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Remotely piloted aircrafts (UAVs) and Structure-from-Motion photogrammetry (SfM) have become a widely used approach for producing high-resolution topographical measurements of river systems. This approach has the benefit of capturing data over large spatial scales while requiring little time in the field. In small, forested rivers, the dense canopy has hindered the use of remote sensing techniques, limiting topographic data collection to more time-consuming and lower-resolution methods. This complicates monitoring the response of these systems to individual floods, as in many situations there is not enough time to complete more time-consuming surveys between events.In this study, we pilot the use of sub-canopy UAV surveys (flown at 1-3 m altitude) to monitor the response of a small mountain stream (1-3 m wide) in British Columbia to a sediment pulse generated by the removal of an upstream culvert. Using eleven surveys flown over a three-year period, we track the downstream propagation of the pulse and the subsequent responses in bed topography and roughness along the 240 m reach. We observe a “build-and-carve” response of the channel, where some channel segments aggrade during the first floods after pulse generation, whereas others undergo little morphologic activity. In subsequent floods, these aggradational segments rework through the carving of well-defined channels that release this aggraded sediment downstream. These “build-and-carve” segments serve as temporary storage reservoirs that caused the pulse to fragment as it progressed downstream. The locations of these storage reservoirs were set by the initial channel morphology and the movement of in-stream wood and debris. This study highlights the importance of temporary sediment storage reservoirs for fluvial morphodynamics and provides some insights and suggestions for the future monitoring of forested river systems using sub-canopy drone surveys.

This repository contains data and one Python Jupyter Notebook. The Jupyter Notebook loads data located in the .zip file (PR Regime Diagram Data) and goes through a series of caluations based on Equation 1 to produce Figure 3 of the associated manuscript. Data included here: The processed DEMs (data and figure) shown in FIgure 2 of the associated manuscript The trended profiles and water surface profiles showin in Figure 2 of the associated manuscript Channel stationing and width data for Phases 1 and 2 of the physical experiment reported in the associated manuscript. 2 videos which show the experimental flume at approximate topographic steady state for elapse times 960 and 1752 minutes.

Introduction The feasibility of arteriovenous fistula (AVF) creation in pediatric patients has long been documented, but few studies have evaluated the forms and long-term outcomes. The aim of this article is to highlight the types, techniques and outcomes of AVFs in pediatric age group. Methods This is multi-center, retrospective, single cohort study, including all the cases of pediatric (less than 18 year old) cases underwent AVF creation during four years (2015–2019). The following data were obtained from the patients' medical records and analyzed; socio-demographics, etiology of renal failure (RF), history of dialysis and transplantation, type and site of AVF, the outcomes and complications. Results The study included 108 patients, 89 patients (82.4%) were female. The mean age was 13 years. The most common cause of RF was the urological causes which were found in 34 cases (31.5%), followed by nephrotic syndrome (32 cases, 29.6%), glomerulonephritis (27 cases, 25%), and polycystic kidney disease (12 cases, 11.1%). The decision for choosing access site was performed only by clinical examination in 96 patients (88.9%) while duplex ultrasound was requested for 12 cases (11.1%). The operation was done under local anesthesia in 81 cases (75%) and general anesthesia in 27 cases (25%). The procedure was performed in the wrist in 58 cases (53.7%) and in cubital fossa in 50 cases (46.2%). The most common early complication was hematoma (12 cases, 11.1%), followed by ecchymosis (10 cases, 9.3%), infection (8, 7.4%), seroma (4, 3.7%) and thrombosis (3, 2.8%). The one-year primary patency rate was found in 95 patients (88%) and two-year patency rate in 86 patients (79.6%). Conclusion Native AVF in pediatric is the first choice dialysis access even in pediatric population. Radiocephalic in the non-dominant hand is the most preferred site., Highlights • The feasibility of AVF creation in pediatric patients has long been documented. • Few studies have evaluated the forms and long-term outcomes. • Physical examination is an effective method for determining the proper site for access creation. • The aim of this article is to highlight the types, techniques and outcome of AVFs in pediatric age group.

