Norges teknisk-naturvitenskapelige universitet
http://hdl.handle.net/11250/223328
Sat, 25 Mar 2017 19:31:45 GMT2017-03-25T19:31:45ZScattering and Uncertainty for discrete models
http://hdl.handle.net/11250/2435346
Scattering and Uncertainty for discrete models
Alvarez-Romero, Isaac
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11250/24353462017-01-01T00:00:00ZInvestigation of CO2 hybrid groundcoupled heat pumping system for warm climate
http://hdl.handle.net/11250/2435297
Investigation of CO2 hybrid groundcoupled heat pumping system for warm climate
Jin, Zhequan
Environmental sustainability, renewable and clean energy utilization and energy conservation have become key issues facing the development of modern society. In the field of refrigeration, air conditioning and heat pumping systems, the combination of sustainable energy technology with environmentally friendly refrigerants, will be an important trend. Carbon dioxide (CO2), or R744, as a natural refrigerant, shows great potential to be a main refrigerant in the future, because of its environmental characteristics and superior thermodynamic properties.
The methods described in this thesis aims to improve the energy efficiency of a CO2 transcritical heat pumping system for indoor air conditioning application under warm climate conditions. Therefore, concepts, like ground borehole heat exchanger, two-phase ejector and separated gas coolers for gas cooling heat recovery, are introduced to the investigated CO2 heat pumping system.
The state-of-the art CO2 as refrigerant, heat pumping system and hybrid ground-coupled heat pumping system indicates the necessity of further development of CO2 refrigerant to the building air conditioning sector. Moreover, it also shows that the transcritical CO2 heat pumping system has potential to integrate the technology concepts, like temperature and humidity independent control for indoor air conditioning and the hybrid ground-coupled heat pumping system, for the better energy efficiency.
Regarding the methodology, steady and quasi-steady state analyses are performed on a CO2 heat pumping system. The steady state analysis provides the fundamental information for the system design procedure for the transient and practical models of the system, and the optimal control strategy of the gas cooler pressure is proposed for the investigated CO2 transcritical refrigeration cycle. The quasi-steady state analysis provides the practical information on the system’s energy performance. In addition, the experimental validation for the transient models shows the difference between the simulation and the measurements of the practical system.
In the research work, the following solutions are suggested to improve the energy efficiency of a CO2 heat pumping system for air conditioning application. These include the technology concepts, i.e. ①.CO2 hybrid heat pumping system by integration of ground soil and ambient air as heat sinks; ②.CO2 two-phase ejector system for the indoor air temperature and humidity independent control; ③.smart utilization of the CO2 heat pumping system by the means of gas cooling heat recovery.
Firstly, the quasi-steady state simulation calculates the annual energy efficiency of R744 hybrid ground-coupled heat pumping system. This hybrid system uses both ground soil and ambient air as heat sinks in the cooling mode. The simulation work also compares the annual energy performance of heat pumping systems using R410A and CO2 as refrigerant. The simulation results show that the seasonal cooling and heating coefficient of performance (COP) of the CO2 hybrid ground-coupled heat pumping (GCHP) system are 3.55 and 3.32. Moreover, the cooling performance is 42% and 23% better than both the CO2 air source heat pumping (ASHP) and the conventional CO2 GCHP systems. Therefore, the CO2 system shows superiority by integrating of two temperature levels of heat sinks, i.e. ground soil and ambient air. Although the annual system performance of the R410A ASHP system is better than that of the CO2 hybrid GCHP system, the cooling performance of the R410A system seriously decreases when the ambient temperature is higher than 30 ºC.
The research work also investigates the CO2 two-phase ejector transcritical system with quasi-steady state simulation, based on the gas cooler outlet split (GOS) ejector refrigeration cycle. This CO2 GOS ejector refrigeration cycle can generate two levels of evaporating temperature for the indoor air temperature and humidity independent control. The theoretical steady state analysis shows that the COP of the CO2 ejector refrigeration cycle is 12~60% better than that of the standard isenthalpic expansion cycle when the humidity load ratio covers from 10~50% under the same boundary conditions. Meanwhile, the quasi-steady state investigation explains the operating characteristics with the different control strategies, and the low evaporating temperature control strategy is much more challenging than the high evaporating temperature one. The quasi-steady state performance of the CO2 GOS ejector system under different load conditions shows that the average energy performance is 30% better than that of the R410A system under hottest summer weather conditions. Finally, compared to the traditional standard ejector refrigeration cycle, with the GOS cycle, it is much easier to control the system performance and easy to switch the operating mode for the different ejector refrigeration cycles.
