No description available.

Electric power generation using wind turbines is on the rise in not only the United States but the entire globe. While the benefits from such methods of generation include clean and renewable energy, wind turbines may pose a potential risk to the stability of grid operation. Wind turbine generators are similar to conventional generators; however, the manner with which the wind turbine is coupled to the grid may reduce system inertia and increase the magnitude of transient stability problems. This study empirically examines the effect of wind generation on ERCOT system response characteristics following unit trips such as frequency drop, and phasor oscillation frequency and damping. It is shown with a high degree of certainty that an increase in wind generation is leading to a greater phasor oscillation frequency and lesser system inertia. Wind generation may also be leading to less system damping and smaller power frequency drops. / text

The GB power system will see huge growth in transmission connected wind farms over the next decade, driven by European clean energy targets. The majority of the UK’s wind development is likely to be offshore and many of these wind farms will be interfaced to the grid through power converters. This will lead to a loss of intrinsic inertia and an increasing challenge for the system operator to keep grid frequency stable. Given this challenge, there is increasing interest in understanding the capabilities of converter control systems to provide a synthesised response to grid transients. It is interesting to consider whether this response should be demanded of wind turbines, with a consequential reduction in their output, or if advanced energy storage can provide a viable solution. In order to investigate how large offshore wind farms could contribute to securing the power system, wind turbine and wind farm models have been developed. These have been used to design a patented method of protecting permanent magnet generator’s converters under grid faults. Furthermore, these models have enabled investigation of methods by which a wind turbine can provide inertial and frequency response. Conventionally inertial response relies on the derivative of a filtered measurement of system frequency; this introduces either noise, delay or both. This research proposes alternative methods, without these shortcomings, which are shown to have fast response. Overall, wind farms are shown to be technically capable of providing both high and low frequency response; however, holding reserves for low frequency response inevitably requires spilling wind. Wind’s intermittency and full output operation are in tension with the need of the power system for reliable frequency response reserves. This means that whilst wind farms can meet the technical requirements to hold reserves, they bid uncompetitive prices in the market. This research shows that frequency response market prices are likely to rise in future suggesting that the Vanadium Redox Flow Battery is one technology which could enter this market and also complement wind power. Novel control incorporating fuzzy logic to manage the battery is developed to allow a hybrid wind and storage system to aggregate the benefits of frequency response and daily price arbitrage. However, the research finds that the costs of smoothing wind power output are a burden on the store’s revenue, leading to a method of optimising the combined response from an energy store and generator that is the subject of a patent application. Furthermore, whilst positive present value may be derived from this application, the long payback periods do not represent attractive investments without a small storage subsidy.

No description available.

This thesis presents a unique finite element investigation of the seismic behaviour of 2 retaining wall types – a rigid retaining wall and a cantilever retaining wall. The commercial finite element program PLAXIS2D was used to develop the numerical simulation models. The research includes: (1) validating the finite element model with the results of 3 previously existing centrifuge tests taken from literature; (2) investigating the seismic response of rigid and cantilever retaining walls including studying the effects of contribution of wall displacement, wall and backfill seismic inertia and stiffness of the foundation soil; (3) developing analytical methods to concrete the findings of the numerical models. Based on the results of the seismic response of a rigid retaining wall, a unique relationship between the seismic earth pressure and wall displacement has been developed for the active and passive modes of failure. The seismic active earth pressure has been found to be not dependent on the wall displacement while the seismic passive earth pressure has been found to be highly affected by the wall displacement. The maximum seismic passive earth pressure force and relative horizontal displacement are predicted when the ground earthquake acceleration is applied with maximum amplitude and minimum frequency content. The seismic response of the wall was not affected by the ratio of the frequency content of the earthquake to the natural frequency of the wall-soil system. For the cantilever retaining wall detailed structural integrity and global analyses have been carried out. It has been observed that the seismic earth pressure, computed at the stem and along a vertical virtual plane are found to be out of phase with each other during the entire duration of the earthquake, and hence, the structural integrity and global stability should be evaluated and assessed individually. A critical case for the structural integrity is observed when the earthquake acceleration is applied towards the backfill soil and has frequency content close to the natural frequency of the retaining wall, while, for the global stability, the critical case is observed when the earthquake acceleration has maximum amplitude and is applied towards the backfill soil with minimum frequency content. The structural integrity is also found to be highly dependent on the ratio between the frequency content of earthquake acceleration to the natural frequency of the cantilever retaining wall. The relative horizontal displacement of a rigid and cantilever retaining wall is found to be highly affected by the duration of the earthquake in contrast to what has been observed for the seismic earth pressure force. The structural integrity of a rigid and cantilever retaining wall reduces when the backfill soil has a higher relative density, while the global stability increases when the backfill soil has a high relative density during an earthquake. The results obtained from the analytical methods reveal that the wall seismic inertia force has a significant effect on the structural integrity only for the top of the stem while the base of the stem does not get affected significantly. The modified Newmark sliding block method provided a more reasonable estimation of the relative horizontal displacement of a rigid retaining wall and a cantilever retaining wall compared with the classic Newmark sliding block method.

In the present study, the influence of geometric scale, unit orientation (alternatively, flow direction), and the dimensionless Reynolds and Keulegan-Carpenter quantities on the hydrodynamic loading of Core-Loc armour units is explored through a series of physical modelling tests under unsteady and oscillatory flow conditions.

