Robust aerodynamic design of variable speed wind turbine rotors

This study focuses on the robust aerodynamic design of the bladed rotor of small horizontal axis wind turbines. The optimization process also considers the effects of manufacturing and assembly tolerances on the yearly energy production. The aerodynamic performance of the rotors so designed has reduced sensitivity to manufacturing and assembly errors. The geometric uncertainty affecting the rotor shape is represented by normal distributions of the pitch angle of the blades, and the twist angle and chord of their airfoils. The aerodynamic module is a blade element momentum theory code. Both Monte Carlo-based and the Univariate ReducedQuadrature technique, a novel deterministic uncertainty propagationmethod, are used. The performance of the two approaches is assessed both interms of accuracy and computational speed. The adopted optimization method is based on a hybrid multi-objective evolutionary strategy. The presented results highlight that the sensitivity of the yearly production to geometric uncertainties can be reduced by reducing the rotational speed and increasing the aerodynamic blade loads

Multi-disciplinary robust design of variable speed wind turbines

This paper addresses the preliminary robust multi-disciplinary design of small wind turbines. The turbine to be designed is assumed to be connected to the grid by means of power electronic converters. The main input parameter is the yearly wind distribution at the selected site, and it is represented by means of a Weibull distribution. The objective function is the electrical energy delivered yearly to the grid. Aerodynamic and electrical characteristics are fully coupled and modelled by means of low- and medium-fidelity models. Uncertainty affecting the blade geometry is considered, and a multi-objective hybrid evolutionary algorithm code is used to maximise the mean value of the yearly energy production and minimise its variance

Comparative turbulent three-dimensional Navier-Stokes hydrodynamic analysis and performance assessment of oscillating wings for renewable energy applications

Oscillating wings can extract energy from an oncoming water or air stream, and first large-scale marine demonstrators are being tested. Oscillating wing hydrodynamics is highly unsteady, may feature dynamic stall and leading edge vortex shedding, and is significantly three-dimensional due to finite-wing effects. Understanding the interaction of these phenomena is essential for maximizing power generation efficiency. Much of the knowledge on oscillating wing hydrodynamics stemmed from two-dimensional low-Reynolds number computational fluid dynamics studies and laboratory testing; real installations, however, will feature Reynolds numbers higher than 1 million and unavoidable finite-wing-induced losses. This study investigates the impact of flow three-dimensionality on the hydrodynamics and the efficiency of a realistic aspect ratio 10 device in a stream with Reynolds number of 1.5 million. The improvements achievable by using endplates to reduce finite-wing-induced losses are also analyzed. Three-dimensional time-dependent Navier-Stokes simulations using the shear stress transport turbulence model and a 30 million-cell grid are performed. Detailed comparative hydrodynamic analyses of the finite and the infinite wings reveal that flow three-dimensionality reduces the power generation efficiency of the finite wing with sharp tips and that with endplates by about 17% and 12% respectively. Presented analyses suggest approaches to further reducing these power losses

Comparative assessment of the harmonic balance Navier Stokes technology for horizontal and vertical axis wind turbine aerodynamics

Several important wind turbine unsteady flow regimes, such as those associated with the yawed wind condition of horizontal axis machines, and most operating conditions of all vertical axis machines, are predominantly periodic. The harmonic balance Reynolds-averaged Navier-Stokes technology for the rapid calculation of nonlinear periodic flow fields has been successfully used to greatly reduce runtimes of turbomachinery periodic flow analyses in the past fifteen years. This paper presents an objective comparative study of the performance and solution accuracy of this technology for aerodynamic analysis and design applications of horizontal and vertical axis wind turbines. The considered use cases are the periodic flow past the blade section of a utility-scale horizontal axis wind turbine rotor in yawed wind, and the periodic flow of a H-Darrieus rotor section working at a tip-speed ratio close to that of maximum power. The aforementioned comparative assessment is based on thorough parametric time-domain and harmonic balance analyses of both use cases. The paper also reports the main mathematical and numerical features of a new turbulent harmonic balance Navier-Stokes solver using Menter’s shear stress transport model for the turbulence closure. Presented results indicate that a) typical multi-megawatt horizontal axis wind turbine periodic flows can be computed by the harmonic balance solver about ten times more rapidly than by the conventional time-domain analysis, achieving the same temporal accuracy of the latter method, and b) the harmonic balance acceleration for Darrieus rotor unsteady flow analysis is lower than for horizontal axis machines, and the harmonic balance solutions feature undesired oscillations caused by the wide harmonic content and the high-level of stall predisposition of this flow field type

Robust aerodynamic design optimization of variable speed wind turbine rotors

Background: Manufacturing and assembly tolerances may cause wind turbine (WT) energy production to differ significantly from nominal design targets. Issue can be alleviated in two ways: A. Reducing tolerances. This may be expensive. B. Developing reliable and computationally affordable robust analysis and design optimization technologies. Robust WT rotor is one yielding minimal variations of aerodynamic performance arising due to errors affecting rotor geometry. Objectives: To develop/demonstrate computational framework for the robust design optimization of WT rotors. Main aim is to 1) maximize expectation and 2) minimize standard deviation of yearly energy production. Highlight computational effectiveness of the Univariate Reduced Quadrature approach to ‘deterministic’ uncertainty propagation. Highlight capabilities of a recently developed multi-level evolution-based optimizer

