving performance in other domains including business analytics, graph processing, and program analysis

Book review: war in 140 characters: how social media is reshaping conflict in the twenty-first century by David Patrikarakos

In War in 140 Characters: How Social Media is Reshaping Conflict in the Twenty-First Century, David Patrikarakos explores how social media is shifting the power balance from governments and institutions towards individuals and networks and the impact this is having on contemporary warfare. Relating the personal stories of individuals caught up in conflict, this book underscores the centrality of narratives and storytelling to understanding the changing face of war today, writes Madeline McSherry

Book review: at home in two countries: the past and future of dual citizenship by Peter J. Spiro

Dual citizenship is a concept that has greatly transformed since the term’s first use in the early 1900s. As misunderstandings nonetheless abound about its legal status and implications, Peter J. Spiro traces the historical emergence of dual citizenship and offers a defence of its value in the twenty-first century in At Home in Two Countries: The Past and Future of Dual Citizenship. While she would welcome more reflection on dual citizenship outside of the book’s US focus, Madeline McSherry finds this a comprehensive, thought-provoking and vital read that will be of interest to policymakers, lawyers and dual nationals alike

Book review: lovecidal: walking with the disappeared by Trinh T. Minh-ha

In Lovecidal: Walking with the Disappeared, the feminist filmmaker and postcolonial theorist Trinh T. Minh-ha offers a lyrical and poetic meditation on a wounded world mired in perpetual conflict. This artistic book, defying conventional categorisation, draws attention to everyday gestures of dissent by the seemingly powerless that invite us to see love as a creative, transformative act that has the capacity to challenge our conversations on militarism, violence and the meaning of ‘victory’, writes Madeline McSherry

Written under September Skies for a Quiet Hero of Our Time: A Tribute to the Honorable Walter H. Rice

Special issue: Intellectual Property and Technolog