At last, the research work involves the smart utilization of the CO2 heat pumping system for service hot water heating by the means of heat recovery. The developed system is able to operate under space cooling or heating mode, while simultaneously servicing hot water heating. The energy performance of the heat pumping system is investigated under both full and partial capacity, and the quasi-steady state simulation result shows that the combined COP for indoor air conditioning and service hot water varies from 3.0 to 5.5 with 65 ºC of service hot water supply. Moreover, the annual ground thermal imbalance for the CO2 hybrid GCHP system is studied for three cities in a warm climate. It is proved that the proposed CO2 system is suitable for reference buildings under two climate conditions without the problem of ground thermal energy accumulation due to overheating of the underground borehole.
To summarize, the hybrid utilization of the heat sinks based on the CO2 transcritical refrigeration cycle improves the heat pumping system’s energy efficiency under warm climate conditions. In addition, some smart technologies, like work recovery from an expansion process by an ejector and heat recovery from a gas cooling process for servicing hot water heating, further improves the energy efficiency of the CO2 heat pumping system. In particular, the CO2 hybrid GOS ejector GCHP system offers the future air conditioning sector many possibilities for using the environmentally friendly CO2 refrigerant.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11250/24352972017-01-01T00:00:00ZShip routing and scheduling with speed optimization and voyage separation requirements
http://hdl.handle.net/11250/2435293
Ship routing and scheduling with speed optimization and voyage separation requirements
Norstad, Inge
Summary of the thesis
This PhD thesis discusses how operations research (OR) can be used to help ship operators with finding good solution for their tactical planning problems. The research has been a part of the larger research project DESIMAL (Decision support in maritime logistics) at NTNU.
The focus of this thesis has been on two important application-driven extensions of the ship routing and scheduling problem. The first extension is to include sailing speed as decision variables in the model. The second one is voyage separation requirements.
This thesis consists of five scientific papers. The first three deal with introducing sailing speed as decision variables in maritime routing and scheduling. Traditionally, mathematical models of ship routing problems assume that a ship has a fixed speed and fuel consumption rate. This leads to fixed sailing times and fixed sailing costs for a given distance. However, in real life, it is possible for the ship to adjust the speed within an interval, and hence change the sailing time. Furthermore, the fuel consumption rate is exponentially dependent on the speed. By optimizing the speed of each leg on a ship's route, without violating required time windows, the total fuel consumption can be reduced. The papers presents both exact and heuristic solution methods for the speed optimization problem. Computational experiments show that solving the speed optimization and the tramp ship routing and scheduling problem simultaneously, significantly improves the total fleet profit, and also reduces harmful emissions.
The last two papers consider the real tactical planning problem of a tramp ship routing company. The problem is to allocate ships of a fleet to a set of voyages. The contracts between the shipping company and the cargo owners state that voyages on the same trade line should be fairly evenly spread. This leads to a maritime fleet deployment problem with voyage separation requirements. One arc flow and two path flow formulations are presented and compared. A path flow model with routes as variables is solved by a priori column generation, while another path flow model with schedules as variables is solved by a branch-and-price approach.; Sammendrag av avhandlingen
Denne PhD-avhandlingen tar for seg hvordan operasjonsanalyse (OA) kan brukes for å hjelpe skipsplanleggere med å finne gode løsninger for sine taktiske planleggingsproblem. Forskningen har vært en del av et større forskningsprosjekt DESIMAL (Decision support in maritime logistics) ved NTNU.
Fokuset for denne avhandlingen har vært på to viktige anvendelsesstyrte utvidelser av skipsrutingsproblemet. Den første utvidelsen er å ta med seilingshastighet som beslutningsvariabler i modellen. Den andre er krav til tidsseparasjon mellom reiser.
