Matthew Lennie

Finite micro-tab system for load control on a wind turbine

Finite micro-tabs have been investigated experimentally to evaluate the potential for load control on wind turbines. Two dimensional full span, as well as multiple finite tabs of various aspect ratios have been studied on an AH93W174 airfoil at different chord wise positions. A force balance was used to measure the aerodynamic loads. Furthermore, the wake vortex system consisting of the Karman vortex street as well as the tab tip vortices was analyzed with a 12-hole probe and hot wire anemometry. Finally, conventional oil paint as well as a quantitative digital flow analysis technique called SMARTviz were used to visualize the flow around the finite tab configurations. Results have shown that the devices are an effective solution to alleviate the airfoils overall load. The influence of the tab height, tab position as well as the finite tab aspect ratio on the lift and lift to drag ratio have been evaluated. It could be shown, that the lift difference can either be varied by changing the tab height as well as by altering the aspect ratio of the finite tabs. The drag of a two-dimensional flap is directly associated with the vortex street, while in the case of the finite tab, the solidity ratio of the tabs has the strongest effect on the drag. Therefore, the application of a finite tab system showed to improve the lift to drag ratio.

Modern methods for investigating the stability of a pitching floating platform wind turbine

The QBlade implementation of the Lifting Line Free Vortex Wake method(LLFVW) was tested in conditions analogous to floating platform motion. Comparisons against two independent test cases, using a variety of simulation methods show excellent agreement in thrust forces, rotor power, blade forces and rotor plane induction. Along with the many verifications already undertaken in literature, it seems that the code performs solidly even in these challenging cases. Further to this, the key steps are presented from a new formulation of the instantaneous aerodynamic thrust damping of a wind turbine rotor. A test case with harmonic platform motion and collective pitch is used to demonstrate how combining such tools can lead to better understanding of aeroelastic stability.

The Unsteady Aerodynamic Response of an Airfoil with Microtabs and it's Implications for Aerodynamic Damping

In this study, experimental airfoil data was taken in the Laminar Wind Tunnel of the Technical University of Berlin. A NACA0021 airfoil was tested with various sizes and positions of static microtabs while the foil underwent pitching oscillations. The resulting data was analysed using the novel Hilbert Transform method to yield the instantaneous aerodynamic damping. From this analysis it was possible to determine that in light and deep stall, microtabs increase the cycle averaged aerodynamic damping but in the case of light stall, the microtabs amplify the sub-cycle negative damping regions as well. This yields the conclusion that sub-cycle actuation of the microtab in light stall could further optimise the aerodynamic damping. The aforementioned effects get stronger with increasing tab size and as the microtabs are positioned further forward.

A Review of Wind Turbine Polar Data and its Effect on Fatigue Loads Simulation Accuracy

To certify a Wind Turbine the standard processes set out by the GL guidelines and the IEC61400 demand a large number of simulations in order to justify the safe operation of the machine in all reasonably probable scenarios. The result of this rather demanding process is that the simulations rely on lower fidelity methods such as the Blade Element Momentum (BEM) method. The BEM method relies on a number of simplified inputs including the coefficient of lift and drag polar data (usually referred to as polars). These polars are usually either measured experimentally, generated using tools such as XFoil or, in some cases obtained using 2D CFD. It is typical to then modify these polars in order to make them suitable for aeroelastic simulations. Some of these modifications include 360° angle of attack extrapolation methods and polar modifications to account for 3D effects. Many of these modifications can be perceived to be a black art due to the manual selection of coefficients. The polars can misrepresent reality for many reasons, for example, inflow turbulence can affect measurements obtained in wind tunnels. Furthermore, on real wind turbine blades leading edge erosion can reduce performance. Simulated polars can even vary significantly due to the choice of turbulence models. Stack these effects on top of the uncertainties caused by yaw error, pitch error and dynamic stall and one can clearly see an operating environment hostile to accurate simulations. Colloquial evidence suggests that experienced designers would account for all of these sources of errors methodically, however, this is not reflected by the certification process. A review of experimental data and literature was performed to identify some of the inaccuracies in wind turbine polars. Significant variations were found between a range of 2D polar techniques and wind tunnel measurements. A sensitivity study was conducted using the aeroelastic simulation code FAST (National Renewable Energy Laboratory) with lift and drag polars sourced using different methods. The results were post-processed to give comparisons the rotor blade fatigue damage; variations in accumulated damages reached levels of 164%. This variation is not disastrous but is certainly enough to motivate a new approach for certifying the aerodynamic performance of wind turbines. Such an approach would simply see the source of polar data and all post-processing steps documented and included in the checks performed by certification bodies.Copyright

The Use of a New Fatigue Tool (ALBdeS) to Analyse the Effects of Vortex Generators on Wind Turbines

Wind turbines are classically designed for an extremely long lifetime in machinery design terms, for example, the Siemens SWT-6.0-154 was recently certified for 25 years [1]. The implication is that wind turbines accumulate damage via a number of mechanisms. A primary concern is naturally fatigue, exacerbated by a long life (high number of cycles); however, environmental effects such as bio-fouling and leading edge erosion damage the structure but also modify the Lift and Drag characteristics, particularly the stall behaviour. Vortex Generators (VGs), more commonly known from the aviation industry, have been demonstrated to delay stall and improve the stall region characteristics. This restoration of properties has been associated with reduced fatigue loading following the logic that the rotor blade will undergo stall less severely and less often. This hypothesis was tested in this study using a newly developed fatigue tool ALBdeS (named after W. Albert the first author to write a paper considering fatigue) to post-process aeroelastic simulations conducted in FAST (from NREL/NWTC) [2]. The post processing tool is an extension of the PMV custom section rotorblade analysis tool of SMART BLADE GmbH. The ALBdeS tool calculates the cumulative damage value in each individual layer of the blade section laminates and determines whether or not failure will occur over the course of 20 years, following the GL Guidelines [3]. Sensitivity studies showed that by de-constructing the main oscillation into 30 analysis points, the accumulated damage converges to a stable result, thus increasing confidence in the stability of the method. The FAST simulations were conducted with modified versions of the NREL 5MW reference turbine. The inboard lift and drag polars of the 5MW were modified in order to simulate the effect of adding VGs to the design. The polar modifications were made in the absence of 3D stall delay effects although literature does indicate the effects are somewhat additive. However, the resulting simulations did demonstrate that VGs did in fact change the fatigue characteristics of the rotor blades but by an inconsiderate amount.Copyright

Development and Validation of a Modal Analysis Code for Wind Turbine Blades

ABSTRACT QBlade is an open source wind turbine design and simula-tion tool developed at the Berlin Institute of Technology. To en-able a coupling with the aeroelastic simulation tool FAST fromNREL an aditional module, called QFEM, was created and in-tegrated with QBlade. This module performs a modal analysison rotor blades designed in QBlade using isotropic tapered Eu-ler Beam elements. The newly developed module now providesstructural properties to the National Renewable Energy Labo-ratorys aeroelasticity simulation tool FAST. The 2D structuralproperties of the beam elements are created using integrationmethods. A number of test cases show that the 2D integrationmethods and beam element code work with adaquete accuracy.The integration of the modal analysis code greatly facilitates thestructural design and analysis of rotor blades and will be madeavailable to the public under an open source license. NOMENCLATURE [EA] Longitudinal Stiffness[ES xR ] Moment of Stiffness about the x Ref. Axis[ES