The research work reported in this thesis stems from the development of an accurate and computationally efficient Reynolds-Averaged Navier-Stokes (RANS) research code, with a particular emphasis on the steady and unsteady aerodynamics analysis of complex low speed turbulent flows. Such turbulent flow problems include horizontal axis wind turbine (HAWT) and vertical axis wind turbine (VAWT) operating at design and off-design conditions. On the algorithmic side, the main contribution of this research is the successful development of a rigorous novel approach to low-speed preconditioning (LSP) for the multigrid fully coupled integration of the steady, time-domain and harmonic balance RANS equations coupled to the two-equation shear stress transport (SST) turbulence model. The design of the LSP implementation is such that each part of the code affected by LSP can be validated individually against the baseline solver by suitably specifying one numerical input parameter of the LSP-enhanced code. The thesis has investigated several important issues on modelling and numerical aspects which are seldom thoroughly analysed in the computational fluid dynamics problems of the type presented herein. The first and most important modelling issue is the necessity of applying the low speed preconditioning to both RANS and SST equations and maintaining the turbulent kinetic energy in the definition of the total energy, which, to the best knowledge of author, has never been seen in any published literature so far. Based on the results obtained in the analysis of the vertical axis wind turbine application, we have demonstrated that by preconditioning the SST turbulence equations, one can significantly improve the convergence rate; and keeping the turbulence kinetic energy in the total energy has a great positive effect on the solution accuracy. The other modelling issue to be analysed is the sensitivity of the flow solution to the farfield boundary conditions, particularly for low speed problems. The analyses reported in the thesis highlight that with a small size of the computational domain, the preconditioned farfield boundary conditions are crucial to improve the solution accuracy. As for the numerical aspects, we analyse the impact of using the relative velocity to build the preconditioning parameter on the flow solutions of an unsteady moving-grid problem. The presented results demonstrate that taking into account the grid motion in building the preconditioning parameter can achieve a noticeable enhancement of the solution accuracy. On the other hand, the nonlinear frequency-domain harmonic balance approach is a fairly new technology to solve the unsteady RANS equations, which yields significant reduction of the run-time required to achieve periodic flows with respect to the conventional time-domain approach. And the implementation of the LSP approach into the turbulent harmonic balance RANS and SST formulations is another main novelty presented herein, which is also the first published research work on this aspect. The newly developed low speed turbulent flow predictive capabilities are comprehensively validated in a wide range of tests varying from subsonic flow with slight compressibility to user-defined extremely low speed incompressible flows. The solutions of our research code with LSP technology are compared with experiment data, theoretical solutions and numerical solutions of the state-of-the-art CFD research code and commercial package. The main computational results of this research consist of the analyses of HAWT and VAWT applications. The first one is a comparative analysis of 30% and 93.5% blade sections of a VESTAS multi-megawatt HAWT working in various regimes. The steady, time-domain and frequency-domain results obtained with the LSP solver are used to analyse in great detail the steady and unsteady aerodynamic characteristics in those regimes. The main motivation is to highlight the predictive capabilities and the numerical robustness of the LSP-enhanced turbulent steady, time-domain and frequency domain flow solvers for realistic complex and even more challenging problems, to quantify the effects of flow compressibility on the steady and yawed wind-induced unsteady aerodynamics in the tip region of a 82-m HAWT blade in rated operating condition, and to assess the computational benefits achieved by using the harmonic balance method rather than the conventional time-domain method. The second application is the comparative aerodynamic analyses of the NREL 5MW HAWT working in the inviscid steady flow condition. The main motivation of this analysis is to further demonstrate the predictive capabilities of the LSP solver to simulate the threedimensional wind turbine flows. The last application is the time-domain turbulent flow analysis of the VAWT to the aim of demonstrating the accuracy enhancement of the LSP solver for this particular problem, the necessity of applying the full preconditioning strategy, the important effect of the turbulent kinetic energy on the solution accuracy and the proper implementation of the preconditioning parameter required for an accurate numerical solution to an unsteady moving grid low-speed problem.