Avhandlingen består av fem vitenskapelige artikler. De tre første handler om innføring av seilingshastighet som beslutningsvariabel i maritim ruting og schedulering. Tradisjonelt sett har matematiske modeller for skipsrutingsproblemer antatt at et skip alltid seiler med samme hastighet og har en fast rate for drivstofforbruk. I virkeligheten kan imidlertid skipet variere hastigheten innenfor et gitt intervall og dermed endre seilingstiden. Videre, så er forbruket av drivstoff eksponentielt avhengig av hastigheten. Ved å optimere hastigheten på hvert leg av skipets rute, uten å bryte tidsvinduene, kan det totale drivstofforbruket reduseres. Artiklene presenterer både eksakte og heuristiske løsningsmetoder for hastighetsoptimeringsproblemet. Beregninger viser at ved å optimere hastigheten samtidig som man løser rutingsproblemet, kan man oppnå betydelig forbedring i totalt resultat for flåten, og også redusere skadelige utslipp.
De to siste artiklene omhandler et virkelig taktisk planleggingsproblem for et trampshippingsrederi. Problemet er å allokere flåtens skip til et sett av reiser. Kontraktene mellom rederiet og vareeierne sier at reiser på samme rute skal være rimelig jevnt fordelt i tid. Dette gir oss et flåtedeployeringsproblem med krav til reiseseparasjon. En bueflyt-modell og to ulike stiflyt-modeller presenteres og sammenlignes. Den ene stiflyt-modellen bruker ruter som beslutningsvariable og løses med a priori kolonnegenerering, mens den andre bruker schedules som variabler og blir løst med en branch-and-price-tilnærming.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11250/24352932017-01-01T00:00:00ZTechno-economic models in Smart Grids: Demand side flexibility optimization for bidding and scheduling problems
http://hdl.handle.net/11250/2435291
Techno-economic models in Smart Grids: Demand side flexibility optimization for bidding and scheduling problems
Ottesen, Stig Ødegaard
Summary of the thesis
Introduction of power-intensive appliances such as electric vehicle chargers and induction cooktops, as well as technologies for local renewable electricity generation from solar panels and wind turbines will provide challenges for distribution in the coming years. High power peaks, rapid power changes and less predictability will increase the need for transmission capacity and reserves. Traditionally, such problems are met with costly investments in new capacity. An alternative approach is to use flexibility from the end users, which means that generation and consumption of electricity is changed as a response to prices or other signals. Introduction of batteries in buildings, advanced metering infrastructure (AMI) and the Internet of Things (IoT) increase the potential for demand side flexibility. Altogether, these technologies constitute the concept denoted the Smart Grid.
To realize this increased flexibility potential, financial incentives must be introduced. Major changes are therefore expected in the electricity market in the coming years, including introduction of new, innovative contract types and business models, changes in market designs and the establishment of new market roles.
To maximize the benefit of demand side flexibility, there is a need for development of new decision support models. This thesis proposes and analyzes models for trading in different markets and for the scheduling of flexible devices in an operational situation. The models are based on operations research. The decision problems are mathematically formulated, and a particular focus is on how to handle uncertain parameters, such as consumption, generation and market prices. Stochastic programming is used for this purpose.
The thesis consists of four articles. In Article 1 a basic model is established where flexibility is divided into different classes. The article analyzes a prosumer in the retail market, where flexibility gives cost savings by exploiting price variations over a day, between energy carriers and by reducing the demand charge at the grid tariffs. In Article 2 several prosumers are coordinated via an aggregator who buys and sells electricity in a spot market and where imbalances are settled in a balancing market. Article 3 focuses on flexibility trade, where the value of an aggregated flexibility portfolio is maximized by trading in three sequential markets. The last article analyzes the decision problem to a service provider who operates a charging site for electric vehicles, where the capacity is limited. All articles contain case studies that have been conducted in close cooperation with companies in the Norwegian electricity market.; Sammendrag av avhandlingen
Innføring av effektkrevende forbruksapparater som elbiler-ladere og induksjonskoketopper, samt teknologi for lokal, fornybar elektrisitetsproduksjon fra sol og vind vil gi utfordringer for distribusjonsnettet i årene som kommer. Høye effekttopper, raske effektendringer og mindre forutsigbarhet vil gi økt behov for overføringskapasitet og reserver. Tradisjonelt vil slike problemer møtes med kostnadskrevende investeringer i ny kapasitet. En alternativ tilnærming er å benytte fleksibilitet hos sluttbrukerne, som innebærer at innmating og uttak av elektrisitet endres som følge av priser eller andre signaler. Innføring av batterier i bygg, avanserte målings- og styringssystemer (AMS) og tingenes internett (IoT) gjør at potensialet for sluttbrukerfleksibilitet er økende. Til sammen utgjør disse teknologiene konseptet som benevnes det smarte nettet, Smart Grid.
