Dale E. Berg

Computational Investigations of Small Deploying Tabs and Flaps for Aerodynamic Load Control

The cost of wind-generated electricity can be reduced by mitigating fatigue loads acting on the blades of wind turbine rotors. One way to accomplish this is with active aerodynamic load control devices that supplement the load control obtainable with current full-span pitch control. Techniques to actively mitigate blade loads that are being considered include individual blade pitch control, trailing-edge flaps, and other much smaller trailing-edge devices such as microtabs and microflaps. The focus of this paper is on the latter aerodynamic devices, their time-dependent effect on sectional lift, drag, and pitching moment, and their effectiveness in mitigating high frequency loads on the wind turbine. Although these small devices show promise for this application, significant challenges must be overcome before they can be demonstrated to be a viable, cost-effective technology.

ACTIVE AERODYNAMIC LOAD CONTROL OF WIND TURBINE BLADES

The cost of wind-generated electricity can be reduced by mitigating fatigue loads acting on the rotor blades of wind turbines. One way to accomplish this is with active aerodynamic load control devices that supplement the load control obtainable with current full-span pitch control. Thin airfoil theory suggests that such devices will be more effective if they are located near the blade trailing edge. While considerable effort in Europe is concentrating on the capability of conventional trailing edge flaps to control these loads, our effort is concentrating on very small devices, called microtabs, that produce similar effects. This paper discusses the work we have done on microtabs, including a recent simulation that illustrates the large impact these small devices can exert on a blade. Although microtabs show promise for this application, significant challenges must be overcome before they can be demonstrated to be a viable, cost-effective technology.© 2007 ASME

Aerodynamic and aeroacoustic properties of flatback airfoils.

In 2002, Sandia National Laboratories (SNL) initiated a research program to demonstrate the use of carbon fiber in wind turbine blades and to investigate advanced structural concepts through the Blade Systems Design Study, known as the BSDS. One of the blade designs resulting from this program, commonly referred to as the BSDS blade, resulted from a systems approach in which manufacturing, structural and aerodynamic performance considerations were all simultaneously included in the design optimization. The BSDS blade design utilizes “flatback” airfoils for the inboard section of the blade to achieve a lighter, stronger blade. Flatback airfoils are generated by opening up the trailing edge of an airfoil uniformly along the camber line, thus preserving the camber of the original airfoil. This process is in distinct contrast to the generation of truncated airfoils, where the trailing edge the airfoil is simply cut off, changing the camber and subsequently degrading the aerodynamic performance. Compared to a thick conventional, sharp trailing-edge airfoil, a flatback airfoil with the same thickness exhibits increased lift and reduced sensitivity to soiling. Although several commercial turbine manufacturers have expressed interest in utilizing flatback airfoils for their wind turbine blades, they are concerned with the potential extra noise that such a blade will generate from the blunt trailing edge of the flatback section. In order to quantify the noise generation characteristics of flatback airfoils, Sandia National Laboratories has conducted a wind tunnel test to measure the noise generation and aerodynamic performance characteristics of a regular DU97-300-W airfoil, a 10% trailing edge thickness flatback version of that airfoil, and the flatback fitted with a trailing edge treatment. The paper describes the test facility, the models, and the test methodology, and provides some preliminary results from the test.

Active Aerodynamic Blade Distributed Flap Control Design Procedure for Load Reduction on the UpWind 5MW Wind Turbine

This paper develops a system identification approach and procedure that is employed for distributed control system design for large wind turbine load reduction applications. The primary goal of the study is to identify the process that can be used with multiple sensor inputs of varying types (such as aerodynamic or structural) that can be used to construct state-space models compatible with MIMO modern control techniques (such as LQR, LQG, H1, robust control, etc.). As an initial step, this study employs LQR applied to multiple flap actuators on each blade as control inputs and local deflection rates at the flap spanwise locations as measured outputs. Future studies will include a variety of other sensor and actuator locations for both design and analysis with respect to varying wind conditions (such as high turbulence and gust) to help reduce structural loads and fatigue damage. The DU SWAMP aeroservoelastic simulation environment is employed to capture the complexity of the control design scenario. The NREL 5MW UpWind reference wind turbine provides the large wind turbine dynamic characteristics used for the study. Numerical simulations are used to demonstrate the feasibility of the overall approach. This study shows that the distributed controller design can provide load reductions for turbulent wind profiles that represent operation in above-rated power conditions.

