The prediction of unsteady three-dimensional aerodynamics on wind turbine blades

After introducing the main features of the aerodynamics of wind turbines, a review of key theoretical studies and aerodynamic modelling methods provides the opportunity to focus on predictive methods and the main technical challenges associated with the aerodynamics of HAWTs. The basic aerodynamic method adopted in this study, a classic Blade Element Momentum theory model, BEM, is described next and its extension to yawed flow is detailed for completeness. Analysis then focuses on how stall delay due to three-dimensional effects can be predicted on a HAWT. Implementation of a semi-empirical stall delay model shows sensitivity to blade geometry but no dependency on wind velocity or rotational speed. This seems to be physically incorrect and suggests that a deeper understanding of 3-D effects is still needed if better algorithms are to be developed. The work then examines the onset of dynamic stall. A 2-D semi-empirical correlation of vortex stall onset, developed previously at Glasgow University, is implemented and validated through available field data from the NREL turbine Phases II and IV. The comparison of measured and predicted locations of dynamic stall onset highlights some interesting features of the three-dimensionality of the process; after the local inception, earlier dynamic stall appears to be triggered in adjacent stations. An attempt to study how 3-D stall delay interacts with the onset of dynamic stall, shows that stall delay appears not to influence the inception of dynamic stall in the way it does static stall. Moreover, the firsts signs of dynamic stall onset are generally best characterised by the correlation when it assumes locally 2-D flow. This is a significant result, as it demonstrates that the earliest signs of dynamic stall onset on wind turbines can be correctly predicted using 2-D tools. A closer examination of the discrepancies between the predictions and measurements has highlighted the particular aerodynamic characteristics of the S809 aerofoil, utilised as the blade section of the NREL turbines. The unusual stalling characteristics of this aerofoil bring into question the significance of the static stall angle in relation to dynamic stall. It is show that other features of the static behaviour may provide a more appropriate link to dynamic stall for some aerofoils. Finally, the phenomenon of tower shadow on a downwind turbine is studied. Unaveraged pressure measurements and integrated normal force coefficients from tests conducted at Glasgow University are analysed. The analysis highlights many interesting features of the tower shadow response. In particular, as the blade enters the tower shadow region, there is a rapid reduction in normal force due to the tower wake velocity deficit. As the blade leaves the tower shadow, the recovery is consistently slower and more progressive and apparently extends further than the edge of the velocity deficit region. These observations are then used in a examination of tower shadow modelling. A steady model, based on a cosine shaped velocity deficit is evaluated by comparison with the wind tunnel measurements. Unfortunately neither the phase nor the intensity of the response is adequately captured. This leads to the implementation of a new model, based on classic unsteady thin aerofoil theory that accounts for the aerofoil wake induced velocities. The unsteady model captures, in a satisfactory manner, the global response of the blade through the tower shadow region, with a negligible computational cost

AVATAR HIGH REYNOLDS NUMBER TESTS ON AIRFOIL DU00-W-212

Within EU FP7 AVATAR project (AdVanced Aerodynamic Tools of lArge Rotors), a high Reynolds number and low Mach number wind tunnel test has been performed with the aim to obtain reliable data that can be used to validate existing aerodynamic models for this operating range. The test has been performed at the DNW High Pressure Wind Tunnel in Göttingen (HDG)

Development of OPASS Code for Dynamic Simulation Mooring Lines in Contact with Seabed

The mooring system of a floating platform keeps the structure in the desired sea location and can also contribute to the system stability. It consists of several cables attached to the platform in a point called fairlead, and with the lower ends anchored to the seabed. A new code for the dynamic simulation of mooring lines on wind turbines based on Finite Elements is presented. The code, called OPASS (Offshore Platform Anchoring System Simulator), includes effects as inertia, added mass, hydrodynamic drag, structural damping and contact and friction with seabed. The equations of motion are integrated in time using the Runge-Kutta-Nystr ¿om scheme. It has been loosely coupled with the FAST code for the simulation of offshore floating wind turbine dynamics. OPASS has been verified with semi-empirical expressions and with other codes as 3DFloat and SIMO-RIFLEX. Simulations prescribing a harmonic horizontal motion with 5m amplitude at the fairlead with different periods were performed with the objective of evaluating the importance of dynamic effects. In these simulations, dynamic effects on the lines are more important at moderate line tensions and can increase the tension up to 60% in comparison with the static tension. For taut lines dynamic effects are smaller and the quasi-static model could be an acceptable approximation. A comparison with FAST quasi-static mooring lines model has been also performed for a spar-buoy floating platform. The dynamic model predicts an important increase (30%) in the tension peaks of the cable with respect to the quasi-static approach and the consideration of dynamic effects can also have influence on the global motions of the platform. In particular, the quasi-static model can underestimate the surge damping