Channel morphology of forested, mountain streams in glaciated landscapes is regulated by a complex suite of processes, and remains difficult to predict. Here, we analyze models of channel geometry against a comprehensive field dataset collected in two previously glaciated basins in Haida Gwaii, B.C., to explore the influence of variable hillslope–channel coupling imposed by the glacial legacy on channel form. Our objective is to better understand the relation between hillslope–channel coupling and stream character within glaciated basins. We find that the glacial legacy on landscape structure is characterized by relatively large spatial variation in hillslope–channel coupling. Spatial differences in coupling influence the frequency and magnitude of coarse sediment and woody material delivery to the channel network. Analyses using a model for channel gradient and multiple models for width and depth show that hillslope–channel coupling and high wood loading induce deviations from standard downstream predictions for all three variables in the study basins. Examination of model residuals using Boosted Regression Trees and nine additional channel variables indicates that ~10 to ~40% of residual variance can be explained by logjam variables, ~15–40% by the degree of hillslope–channel coupling, and 10–20% by proximity to slope failures. These results indicate that channel classification systems incorporating hillslope–channel coupling, and, indirectly, the catchment glacial legacy, may present a more complete understanding of mountain channels. From these results, we propose a conceptual framework which describes the linkages between landscape history, hillslope–channel coupling, and channel form. © 2018 John Wiley & Sons, Ltd.

General surveillance is increasingly seen as a cost-effective way to obtain monitoring data about pest and disease status. Different forms of general surveillance are already making a considerable contribution to Australia’s biosecurity system. However, such initiatives can be challenging to instigate and maintain due to interrelated social, institutional, organisational, ecological and infrastructure dimensions. Much of the literature dealing with general surveillance focuses on certain aspects of general surveillance only, such as data management, community engagement or the development of smart phone applications. In addition, there is considerable fragmentation between sectors (plant, animal, marine and environment) in their pursuits to progress general surveillance with limited sharing of lessons learned between them. In response to these challenges this report explores general surveillance from a holistic systems thinking perspective by compiling lessons learned from recent literature to capture key considerations for the different system components and the dynamics between them.

Los troncos en cauces de cuencas forestadas son elementos clave (e intrínsecos) que contribuyen a configurar la morfología de estos ríos. A menudo, estos troncos se acumulan formando barreras que, además de modificar la morfología del río a nivel local, afectan a todo un tramo de río por el impacto que tienen sobre el transporte sólido. Las barreras de troncos o presas de madera suponen un freno para el transporte de fondo que consecuentemente se acumula aguas arriba de ellas. Esta acumulación conlleva asimismo una disminución de la disponibilidad de sedimento hacia tramos inferiores que conduce a la incisión. Sin embargo, está aceptado que, en general, las presas de madera contribuyen a la acumulación de sedimento en los tramos de ríos donde se ubican. Es por este motivo por el que la introducción de troncos y presas de madera en cauces es, desde tiempos recientes, una herramienta para la restauración fluvial. Este trabajo presenta un modelo matemático simplificado para cuantificar, de forma aproximada, cuánto sedimento se acumula en un tramo de río con presas de madera. El modelo desarrollado se ha aplicado en un río de montaña de la Columbia Británica (Canadá).

This preprint provides a design framework for river restoration and enhancement efforts. The preprint will be of interest to watershed stakeholder groups, jurisdictional permitting staff and river restoration practitioners.