Hardware and software developments for the Accurate Time-Linked Data Acquisition System

Wind-energy researchers at Sandia National Laboratories have developed a new, light-weight, modular data acquisition system capable of acquiring long-term, continuous, multi-channel time-series data from operating wind-turbines. New hardware features have been added to this system to make it more flexible and permit programming via telemetry. User-friendly Windows-based software has been developed for programming the hardware and acquiring, storing, analyzing, and archiving the data. This paper briefly reviews the major components of the system, summarizes the recent hardware enhancements and operating experiences, and discusses the features and capabilities of the software programs that have been developed.

Design, Fabrication, Assembly and Initial Testing of a SMART Rotor 1

Sandia National Laboratories has designed and built a full set of three 9m blades (based on the Sandia CX-100 blade design) equipped with active aerodynamic blade load control surfaces on the outboard trailing edges. The design and fabrication of the blades and active aerodynamic control hardware and the instrumentation are discussed and the plans for control development are presented. , Albuquerque, NM 87185-1124

Impact of Higher Fidelity Models on Simulation of Active Aerodynamic Load Control For Fatigue Damage Reduction

Active aerodynamic load control of wind turbine blades is being investigated by the wind energy research community and shows great promise, especially for reduction of turbine fatigue damage in blades and nearby components. For much of this work, full system aeroelastic codes have been used to simulate the operation of the activel y controlled rotors. Research activities in this area continually push the limits of the models and assumptions within the codes. This paper demonstrates capabilities of a full system aeroelastic code recently developed by researchers at the Delft Universi ty Wind Energy Research Institute with the intent to provide a capability to serve the active aerodynamic control research effort, The code, called DU_SWAMP, includes higher fidelity structural models and unsteady aerodynamics effects which represent improvement over capabilities used previously by researchers at Sandia National Laboratories. The work represented by this paper includes model verification comparisons between a standard wind industry code, FAST, and DU_SWAMP. Finally, two different types of a ctive aerodynamic control approaches are implemented in order to demonstrate the fidelity simulation capability of the new code.

Aerodynamic and Aeroacoustic Properties of a Flatback Airfoil: An Update

Results from an experimental study of the aerodynamic and aeroacoustic properties of a atback version of the TU Delft DU97-W-300 airfoil are presented for a chord Reynolds number of 3 10 6 . The data were gathered in the Virginia Tech Stability Wind Tunnel, which uses a special aeroacoustic test section to enable measurements of airfoil self-noise. Corrected wind tunnel aerodynamic measurements for the DU97-W-300 are compared to previous solid wall wind tunnel data and are shown to give good agreement. Aeroacoustic data are presented for the atback airfoil, with a focus on the amplitude and frequency of noise associated with the vortex-shedding tone from the blunt trailing edge wake. The effect of a splitter plate attachment on both drag and noise is also presented. Computational Fluid Dynamics predictions of the aerodynamic properties of both the unmodied DU97-W-300 and the atback version are compared to the experimental data. Technical risks associated with the use of atback airfoils for the inboard region of wind turbine blades include increased aerodynamic noise and increased aerodynamic drag. Both of these penalties are the result of the blunt trailing edge shape and the wake that is produced by this shape. The relatively low pressure at the blunt base results in a much larger drag force than for a conventional airfoil shape. The effect of this drag penalty on rotor thrust and torque coefcient for typical inboard twist angles is not severe, and in fact can be offset by the additional lift that a atback airfoil generates. 3 Consideration of drag reducing devices such as splitter plates or trailing edge serrations may be desireable to further boost performance, however. The increased noise from the atback is due primarily to the vortex shedding phenomenon associated with bluff- body wakes. The vortex shedding often leads to tonal noise, similar to the Aeolian tones of o w past circular cylinders. The intensity of bluff-body vortex shedding tones at low Mach number scales with the sixth power of the relative o w velocity. Broadband aeroacoustic noise sources associated with turbulent boundary layer-trailing edge interaction scale with the fth power of the relative o w velocity. Since outboard o w velocities are much higher than those encoun- tered inboard, the overall aerodynamic noise levels of a rotor incorporating inboard atback shapes will likely continue to be dominated by outboard trailing edge noise. However, two aspects of the atback noise source may be cause for concern. First, the vortex-shedding noise from atbacks is likely to be contained in a relatively low-frequency band (50-200 Hz). Some community noise regulations have separate low-frequency noise standards apart from considera- tion of A-weighted sound, which emphasize higher frequencies to which the human ear is more sensitive. Second, the

