Session H27: Focus Session: Wind Energy Fluid Dynamics IV

Induction factor optimization through variable lift control

Due to practical design limitations coupled with the detrimental effects posed by complex wind regimes, modern wind turbines struggle to maintain or even reach ideal operational states. With additional gains through traditional approaches becoming more difficult and costly, active lift control represents a more attractive option for future designs. Here, plasma actuators have been explored experimentally in trailing edge applications for use in attached flow regimes. This authority would be used to drive the axial induction factor toward the ideal given by the Betz limit through distributed lift control thereby enhancing energy capture. Predictions of power improvement achievable by this methodology are made with blade - element momentum theory but will eventually be demonstrated in the field at the Laboratory for Enhanced Wind Energy Design, currently under construction at the University of Notre Dame.   

Influence of transition on steady and unsteady wind-turbine airfoil aerodynamics

Laminar--flow airfoils for large stall--regulated horizontal--axis wind turbines are designed to achieve a restrained maximum lift coefficient and a broad laminar low- drag bucket under steady flow conditions and at specific Reynolds numbers. Blind- comparisons of the 2000 NREL Unsteady Aerodynamics Experiment showed large discrepancies and illustrated the need for improved physics modeling. We have studied the S809 airfoil under static and dynamic (ramp-up, ramp-down, and oscillatory) conditions, using the four-equation transition model of Langtry and Menter (2009), which has been implemented as a library accessible by an OpenFOAM RANS solver. Model validation is performed using surface--pressure and lift/drag data from U. Glasgow (2009) and OSU (1995) wind tunnel experiments. Performance of the transition model is assessed by analyzing integrated performance metrics, as well as detailed surface pressure and pressure gradient, wall--shear stress, and boundary--layer profiles and separation points. Demonstration of model performance in the light-- and deep--stall regimes of dynamic stall is an important step in reducing uncertainties in full 3D simulations of turbines operating in the atmospheric boundary layer.   

Optimization of Airfoil Design for Flow Control with Plasma Actuators

Using computer simulations and design optimization methods, this research examines the implementation of active flow control devices on wind turbine blades. Through modifications to blade geometry in order to maximize the effectiveness of flow control devices, increases in aerodynamic performance and control of aerodynamic performance are expected. Due to this compliant flow, an increase in the power output of wind turbines is able to be realized with minimal modification and investment to existing turbine blades. This is achieved through dynamic lift control via virtual camber control. Methods using strategic flow separation near the trailing edge are analyzed to obtain desired aerodynamic performance. FLUENT is used to determine the aerodynamic performance of potential turbine blade design, and the post-processing uses optimization techniques to determine an optimal blade geometry and plasma actuator operating parameters. This work motivates the research and development of novel blade designs with flow control devices that will be tested at Notre Dame's Laboratory for Enhanced Wind Energy Design.   

Harvesting energy via fluttering piezoelectric beams in viscous flow

This work explores the idea of harvesting energy from ambient flows using flexible piezoelectric beams. Beams lose their stability and flutter above a critical length or flow speed or below a critical stiffness. During flutter, beams oscillate in increasing amplitude until they enter a self-sustained limit cycle oscillation, which could be exploited to harvest energy. The objectives of this study are to: (i) identify the flutter boundary of a flexible beam in viscous flow; (ii) explore the energy harvesting potential; and (iii) identify critical non-dimensional parameters and parametric relations that govern the response and stability of thin composite beams vibrating in a viscous fluid. Two-dimensional Navier-Stokes equations are solved with a nonlinear beam model coupled with a linear piezoelectric material constitutive model. The harvested energy potential for various solid/fluid combinations is investigated by varying the critical non-dimensional parameters, which are defined in terms of beam length, density, thickness, and stiffness; fluid speed and density; and piezoelectric material properties.   

Improving the Efficiency of Piezoelectric Fluidic Energy Harvesters

Piezoelectric systems can harvest electrical energy from fluid flows. Maximizing the power output and increasing their overall energy conversion efficiency is a formidable challenge. We compare the conversion efficiencies of various harvester configurations. The first experimentally tested configuration is a cantilevered beam in the wake of a circular cylinder. An efficiency of only 0.03 per cent in converting fluidic to mechanical energy (i.e. ``aeroelastic efficiency'') was estimated while the efficiency of mechanical to electrical energy conversion (i.e. ``electromechanical efficiency'') was close to 11 per cent. In our latest experiments, we attached a circular cylinder to the free end of a piezoelectric beam. The aeroelastic efficiency increased to 2.5 per cent and the electromechanical efficiency is increased to 30 per cent. Placing a stationary circular cylinder upstream of this configuration caused a dramatic increase in harvested power and efficiency. Furthermore, attaching a cylinder of a half-circle cross section resulted in a much larger power and efficiency as compared to attaching a circular cylinder to the tip of beam.   

Preliminary Investigation of the Active Flow Control Benefits on Wind Turbine Blades

This study investigates the benefit of flow control over a 2D airfoil specially designed for wind turbine applications. The experiments were carried out in Syracuse University's Anechoic Wind Tunnel, both with and without large scale unsteadiness in the freestream. When there is no large scale unsteadiness introduced in the flow, under open loop flow control conditions with unsteady blowing, the leading edge separation was delayed and maximum lift coefficient was increased. For the cases where large scale unsteadiness was introduced into the flow, the experiments showed that both open loop and closed loop control cases were able to reduce lift fluctuations by a measurable amount. However, only the closed loop control case which utilized surface pressure information from the airfoil near the leading edge was capable of consistently mitigating the fluctuating load.   

Energy-harvesting potential of multiple elastic structures in tandem arrangement

Vortex-induced flapping vibrations of elastic structures attached with piezoelectric materials, i.e., ``piezo-leaves'', have recently been explored for its potential application in wind energy harvesting (e.g., Li, Yuan, and Lipson, J. Appl. Phys., 2011). In this work, we explore the possibility of enhancing the structural vibration and energy harvesting performance of the generator by putting the leaves in tandem arrangement and within close range of hydrodynamic interaction. A two-dimensional model is developed, where two or more elastic plates are mounted in a cross flow. In the case of two plates, the numerical simulation shows that at a particular distance, the vibration of the downstream plate is greatly increased, and so is the energy level of the entire system. For multiple plates, we observed both synchronized and apparently chaotic vibration modes. The characteristics of the vortex interaction, plate deformation, and energetics will be presented for those coupling modes.