Mac Gaunaa

Wind Tunnel Test on Wind Turbine Airfoil with Adaptive Trailing Edge Geometry

,A wind tunnel test of the wind turbine airfoil Ris oe-B1-18 airfoil equipped with an Adaptive Trailing Edge Geometry (ATEG) was carried out. The ATEG was made by piezo electric actuators attached to the trailing edge of a non-deformable airfoil and controlled by an amplifier. The airfoil was tested at Re = 1.66x10 6 . Steady state and dynamic tests were carried out with prescribed deflections of the ATEG. The steady state tests showed that deflecting the ATEG towards the pressure side (posi tive β) translated the lift curve to higher lift values and deflecting the ATEG towards the suc tion side (negative β β β β) translated the lift curve to lower lift values. Furthermore, cd was almost unaffected by the ATEG actuation. Testing the airfoil for a step change of the ATEG f rom β=-3.0 to +1.8 showed that the obtainable Δcl was 0.10 to 0.13 in the linear part of the lift cu rve. Modeling the step response with an indicial function formulation showed that t he time constant in the step change and in sinusoidal deflections in dimensionless terms was T0* =0.6. Testing the ability of the ATEG to cancel out the load variations for an airfoil in si nusoidal pitch motion showed that it was possible to reduce the amplitude with around 80% from Δ Δ Δ Δcl=0.148 to Δcl=0.032.

Determination of the Maximum Aerodynamic Efficiency of Wind Turbine Rotors with Winglets

The present work contains theoretical considerations and computational results on the nature of using winglets on wind turbines. The theoretical results presented show that the power augmentation obtainable with winglets is due to a reduction of tip-effects, and is not, as believed up to now, caused by the downwind vorticity shift due to downwind winglets. The numerical work includes optimization of the power coefficient for a given tip speed ratio and geometry of the span using a newly developed free wake lifting line code, which takes into account also viscous effects and self induced forces. Validation of the new code with CFD results for a rotor without winglets showed very good agreement. Results from the new code with winglets indicate that downwind winglets are superior to upwind ones with respect to optimization of Cp, and that the increase in power production is less than what may be obtained by a simple extension of the wing in the radial direction. The computations also show that shorter downwind winglets (>2%) come close to the increase in Cp obtained by a radial extension of the wing. Lastly, the results from the code are used to design a rotor with a 2% downwind winglet, which is computed using the Navier-Stokes solver EllipSys3D. These computations show that further work is needed to validate the FWLL code for cases where the rotor is equipped with winglets.

Design and verification of airfoils resistant to surface contamination and turbulence intensity

This paper presents the design of high performance airfoils for incompressible ∞ow and for Reynolds numbers at 6mio with a lift performance which is resistant to surface contamination and turbulence intensity. The Ris?-C2 airfoil family is dedicated for MW-size wind turbines, which are exposed to varying in∞ow conditions and surface contamination from bugs and dust. The airfoils were designed to have high maximum lift coe‐cient, while maintaining high aerodynamic e‐ciency. Given these characteristics the airfoils were designed with maximum stifiness. The design was carried out with a quasi 3D multi disciplinary optimization tool to take into account the complete blade shape. The design of the Ris?-C2-18 airfoil was verifled in the LM Glasflber wind tunnel, Denmark and showed good agreement with predicted characteristics.

Analysis of aeroelastic loads and their contributions to fatigue damage

The paper presents an analysis of the aeroelastic loads on a wind turbine in normal operation. The characteristic of the loads causing the highest fatigue damage are identified, so to provide indications to the development of active load alleviation systems for smart- rotor applications. Fatigue analysis is performed using rain-flow counting and Palmgren-Miner linear damage assumption; the contribution to life-time fatigue damage from deterministic load variations is quantified, as well as the contributions from operation at different mean wind speeds. A method is proposed to retrieve an estimation of the load frequencies yielding the highest fatigue contributions from the bending moment spectra. The results are in good agreement with rain-flow counting analysis on filtered time series, and, for the blade loads, show dominant contributions from frequencies close to the rotational one; negligible fatigue contributions are reported for loads with frequencies above 2 Hz.