For å realisere dette økte fleksibilitetspotensialet, må økonomiske insentiver innføres. Det forventes derfor store endringer i elektrisitetsmarkedet i årene som kommer, blant annet ved introduksjon av nye, innovative kontraktstyper og forretningsmodeller, endringer i markedsdesign og etablering av nye markedsroller.
For å maksimere nytteverdien av sluttbrukerfleksibilitet, vil det være behov for utvikling av nye beslutningsstøttemodeller. Denne avhandlingen foreslår og analyserer modeller for handel i ulike markeder og for planlegging av hvordan fleksible enheter skal benyttes i en driftssituasjon. Modellene baseres på operasjonsanalyse. Beslutningsproblemene modelleres matematisk, og det fokuseres spesielt på hvordan optimale beslutninger skal tas under forutsetning av at enkelte parametere, som forbruk, produksjon og markedspriser, kan være usikre på beslutningstidspunktet. Til dette benyttes stokastisk programmering.
Avhandlingen består av fire artikler. I artikkel 1 etableres en grunnleggende modell der fleksibilitet deles inn i ulike klasser. Artikkelen analyserer en prosument i sluttbrukermarkedet, der fleksibiliteten gir kostnadsbesparelser ved å utnytte prisvariasjoner over døgnet, mellom energibærere og ved å redusere effektledd på nettleiekontrakter. I artikkel 2 koordineres flere prosumenter via en aggregator som kjøper og selger elektrisitet i et spotmarked og der ubalanser avregnes i et balansemarked. Artikkel 3 rendyrker fleksibilitetshandel, der verdien av en aggregert fleksibilitetsportefølje maksimeres ved å handle i tre sekvensielle markeder. Den siste artikkelen analyserer beslutningsproblemet til en tjenestetilbyder som drifter et ladested for elbiler, der kapasiteten er begrenset. Alle artiklene inneholder case-studier som er gjennomført i tett samarbeid med aktører i det norske kraftmarkedet.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11250/24352912017-01-01T00:00:00ZModel predictive load-frequency control
http://hdl.handle.net/11250/2435279
Model predictive load-frequency control
Ersdal, Anne Mai
The decrease in frequency quality seen in the Nordic power system over the past
two decades is a clear token of the major changes that power systems all around the
world are facing. These changes are to a large extent connected to the green shift in
energy production, which results in less controllable power production. Additionally,
there are bottlenecks in the Nordic transmission grid, which at times exclude
some of the resources from participating in frequency control, and the power trading
between the Nordic and the Continental European system is increasing, which
means that the Nordic system is being subjected to higher and more unpredictable
consumption.
One important mean for improving the frequency quality is to improve the load
frequency control (LFC), which is the continuous operation of keeping produced
and consumed power equal all times. With the Nordic power system in mind,
an important task will be to implement a fully operable automatic generator control
(AGC), which automatically controls the power-production set point of each
generator. AGC was _rst implemented in the Nordic system in 2013, and due to
unexpectedly high expenses, it is still not fully up and running. This thesis aims at
investigating model predictive control (MPC) as a control design method for AGC,
with application to the Nordic power system. It is believed that the natural handling
of multiple inputs and system constraints, as well as the optimizing nature of
MPC makes it a promising candidate for AGC.
