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

LES of flow through and around a finite patch of thin plates

Large eddy simulations (LESs) are performed for turbulent flow through and around a porous patch of thin vertical plates at a plate Reynolds number of Rep=5,800. The plates are arranged in a staggered pattern, presenting an elliptical planform and mimicking streamwise‐oriented blades of emergent vegetation. The immersed boundary method is employed to explicitly resolve the interaction between flow and plates. Three flow cases, each with a different number of plates within the same planform area, that is, different patch density, are studied. The Reynolds number based on freestream velocity and plate length is the same in all cases. Inspection of the distribution of velocity and vorticity in the horizontal plane reveals that downstream plates are significantly impacted by the wakes from upstream plates. It is therefore proposed that the plates can be divided into two groups based on the local flow characteristics, which are a function of position within the patch: a free group and a wake group. This classification is subsequently used in the quantitative analysis of boundary layer development and drag force at plate scale. The thickness and character of the simulated boundary layers on the plates differ significantly from predictions based on analytical or empirical relationships, which is due to wake effects and the finite length of the plates. The simulations demonstrate the so‐called sheltering effect; that is, the drag forces acting on downstream plates (in the wake group) are significantly lower than those acting on upstream plates, a result of the lower approach flow speed. Although the front‐area‐to‐lateral‐area ratio of the plates is low (1/40), pressure drag is observed to be larger than friction drag for each plate. The ratio of pressure drag to the total drag at patch scale shows only very little dependence on the plate density of the patch

Free surface flow over square bars at intermediate relative submergence

Results from large-eddy simulations and complementary flume experiments of turbulent open channel flows over bed-mounted square bars at intermediate submergence are presented. Scenarios with two bar spacings, corresponding to transitional and k-type roughness, and three flow rates, are investigated. Good agreement is observed between the simulations and the experiments in terms of mean free surface elevations and mean streamwise velocities. Contours of simulated time-averaged streamwise, streamfunction and turbulent kinetic energy are presented and these reveal the effect of the roughness geometry on the water surface response. The analysis of the vertical distribution of the streamwise velocity shows that in the lowest submergence cases no logarithmic layer is present, whereas in the higher submergence cases some evidence of such a layer is observed. For several of the flows moderate to significant water surface deformations are observed, including weak and/or undular hydraulic jumps which affect significantly to the overall streamwise momentum balance. Reynolds shear stress, form-induced stress and form drag are analysed with reference to the momentum balance to assess their contributions to the total hydraulic resistance of these flows. The results show that form-induced stresses are dominant at the water surface and can contribute significantly to the overall drag, but the total resistance in all cases is dominated by form drag due to the presence of the bars

Large eddy simulation of free-surface flows

This paper introduces and discusses numerical methods for free-surface flow simulations and applies a large eddy simulation (LES) based free-surface-resolved CFD method to a couple of flows of hydraulic engineering interest. The advantages, disadvantages and limitations of the various methods are discussed. The review prioritises interface capturing methods over interface tracking methods, as these have shown themselves to be more generally applicable to viscous flows of practical engineering interest, particularly when complex and rapidly changing surface topologies are encountered. Then, a LES solver that employs the level set method to capture free-surface deformation in 3-D flows is presented, as are results from two example calculations that concern complex low submergence turbulent flows over idealised roughness elements and bluff bodies. The results show that the method is capable of predicting very complex flows that are characterised by strong interactions between the bulk flow and the free-surface, and permits the identification of turbulent events and structures that would be very difficult to measure experimentally

Effect of floodplain obstructions on the discharge conveyance capacity of compound channels

Results of an experimental study into steady uniform flows in compound open channels with cylindrical obstructions designed to mimic emergent vegetation is presented. Two configurations—fully covered floodplain and one-line obstructions—are considered, and the hydraulic properties are compared to those of a smooth, unobstructed compound channel. Particular attention is given to the effect of obstruction (i.e., vegetation) density on the rating curve, drag coefficients, and spanwise profiles of streamwise velocity. Flow resistance is estimated using an established approach, and the results are in agreement with other experimental studies. It was shown that the obstruction configuration significantly influences the flow velocity in the main channel, and in the case of one-line obstructions the floodplain velocity is higher than for an unobstructed channel for a given flow rate. Spanwise velocity profiles exhibit markedly different characters in the one-line and fully covered configurations