The use of Computational Fluid Dynamics (CFD) has become a well-established approach in the analysis and optimisation of impulse hydro turbines. Recent studies have shown that modern CFD tools combined with faster computing processors can be used to accurately simulate the operation of impulse turbine runners and injectors in timescales suitable for design optimisation studies and which correlate well with experimental results. This work has however focussed mainly on Pelton turbines and the use of CFD in the analysis and optimisation of Turgo turbines is still in its infancy, with no studies showing a complete simulation of a Turgo runner capturing the torque on the inside and outside blade surfaces and producing a reliable extrapolation of the torque and power at a given operating point. Although there have been some studies carried out in the past where injector geometries (similar for both Pelton and Turgo turbines) have been modified to improve their performance, there has been no thorough investigation of the basic injector design parameters and the influence they have on the injector performance. The aim of this research is to use modern CFD tools to develop models which aid the better understanding of Turgo impulse turbine runners and injectors and facilitate the optimisation of existing designs. CFD is used to model and optimise both the injectors and the runner of a modern commercial Turgo impulse turbine and the accuracy of the models are verified by carrying out experimental tests on the original and optimised designs. The original designs together with experience in the operation of these turbines were provided by the industrial sponsors of this research Gilbert Gilkes and Gordon Ltd. The research described in this thesis can be split into five main parts: 1.Development of a numerical model to analyses the flow through the Turgo runner using modern CFD tools combined with a series of assumptions to reduce the computational time while still retaining the accuracy of the model. Using this model to optimise the design of the Turgo runner provided by Gilkes. 2.Development of a similar numerical model for a simplified 2D injector design to facilitate a study of the impact of the basic design parameters on the performance over a range of operating conditions. Applying these optimisations to the existing Gilkes design and taking the numerical analysis further by including the full injector geometry as well as the branch pipe and guide vanes. 3.Manufacture and experimental testing of the original and optimised Turgo runners. 4.Manufacture and experimental testing of the original and optimised injector designs. 5.Verification of the numerical models developed in 1.) and 2.) by comparison with the experimental results. The numerical model developed in 1.) includes several simplifying assumptions in order to reduce the computational time and produce models which could solve in reasonable timescales allowing many design variations to be analysed. As the runner simulations require a transient analysis of complex multi-phase free surface flow with a rotating frame of reference they are already computationally costly and efforts have to be made to reduce this computational cost if the models are to be effective for optimisation purposes. The runner model simplifications were the exclusion of any casing interactions by not modelling the casing and the use of a 2 blade model analysing only a single blade passage in order to reduce the size of the computational domain. Several modelling assumptions were also introduced and attempts are made to quantify the effects of these assumptions through unit tests. For discretisation of the domain two mesh sizes were used, a coarse mesh which slightly under predicts the efficiency but was suitable for comparing designs and a fine mesh which gave mesh independent results. The fine mesh took over 4 times longer to solve rendering it unfeasible for optimisation purposes and it was therefore used only at key points to verify the design changes made using the coarse mesh. The analysis and optimisation of the injectors carried out in 2.) use similar CFD tools as the runner analysis however the geometry (excluding the branch pipe and guide vanes) could be simplified into a 2D axisymmetric case operating at steady state conditions. This drastically reduces the solve time and allows the use of a mesh independent model and the analysis of hundreds of designs and operating conditions. Once the optimisations had been carried out, the design changes were verified by extending the model to analyse the 3D case with a straight pipe upstream of the injector and a 3D full case including the branch pipe and guide vanes. In 3.), following the optimisation of the runner in 1.), a Finite Element Analysis (FEA) of the runner was carried out to ensure the optimised runner had sufficient strength for operation at the highest heads recommended for a runner of this size. The design was strengthened based on the results of the FEA and CFD was carried out in conjunction with these changes to ensure minimal loss in hydraulic efficiency. The manufacturing process was also researched and Design for Manufacture and Assembly (DFMA) was applied to the strengthened design identifying two optimised designs (LE4 and LE1) which will be tested before and after additional dressing of the leading edges. Both optimised runner designs were manufactured and tested at the Laboratory of Hydraulic Machines, National Technical University of Athens (NTUA). Following the injector analysis and optimisations in 2.), the optimised injectors were manufactured for experimental testing using both the Pelton and the Turgo test rig at NTUA in 4.). As the design changes made were not critical to the strength of the injectors there was no need to carry out a FEA. The CFD model verification in Part 5.) looks initially at the full Turgo system in order to compare the absolute difference between the numerical efficiency and the experimental efficiency of the original Turgo runner at the best efficiency point. The mechanical losses of the test rig are estimated to determine the experimental hydraulic efficiency. The numerical hydraulic efficiency is then determined by calculating the losses upstream of the injector, using standard pipe flow equations and combing these with the losses through the injector, as well as the numerical efficiency of the runner by simulating the runner using the ‘real jet’ profile produced by the full injector simulations. The results showed the numerical model to be over-predicting the efficiency by 1.26%. The numerical difference in the performance of the two injectors is then compared to the experimental difference measured during testing. This is done by importing the ‘real jet’ profiles produced by the full 3D injector simulations into the LE1 runner simulation. This allows the difference in total efficiency between the injectors combined with the runner to be compared to the experimental differences which also includes the impact of the jet on the runner performance. The comparison between the injectors is less accurate as more uncertainties are introduced when combining these models and the differences are smaller however the CFD was able to predict the improvements to within 0.4%. Finally, the numerical differences between the runner designs and the experimental differences are compared showing that the runner model is able to predict differences in hydraulic efficiency to within 0.1%. This accuracy is largely down to that fact that many of the systematic experimental and modelling errors are cancelled out when comparing only the runners. / The CFD model verification has shown that although the absolute performance of the Turgo system can be modelled numerically to within a good degree of accuracy, it requires combining injector and runner models as well as estimating additional losses in the pipework which can prove time consuming. However for design comparison and optimisations the CFD models have been shown to be far more accurate suggesting that this is where these numerical models are most useful.