The main contribution of this thesis is an MPC-based solution to the LFC/AGC
problem, where state feedback is achieved through a state estimator and a simpli_ed
system model is used both in the MPC predictions and the state estimator. System
constraints include production limits and limits on generation rate of change, as well
as constraints on tie-line power transfer capacity. In order to include constraints
on the individual generating units, and not only on the aggregated generating units
of the simpli_ed model, the participation factors of each generator are included as
optimization variables. Simulations on a proxy model show that the MPC-based
solution outperforms a traditional PI-based solution. In order to make the controller
more robust against uctuations in produced wind power, a multi-stage nonlinear
MPC (MNMPC) is also presented. Based on estimates of the worst-case deviation
in produced wind-power, the MNMPC makes sure there is enough available transfer
capacity on tie lines to make use of all resources in case of large deviations in windpower
production. The approach of stochastic NMPC (SNMPC) is also tested as an
alternative to the MNMPC. The SNMPC has the theoretical advantage of stochastic
guarantees for constraint ful_llment in the presence of disturbances (deviations in produced wind power), while the MNMPC shows better tractability and is less
likely to encounter feasibility issues.
Using the power transfer in high voltage direct current (HVDC) lines as controllable
inputs to the system is also investigated as a method for improving angle
stability, which is a di_erent part of power system stability. The method of backstepping
was applied in this part of the thesis, which is a control-design method that
is not based on online optimization, contrary to the MPC. The work shows that
HVDC-lines can contribute in stabilizing the overall stability of a power system.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11250/24352792017-01-01T00:00:00ZModel-based Control of the Czochralski Silicon Crystal Pulling Process
http://hdl.handle.net/11250/2435273
Model-based Control of the Czochralski Silicon Crystal Pulling Process
Rahmanpour, Parsa
This study explores the use of mathematical modeling as an aid in automatic control of the standard Czochralski process (Cz). The aim of the modeling has been to develop a simple model, while covering the essential process dynamics, with the ability to update the model from measurements (on-line, during and between runs), rather than developing a highly accurate and detailed complex model. This should result in a model-based controller with acceptable performance, both in accuracy and calculation time.
An overview of the process, with emphasis on aspects related to control theory, is presented. The most challenging aspects of the growth process are identified. The conventional method of control is described along with its limitations and disadvantages. A thorough review of existing publications related to modeling and control of the Czochralski process is presented, before introducing a mathematical description of the process. With proper choice of model parameters, the represented nonlinear model is able to express the main hydrodynamic-geometrical dynamics, as well as the essential thermal dynamics of the Czochralski process used for production of Si crystals with specific crystal diameters. Furthermore, the model is assessed based on properties such as local controllability and observability.
For the purpose of estimating the actual crystal radius, standard Kalman filter (KF) and Extended Kalman filter (EKF) are explored. Based on simulations, the EKF gave better results in terms of estimation error when compared to the KF. This might indicate that describing the essential and complex dynamics of the Cz process acceptably accurately with a linear model can be a difficult task. Additionally, in order to evaluate the amount of model accuracy loss due to the linearization process in the EKF, Unscented Kalman Filter (UKF) was also applied. Most application studies on state estimation only have on-line, noise corrupted/uncertain measurements available. In this case, the crystal radius, as the main process variable, can be measured with much greater accuracy after the crystal is produced. Thus, based on collected data from actual plant operation, the estimators' performance is assessed, which shows a very similar estimation quality when using EKF and UKF. This might indicate that the nonlinearity in the model is not strong enough to benefit very much from using the UKF. However, the fact that the use of the posteriori radius measurements confirm the findings of the preceding simulation study, is interpreted as a confirmation that the model used represents the nonlinear effects in the meniscus/radius dynamics with reasonable accuracy.
The tightly coupled dynamics of the Cz process and the existence of physical constraints on states and manipulators, motivate the application of a model-based control approach that can handle the nonlinearities, as well as the imposed operational constraints, in an optimal fashion. Hence, the nonlinear model is also used to apply Nonlinear Model Predictive Control (NMPC) method for automatic radius and melt temperature control.
For an open-loop stable linear system, the stability can be guaranteed by the MPC, without the need for terminal constraints. However, if the system is unstable or has an integrating mode (which suffer from infinitely large time constants), the solution will not converge to a stable point at the end of the control horizon. Based on numerical analysis, the introduced nonlinear model for the Cz process has a close to integrating behavior. In order to effectively deal with this challenge, the widely known and established prestabilization technique is applied. What is involved is a kind of re-parametrization of the controller. The problem can be overcome by expressing predictions as perturbations on a stabilizing linear feedback law.
