The Impact Of Inertia Forces On A Morphing Wind Turbine Blade In A Vertical Axis Configuration

There is a growing global demand for "clean" energy due to an increased mandate to reduce greenhouse gases. Wind energy has established itself as an economically competitive source due to major developments made in the efficiency and reliability of conversion systems. Currently, horizontal axis wind turbines (HAWTs) dominate the wind energy conversion market because of their high efficiency. However, recent advances in vertical axis conversion systems are closing the gap in efficiency. A novel flexible blade concept with the ability to morph and adapt to changing flow conditions was proposed by A. Beyene and T. Ireland, to address part load and performance issues encountered in wind energy conversion systems. The extension of these benefits to a vertical axis wind turbine (VAWT) would make wind technology a more competitive player in the energy market. A straight bladed vertical axis wind turbine (SB-VAWT) rotor was manufactured, to accommodate flexible and rigid blades. The performance and flexible behavior was studied in the department of mechanical engineering's low speed wind tunnel using a test rig that was built for this study. A mathematical model, validated using a high speed camera and finite element analysis, was developed to predict the magnitude and direction of blade morph. The results show that the coefficient of performance (CP) greatly depends on the tip speed ratio (TSR), i.e., the rigid blade has CP of 0.11 for a TSR of 1.6, whereas the morphing blade achieved a CP of 0.06 at a TSR of 1.13. Overall, the modified morphing blade has better performance at low RPMs, but the rigid blade performed better at high RPMs. It was observed that the VAWT equipped with flexible blades self-started in the majority of the experiments. The flexible blade's production of power at relatively low TSRs is a rare occurrence in the field. At high RPM, the centrifugal force overwhelmed the lift force, bending the blade out of phase in an undesired direction increasing drag and therefore reducing the CP. These results suggest that alterations to the current design must be made in order to account for the inertial forces experienced by blades in a vertical axis configuration.

The simulation of flow through vertical axis wind turbine (VAWT) is characterized by unsteady flow where the blade experiences varying angles of attack and Reynolds number as it completes a cycle. Therefore, the lift generated also varies as a function of its rotational position relative to the incoming freestream velocity. In order to improve the performance of these turbines the blade can take advantage of smart materials developed for control surface actuation. The aim of this paper is to investigate the effect of morphing blades on the aerodynamic performance of the turbine blades. The study uses commercial software Ansys Fluent pressure-based solver to investigate the flow past the turbine blades by solving the 2D Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations. In order to simulate the morphing blade for VAWT, a sliding mesh method is used to simulate the VAWT rotation while a user-defined function (UDF) is written for the blade morphing flexure motion. This entails the use of dynamic mesh smoothing to prevent the mesh from having negative cell volumes. Although the dynamic mesh strategy has been successful in preserving the cell quality, it has been shown that the proposed method of simulating the morphing blade on VAWT is inadequate due to unphysical solutions. Finally, the effect of morphing the blade is tested on a static airfoil case instead, where it is shown that stall is alleviated by morphing the blade trailing edge.

The increasing demand for "clean" energy has become a global issue that triggers the rapid growth of renewable energy development. Engineers and designers envisage new technology to improve the performance of wind energy systems. In order to contribute to the development of renewable energy, it is important to discover the most effective type of morphing wind turbine blade. To optimize the design of a wind turbine, it requires extensive study of the interaction between the aerodynamic forces induced by the wind field and the structural response of the wind turbine blade. Coupled fluid-structure interaction (FSI) modeling can provide details about the behaviors of a morphing wind turbine blade. The two main techniques for solving the fluid-structure interaction (FSI) problem are the segregated-coupled and fully-coupled approaches. Since most of the past studies are mainly focused on the segregated-coupled approach, this study attempts to investigate the aerodynamics and structural impact of blade thickness on a morphing wind turbine blade using the fully-coupled approach. Three symmetrical morphing airfoils selected in this study are NACA 0012, NACA 0018 and NACA 0024 representing different thicknesses. These three different airfoil profiles are subject to a fixed wind speed at various attack angles from 2 to 20 degree in twodegree increments. Reynolds number of Re = 5.0 x 105 is used in all cases. Three different Young's modulus values are E1 = 1.64 x 105 Pa, E2 = 2.46 x 105 and E3 = 3.29 x 105 used to represent different material flexibility. There are a total of 90 simulation morphing airfoil models being generated. This study shows that symmetrical morphing airfoil in HAWT can improve drag and lift-drag ratio in the post-stall region. However, the benefits of the morphing effect diminish as airfoil thickness increases due to the higher bending resistance in thicker airfoil. The presented results prove that it is unnecessary to have thick root section for a wind turbine since a morphing airfoil can both increase aerodynamics performance in the post-stall region and alleviate stress loading to withstand strong wind.