Bu çalışmada, toprak ve çelik boru hatları arasındaki etkileşim üzerine bir sayısal analiz yapıldı. Toprak ve boru özellikleri gömülü boru hatlarının hareketleri üzerinde önemli etkilere neden olabilir. Dinamik ve statik yüklemeye maruz boru hatlarının davranışını inç elemek için, farklı petrol ve gaz boruları bu çalışmada dikkate alınmıştır. Süresi 10 sn ve büyüklüğü 5.4 olan deprem yükü ve 50, 100, 150 ve 200 kPa değişken yüzey yükleri kullanılmıştır. Bu etkileri simüle etmek için, 2D sonlu elemanlar sayısal metodu, PLAXIS, kullanılmıştır. Sonuçlar tartışılmış ve tek değişkenli doğrusal ve doğrusal olmayan matematiksel modelleme ile ifade edilmiştir. Toprak tipleri (kil, gevşek ve yoğun kum), toprak katmanları (bir, iki ve üç toprak katmanları), yeraltı su tablası, statik su yükleri (toprak üstünde yükseklik 20m), gömme derinliği, boru çapı, boru kalınlığı gibi faktörler çalışıldı. Sonuçları dikkate alındığında, bu faktörlerin hem statik hem de dinamik yükler etkisinde boru hattının deplasmanına sebep olabilecek önemli öğeler olduğu sonucuna varılabilir. Bazı önemli karşılaştırmalar ve sonuçlar verilmiştir. In this study, a numerical calculation on interaction between soil and steel pipelines was performed. Properties of soil and pipe may cause significant effects on the movements of buried pipelines. To improve the understanding of the behavior of buried pipelines subjected to dynamic and static loading, different oil and gas pipes have been considered in this study. Earthquake load of magnitude 5.4 with time shaking of 10 sec and surface loads (50, 100, 150 and 200 kPa) have been used. To simulate numerically this effects, 2D finite element method PLAXIS is performed. The results are discussed and fitted by univariate linear and non-linear analysis. Some influential factors such as soil types (clay, loose and dense sand), soil layers (one, two and three soil layers), underground water table, static water loads (of height 20m above soil), burying depth, pipe diameter and pipe thickness are discussed in details. Based on the results, it can be concluded that these factors are important items on pipeline displacement for both static and dynamic loads. Some significant comparisons and conclusions are drawn. 133

Tube failure due to excessive flow-induced vibrations is a major concern with regards to the operation of heat exchangers in nuclear power and chemical process plants. Typical consequences include unplanned and expensive plant shutdowns to plug the failed tubes. Fretting wear resulting from tube/support impact is considered as a major contributor to tube failure. Impact forces, which occur when tube vibration amplitudes exceed the local support clearance, play a vital role in determining tube wear. Turbulence is one of the possible excitation mechanisms which can drive tube vibrations and is of particular concern in heat exchangers. Unlike other excitation mechanisms, turbulence has a persistent effect and thus determines the long term reliability of the heat exchangers. To address these issues, a point contact algorithm describing the tube/support interaction was implemented in an in-house finite element program and validated by several published examples. Pseudo-forces were utilized in conjunction with modal superposition in solving the nonlinear equations of tube motion. The equilibrium equations were solved iteratively to calculate the contact forces required to oppose any tube/support overlaps. The impact model was further modified to consider more realistic tube/support contact configurations. The new model considers a finite support width. The contact forces arising from the tube/support overlap are due to two different contact situations that may be encountered (point and segment contact). A distributed stiffness along the tube/support interface was utilized to model the segment contact. The resulting contact pressure was calculated using the displacement profile along the contact segment. The general point contact model considers any tube/support overlap that may occur between the principal contact node and the neighbouring node. Time-domain simulations of the nonlinear response of the tube are presented to determine the effect of various tube/support parameters on the system's vibratory characteristics. Special attention was paid to the effect of clearance enlargement due to fretting wear on the response of tubes in lattice-bar supports. The tube response, the impact force, and the contact ratio (ratio of the contact time to the total time) were analysed and presented in a dimensionless form. The dimensionless parameters utilized proved to be effective at collapsing all the data pertaining to different flow velocities over a single curve. This aids in identifying the role of support variables in influencing tube dynamics. Moreover, these parameters may also be used to scale the results in order to account for differences in geometrical and material properties. In addition, simulations were conducted to investigate the effect of the support type, the flow orientation, and the lattice-bar offset on the tube dynamics. The study indicates that some flow orientations, support types, and support offsets provide a favourable support geometry for higher normal work rates. This, in turn, increases susceptibility to fretting wear damage. These results provide new insights and a better understanding of the underlying phenomena of nonlinear tube behaviour in loose lattice-bar supports. Doctor of Philosophy (PhD)