Moreover, two types of system dynamics are identified during the controller tuning phase. The fast dynamics (quick response to input changes) are represented by the geometrical properties of the process such as the radius and the meniscus shape. On the other hand, the bulk part of the process is responsible for the slow dynamics due to delays in the heat transportation. Therefore, it is chosen to divide the control part into two separate controllers where the puller MPC handles the fast dynamics while the heater MPC deals with the slow dynamics. This structure makes it possible to choose different tuning parameters for each of the controllers based on their model dynamics, and to use different sampling rates and prediction horizons in the two MPCs. However, there exists a link of communication between them in terms of the melt temperature.
As the simulation results show, the MPC controllers can follow their predefined reference trajectories acceptably accurately, while respecting the physical constraints on both the manipulators and the states. Since the trajectories are designed by experienced metallurgists, an adequate trajectory tracking should most probably result in crystals with less structural defects. Therefore, in addition to reducing the cut-offs from the crystal for wafer production, the enhanced control scheme also improves the crystal qualities required for solar cell productions. Similar to the separation of the controllers, two EKFs are applied for reconstruction of the noisy or unmeasured states. Each estimator has it's own specific sets of tasks that must be fulfilled. The first EKF estimates the real radius, growth angle and a parameter related to calculation of crystallization rate. The second EKF estimates the necessary temperatures at the hot-zone such as the melt temperature, in addition to a parameter required for calculating the entering heat energy from the heater elements into the melt.
In order to evaluate the parameter estimation ability of these observers, model mismatch has been introduced in the simulations. That means the applied model for the MPC controllers and the EKF estimators are considerably different from the plant model, in terms of two important model parameter. Additionally, this is done to illustrate the performance ability of the MPC controllers in situations where the model cannot describe the plant dynamics accurately. As illustrated by the simulation results, the estimated parameters seem to converge towards their respective true values, which indicates the satisfactory performance of the applied observers in this study.
For commercial Czochralski ingot pullers using the conventional control scheme, feedforward control is an essential part of the overall control system. For instance, the conventional pullers depend to a large extent on the feed-forward temperature reference trajectory to keep the desired pulling rate. A common method in practice is to determine the feedforward control by careful analysis of repeated growth runs, resulting in a trajectory for the control inputs which can then be used as part of the recipe. Although this method is widely accepted, it suffers from the fact that it is extremely time consuming, and thus expensive. Besides that, it works well if the same conditions are repeated and there are no significant variations from run to run. However, any changes in plant setup, process equipments or in desired crystal diameter, means repeating this procedure. Moreover, when using a simplified lumped-parameter model to express the Cz process, the model accuracy depends on updating the time-varying parameters (during, as well as between runs) used in the system equations. Therefore, adaptation of model quantities and trajectories to the real crystal-growing process is essential in terms of controller performance enhancement and ingot quality. Thus, Run-To-Run (R2R) control strategy is suggested to update the process model parameters in a recursive way when output from the previous run becomes available, and thereby help generate better recipes for future runs. Combining MPC, estimation and Run-To-Run control has enabled robust, effective control of the Czochralski crystallization process. Using this approach, the traditional feed-forward control of the growth rate is replaced with a more direct control of the melt temperature. Consequently, for each completed run, the growth rate reference tracking becomes tighter, which results in faster and better control of the crystal diameter.
As suggestion for future research, better technique for melt temperature measurement is proposed. It is suggested that better measurement techniques and instrumentation could potentially increase the model accuracy, thus, improving the overall performance of the controllers.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11250/24352732017-01-01T00:00:00ZThree-dimensional streaming in sea bed boundary layer
http://hdl.handle.net/11250/2435270
Three-dimensional streaming in sea bed boundary layer
Afzal, Mohammad Saud
The objective of the work carried out in the present thesis is to obtain new insights into wave- current seabed boundary layers by studying the wave-current interaction for the general case of a non-zero angle between the waves and the current using numerical modelling. The first part of this work focuses on the hydrodynamics, while the second part deals with the resulting sea bed sediment transport.
