A numerical investigation of using flap control on VAWTs

Abstract

The renewable energy need facilitates the development of offshore wind power. The abundant wind resource has pushed the wind farms into deep water sites. The floating VAWT concept is emerges as the times require. This research is part of Deepwind concept development. The project focuses on verifying the feasibility of using flap control to increase the power conversion of a VAWT through the numerical investigation. As a preliminary implementation, only a 2-D section is taken out from the Darrieus rotor as the modeled object. The report concentrates on the purely aerodynamic modeling of a VAWT. All the blades are assumed to be fully rigid in order to exempt the structural response. VAWTs present a distinct aerodynamic pattern from HAWT. To start up, a historical review on the aerodynamic models of VAWTs is performed to recap the critical issues in the aerodynamic modeling. The study mainly emphasizes on the momentum models, covers SST, MST, DMST and Actuator Cylinder (AC) models. AC mainly advantages on including the lateral inductions, and the wake interaction. Its precise prediction was confirmed in the simulation results of the aerodynamic model comparison study [10]. Hence, AC has been chosen as the numerical tool for VAWT modeling, both for the self-developed routine and HAWC2. A promising solution to determine the load-form for optimal power is constructing a uniform distribution so that the power coefficient could reach Betz limit. The normal load distribution on the blade path is the link between the power conversion and the blade force. At the moment of the determination of the load distribution that leads to the optimal power, the required blade force at the different azimuth positions are also settled. The simulations are performed with TSR=2, 3, 4, and 5 to verify the possibility on achieving the ideal power production in order to draw up an optimized control option. The result of the study is presented as a form of different flap angles at discretized azimuth positions. Even though the load-form is universal, the resultant flap variation only works for the specific rotor because of the introduction of airfoil data in the computation. An H-rotor VAWT with aps is set up in HAWC2 to validate the control strategy. As the supplement to AC model, a dynamic stall model called ATEFlap is integrated in HAWC2 to calculate the dynamic stall impact, and the influences from flap detection for the given airfoil. The model is supposed to return the aerodynamic loading of the blades under the dynamic stall. Hence, the deviations between the target load distribution obtained from the quasi-steady environment and the semi-dynamic one is observed as expected. In the end, the aps running with the given function could almost give the desired loading, but the deviations are observed in downwind part. It is mainly the hysteresis of the lift coefficient causes the difference. It is proven to be partly made up by increasing the flap deflections in the downwind side.