Approach

IEA Task 29 is carried out in four phases. The first three phases are finished, the fourth phase started on January 1st 2018 and ends on December 31st 2020. The approaches in all four phases of Task 29 are similar although the fact that the first three phases are devoted to wind tunnel measurements where the fourth phase is devoted to field measurements gives different focal points (e.g. the first phases investigated tunnel effects and PIV flow field measurements where the fourth phase puts emphasis on aero-elastic effects and turbulent inflow). Still the WP's IV.1 to IV.3 have a similar character as the corresponding Work Packages in the previous phase.

WPIV.1: Processing/presentation of data, uncertainties, tunnel effects
Data will be processed and uncertainties will be assessed.

WPIV.2: Comparison of calculational results from different types of codes with measurement data
In this WP the calculational results from the codes which are used by the participants are compared with the data from the various experiments. It is meant to be a thorough validation of different codes and it provides insights into the phenomena which need further investigation (see WP2).

WPIV.3: Deeper investigation into phenomena
In this Work package a deeper investigation of different phenomena will take place.
The phenomena will be investigated with isolated submodels, simple analytical tools or by physical rules.

Task IV.3.1: Aerodynamic response to turbulent inflow
In the former phases of IEA Task 29 measurement at non-turbulent wind tunnel conditions were considered. It is well known that modelling the aerodynamic response at turbulent conditions leads to additional complications. The DanAero database will be searched for measurements at different turbulence levels to assess the impact of turbulence on the response. A possibility is to sort measurement into IEC 61400 turbulence classes i.e. turbulence intensity bins of 2% wide.

Task IV.3.2: Sheared and yawed flow
In the former phases of IEA Task 29 measurements at non-sheared conditions were considered only. It is well known that the induction modelling and the aerodynamic response from BEM methods at sheared conditions leads to results which differ from those of higher fidelity codes. The same holds for yawed conditions. The starting point for the present analysis will then be formed by the DanAero measurement cases which were selected at minimal shear and different yaw angles. At a later stage higher shear cases are selected.

Task IV3.3: Wake inflow
Nowadays wind turbines are clustered together in wind farms. In such an operating environment wind turbines are exposed to wake inflow with increased (horizontal and vertical) shear and increased turbulence. Wake inflow could not be considered in the former phases of Task 29 but in the DanAero experiment pressure distributions, pilot probe measurements and load measurements at wake inflow have been measured. They will be analysed to understand the turbine response at wake conditions.

It is noted that wake flow is also considered in another IEA Task, i.e. IEA Task 31. However until now Task 31 is focussed on the power production where IEA Task 29 is focussed on the load response.

Task IV.3.4: 3D effects (both lift and drag)
In this task the 3D effects on the rotating airfoil data are being analysed. Sectional data as measured on the rotating DanAero blade are compared with 2D airfoil data from task IV.2. in order to distinguish the rotational effects on the airfoil data. Most of these studies until now only addressed the lift coefficient. In this task the drag coefficient will also be considered.

It will be attempted to define a common Task 29 engineering model for both the lift as well as the drag coefficient.

Task IV.3.5: Aero-elastic effects
In the former phases of IEA Task 29 measurements were considered on small wind turbine models which were placed in the wind tunnel. All structural components of these models were relatively stiff by which the aerodynamic response of these turbine models could be seen independent of the aero-elastic response. However, the Danaero turbine with blade lenghts of 40 meters will have much more significant blade deflectations. Although and analysis carried out in the EU project AVATAR showed that local aerodynamic loads themselves were not too heavily influenced by the aero-elasticity but the total blade and turbine loads are affected. Therefore within this task a further analysis of aero-elastic effects will be carried out. For this purpose a full aero-elastic behavior of the blades and turbine will be made available.

Task IV.3.6: Boundary layer transition
Precise knowledge of the transition location on an airfoil of a rotating wind turbine blade is extremely important since it determines the airfoil characteristics (in particular the drag) and so the power and loads to a large extent. Airfoil data used in BEM generally originate from wind tunnel measurements which normally are performed at very low turbulence levels. Within the wind energy society the usefulness of 'smooth' wind tunnel data for the calculation of the power has often been questioned because it is believed that the turbulent environment in which a wind turbine operates causes a significant forward shift of the transition point on suction sides, "by-pass transition". In the Dan-Aero information on transition obtained from microphones as the measurement on the rotor were complimented by 2D measurements in the LM wind tunnel on an exact copy of the blade section on the rotor and with the same instrumentation this constitutes a unique basis for improved understanding of transition.

Task IV.3.7: Acoustics
Wind turbine noise forms one of the major hindrances for the social acceptances of wind energy. It is well known that the dominant noise source is aerodynamically driven. Within the Danaero experiment high frequency surface pressure data from microphones makes it possible to characterise the noise sources at different conditions. Acoustic turbulent inflow modelling and trailing edge noise models will be studies on this basis.

Moreover Phase IV has a Work Package (WPIV.4) in which large scale field experiment is designed. This WP has been introduced because the seemingly large amount of data as analysed in the IEA task does not overcome the most important problem in the wind energy society are still very scarce. To support validation of unsteady, non-uniform inflow conditions combined with flexible blades, which is necessary to design the large rotors of the future, a new experiment on an even larger scale is urgently needed. The design of the experiment should be as complete as possible and it should include goal, requirements, target turbine but also details on sensors (prices, preferred suppliers, etc.), test matrix, supporting experiments, etc.

Finally WPVI.5 considers communication to other IEA Tasks. The IEA tasks which are considered to have closest relation to aerodynamic are:

  • IEA Task 19: "Wind Energy in Cold Climates" (https://community.ieawind.org/task19/home (The connection to Task 29 lies in the Task 29 models which can assess the aerodynamic effects of iced turbine blades)
  • IEA Task 28: "Social Acceptance of wind energy projects" (https://communit.ieawind.org/task28/home (The connection lies in the acoustic characterization of wind turbine noise sources which become available from Task 29
  • IEA Task 30: "Comparison of Dynamic Computer Codes and Models for Offshore Wind Energy" (https://community.ieawind.org/task30/home) (The connection task 29 lies in the fact that part of the Task 30 computer codes are aerodynamically driven using models being improved in Task 29)
  • IEA Task 31: "Wakebench" (https://windbench.net/wakebench (The connection to Task 29 lies in the near wake which is considerd in Task 29 and which forms the starting point of the far wake considered in WakeBench. Moreover Task 29 will consider the turbine response to waked inflow with an impact on the loading where WakeBench considers the power output at wake inflow)
  • IEA Task 37: "wind Energy System Engineering: Integrated R, D&D" (https://windbench.net/iea37 (The connection to Task 29 lies in the Task 29 aerodynamic models which are integrated into Task 37)
  • IEA Task 38: "Downwind turbines (The connection to Task 29 lies in the Task 29 aerodynamic models which are needed to design download turbines)
  • IEA Task 39: "Quiet noise technologies" (https://community.ieawind.org/task39/home (The connection to Task 29 lies in the improved understanding of aerodynamic noise provided in Task 29)