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Large eddy simulations are considered for wind plants with varied spanwise and streamwise spacing. Data from five different configurations of staggered and aligned LES wind turbine arrays with a neutrally stratified atmospheric boundary layer are employed for analysis. Fields are analyzed by evaluating the anisotropy stress invariants based on the Reynolds shear stresses and dispersive stress tensor. The relationship between quantities are drawn as a function of the wind plant packing. Reynolds stresses and dispersive stresses are investigated alongside a domain altered version of the second and third scalar invariants, ξ, η, as well as the combination of the two invariants described by the function F = 1−27η 2 + 54ξ 3. F is a measure of the approach to either a two-component turbulence (F=1) or an isotropic turbulence (F=0). The invariant η describes the degree of anisotropy while ξ describes the characteristic shape. For the purposes of this study, the LES data is analyzed to understand the effects of canopy density on anisotropy and dispersive stresses, adding further insight and detail for future modeling techniques.

This thesis covers the analysis, dynamic modelling and control of an isolated selfexcited induction generator (SEIG) driven by a variable speed wind turbine. The voltage build up process of an isolated induction generator excited by AC capacitors starts from charge in the capacitors or from a remnant magnetic field in the core. A similar voltage build up is obtained when the isolated induction generator is excited using an inverter/rectifier system with a single DC capacitor on the DC link of the converter. In this type of excitation the voltage build up starts from a small DC voltage in the DC link and is implemented using vector control. The dynamic voltage, current, power and frequency developed by the induction generator have been analysed, simulated and verified experimentally for the loaded and unloaded conditions while the speed was varied or kept constant. Results which are inaccessible in the experimental setup have been predicted using the simulation algorithm. To model the self excited induction generator accurate values of the parameters of the induction machine are required. A detailed analysis for the parameter determination of induction machines using a fast data acquisition technique and a DSP system has been investigated. A novel analysis and model of a self-excited induction generator that takes iron loss into account is presented in a simplified and understandable way. The use of the variation in magnetising inductance with voltage leads to an accurate prediction of whether or not self-excitation will occur in a SEIG for various capacitance values and speeds in both the loaded and unloaded cases. The characteristics of magnetising inductance, Lm, with respect to the rms induced stator voltage or magnetising current determines the regions of stable operation as well as the minimum generated voltage without loss of self-excitation. In the SEIG, the frequency of the generated voltage depends on the speed of the prime mover as well as the condition of the load. With the speed of the prime mover of an isolated SEIG constant, an increased load causes the magnitude of the generated voltage and frequency to decrease. This is due to a drop in the speed of the rotating magnetic field. When the speed of the prime mover drops with load then the decrease in voltage and frequency will be greater than for the case where the speed is held constant. Dynamic simulation studies shows that increasing the capacitance value can compensate for the voltage drop due to loading, but the drop in frequency can be compensated only by increasing the speed of the rotor. In vector control of the SEIG, the reference flux linkage varies according to the variation in rotor speed. The problems associated with the estimation of stator flux linkage using integration are investigated and an improved estimation of flux linkage is developed that compensates for the integration error. Analysis of the three-axes to two-axes transformation and its application in the measurement of rms current, rms voltage, active power and power factor from data obtained in only one set of measurements taken at a single instant of time is discussed. It is also shown that from measurements taken at two consecutive instants in time the frequency of the three-phase AC power supply can be evaluated. The three-axes to twoaxes transformation tool simplifies the calculation of the electrical quantities.

Thesis (M.D.S.)--University of Hong Kong, 1998. / Includes bibliographical references (leaves 96-105) Also available in print.

This thesis presents and examines the concept that the output of a wound-rotor induction generator (WRIG) can be limited by means of linking to external impedances for wind-turbine generating system application. An 80kW-WRIG is simulated as a model to examine the control of the output power vs. speed characteristic. Model of WRIG derived from per phase equivalent circuit is organized, then it is estimated how much external impedances affect the characteristic of output power for it to approach to a typical wind-turbine curve. Practical tests are performed using 80kW-WRIG in testing lab to validate the simulation data. In addition, a smaller WRIG, connected on same shaft as 80kW-WRIG, is designed to extend the range of wind speed. Also external impedances with smaller WRIG are chosen to extract optimum power from wind-turbine. Finally, passively controlled tandem WRIGs are shown to have the capability to optimize wind-turbine energy extraction when controlled entirely by external impedances. / Graduation date: 2003