This study compares the performance of a Vertical Axis Wind Turbine with and without using morphing capabilities applied to its blades. It also explores the feasibility of applying moving mesh to model the morphing capability inside the software package STAR CCM+© in order to use Computational Fluid Dynamics (CFD) to analyze the flow's behavior. Particularly it is important to capture the presence of dynamic stall and vortex shedding at certain regions over the blade's path, which are associated with a decreased in the overall power coefficient. This work developed a methodology to analyze these morphing capabilities when applied over airfoils in 2D simulations, by using a combination of overset meshes and the morphing approach. The accuracy is verified by creating a baseline scenario and compare it against a benchmark case, while also testing for grid and time step sensitivity. The use of Reynold Averaged Navier Stokes equations was chosen, with Menter's SST k-omega as the turbulence model. Afterward, a maximum power coefficient curve was plotted by testing three airfoil's shapes as references, one forming the baseline case, while the other two delimiting the maximum deformation, marked as outward and inward cases. A final optimized case was tested, where the morphing was applied to strategic regions where the dynamic stall was highest, and where the shapes could ensure the maximum possible power output.This resulted in an improvement of 46.2% of the overall power coefficient.

Worldwide interest in renewable energy systems has increased dramatically, due to environmental concerns like climate change and other factors. Wind power is a major source of sustainable energy, and can be harvested using both horizontal and vertical axis wind turbines. This thesis presents studies of a vertical axis wind turbine performance for applications in urban areas. Numerical simulations with FLUENT software are presented to predict the fluid flow through a novel Zephyr vertical axis wind turbine(VAWT). Simulations of air flow through the turbine rotor were performed to analyze the performance characteristics of the device. Major blade geometries were examined. A multiple reference frame (MRF) model capability of FLUENT was used to express the dimensionless form of power output of the wind turbine as a function of the wind freestream velocity and the rotor's rotational speed. The simulation results exhibit close agreement with a stream-tube momentum model.