Sediment transport in suspension can represent more than 90% of a river's total annual flux of sediment. In the case of the Yangtze River, more than 99% of the sediment supplied to the sea is suspended load. Suspended sediment is thus an important component of the total sediment load, with implications for channel dynamics, landscape evolution, ecology, and human-related activities. For hydrological management of large basins such as the Yangtze River, knowledge of the processes governing suspended sediment concentration (SSC) is essential. An analysis of the temporal variation of SSC for the Upper Yangtze basin (defined at Pingshan station) is presented here. For this purpose, a database of 50years of concurrent discharge and SSC measurements, made by the Yangtze River Commission, is used. The analysis is made using a novel probabilistic data-driven technique, the Generalized Pareto Uncertainty (GPU). This technique allows for the testing of several strategies of prediction and forecast applied to a time series of SSC and streamflow. Changing between local or seasonal variables to feed these strategies, we inferred that although the main driver of the SSC transport is flow (as reported by previous authors), sediment storage is also a major control. Furthermore, the maximum necessary time lag for forecasts made with the data is on the order of one week, which provides one indication of the time scale of the local processes of SSC transport in the Upper Yangtze. In this paper, limitations and data requirements of the GPU methodology are also discussed., Plain Language Summary In this manuscript we use a probabilistic technique to study what controls the dynamics of the sediments in suspension in Upper Yangtze River. For that, a database of 50 years of measurements of streamflow and sediments in suspension collected at Pingshan station by the Yangtze River Commission was analyzed with a new technique. Sediments in suspension represent the major portion of total sediment load in most river systems, and the Yangtze River is no exception. Fine sediment dynamics is an important component of many physical, chemical, and biological processes in rivers.

Sediment transport in suspension may represent 90% or more of the global annual flux of sediment. For instance, more than 99% of the sediment supplied to the sea by the Yangtze River is suspended load. Suspended load is an important component for understanding channel dynamics and landscape evolution. Sediments transported in suspension are a major source of nutrients for aquatic organisms in riparian and floodplain habitats, and play a beneficial role acting as a sink in the carbon cycle. Excess of fine sediments may also have adverse effects. It can impair fish spawning by riverbed clogging, disturb foraging efficiency of hunting of river fauna, cause algae and benthos scouring, reduce or inhibit exchanges through the hyporheic region. Accumulation of fine sediments in reservoirs reduces storage capacity. Although fine sediment dynamics has been the focus of many studies, the current knowledge of sediment sources, transfer, and storage is inadequate to address fine sediment dynamics in the landscape. The theoretical derivation of a complete model for suspended sediment transport at the basin scale, incorporating small scale processes of production and transport, is hindered because the underlying mechanisms are produced at different non-similar scales. Availability of long-term reliable data on suspended sediment dynamics is essential to improve our knowledge on transport processes and to develop reliable sediment prediction models. Over the last 60 years, the Yangtze River Commission has been measuring the daily Suspended Sediment Concentration (SSC) at the Pingshan station. This dataset provides a unique opportunity to examine temporal variability and controls of fine sediment dynamics in the Upper Yangtze basin. The objective of this study is to describe temporal variation of fine sediment dynamics at the Pingshan station making use of the extensive sediment monitoring program undertaken at that location. We test several strategies of prediction and forecast applied to the long time series of SSC and streamflow. By changing the base variables between strategies, we improve our understanding of the phenomena driving SSC. Prediction and forecasts are obtained from the various input data sets based on a novel probabilistic data-driven technique, the Generalized Pareto Uncertainty (GPU), which requires very little parametrization. Addressing uncertainty explicitly, this methodology recognizes the stochastic nature of SSC. The GPU was inspired in machine learning concepts and benefits from advances in multi-objective optimization techniques to discard most explicit assumptions about the nature of the uncertainty being modeled. Assumptions that do remain are the need to specify a model for eventual non-stationarity of the series and that there are enough observations to conveniently model the uncertainty. In this contribution, several models are tested with conditioned inputs to focus on specific processes leading affecting SSC. For example, the influence of seasonal and local contributions to SSC can be separated by conditioning the probability estimation on seasonal and local drivers. Probabilistic forecasting models for SSC that account for different drivers of the phenomena are discussed.