The effects of the orifice geometry and the surface boundary condition on the heat transfer distribution to a flat surface of an impinging jet array were investigated. The jet array impinged normally onto the surface which was either isothermal or had a uniform heat flux. The experiments were performed for the flow rate range from 0.0039 to 0.0070 m��/s corresponding to jet Reynolds numbers of 5000 to 11000. The jet-to-surface spacings varied from 1 to 4 jet diameters. After impinging, the air jet was constraine4 to exit in one direction creating a "crossflow". condition. The isothermal surface results are presented in terms of the average heat transfer coefficient. For the uniform heat flux surface, both average and local values are presented. The average and local heat transfer distributions were mapped using thermochromic liquid crystals. Results are presented for two jet geometries: circular and cusped ellipse. The cusped ellipse jets show better heat transfer performance compared to the circular jets for both surface boundary conditions. This is thought to be a result of increased turbulence and the axis-switching phenomenon. Results for the uniform heat flux surface boundary higher than for the isothermal surface boundary condition. This result can be explained by the difference between the surface temperature and the jet temperature for both surface boundary conditions. Correlations of Nusselt versus Reynolds numbers are presented for both jet geometries and surface boundary conditions. / Graduation date: 2000

Thesis (M.S.)--Virginia Polytechnic Institute, 1953. / Typewritten. Vita. Bibliography: leaves 90-91. Also available via the Internet.

The primary goal for this PhD project has been to investigate instability of reversiblepump turbines (RPTs) as a phenomenon and to find remedies to solve it. The instability occurs for turbines with s-shaped characteristics, unfavourable waterways and limited rotating inertia. It is only observed for certain operation points at either high speed or low load. These correspond to either high values of Ned or low values of Qed. The work done in this PhD thesis can be divided into the three following categories. Investigate and understand the behaviour of a pump turbine: A model was designed in order to investigate the pump turbine behaviour related to its characteristics. This model was manufactured and measurements were performed in the laboratory. By using throttling valves or torque as input the full s-shaped characteristics was measured. When neither of these techniques is used, the laboratory system has unstable operation points which result in hysteresis behaviour. Global behaviour of the RPT in a power plant system was investigated through analytical stability analysis and dynamic system simulations. The latter included both rigid and elastic representation of the water column. Turbine internal flow: The flow inside the runner was investigated by computer simulations (CFD). Two-dimensional analysis was used to study the inlet part of the runner. This showed that a vortex forming at the inlet is one of the causes for the unstable characteristics. Three-dimensional analyses were performed and showed multiple complex flow structures in the unstable operation range. Measurements at different pressure levels showed that the characteristics were dependent on the Reynolds number at high Ned values in turbine mode. This means that the similarity of flows is not sufficiently described by constant Qed and Ned values at this part of the characteristics. Design modifications: The root of the stability problem was considered to be the runner’s geometric design at the inlet in turbine mode. Therefore different design parameters were investigated to find relations to the characteristics. Methods used were measurements, CFD modelling and analytical models. The leading edge profile was altered on the physical model and measurements were performed in the laboratory. Results showed that the profiles have significant influence on characteristics and therewith stability at high speed operation points. Other design parameters were investigated by CFD analysis with special focus on the inlet blade angle.

With the present drive in turbomachine engine developmenttowards thinner and lighter bladings, closer spaced blade rowsand higher aerodynamic loads per blade row and blade, advanceddesign criteria and accurate prediction methods for vibrationalproblems such as forced response become increasingly importantin order to be able to address and avoid fatigue failures ofthe machine early in the design process. The present worksupports both the search for applicable design criteria and thedevelopment of advanced prediction methods for forced responsein transonic turbine stages. It is aimed at a betterunderstanding of the unsteady aerodynamic mechanisms thatgovern forced response in transonic turbine stages and furtherdevelopment of numerical methods for rotor stator interactionpredictions. The investigation of the unsteady aerodynamic excitationmechanisms is based on numerical predictions of thethree-dimensional unsteady flow field in representative testturbine stages. It is conducted in three successive steps. Thefirst step is a documentation of the pressure perturbations onthe blade surface and the distortion sources in the bladepassage. This is performed in a phenomenological manner so thatthe observed pressure perturbations are related to thedistortion phenomena that are present in the blade passage. Thesecond step is the definition of applicable measures toquantify the pressure perturbation strength on the bladesurface. In the third step, the pressure perturbations areintegrated along the blade arc to obtain the dynamic bladeforce. The study comprises an investigation of operationvariations and addresses radial forcing variations. With thehelp of this bottom-up approach the basic forcing mechanisms oftransonic turbine stages are established and potential routesto control the aerodynamic forcing are presented. For the computation of rotor stator interaction aerodynamicsfor stages with arbitrary pitch ratios a new numerical methodhas been developed, validated and demonstrated on a transonicturbine test stage. The method, which solves the unsteadythree-dimensional Euler equations, is formulated in thefour-dimensional time-space domain and the derivation of themethod is general such that both phase lagged boundaryconditions and moving grids are considered. Time-inclination isutilised to account for unequal pitchwise periodicity bydistributing time co-ordinates at grid nodes such that thephase lagged boundary conditions can be employed. The method isdemonstrated in a comparative study on a transonic turbinestage with a nominal non integer blade count ratio and anadjusted blade count ratio with a scaled rotor geometry. Thepredictions show significant differences in the blade pressureperturbation signal of the second vane passing frequency, whichwould motivate the application of the new method for rotorstator predictions with non-integer blade count ratios.