Jose R. Zayas

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.

Multiplexed Fiber Bragg Grating Interrogation System Using a Microelectromechanical Fabry–Perot Tunable Filter

This paper describes a fiber Bragg grating strain sensor interrogation system based on a microelectromechanical systems tunable Fabry-Perot filter. The shift in the Bragg wavelength due to strain applied to a sensor fiber is detected by means of a correlation algorithm which was implemented on an embedded digital signal processor. The instrument has a 70 nm tuning range, allowing multiple strain sensors to be multiplexed on the same fiber. The performance of the interrogator was characterized using an optical fiber containing six grating strain sensors embedded in a fiberglass test specimen. The measured root mean square (RMS) strain error was 1.5 microstrain, corresponding to a 1.2 pm RMS error in the estimated wavelength shift. Strain measurements are produced with an update rate of 39 samples/s.

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.

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.

Evaluation of NASA PZT Sensor/Actuator for Structural Health Monitoring of a Wind Turbine Blade

A collaborativ e effort between Sandia National Laboratories (SNL) , and NASA KSC has been performed to evaluate the viability of a NASA developed piezo sensor/a ctuator for structural health monitoring (SHM) of wind turbine blades. The innovation behind the NASA develope d sensor is in the combination of signal data processing with the development of a unique sensor/actuator consisting of piezoelectric materials in a thin and highly sensitive configuration. Two similar 9 -meter composite blade s were heavily instrumented wi th traditional foil strain gauges and the NASA PZT sensor. A fatigue to failure test was pe rformed independently on both blades at the NWTC lab testing facility. During the test, data from both the foil strain gauges and the NASA PZT’s was collected. Th e SNL developed ATLAS II DAS was used to collect the data from the PZT’ s at a rate of 5000 Hz, while t he strain gauge data was collected using the NREL DAS. The fatigue test ran for approximately one month and preliminary data analysis has been performed. Data from the PZT’s sensors show noticeable structural changes well prior to the failure, approximately 300,000 cycles. I. Nomenclature

Low-cost fiber Bragg grating interrogation system for in situ assessment of structures

A project targeted at developing a low-cost fiber optic interrogator system for fiber Bragg grating (FBG) sensors has been completed, and has resulted in a stand-alone system that can be used in multiple applications. The interrogator system, tailored as a potential solution for embedded strain sensing in composite wind turbine blades, was recently tested and its performance validated at the Infrastructure Assurance & Non-Destructive Inspection (NDI) department at Sandia National Laboratories (SNL). The test specimen used to test the system consisted of a single fiber optic cable with six FBG sensors embedded in a 36-ply fiberglass composite specimen. The FBG sensors were installed around a series of known engineered flaws. Six foil type resistive strain gauges were bonded to the composite specimen surface and co-located with the six embedded FBG sensors. The fiber optic interrogator was used to sample the FBG sensors and an independent data acquisition system was used to sample the foil strain gauges. The test specimen was subjected to a series of static loads and the results from both the foil strain gauges and the FBG sensors were compared. Results from the analysis show a good correlation between the embedded FBG sensors and the foil strain gauges.

ATLAS - A small, light weight, time-synchronized wind-turbine data acquisition system

Wind energy researchers at Sandia National Laboratories have developed a small, lightweight, time- synchronized, robust data acquisition system to acquire long-term time-series data on a wind turbine rotor. A commercial data acquisition module is utilized to acquire data simultaneously from multip!e strain-gauge, analog, and digital channels. Acquisition of rotor data at precisely the same times as acquisition of ground data is ensured by slaving the acquisition clocks on the rotor- based data unit and ground-based units to the Global Positioning Satellite (GPS) system with commercial GPS receiver units and custom-built and programmed programmable logic devices. The acquisition clocks will remain synchronized within two microseconds indefinitely. Field tests have confirmed that synchronization can be maintained at rotation rates in excess of 350 rpm, Commercial spread-spectrum radio modems are used to transfer the rotor data to a ground- based computer concurrently with data acquisition, permitting continuous acquisition of data over a period of several hours, days or even weeks.