Experimental Validation of a Dynamic Mooring Lines Code with Tension and Motion Measurements of a Submerged Chain

This paper shows a complete study of the dynamics of a mooring line: first, a simulation code based on a lumped mass formulation was developed and tested under different setups and second, an equivalent experimental campaign was performed to compare against the numerical predictions. The tests consisted of a suspended chain submerged into a water basin, where the suspension point of the chain was excited with horizontal harmonic motions of different periods in the plane of the catenary. The code is able to predict the tension at the suspension point and the motions of the line with accuracy. For those cases where the line loses and subsequently recovers tension, the resulting snap load and motions are well captured with a slight overprediction of the maximum tension. The added mass and drag coefficients for chains used in the computations have been taken from guidelines and, in general, predict correctly the hydrodynamic loads. In addition, sensitivity studies and verification against another code show that highly dynamic cases are sensitive to the seabed-cable contact and friction models. The results show the importance of capturing the evolution of the mooring dynamics for the prediction of the line tension, especially for the high frequency motions

Impact of Mooring Lines Dynamics on the Fatigue and Ultimate Loads of Three Offshore Floating Wind Turbines Computed with IEC 61400-3 Guideline

The calculation of loads for floating offshore wind turbines requires time-domain integrated simulation tools where most of the physical concepts involved in the system dynamics are considered. The loads at the different components are used for the structural calculation and influence the design noticeably. This study quantifies the influence of mooring dynamic models on the calculation of fatigue and ultimate loads with integrated tools and compares its performance with a lower computational cost quasi-static mooring model. Three platforms representing the principal topologies (spar, semisubmersible and tension-leg platform) were assumed to be installed at the same 200 m depth location in the Irish coast. For each platform, the fatigue and ultimate loads were computed with an integrated floating wind turbine simulation code using both, a quasi-static and a fully dynamic moorings model. More than 3500 simulations for each platform and mooring model were launched and post-processed according to the IEC 61400-3 guideline in an exercise similar to what a certification entity may require to an offshore wind turbine designer. The results showed that the impact of mooring dynamics in both fatigue and ultimate loads increases as elements located closer to the platform are evaluated; the blade and the shaft loads are only slightly modified by the mooring dynamics in all the platform designs; the tower base loads can be significantly affected depending on the platform concept; and the mooring lines tensions strongly depend on the lines dynamics, both in fatigue and extreme loads for all the platform concepts evaluated

Summary of the Blind Test Campaign to predict the High Reynolds number performance of DU00-W-210 airfoil

This paper summarizes the results of a blind test campaign organized in the AVATAR project to predict the high Reynolds number performance of a wind turbine airfoil for wind turbine applications. The DU00-W-210 airfoil was tested in the DNW-HDG pressurized wind tunnel in order to investigate the flow at high Reynolds number range from 3 to 15 million which is the operating condition of the future large 10MW+ offshore wind turbine rotors. The results of the experiment was used in a blind test campaign to test the prediction capability of the CFD tools used in the wind turbine rotor simulations. As a result of the blind test campaign it was found that although the codes are in general capable of predicting increased max lift and decreased minimum drag with Re number, the Re trend predictions in particular the glide ratio (lift over drag) need further improvement. In addition to that, the significant effect of the inflow turbulence on glide ratio especially at high Re numb ers is found as the most important parameter where the prediction as well as the selection of the correct inflow turbulence levels is the key for correct airfoil designs for the future generation 10MW+ wind turbine blades