No description available.

Horizontal Axis Tidal Turbines (HATTs) can experience amplified, time varying hydrodynamic loads during operation due to dynamic stall. Elevated hydrodynamic loads impose high structural loads on turbine blades, thus appreciably shortening machine service life. An improved characterization of the unsteady hydrodynamic loads on tidal turbine blades is therefore necessary to enable more reliable predictions of their fatigue life and to avoid premature failures. This thesis reports on a Computational Fluid Dynamics (CFD) analysis of the unsteady blade loading of a scale-model HATT taking dynamic stall into account. Numerical simulations are performed both in two-dimensional (2-D) and three-dimensional (3-D) using the commercial CFD solver ANSYS Fluent. After a brief description of the theories and methods involved, the behaviour of flow at low Reynolds number around a NACA-0012 aerofoil pitching in a sinusoidal pattern that induces dynamic stall is studied firstly to validate the numerical method and the choice of turbulence models. Then full 3-D computations of a rotating scale-model HATT rotor are presented for steady and periodic unsteady inflow situations, respectively. The reliability of the 3-D numerical method is evaluated by comparing the blade loads, especially the out-of-plane blade-root bending moment (defined as being about an axis normal to the rotor axis), with measurement data obtained from experimental tests conducted at the University of Strathclyde’s Kelvin Hydrodynamics Laboratory towing tank. Analyses in the steady velocity study are documented for a broad range of rotor speeds and flow velocities. Furthermore, investigations of 3-D flow separation and scale effects on blade loads are also performed. The periodic unsteady velocity study aims to examine the out-of-plane blade-root bending moment response to harmonic axial motion, deemed representative of the free-stream velocity perturbations induced by the unsteady flow. Parametric tests on oscillatory frequencies and amplitudes are carried out in order to analyse the HATT blade hydrodynamic behaviour under different flow patterns. Detailed flow field data is analysed to understand 3-D dynamic stall from a modelling perspective. It is concluded that the results by the present study provide significant insights into the flow physics occurring around the HATT rotor blades under various flow conditions. The CFD method can be used for designing more advanced HATT rotors, it also can be used to fine tune the computationally faster lower order Blade Element Momentum (BEM) methods for parametric design studies where experimental data is not available, particularly at the challenging rotor operating conditions involving flow separation and dynamically varying hydrodynamic behaviours.