The effect of three-dimensional wave-induced streaming on seabed boundary layers is investigated for following and opposing waves and current where the wave propagation forms a non-zero angle with the current. It is shown that the sea bed boundary layer flow results from an interaction between the classical wave-current interaction (reducing the mean velocity relative to current alone), Longuet-Higgins streaming (forcing the flow in the wave propagation direction) and streaming caused by turbulence asymmetry in successive wave half-cycles (forcing the flow against the wave propagation direction). For waves and current which are not colinear, the mean velocity profile exhibits a veering behaviour which is strongly affected by streaming, particularly for the most wave-dominated situations. The effect of streaming on the boundary layer flow has been investigated for different wave-current conditions and bottom roughnesses. Visualisations are given by mean Eulerian and Lagrangian velocity profiles, as well as three-dimensional seabed boundary layer particle trajectories. The effect of streaming decreases as the flow becomes more current-dominated. The mean velocity in the current direction decreases as the roughness increases. However, the mean velocity orthogonal to the current direction increases as the roughness increases due to the lack of wave-current interaction in this direction. An excellent agreement between the predicted and recently measured velocity profiles (Yuan and Madsen, 2015) beneath horizontally uniform asymmetric forcing is obtained.
The effect of three-dimensional wave-induced streaming on the resulting seabed boundary layer sediment transport (i.e. bedload and suspended sediment transport) has been investigated for following and opposing waves and current where the wave propagation forms a nonzero angle with the current. The mean sediment transport results from an interaction between Longuet-Higgins streaming, asymmetric streaming and the current. It appears that the mean sediment transport decreases as the angle between the waves and the current increases and for a given angle, the sediment transport is largest for second order Stokes waves, followed by linear propagating waves, horizontally uniform Stokes forcing and horizontally uniform linear forcing. The mean sediment transport direction is rotated from the wave propagation direction towards the current and this rotation is largest for horizontally uniform linear forcing, followed by horizontally uniform asymmetric forcing, linear propagating waves and second order Stokes waves. The effect of wave asymmetry on the mean sediment transport has been investigated showing that the mean sediment transport increases as the wave asymmetry increases. An excellent agreement between predictions and existing measurements of the mean bedload transport beneath second order Stokes waves is obtained, while the mean suspended sediment concentration is well predicted near the bed and poorer predicted farther up in the water column.
To authors knowledge, the numerical model developed in the current thesis is the first one- dimensional vertical (1DV) Reynolds-averaged Navier-Stokes (RANS) solver that accounts for wave-current interaction at an arbitrary angle between the waves and the current.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11250/24352702017-01-01T00:00:00ZA Fault-Tolerant and Consistent SDN Controller
http://hdl.handle.net/11250/2435254
A Fault-Tolerant and Consistent SDN Controller
Kamisinski, Andrzej; Gonzalez, Andres Javier; Helvik, Bjarne Emil; Nencioni, Gianfranco
Software-Defined Networking (SDN) is a new paradigm that promises to enhance network flexibility and innovation. However, operators need to thoroughly assess its advantages and threats before they can implement it. Robustness and fault tolerance are among the main criteria to be considered in such assessment. The currently available SDN controllers offer different fault tolerance mechanisms, but there are still many open issues, especially regarding the trade-off between consistency and performance in a fault- tolerant SDN platform. In this paper, we describe existing fault-tolerant SDN controller solutions, and propose a mechanism to design a consistent and fault-tolerant Master-Slave SDN controller that is able to balance consistency and performance. The main objective of this paper is to bring the performance of an SDN Master-Slave controller as close as possible to the one offered by a single controller. This is obtained by introducing a simple replication scheme, combined with a consistency check and a correction mechanism, that influence the performance only during the few intervals when it is needed, instead of being active during the entire operation time.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11250/24352542016-01-01T00:00:00ZSelf-reported visual impairment, physical activity and all-cause mortality: The HUNT Study
http://hdl.handle.net/11250/2435249
Self-reported visual impairment, physical activity and all-cause mortality: The HUNT Study
Brunes, Audun; Flanders, William Dana; Augestad, Liv Berit