Wind turbine design optimization under environmental uncertainty

Wind turbine design optimization is typically performed considering a given wind distribution. However, turbines so designed often end up being used at sites characterized by different wind distributions, and this results in significant performance penalties. This paper presents a probabilistic integrated multidisciplinary approach to the design optimization of multimegawatt wind turbines accounting for the stochastic variability of the mean wind speed. The presented technology is applied to the design of a 5 MW rotor for use at sites of wind power class from 3 to 7, where the mean wind speed at 50 m above the ground ranges from 6.4 to 11.9 m/s. Assuming the mean wind speed to vary stochastically in such range, the rotor design is optimized by minimizing mean and standard deviation of the levelized cost of energy. Airfoil shapes, spanwise distributions of blade chord and twist, blade internal structural layup and rotor speed are optimized concurrently, subject to structural and aeroelastic constraints. The probabilistically designed turbine achieves a more favourable probabilistic performance than the initial baseline turbine. The presented probabilistic design framework is portable and modular in that any of its analysis modules can be replaced with counterparts of user-selected fidelity

Numerical and experimental investigation of a vortical flow-inducing jet pump

Experimental analyses and CFD simulations are performed on a vortical flow-inducing jet pump. The device is a multi-nozzle annular jet pump, in which a high-pressure fluid is injected into a bore through circumferentially distributed nozzles. The nozzles are angled axially and radially so that the injected primary fluid produces both suction and a vortical flow pattern. Analysis of the pump is considered as single phase, using compressed air to pump atmospheric air. Experiments are carried out on two jet pump designs, working at different conditions with results used to validate CFD simulations. CFD turbulence model analyses is used to determine the optimal numerical method, with hybrid turbulence models shown to be effective in predicting the pressure produced by the swirling flow phenomena. Suction pressure induced by the jets is shown to be highly dependent on the axial angle of the nozzles, which has considerable impact on the radial and tangential components of the resulting flow field, consequently affecting the pump performance

Harmonic balance Navier–Stokes aerodynamic analysis of horizontal axis wind turbines in yawed wind

Multi–megawatt horizontal axis wind turbines often operate in yawed wind transients in which the resulting periodic loads acting on blades, drive–train, tower and foundation adversely impact on fatigue life. Accurately predicting yawed wind turbine aerodynamics and resulting structural loads can be challenging, and would require the use of computationally expensive high–fidelity unsteady Navier–Stokes Computational Fluid Dynamics. The high computational cost of this approach can be significantly reduced by using a frequency–domain framework. The paper summarizes the main features of the COSA Harmonic Balance Navier–Stokes solver for the analysis of open rotor periodic flows, presents initial validation results based on the analysis of the NREL Phase VI experiment, and it also provides a sample application to the analysis of a multi–megawatt turbine in yawed wind. The reported analyses indicate that the harmonic balance solver determines the considered periodic flows from 30 to 50 times faster than the conventional time–domain approach with negligible accuracy penalty to the latter. Copyright c2017 John Wiley & Sons, Ltd

Numerical and Experimental Analysis of the Cold Flow Physics of a Non Pre-mixed Industrial Gas Burner

The flow field of a non-premixed industrial gas burner is analysed with Reynolds-averaged Navier Stokes computational fluid dynamics validated against velocity and pressure measurements. Combustion is not modeled because the aim is optimizing the predictive capabilities of the cold flow before including chemistry. The system's complex flow physics, affected by a 90° turn, backward and forward facing steps, and transversal jets is investigated at full and partial load. The sensitivity of the computed flow field to inflow boundary condition setup, approach for resolving/modeling wall bounded flows, and turbulence closure are assessed. In the first sensitivity analysis, the inflow boundary condition is prescribed using measured total pressure or measured velocity field, and the boundary layers are resolved down to the wall or modeled with wall functions. In the second sensitivity analysis, the turbulence closure uses the k-ω shear stress transport eddy viscosity model or two variants of the Reynolds stress model. The agreement between the predictions of most simulation set-ups among themselves and with the measurements is good. For given type of inflow condition and wall flow treatment, the ?-based Reynolds stress model gives the best agreement with measurements among the considered turbulence models at full load. At partial load, the comparison with measured data highlights some scatter in the predictions of different patterns of the flow measurements. Overall, the findings of this study provide insight into the fluid dynamics of industrial gas burners, and guidelines for their simulation-based analysis

Darrieus wind turbine blade unsteady aerodynamics:a three-dimensional Navier-Stokes CFD assessment

Energized by the recent rapid progress in high-performance computing and the growing availability of large computational resources, computational fluid dynamics (CFD) is offering a cost-effective, versatile and accurate means to improve the understanding of the unsteady aerodynamics of Darrieus wind turbines, increase their efficiency and delivering more cost-effective and structurally sound designs. In this study, a Navier-Stokes CFD research code featuring a very high parallel efficiency was used to thoroughly investigate the three-dimensional unsteady aerodynamics of a Darrieus rotor blade. Highly spatially and temporally resolved unsteady simulations were carried out using more than 16,000 processor cores on an IBM BG/Q cluster. The study aims at providing a detailed description and quantification of the main three-dimensional effects associated with the periodic motion of this turbine type, including tip losses, dynamic stall, vortex propagation and blade/wake interaction. Presented results reveal that the three-dimensional flow effects affecting Darrieus rotor blades are significantly more complex than assumed by the lower-fidelity models often used for design applications, and strongly vary during the rotor revolution. A comparison of the CFD integral estimates and the results of a blade-element momentum code is also presented to highlight strengths and weaknesses of low-fidelity codes for Darrieus turbine design. The reported CFD results may provide a valuable and reliable benchmark for the calibration of lower-fidelity models, which are still key to industrial design due to their very high execution speed