Wind turbines are one of the fastest-growing energy sources. Based on their axis of rotation they fall into two basic categories: horizontal-axis wind turbines (HAWTs) and vertical-axis wind turbines (VAWTs). Darrieus VAWTs exploit aerodynamic lift. This study entails the vibration analysis of large vertical-axis Darrieus wind turbine blades. Very large wind turbines are becoming more abundant due to their ability to harvest greater wind power. VAWTs are less common than HAWTs for large wind applications, but have some favorable characteristics, for example in offshore applications, and so further development of large VAWTs is anticipated. However, VAWTs are known to have vibration issues. VAWT blade vibration is the focus of this work.The straight-bladed H-rotor/Giromill is the simplest type of VAWT. We first derive the equations of motion of a H-rotor blade modeled as a uniform straight elastic Euler-Bernoulli beam under transverse bending and twist deformation. The reduced-order model suggests the existence of periodic damping, periodic stiffness, and direct excitation generated by a cyclic aeroelastic load. The model also indicates spin softening, which could be detrimental as the turbines become large. Periodic damping and stiffness are examples of parametric excitation and are likely to carry over to other types of VAWT blades. Systems with parametric excitation have been studied with various methods. Floquet theory has been classically used to study the stability characteristics of linear systems with periodic coefficients, and has been commonly applied to Mathieu's equation, which represents a vibration system with periodic stiffness. We apply the Floquet theory combined with the harmonic-balance method to a linear vibration system with a periodic damping coefficient. Based on this theory, the approximated solution includes an exponential part, with an unknown exponent, and a periodic part. Our analysis investigates the initial conditions response, the boundaries of instability, and the characteristics of free response solution of the system. The coexistence phenomenon, in which some of the transition curves overlap so that the instability wedges disappear, is recovered in this approach, and is examined closely.An additional case of the parametric excitation is the combination of parametric damping and parametric stiffness. The Floquet-based analysis shows that the combined parametric excitation reshapes the stability characteristics, compared to the system with only parametric damping or stiffness and disrupts the coexistence which is observed in the parametric damping case.The aeroelastic forces encountered by the wind turbines can cause self-excitation in blades, the mechanism of which can be loosely modeled with van-der-Pol-type nonlinearity. We seek to understand the combined effect of parametric excitation and van der Pol nonlinearity, as both can induce instabilities and oscillations. The oscillator is studied under nonresonant conditions and secondary resonances, with and without external excitation. We analyze the system using the method of multiple scales and numerical solutions. For the case without external excitation, the analysis reveals nonresonant phase drift (quasi-periodic responses), and subharmonic resonance with possible phase drift or phase locking (periodic responses). Hard excitation is treated for nonresonant conditions and secondary resonances, and similar phenomena are uncovered.Some Darrieus VAWTs consist of curved blades. We lastly study the modal analysis of curved Darrieus wind-turbine blades and obtain the mode shapes and modal frequencies. The governing equations are derived using the fundamental deformation mechanics, and thin beam approximations are employed to express the strain and kinetic energies. The assumed-modes method is applied to the energies, and the Euler-Lagrange equation is used to discretize the equations of motion. Implementing these equations, mode shapes are calculated and mapped back onto the curved beam for visualization. This analysis is conducted for pinned-pinned and hinged-hinged blades. The results are compared with Finite element analysis using Abaqus and with the literature.

This chapter deals with loads on wind turbine blades. It describes the load generating process, wind fields, and the concepts of stresses and strains. Aerodynamic loads, loads introduced by inertia, gravitation and gyroscopic effects, and actuation loads are discussed. The loading effects on the rotor blades and how they are interconnected with the dynamics of the turbine structure are highlighted. There is a discussion on how stochastic loads can be analysed and an outline of cycle counting methodology. The method of design verification testing is briefly described.

Wind turbines are designed to operate optimally at a certain set of ambient conditions, termed the design point. An unavoidable consequence of wind turbine design is that away from this point, efficiency can drop drastically which inhibits energy conversion, especially in highly varying winds. Numerous strategies have been employed, for example blade pitch and torque control systems, which mitigate these losses, but such systems are economically costly and are usually only present in larger scale applications. For small Horizontal-Axis Wind Turbines (HAWTs) as well as most Vertical Axis Wind Turbines (VAWTs), pitch control strategies in particular are either too complex or simply not economically feasible. Recent research into flexible bladed wind turbine rotor design has shown that continuous, passive blade morphing can increase aerodynamic lift, reduce drag, and even delay stall for two-dimensional airfoil sections. These studies, however, do not take into account centrifugal or gravitational loadings which can change the morphing direction of the flexible material. In this thesis, a fully three-dimensional Fluid-Structure Interaction (FSI) solver is developed and used to simulate a flexible HAWT rotor, with comparisons to experimental results, also conducted herein. The experimental findings, which compare geometrically identical rigid and flexible rotors, show a marked increase in average torque and operational envelope over a wide range of flow conditions. Through the analysis of the associated FSI simulations, these performance increases are attributed to small changes in local attack angles, acting to mitigate losses associated with flow separation. After validation with experimental data, the FSI solver is then utilized to investigate the effects of flexible blade design on a VAWT, the first analysis of its kind. Results indicate that efficiency increases are realized not by improving the maximum rotor torque, which varies along VAWT rotation, but by increasing the torque minima through passive deflection of the blades. This morphing action augments the local attack angle to decrease the magnitude of low-pressure regions created due to blade stall, acting to improve average rotor torque and increase energy capture of the flexible VAWT design when compared to a geometrically identical rigid one.