3D Navier-Stokes Simulations of a rotor designed for Maximum Aerodynamic Efficiency

The present paper describes the design of a three-bladed wind turbine rotor taking into account maximum aerodynamic efficiency only and not considering structural as well as offdesign issues. The rotor was designed assuming constant induction for most of the blade span, but near the tip region a constant load was assumed. The rotor design was obtained using an Actuator Disc model and was subsequently verified using both a free wake Lifting Line method and a full 3D Navier-Stokes solver. Excellent agreement was obtained using the three models. Global mechanical power coefficient, CP, reached a value of slightly above 0.51, while global thrust coefficient, CT, was 0.87. The local power coefficient, Cp, increased to slightly above the Betz limit on the inner part of the rotor as well as the local thrust coefficient, Ct, increased to a value above 1.1. This agrees well with the theory of de Vries which states that including the effect of the low pressure behind the centre of the rotor stemming from the increased rotation both Cp and Ct will increase towards the root. Towards the tip both Cp and Ct decrease due to tip corrections as well as drag.

First-order aerodynamic and aeroelastic behavior of a single-blade installation setup

Limitations on the wind speed at which blade installation can be performed bears important financial consequences. The installation cost of a wind farm could be significantly reduced by increasing the wind speed at which blade mounting operations can be carried out. This work characterizes the first-order aerodynamic and aeroelastic behavior of a single blade installation system, where the blade is grabbed by a yoke, which is lifted by the crane and stabilized by two taglines. A simple engineering model is formulated to describe the aerodynamic forcing on the blade subject to turbulent wind of arbitrary direction. The model is coupled with a schematic aeroelastic representation of the taglines system, which returns the minimum line tension required to compensate for the aerodynamic forcing. The simplified models are in excellent agreement with the aeroelastic code HAWC2, and provide a solid basis for future design of an upgraded single blade installation system able to operate at higher wind speeds.

Toward an Engineering Model for the Aerodynamic Forces Acting on Wind Turbine Blades in Quasisteady Standstill and Blade Installation Situations

The crossflow principle is one of the key elements used in engineering models for prediction of the aerodynamic loads on wind turbine blades in standstill or blade installation situations, where the flow direction relative to the wind turbine blade has a component in the direction of the blade span direction. In the present work, the performance of the crossflow principle is assessed on the DTU 10MW reference blade using extensive 3D CFD calculations. Analysis of the computational results shows that there is only a relatively narrow region in which the crossflow principle describes the aerodynamic loading well. In some conditions the deviation of the predicted loadings can be quite significant, having a large influence on for instance the integral aerodynamic moments around the blade centre of mass; which is very important for single blade installation applications. The main features of these deviations, however, have a systematic behaviour on all force components, which in this paper is employed to formulate the first version of an engineering correction method to the crossflow principle applicable for wind turbine blades. The new correction model improves the agreement with CFD results for the key aerodynamic loads in crossflow situations. The general validity of this model for other blade shapes should be investigated in subsequent works.

Indicial response function for finite-thickness airfoils, a semi-empirical approach

Many wind turbines aeroelastic codes recourse to indicial lift response formulations to evaluate unsteady aerodynamics in attached flow. The indicial response of a finitethickness airfoil differs from the flat plate one, which is usually adopted through Jones’s approximation. The lift response of airfoils with different geometries is determined with a panel code, and approximated by two term exponential functions. An empirical relation is then outlined in order to estimate the indicial response from the profile geometry. Unsteady lift computations are compared to CFD simulations for a harmonic pitching airfoil; the agreement with CFD results is improved by using the estimated indicial response instead of Jones’s flat plate expression. Finally, the effects on fatigue and ultimate loads are assessed by using the different indicial lift response function approximations in aeroelastic simulations.

Investigation of Stability Issues for an Adaptive Trailing Edge System

This work presents a method to determine ∞utter and divergence instability limits for a 2D airfoil section fltted with an actively controlled trailing edge ∞ap. This ∞ap consists of a deformable trailing edge, which deformation is governed by control algorithms based on measurements of either heave displacement or local angle of attack. The purpose of the controlled deformable ∞ap is to reduce ∞uctuations in the aerodynamic forces on the airfoil, which according to recent studies have a signiflcant potential for fatigue load alleviation. The structural model of the airfoil section contains three degrees of freedom: heave translation, pitch rotation and ∞ap de∞ection. A potential ∞ow model provides the aerodynamic forces and their distribution. The unsteady aerodynamics are described using an indicial function approximation. Stability of the full aeroservoelastic system is determined through eigenvalue analysis by state-space formulation of the indicial approximation. Validation is carried out against an implementation of the recursive method by Theodorsen and Garrick for ‘∞exure-torsion-aileron’ ∞utter. The implemented stability tool is then applied to an airfoil section representative of a wind turbine blade with active ∞ap control. It is thereby observed that the airfoil stability limits are signiflcantly modifled by the presence of the ∞ap, and they depend on several parameters: ∞ap structural characteristics, type of control, control gain factors and time lag.