Two inverse methods for turbomachinery blade design are developed. In these inverse design approaches blade shapes are computed for a specified design parameter such as mass-averaged tangential velocity or pressure loading distribution. These inverse methods directly define a geometry needed to obtain these prescribed target design parameters which are related to the performance of turbomachinery blades. The first method is based on the prescription of pressure loading on the blade while the second method is based on the prescription of mass-averaged tangential velocity on the blade. In both methods the blade thickness is also prescribed. These choices of target design prescription allow the designer to control the blade work distribution and the overall flow field effectively. It also prevents the generation of unrealistic blades as the designer directly control the blade thickness. Mesh movement algorithm is an integral part of the current inverse design method as once the blade surface is modified during the design iterations the corresponding unstructured mesh also has to be altered. The mesh movement algorithm is based on a linear tension spring analogy which is a very fast and robust mesh movement method. The capabilities of these design methodologies have been verified for inverse design of two dimensional turbomachinery blades. The flow analysis algorithm is an integral part of the current methodologies. It is based on the incompressible Navier-Stokes flow equations on unstructured meshes. The capability of the flow analysis algorithm is verified for three-dimensional external and internal incompressible flow solutions. Indeed the current method is applied for simulation of flow over marine propeller blades in open water. Also it is applied for the flow analysis of the stator and rotor blades of a low-speed axial turbine.

The generation of electricity from ocean waves using oscillating water column (OWC) wave energy converters is currently uneconomic due to the high capital cost and low efficiencies of such devices. The bidirectional air turbines utilised in OWCS are one of the principal sources of inefficiency and a significant increase in their performance would improve the prospects of commercial scale wave power generation. The ability of computational fluid dynamics (CFD) to predict the performance of both Wells and impulse type bidirectional turbines for use in OWCS was examined by comparison with experimental results taken from the literature. A design process was then undertaken for a datum impulse turbine and a novel high-efficiency impulse turbine arrangement. Numerical performance predictions are presented with a comparison against experimental data from a large-scale oscillating-flow test rig. An automated design and aerodynamic optimisation system was subsequently developed for application to this novel impulse turbine design. The optimiser employs a hybridised genetic algorithm along with Kriging meta¬models to significantly decrease the number of expensive calls to the 3D-CFD code used to evaluate the objective function. Comparisons to a number of state of the art optimisation algorithms from the literature on some mathematical test functions indicated that the optimiser had equivalent or better performance for most problems. A parameter study was carried out to investigate the effect of various turbine in design variables, before undertaking a 14-variable global design optimisation. A 5-variable optimisation exercise was then performed to investigate the effi- ciency gains that could be achieved by using three-dimensional rotor blades. Substantial gains in performance were attained and the predicted levels of efficiency are significantly higher than those previously reported in the literature for other bidirectional impulse turbine designs.