Growing interest in supraglacial channels, coupled with the increasing availability of high-resolution remotely sensed imagery of glacier surfaces, motivates the development and testing of new approaches to delineating surface meltwater channels. We utilized a high-resolution (2 m) digital elevation model of parts of the western margin of the Greenland Ice Sheet (GrIS) and retention of visually identified sinks (i.e., moulins) to investigate the ability of a standard D8 flow routing algorithm to delineate supraglacial channels. We compared these delineated channels to manually digitized channels and to channels extracted from multispectral imagery. We delineated GrIS supraglacial channel networks in six high-elevation (above 1000 m) and one low-elevation (below 1000 m) catchments during and shortly after peak melt (July and August 2012), and investigated the effect of contributing area threshold on flow routing performance. We found that, although flow routing is sensitive to data quality and moulin identification, it can identify 75% to 99% of channels observed with multispectral analysis, as well as low-order, high-density channels (up to 15.7 km/km² with a 0.01 km² contributing area threshold) in greater detail than multispectral methods. Additionally, we found that flow routing can delineate supraglacial channel networks on rough ice surfaces with widespread crevassing. Our results suggest that supraglacial channel density is sufficiently high during peak melt that low contributing area thresholds can be employed with little risk of overestimating the channel network extent.

Low-frequency vibration is harmful and difficult to attenuate due to resonance. Since the natural frequency of a system is directly proportional to stiffness, reducing stiffness is advantageous. However, there are limitations to how far stiffness can be reduced while supporting a given mass. The addition of multiple degrees of freedom (MDOF), as well as nonlinear springs displaying high static low dynamic (HSLD) stiffness have been shown to partially overcome these limitations. This thesis combines the two techniques and completes an in-depth mathematical and experimental investigation of MDOF HSLD stiffness isolators. MDOF HSLD stiffness systems are shown to provide superior performance compared to other passive isolators. The sensitivity of design parameters is investigated, and the isolator's applicability is showcased by incorporating it into a heavy equipment seat cushion. Friction within the seat is modeled using the Force Balance Friction Model and is shown to change the dynamics of the system substantially.

One of the most important parts of a nuclear reactor is the fuel bundle in which the nuclear reaction takes place and heat is generated. The heat is transported through the flow of the coolant to the steam generator. During such process, the fuel bundles are subjected to severe operation conditions, such as highly turbulent coolant flow, high temperatures and excessive irradiation doses. These severe conditions significantly affect the integrity of the fuel bundle in terms of flow-induced vibrations (FIV). The FIV are produced by various excitation mechanisms such as, the turbulence buffeting, the fluidelastic forces (Motion-dependent forces) and the acoustic pressure pulsations coming from the primary heat pump. In the current study, a numerical approach is presented to characterize the motion-dependent forces. The model was used to predict these forces in flexible fuel kernel. In addition, an analytical model was developed utilizing the force model to predict the dynamic response of the fuel bundle. The dynamic response was compared with the available experimental data. In the second part, a fully-flexible fuel bundle structural model was developed to investigate the dynamic response of the fuel bundle under various excitation mechanisms. The model is capable of predicting the vibration response of fuel bundles with a large number of elements and various end conditions, such as flexible endplates. The fuel bundle and the supporting structure were modelled utilizing finite beam and plate elements. The contacts between the system components were modelled using the single point contact method (SPC). Fluid excitations, such as turbulence, pressure pulsation, and motion-dependent forces, were included in the model. Finally, the irradiation effects on the mechanical behavior of the fuel bundle is investigated using the same structural model. This was accomplished by utilizing a constitutive model that describes the thermal and irradiation effects on the mechanical properties. The current work represents a major advancement step towards a realistic modelling of the complex dynamics of fuel bundles. 2022-09-22

The purpose of this work was to numerically study the area perturbation and phase lag in a staggered tube array, which comprised of an oscillating tube with a prescribed motion in the streamwise direction. CFD simulations were conducted for a parallel triangle array over a range of reduced flow velocities, varying displacement ratios, and pitch to diameter ratios. The velocity field was post-processed and the flow perturbations were extracted using an image processing technique. The obtained perturbation and the associated phase lag profiles were used to formulate and propose simplified ideal profiles for the area perturbation amplitude and phase lag. The proposed ideal perturbation amplitude and phase lag profiles were then utilized in the model of Hassan and Weaver (2016) to predict the stability threshold of Streamwise Fluidelastic Instability. 2021-08-18