ACCURATE TIME-LINKED DATA ACQUISITION SYSTEM FIELD DEPLOYMENT AND OPERATIONAL EXPERIENCE^

The Accurate Time-Linked Data Acquisition System (ATLAS) became fully operational on the Long-term Inflow and Structural Test (LIST) turbine at Bushland, Texas in May of 2000. In the LIST configuration, one data acquisition unit is mounted on the rotor and two additional acquisition units are mounted near the base of the turbine. All communication between the rotor unit and the ground is via telemetry. Data acquisition on all three units is synchronized (within +/- 1 microsecond) by slaving the units to universal time with the Sandia-developed Programmable Accurate Time Synchronization Module. A total of 74 channels of instrumentation is monitored by the three acquisition units. Data acquisition occurs at a 30 Hz rate for a continuous data throughput of over 35,000 bits per second, resulting in over 2 GB of ASCII data per day. Implementation of the system is discussed and operational experience is reviewed.

Fabrication Integration and Initial Testing of a SMART Rotor.

EDUCING ultimate and oscillating (or fatigue) loads on the wind turbine rotor can lead to reductions in loads on other turbine components such as the drive train, gearbox, and generator. This, in turn, is expected to reduce maintenance costs and may allow a given turbine to user longer blades to capture more energy. In both cases, the ultimate impact is reduced cost of wind energy. With the ever increasing size of wind turbine blades and the corresponding increase in non-uniform loads along the span of those blades, the need for more sophisticated load control techniques has produced great interest in the use of aerodynamic control devices (with associated sensors and control systems) distributed along each blade to provide feedback load control (often referred to in popular terms as „smart structures‟ or „smart rotor control‟). A recent review of concepts and feasibility and an inventory of design options for such systems have been performed by Barlas and van Kuik at Delft University of Technology (TUDelft) 1 . Active load control utilizing trailing edge flaps or deformable trailing edge geometries (referred to here as Active Aerodynamic Load Control or AALC) is receiving significant attention, because of the direct lift control capability of such devices and recent advances in smart material actuator technology. Researchers at TUDelft 2-3 , Riso/Danish Technical University Laboratory for Sustainable Energy (Riso/DTU) 4-10 and Sandia National Laboratories (SNL) 11-17 have been very active in this area over the past few years. The SNL work has focused on performing extensive simulations of AALC on several turbine configurations and has analyzed the simulation results to estimate the fatigue damage reduction on the rotor and gearbox and the costof-energy benefits of integrating trailing edge technology into the tip region of the turbine blades. These simulation results show the potential for significant impacts on fatigue damage and cost of energy, but experimental data is badly needed to confirm the simulation-based analyses. To the best of our knowledge, no research group has yet built and field tested a rotor with a full smart blade set. SNL has built a set of blades for its 100 kW test turbine in order to test AALC concepts. The main thrusts of this effort are to develop and validate a highly accurate structural dynamics model of the operating rotor, to work through the implementation details involved with developing appropriate control algorithms for such a rotor, and to obtain experimental verification of simulation runs; we are not attempting to design the optimal rotor for integration of AALC control capability or to develop the optimal control strategy. The design of the blade set is covered briefly in this paper and the reader is directed to a previous AIAA paper 18 for additional information. The fabrication, integration, and test results to date for this smart blade set are the subjects of this paper.