Bladed discs are very sensitive structures and the amplitude vibration of each blade can vary significantly from blade to blade due to a series of factors such as geometrical inhomogeneity between blades or material properties. These factors lead to bladed disks mistuned thus the forced response amplitudes can be much higher than the level predicted for a tuned assembly. Designed models need to be “validate” to predict the response of a real bladed disc within the tolerances set by the manufactures and this process is very expensive as well as difficult. The validation process needs “reference data” as fundamental input against what all predictions can be compared and validated. Data that can be provided both under stationary conditions and under rotating conditions and the latter is the most difficult to achieve, especially for bladed disc assemblies which are very sensitive to any structural modification as it could be attaching a transducer to measure vibrations. There are contact-less measurement techniques available which, however, provide limited information because they can measure only limited areas of the vibrating structures. The aim of this study is to design measurement methods, using a standard Scanning Laser Doppler Vibrometer (SLDV) and to integrate it into a software platform which will be able to handle a series of measurement tasks both under stationary and rotating conditions. The main contribution of this thesis is to extend the use of Continuous Scanning LDV (CSLDV) to the rotating structures, such as bladed discs, thus to perform synchronous measurements. Hence, a bladed disc is needed to be designed to perform vibration predictions and measurements and a mathematical model of the measurement test to control, critically, all possible sources of errors involved in measurement under rotating conditions; all these to produce a robust measurement method. While the primary focus is the measurement method, the study also extends to evaluation of the sensitivity properties of the bladed disk test pieces that are the object of the measurement tool.

No description available.

The thesis describes a set of studies carried out in parallel with a tidal turbine design program which was undertaken by the Turbomachinery Group to which the author belonged to during the duration of this project. Therefore the work presented in this thesis constitutes an exploration of the physics of horizontal axis tidal turbines and of the modelling issues associated with the simulation of these devices. Specifically the investigation centered on the behaviour of the turbine when exposed to the influence of tidal channel waves. The numerical analysis of the free standing, gravity stabilised horizontal axis tidal turbine (HATT) was carried out with the application of Speziale’s Reynolds Stress Model (hereafter know as Speziale, Sarkar and Gatski or SSG model) available in the CFX CFD code. The use of a number of turbulence models was investigated but poor convergence or starting difficulties led to the employment of the SSG Reynolds Stress turbulence model. Simulations for a range of test cases were undertaken ranging from single blade passage cases to transient simulations which included the motion of the turbine and the variation of the flow velocity due to the combined action of waves and the shearing effects of the sea bed boundary layer. A number of CFD results were compared to experimental data acquired from tests conducted on a scale model in the summer of 2009 at the IFREMER test flume in France. An additional source of comparison is provided by data obtained from a BEMT code produced by the CU consultant, Mr Chris Freeman, (Freeman et al., 2009a). A number of numerical models were assembled to analyse the effects of the presence of the pylon and blade-pylon spacing. These were run as steady and unsteady cases. For the steady cases several angular positions were examined to investigate the effects of the transit of the blades through the pylon potential field and across the sea bed boundary layer shear flow. An idealised no-pylon case was analysed to compare with the equivalent model with pylon. On average there was a performance increase of 2% for this configuration when compared to the case with pylon for the datum spacing. The simulations covered four turbine rotational speed cases. These correspond to a startup condition and to rated power, with an intermediate point, and to an overspeed regime. In the overspeed condition the power is essentially unchanged but the thrust reduction is strong. Additional investigations covered the performance of the turbine when yawed and the influence of inflow turbulence. The comparisons between the solutions obtained from steady-state and transient simulations showed that the unsteady approach is preferable to describe these types of flow. Two waves were employed in conjunction with the transient models. The first corresponded to a moderate sea state (1:5m height and 10:0s time period) of the type occurring more frequently. The second case involved a substantial wave (3:0m height and 14:0s time period) which is associated with storm conditions. The datum models which incorporated the most energetic wave showed similar values for the torque observed on the transient simulations without waves for the overspeed and rated power cases respectively. This is a significant finding given that the 15m diameter turbine is immersed in a 35m tidal channel. The peak value for the axial thrust and torque on the large wave simulation is on average 86% and 90% higher than the steady state thrust and torque respectively for the rated power case. The loads imposed on the rotor system for the large wave are approximately 3% higher than those of the regular wave. Similar detailed studies for tidal turbines are non-existent at time of publication. The work is therefore unique in the scope and breath of the simulations it contains. The computational resources required were vast. Transient simulations including wave effects took three weeks of computation time utilizing sixteen processors. Each of these cases required about several terabytes of storage space to record the intermediate transient results.

No description available.