Fluidelastic instability (FEI) is a major concern in steam generators and continues to be one of the leading causes of structural failure. There has been a general consensus that the phenomenon of FEI occurs only in the transverse direction rather than in the streamwise direction. After a refurbishment of the San Onofre Nuclear Generation Station (SONGS), there was a failure in the steam generator due to streamwise FEI. This has never been reported before and had traditionally not been considered as a factor in the design of nuclear steam generators. Therefore, research efforts are made to understand this phenomenon. In this work, experiments were conducted using a closed-loop wind tunnel. A fully flexible tube array subjected to cross-flow was considered in the current investigation with pitch to diameter ratios of 1.25, 1.4, 1.55, and 1.7. The tubes were mounted over a flexible support in the direction of the flow. Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canadian Nuclear Safety Commission (CNSC) 2021-05-20

Centrifugal pumps are widely used in a variety of applications, accounting for approximately 20% of worldwide energy use. Although highly reliable machines, they are not immune to performance degradation over time, experiencing efficiency losses up to 25% by the end of service life. As the pump degrades, operation deviates from the best efficiency point as volumetric, hydraulic and mechanical efficiency decrease. A state-of-the-art experimental setup including a 7.5 kW centrifugal pump rated for a flow rate of 17 L/s and 23 m of head was used. To enable variable speed operation the motor is paired with a variable frequency drive. This thesis investigates and compares the effect of the wear-ring clearance on various operational (head, flow and power) parameters and performance (efficiency and specific speed) parameters under single speed and variable speed operation. The results from this work demonstrates the pump operational reliability under degraded conditions under different control methods. NSERC, Xylem Inc. 2020-12-05

Centrifugal pumps are used in a variety of engineering applications such as power production, heating, cooling, and water distribution systems. Although centrifugal pumps are considered to be highly reliable hydraulic machines, they are susceptible to a wide range of damages due to several degradation mechanisms, operating away from the best efficiency point and improper installation. The most common forms of condition monitoring for rotating machines performance monitoring coupled with vibration analysis. The goal of this thesis is to document the performance degradation and vibration characteristics for an industrial scale centrifugal pump subjected to specific forms of damage, in particular impeller unbalance. In this work a 7.5 kW centrifugal pump rated for a flow rate of 17 L/s and 23 m of pressure head is tested in a state-of-the-art pump experimental setup, equipped with a wide range of sensors and control devices. Extensive performance and vibration measurements were collected to evaluate the effects of various levels of impeller unbalance using the time and frequency domain techniques coupled with principal component analysis. The results from this work can be used to evaluate and monitor the pump performance under prescribed degradation in order to enhance the preventative maintenance program. NCERC, Xylem Inc.

Whole-Body Vibration (WBV) poses a risk of injury to operators. Long hours seated in an uncomfortable seat can result in fatigue, injury and reduced productivity. To improve ride comfort and minimize injury risk, suspension seats are used to attenuate vibration and active and semi-active suspension seats can outperform passive seat suspensions. Mainstream use of active or semi-active systems is limited due to cost and complexity. This research evaluates a new type of semi active device that could upgrade any passive seat suspension to improve WBV attenuation. In this work the operating principle for the device is investigated to determine the potential for WBV attenuation when installed on an Original Equipment Manufacturer (OEM) seat suspension. A prototype was developed and tested to verify performance. A simple mathematical model was developed for the device and was solved numerically. The results showed that a device with semi-active controlled damping installed onto a passive seat suspension can attenuate vibration effectively over the frequency range of 0.8 to 20 Hz. An OEM tractor seat was selected to be used for experimental testing. Stiffness and damping characteristics were determined for the seat and the device was designed and installed in place of the seat cushion. The device was evaluated by mounting on a 6DOF hexapod platform to provide base excitation. Results from the experiment showed that the device has potential to reduce WBV to the operator by transferring energy to the seat components. The Magneto-Rheological damper used in the device added some non-linear stiffness to the system which affected device control. Recommendations for a second generation prototype include redesigning the form to better fit on the OEM seat as well as designing a controlled damping element that is better suited the systems parameters.