Aims: To examine the associations of self-reported visual impairment and physical activity (PA) with all-cause mortality. Methods: This prospective cohort study included 65,236 Norwegians aged ⩾20 years who had participated in the Nord-Trøndelag Health Study (HUNT2, 1995−1997). Of these participants, 11,074 (17.0%) had self-reported visual impairment (SRVI). The participants’ data were linked to Norway’s Cause of Death Registry and followed throughout 2012. Hazard ratios and 95% confidence intervals (CI) were assessed using Cox regression analyses with age as the time-scale. The Cox models were fitted for restricted age groups (<60, 60−84, ⩾85 years). Results: After a mean follow-up of 14.5 years, 13,549 deaths were identified. Compared with adults with self-reported no visual impairment, the multivariable hazard ratios among adults with SRVI were 2.47 (95% CI 1.94–3.13) in those aged <60 years, 1.22 (95% CI 1.13–1.33) in those aged 60–84 years and 1.05 (95% CI 0.96–1.15) in those aged ⩾85 years. The strength of the associations remained similar or stronger after additionally controlling for PA. When examining the joint associations, the all-cause mortality risk of SRVI was higher for those who reported no PA than for those who reported weekly hours of PA. We found a large, positive departure from additivity in adults aged <60 years, whereas the departure from additivity was small for the other age groups. Conclusions: Adults with SRVI reporting no PA were associated with an increased all-cause mortality risk. The associations attenuated with age.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/11250/24352492016-01-01T00:00:00ZFeed Distribution in Large Scale Sea Cage Aquaculture: Experiments, modelling and simulation
http://hdl.handle.net/11250/2435195
Feed Distribution in Large Scale Sea Cage Aquaculture: Experiments, modelling and simulation
Skøien, Kristoffer Rist
The objective of this thesis was to gain new insight into the process of feeding in
large scale sea cage aquaculture and investigate novel methods of feeding in order
to increase profitability, welfare and minimize environmental impact.
Compared to livestock farming, Atlantic salmon (Salmo salar ) farming is a
young industry which has experienced an almost exponential growth rate and the
product continues to be in high demand. A single cage in Norway may contain
more than a 1000 tonnes of fish in the form of 200.000 individuals. Such a figure is
difficult to comprehend, but one may draw a parallel to the equivalent of 1600 cows
inside a single cage. Feeding of fish kept in sea cages is a complicated endeavour
compared to land based farming for a number of reasons. Thousands of individuals
co-exist in a single three-dimensional dynamic space, observation is restricted to
surface inspections or a submerged camera with limited field of view, feed can
not be given to a specific fish and the location of feed is difficult to predict as a
consequence of currents and fish induced turbulence. In addition, feed which is
not consumed from the time it is distributed over the surface to it passes through
the cage represents a direct economic loss and acts as an unnecessary nutrient
discharge to the environment. Over 10.000 kg of feed may be administered to a
single cage towards the end of a production cycle and is the single largest cost in
Norwegian salmon farming. Even though the process of feeding is a complicated
one, the systems used to distribute feed are simple. Significant effort has been made
in determining the ration size, meal frequency and at what time of day Atlantic
salmon should be fed.
This thesis looks into the temporal feed availability on a meal to meal basis and
goes into depth with respect to the spatial availability of feed within the sea cage.
Many studies on a smaller scale indicate that spatially and temporally restrictive
feeding may lead to unequal feed accessibility, loss of growth potential and elevated
levels of aggression. With respect to controlling the spatial distribution of feed, it
has been shown that current methods cover a small area of the cage surface. In
addition, existing methods have limited ability to increase the feed distribution
without exhibiting other detrimental effects such as increased pellet breakage.
There is also no way of controlling where feed is placed as a consequence of wind or currents. Experimental results are presented to better understand the dynamics
of a feed spreader, a model has been developed and the performance of different
spreader designs investigated. Further experimental results for settling rate and
diffusion of pellets are presented and have been used to parametrize a full sea cage
model. This model enables simulation of environmental factors, feeding methods
and fish to predict the effect on central production parameters. Finally, using these
two models, different feeding regimes are simulated and the consequent effects on
spatiotemporal feed distribution, feed intake and feed loss are commented upon.
It is likely that by increased use of environmental measurements run through
feed distribution models and having more adaptable methods of feed placement,
one can in the future minimize the environmental impact whilst maintaining high
growth rates and good fish welfare.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/11250/24351952017-01-01T00:00:00Z