The ability to understand and predict the flow behaviour of non-Newtonian fluids in swirling flow is industrially important for the efficient design and performance of processes which utilise fluids with complex rheological properties. In particular, fluids with elastic properties are not well described by non-Newtonian constitutive models, such that predictions using such models must be carefully validated. A benchmark problem is proposed here which provides a well defined geometry to study the swirling flow of non-Newtonian fluids as a test case for the validation of constitutive models. The confined swirling flow utilised is a torsionally driven cavity where the test fluid is confined in a cylinder with a rotating bottom lid, and stationary side walls and top lid. The flow field is three-dimensional and consists of both a primary motion, which is directed azimuthally, and a secondary motion, which is located in the radial and axial plane of the cylinder and driven by inertial and/or elastic forces.

Evolutionary Structural Optimisation (ESO) is a relatively new design tool used to improve and optimise the design of structures. It is a heuristic method where a few elements of an initial design domain of finite elements are iteratively removed. Such a process is carried out repeatedly until an optimum design is achieved, or until a desired given area or volume is reached. There have been many contributions to the ESO procedure since its conception back in 1992. For example, a provision known as Bi-Directional ESO (BESO) has now been incorporated where elements may not only be removed, but added. Also, rather than deal with elements where they are either present or not, the designer now has the option to change the element's properties in a progressive fashion. This includes the modulus of elasticity, the density of the material and the thickness of plate elements, and is known as Morphing ESO. In addition to the algorithmic aspects of ESO, a large preference exists to optimise a structure based on a selection of criteria for various physical processes. Such examples include stress minimisation, buckling and electromagnetic problems. In a changing world that demands the enhancement of design tools and methods that incorporate optimisation, the development of methods like ESO to accommodate this demand is called for. It is this demand that this thesis seeks to satisfy. This thesis develops and examines the concept of multicriteria optimisation in the ESO process. Taking into account the optimisation of numerous criteria simultaneously, Multicriteria ESO allows a more realistic and accurate approach to optimising a model in any given environment. Two traditional methods � the Weighting method and the Global Criterion (Min-max) method have been used, as has two unconventional methods � the Logical AND method and the Logical OR method. These four methods have been examined for different combinations of Finite Element Analysis (FEA) solver types. This has included linear static FEA solver, the natural frequency FEA solver and a recently developed inertia FE solver. Mean compliance minimisation (stiffness maximisation), frequency maximisation and moment of inertia maximisation are an assortment of the specific objectives incorporated. Such a study has provided a platform to use many other criteria and multiple combinations of criteria. In extending the features of ESO, and hence its practical capabilities as a design tool, the creation of another optimisation method based on ESO has been ushered in. This method concerns the betterment of the bending and rotational performance of cross-sectional areas and is known as Evolutionary Moment of Inertia Optimisation (EMIO). Again founded upon a domain of finite elements, the EMIO method seeks to either minimise or maximise the rectangular, product and polar moments of inertia. This dissertation then goes one step further to include the EMIO method as one of the objectives considered in Multicriteria ESO as mentioned above. Most structures, (if not all) in reality are not homogenous as assumed by many structural optimisation methods. In fact, many structures (particularly biological ones) are composed of different materials or the same material with continually varying properties. In this thesis, a new feature called Constant Width Layer (CWL) ESO is developed, in which a distinct layer of material evolves with the developing boundary. During the optimisation process, the width of the outer surrounding material remains constant and is defined by the user. Finally, in verifying its usefulness to the practical aspect of design, the work presented herein applies the CWL ESO and the ESO methods to two dental case studies. They concern the optimisation of an anterior (front of the mouth) ceramic dental bridge and the optimisation of a posterior (back of the mouth) ceramic dental bridge. Comparisons of these optimised models are then made to those developed by other methods.

The ability to understand and predict the flow behaviour of non-Newtonian fluids in swirling flow is industrially important for the efficient design and performance of processes which utilise fluids with complex rheological properties. In particular, fluids with elastic properties are not well described by non-Newtonian constitutive models, such that predictions using such models must be carefully validated. A benchmark problem is proposed here which provides a well defined geometry to study the swirling flow of non-Newtonian fluids as a test case for the validation of constitutive models. The confined swirling flow utilised is a torsionally driven cavity where the test fluid is confined in a cylinder with a rotating bottom lid, and stationary side walls and top lid. The flow field is three-dimensional and consists of both a primary motion, which is directed azimuthally, and a secondary motion, which is located in the radial and axial plane of the cylinder and driven by inertial and/or elastic forces.

<p>The research area of recruitment utility analysis (RUA) models has</p><p>been somewhat unexplored for decades, and has earlier been reduced</p><p>to simplified mathematic formulas measuring only dollar term value.</p><p>The need for more dynamic models and theories surrounding the area</p><p>has been voiced numerous times, yet little has been done. The purpose</p><p>of this study was to highlight this need to encourage to further</p><p>research, and to examine the managerial perspective on RUA from a</p><p>semi explorative perspective. Eight interviews were conducted where</p><p>the managers received questions concerning the recruitment process</p><p>and the method used to conduct RUA. The results of this study</p><p>indicate somewhat unimpressed attitudes toward the added value of</p><p>conducting RUA. The reasons for not conducting</p><p>RUA were found to be due to time constraint, lack of skills and</p><p>managerial support. The majority of the RUA was found to be reactive</p><p>and based on gut feeling implying a need for proactive RUA hence</p><p>ensuring the value of the recruitment process.</p>