This thesis provides a comprehensive investigation of flow-sound-structure coupling in spring-loaded valves subjected to air flow. While they are commonly used in a multitude of applications, these types of valves have been found to experience severe vibrations when interaction is present among the structure, the hydrodynamic field, and the acoustic field for a range of operational valve structural characteristics, flow parameters, and connected piping length. The first part of this investigation was aimed at characterizing experimentally the valve’s dynamic behaviour and the parameters affecting the onset of self-excited instability. The occurrence of instability was mainly driven by the presence of acoustic feedback: the connected length of piping had to be sufficiently long, with a longer pipe correlating to more severe vibrations. In addition, it was found that the valve’s oscillation frequency depends on the modal characteristics of the combined valve piping system, rather than the structural natural frequency alone. Furthermore, an increase in the valve’s spring stiffness caused the vibrations to become more severe. Meanwhile, other parameters such as initial spring preload force and valve plate area only had moderate effects on the stability behaviour of the valve. The second part of the investigation sought to develop a theoretical model that could simulate the valve’s response when subjected to air flow while considering the effects of acoustic feedback and impact on the seat and limiter. Thus, a structural model of the valve was developed based on a single-degree-of-freedom model of the system with impact computed based on a pseudo-force method. The hydrodynamic field relied on a one dimensional unsteady Bernoulli description of the flow. Finally, the acoustic interaction was accounted for using the one-dimensional wave equation resolved using a finite difference scheme. The model has demonstrated remarkable agreement with the experimental results. It has shown an ability to predict the modal characteristics of the system as well as correctly predict the effect of increased stiffness or increased piping length on vibration amplitude. The final part of the investigation consisted in designing countermeasures to mitigate the effects of this self-excited instability mechanism. A concentric Helmholtz-type cavity resonator, an orifice plate, and an anechoic termination are placed at the downstream side of a model valve which were seen to be unstable in the experimental and modelling phases of the investigation. All tested devices were able to eliminate the self excited instability mechanism. The applicability and robustness of each of these methods were discussed. Thesis Doctor of Philosophy (PhD)

The CANDU steam generator consists of tube bundles where two phase flow across the bundle takes place. Among many tube vibration mechanisms, the FluidElastic Instability (FEI) is considered the most destructive one. Many studies focused on FEI of two phase flow. However, there is no analytical model until now able to predict FEI threshold velocity. This study presents a novel analytical framework to predict the fluidelastic instability threshold of two phase flow across normal square bundle. The model focuses on the bubbly flow (void fractions up to 35%). A single vibrating tube in a fixed bundle was considered. Flow around the vibrating tube is idealized as one dimensional bubbly flow in two channels. The fluid in each channel is composed of continuous phase and dispersed phase. The dispersed phase is accounted for by spherical bubbles. The motion of each individual bubble was modeled by accounting for the external forces acting on its surface. Bubble-to-bubble interaction, bubble break-up, and bubbles coalescence were taken into account. Bubble re-sponse to the pressure pulsations was integrated in the model by solving Rayleigh-Plesset equation. By tracking each bubble in the flow channels, it is possible to calculate the change in the flow density around the tube. The calculated instantaneous local density is used to solve the unsteady conservation equations to find the fluid forces on the vibrating tube. The tube is then modeled as a single degree of freedom system to predict its displacement. The feedback mechanism of the tube on the fluid was then modeled. Time domain simulations were conducted for air-water mixture and the stability threshold was predicted. Prediction of the stability threshold showed a very promising results when compared with the experimental data. A sensitivity study is performed on the model to test its limitations is presented.

Flow Induced Vibrations (FIV) are a significant threat to the life of major power plant components such as heat exchangers. The most dangerous of these excitation sources is fluidelastic instability (FEI). Early FEI models focused on predicting the stability threshold of heat exchanger tubes subjected to fluidelastic forces based on empirical data, but the inability of empirical predictions to account for the variability in tube array geometry, pitch, and single or two phase flow limited the usefulness of these models. This thesis investigates the properties of unsteady flow in a normal triangle tube array with a pitch-to-diameter ratio of 1.5. Stability simulations are conducted over a mass damping parameter range 9--200, using a reduced order model of the moving tube. This investigation provides a direct observation of the channel area. When implemented into the time domain model the new parameters provide better agreement with the experimental stability threshold data available.