Secondary currents and turbulence over a non-uniformly roughened open-channel bed

Large-eddy simulations (LES) of the flow over a non-uniformly roughened channel bed are carried out to study the effect of non-uniform bed roughness on turbulence driven secondary currents and turbulence statistics. The channel bed is comprised of alternating rough and smooth strips, the width of which corresponds to the water depth. The Reynolds number based on hydraulic radius and bulk velocity is 34,000. The LES are successfully validated using experimental data. The secondary flow and bed roughness have a significant effect on the streamwise velocity and second order turbulence statistics. Turbulence is enhanced over rough strips and suppressed over smooth strips. Significant lateral momentum transfer takes place due to both advection and turbulence. The bed shear stresses over the smooth strips are approximately four times less than over the rough strips a result of near bed low momentum fluid being transported from the rough strips to the smooth strips and high momentum fluid being convected from the surface towards the bed. The most significant terms in the streamwise momentum equation are quantified and discussed with regard to momentum transfer

Friction factor decomposition for rough-wall flows : theoretical background and application to open-channel flows

Financial support was provided by the EPSRC/UK project ‘Bed friction in rough-bed free-surface flows: a theoretical framework, roughness regimes, and quantification’ (grants EP/K041088/1 and EP/K04116/1). I.M. acknowledges the support of the Australian Research Council (grant FL120100017). The large-eddy simulations were carried out at Cardiff University’s high performance computer, which is part of the Supercomputing Wales project. Useful and stimulating discussions with M. Fletcher (Arup), P. Samuels (HR Wallingford), T. Schlicke (Scottish Environment Protection Agency) and J. Wicks (Jacobs) have been instrumental for this project and are gratefully acknowledged. The editor and three reviewers provided insightful comments and helpful suggestions that have been gratefully incorporated in the final version.Peer reviewedPublisher PD

New Zealand Nurses’ Perceptions of Spirituality and Spiritual Care: Qualitative Findings from a National Survey

This paper presents the qualitative findings from the first national survey of New Zealand nurses’ views on spirituality and spiritual care. The importance of spirituality as a core aspect of holistic nursing care is gaining momentum. Little is currently known about New Zealand nurses’ understandings, perceptions and experience of spirituality. Design: A descriptive online survey. Method: A random sample of 2000 individuals resident in New Zealand whose occupation on the New Zealand electoral roll suggested nursing was their current or past occupation were invited via postcard to participate in an online survey. This paper reports on the free response section of the survey. Findings: Overall, 472 invitees responded (24.1%). From the respondents, 63% completed at least one of the optional free response sections. Thematic analysis generated three metathemes: ‘The role of spirituality in nursing practice’, ‘Enabling best practice’, and ‘Creating a supportive culture’. Conclusions: Spirituality was predominantly valued as a core aspect of holistic nursing care. However, clarity is needed surrounding what constitutes spiritual care and how this intersects with professional responsibilities and boundaries. Participants’ insights suggest a focus on improving the consistency and quality of spiritual care by fostering inter-professional collaboration, and improved provision of resources and educational opportunities

El Puchero

Dr. McSherry was a U.S. Marine surgeon during the Mexican War who penned letters and hurried notes as his regiment marched and sometimes during the din of battle. According to the author he wrote about his experiences with no intention of sharing his correspondece with the world. His friend, David Holmes Conrad, to whom the letters were addressed, recommended their publication. The author assented on condition that Mr. Conrad make the story complete by describing, from official documents, the battles of Veracruz and Cerro Gordo.https://rio.tamiu.edu/rarebooks/1002/thumbnail.jp