Latest results from the EU project AVATAR: aerodynamic modelling of 10 MW wind turbines

This paper presents the most recent results from the EU project AVATAR in which aerodynamic models are improved and validated for wind turbines on a scale of 10 MW and more. Measurements on a DU 00-W-212 airfoil are presented which have been taken in the pressurized DNW-HDG wind tunnel up to a Reynolds number of 15 Million. These measurements are compared with measurements in the LM wind tunnel for Reynolds numbers of 3 and 6 Million and with calculational results. In the analysis of results special attention is paid to high Reynolds numbers effects. CFD calculations on airfoil performance showed an unexpected large scatter which eventually was reduced by paying even more attention to grid independency and domain size in relation to grid topology. Moreover calculations are presented on flow devices (leading and trailing edge flaps and vortex generators). Finally results are shown between results from 3D rotor models where a comparison is made between results from vortex wake methods and BEM methods at yawed conditions

Wake Characteristics of Large-scale Wind Turbines

The next generation of large-scale wind turbines will exceed 10 MW of rated power and will reach rotor diameters of about 200 m. Their rotor aerodynamics are also extreme with Reynolds numbers that reach 40 million. The wakes generated by these wind turbines cover a very large area downstream of their installation positions which increases the possibility that the wake vortices generated by these large wind turbines may affect passing-by flying vehicles. In this paper a CFD study of a large wind turbine was carried out to predict the power curves and aerodynamic loads on the rotor blades. Flow control devices of active leading and trailing edge flaps were also considered in the CFD study and the effects of flaps were investigated. The near wake flow was captured in the CFD study and the flaps add more complexity to the wake flow. To study the potential wind turbine wake encounters by aircraft, engineering wake models were developed to predict the wind turbine far wakes. The wake induced velocity fields were integrated into an aircraft flight dynamics model to simulate wind turbine wake encounter scenarios, designed for a light aircraft approaching an airport, where a wind turbine is installed. The severity of the wind turbine wake encounter was analysed using off-line flight simulations. The off-line simulation results indicated that the wake encounter severity was highly dependent on the ways that the wake vortex circulation and the core size were calculated, which suggested that field measurements of large wind turbines wake flow are needed to verify the modelling and CFD results

Wake characteristics of large-scale wind turbines

The next generation of large-scale wind turbines will exceed 10 MW of rated power and will reach rotor diameters of about 200 m. Their rotor aerodynamics are also extreme with Reynolds numbers that reach 40 million. The wakes generated by these wind turbines cover a very large area downstream of their installation positions which increases the possibility that the wake vortices generated by these large wind turbines may affect passing-by flying vehicles. In this paper a CFD study of a large wind turbine was carried out to predict the power curves and aerodynamic loads on the rotor blades. Flow control devices of active leading and trailing edge flaps were also considered in the CFD study and the effects of flaps were investigated. The near wake flow was captured in the CFD study and the flaps add more complexity to the wake flow. To study the potential wind turbine wake encounters by aircraft, engineering wake models were developed to predict the wind turbine far wakes. The wake induced velocity fields were integrated into an aircraft flight dynamics model to simulate wind turbine wake encounter scenarios, designed for a light aircraft approaching an airport, where a wind turbine is installed. The severity of the wind turbine wake encounter was analysed using off-line flight simulations. The off-line simulation results indicated that the wake encounter severity was highly dependent on the ways that the wake vortex circulation and the core size were calculated, which suggested that field measurements of large wind turbines wake flow are needed to verify the modelling and CFD results

On the Aerodynamic Properties of Slotted-Flap Flow-Control Devices for Wind Turbine Applications

Wind energy is growing at a fast pace and utility-scale wind turbines growing in size with increasing rotor diameters. To sustain development of up-scaled wind turbines of tomorrow there is a need for innovation in load control methodologies. This thesis research targets an assessment of the aerodynamic properties of airfoil sections specifically intended for wind turbine applications, featuring them with flow-control devices. The use of fractional chord trailing-edge flaps as slotted-flap devices on aerodynamically active parts of a turbine blade were studied. As modular devices attached externally on existing blade designs, they comfortably form a cost-effective means for active load control with low energy actuation. They also have advantages such as minimal design modifications to the original blade, relatively light-weight device, and lack of retooling of the manufacturing process. The basis for an aerodynamic study was the NREL-5MW Reference Wind Turbine, which is a well-studied benchmark for large utility-scale wind turbines of today. This dissertation presents numerical results for the aerodynamic properties of two modified airfoil sections used on the blades designed for this turbine, NACA 643-618 and DU 93-W-210. These airfoils in their original configuration are used on the aerodynamically active parts of many contemporary wind turbines. A new set of coefficients defining the aerodynamic characteristics of these airfoils equipped with a fractional-chord trailing-edge flap of Clark Y profile are presented here. Defining the effects of flaps on airfoil sections specifically intended for wind turbine applications will suffice as a repository of useful aerodynamic data for a wider research community to develop new blade designs and load mitigation approaches for wind turbine rotors

Performance Enhancement of Small-Scale Wind Turbine Featuring Morphing Blades

The demand for renewable energy is driven by the depletion and adverse environmental impacts of fossil fuels. There is a growing global consensus for research and development of renewable energy, including wind. In the current study, National Renewable Energy Laboratory (NREL) Phase VI wind turbine blade is integrated with morphing trailing-edge, installed on the aft-30% blade chord, across outboard 75% blade span. The morphing trailing-edge generates unique topology for each wind speed such that the glide ratio is maximized along the blade span. Three-dimensional transient computational fluid dynamics (CFD) analyses are conducted over low to medium wind speeds to investigate the blade aerodynamics. The analyses exhibit significant increments in the low-speed shaft torque and power of the morphed blades compared to the baseline. The integration of morphing trailing-edge high-lift flow control mechanism on the NREL Phase VI blade enhanced energy harvesting and reduced the wind turbine cut-in wind speed. Comparative investigations are also conducted to assess the improvements in thrust, bending moment, and aerodynamic load distribution, as well as alterations in the pressure, flow field, turbulence, surface flow, and wake. The aeroacoustics directivity of the wind turbines exhibits marginal far-field noise increment in case of morphing trailing-edge integrated blades

Wind Turbine Aerodynamics II

This Special Issue is a collection of the latest research articles on various topics related to wind turbine aerodynamics, which includes wind turbine design concepts, tip loss correction study, wind turbine acoustics modelling, and the vertical axis wind turbine concept

Application of circulation control aerofoils to wind turbines

Circulation control aerofoils potentially offer an additional means of load and power control for horizontal axis wind turbines by virtue of their rapid response time. Their suitability for these tasks has been assessed with respect to the power which they absorb, their interaction with aerofoils used on modern wind turbines, the infrastructure or hardware which they require and the degree to which they can affect the loads experienced by the turbine blades and other major components. It has been determined that the type of circulation control aerofoil most suited to use on wind turbine blades are those of the jet flap type and it has been realised that an ability to shed, as well as increase loads is advantageous in this application. To this end the behaviour of both negatively and positively deflected jets have been investigated with a two-dimensional computational fluid dynamics code, validated in the course of this work for such modelling. Particular emphasis has been placed on minimising the input power requirements of the circulation control aerofoils and in proposing an overall system that has the required level of robustness and reliability. A 2MW turbine has been modelled with a blade element momentum theory code in order to compare performance with and without circulation control aerofoils. These initial results show that there may be some positive benefits to be gained, but that the energy demands of the system place a hard limit on the degree to which circulation control aerofoils can determine the forces experienced by the turbine