G.A.M. Van Kuik

Closed-loop control wind tunnel tests on an adaptive wind turbine blade for load reduction

Wind tunnel tests on a non-rotating, dynamically scaled wind turbine blade equipped with variable trailing edge geometry were carried out. The effectiveness of the system for active load reduction purposes, with the interaction between structural dynamics, aerodynamics and control was tested. The actuation of the adaptive trailing edge was based on a piezoelectric bender actuator. The full aeroservoelastic system was identified based on input and output measurement signals. A feedback controller, using strain signals on the blade root, was designed, tuned and applied on the system in order to minimize root bending moments. The results show remarkable performance in reduction of blade root strains for both open-loop and closed-loop tests. The sensitivity of various design and control parameters are analyzed both in the prescribed cases and in the feedback-controlled system.

The relationship between loads and power of a rotor and an actuator disc

Most state of the art rotor design methods are based on the actuator disc theory developed about one century ago. The actuator disc is an axisymmetric permeable surface carrying a load that represents the load on a real rotor with a finite number of blades N. However, the mathematics of the transition from a real rotor load to an axisymmetrically loaded disc is not yet presented in literature. By formulating an actuator disc equation of motion in which the Bernoulli constant H is expressed in kinematical terms, a comparison of the power conversion and load on the disc and rotor is possible. For both the converted power is expressed as a change of angular momentum times rotational speed. The limits for N ? ? while the chord c ? 0, the rotational speed ? ? ?, the load F becoming uniform by ?F/?r ? 0 and the thickness ? ? 0 confirm that the classical disc represents the rotor with an infinite number of blades. Furthermore, the expressions for the blade load are compared to the expressions in current design and analysis tools. The latter do not include the load on chord-wise vorticity. Including this is expected to give a better modelling of the tip and root flow.

Active flap control on an aeroelastic wind turbine airfoil in gust conditions using both a CFD and an engineering model

In the past year, smart rotor technology has been studied significantly as solution to the ever growing turbines. Aeroservoelastic tools are used to asses and predict the behavior of rotors using trailing edge devices like flaps. In this paper an unsteady aerodynamic model (Beddoes-Leishman type) and an CFD model (URANS) are used to analyze the aeroservoelastic response of a 2D three degree of freedom rigid body wind turbine airfoil with a deforming trailing edge flap encountering deterministic gusts. Both uncontrolled and controlled simulations are used to asses the differences between the two models for 2D aerservoelastic simulations. Results show an increase in the difference between models for the y component if the deforming trailing edge flap is used as control device. Observed flap deflections are significantly larger in the URANS model in certain cases, while the same controller is used. The pitch angle and moment shows large differences in the uncontrolled case, which become smaller, but remain significant when the controller is applied. Both models show similar reductions in vertical displacement, with a penalty of a significant increase in pitch angle deflections.

PIV visualization of dynamic stall VAWT and blade load determination

The increasing awareness of the need for environmentally sustainable housing and cities has driven the promotion of wind energy conversion systems for the built environment. One of the results of the development of solutions for the built environment is the reappearance of Vertical Axis Wind Turbines (VAWTs). In the built environment, the VAWT presents several advantages over the more common Horizontal Axis Wind Turbines (HAWTs), namely: its low sound emission (consequence of its operation at lower tip speed ratios), better esthetics due to its three-dimensionality, its insensitivity to yaw wind direction and its increased power output in skewed ∞ow (see Mertens et al 1 and Sim~ao Ferreira et al 2 ). The phenomenon of dynamic stall is an inherent efiect of the operation of a VAWT at low tip speed ratios (‚). The presence of dynamic stall has signiflcant impact on both load and power. The paper focuses on evaluating the feasibility of estimating loads on Vertical Axis Wind Turbine (VAWT) blades in dynamic stall by velocity data acquired with Particle Image Velocimetry (PIV). The work uses both numerical and experimental data. Simulated velocity data from a Detached Eddy Simulation (DES) at space and time reflnement equivalent to that obtained with PIV is used to estimate the error associated with the method. The method is then applied to experimental data to verify the in∞uence of the complexity of the ∞ow and determination of space and time derivatives. The acquired data over the entire rotation is used to calculate the blade forces from the velocity data and its derivatives (solving the momentum equation), following the methodology presented by Noca et al 3 and Scarano et al. 4 The integration of the forces from the velocity fleld should overcome the di‐culties and limitations presented by pressure sensors for local section loads, but involves the referred di‐culties in determining the correct time-derivatives.

IEA wind annex XX: HAWT aerodynamic and models from wind tunnel measurements - final report

This work characterizes undocumented physical relationships that govern aerodynamic force time variations that take place in connection with rotational augmentation on rotating wind turbine blades.

Individual blade pitch for yaw control

Individual pitch control (IPC) for reducing blade loads has been investigated and proven successful in recent literature. For IPC, the multi-blade co-ordinate (MBC) transformation is used to process the blade load signals from the rotating to a stationary frame of reference. In the stationary frame of reference, the yaw error of a turbine can be appended to generate IPC actions that are able to achieve turbine yaw control for a turbine in free yaw. In this paper, IPC for yaw control is tested on a high-fidelity numerical model of a commercially produced wind turbine in free yaw. The tests show that yaw control using IPC has the distinct advantage that the yaw system loads and support structure loading are substantially reduced. However, IPC for yaw control also shows a reduction in IPC blade load reduction potential and causes a slight increase in pitch activity. Thus, the key contribution of this paper is the concept demonstration of IPC for yaw control. Further, using IPC for yaw as a tuning parameter, it is shown how the best trade-off between blade loading, pitch activity and support structure loading can be achieved for wind turbine design.

Integrating robust lidar-based feedforward with feedback control to enhance speed regulation of floating wind turbines

The structural cost of offshore wind energy can be drastically reduced by the development of floating wind turbines. However, the design of a feedback controller for rotor speed control of such turbines faces fundamental bandwidth limitations because of the presence of nonminimum phase zeros. New developments in lidar technology enable turbines to measure the incoming wind and use the measurements for feedforward control. This paper explores the possibility of combining lidar feedforward with feedback control for floating wind turbines to enhance the speed control performance and increase controller bandwidth without affecting stability. Robust stability and performance of the controllers are investigated. The controllers are validated using the high-fidelity simulation environment FAST for floating turbines with lidar, and enhanced control performance is achieved.

Influence of the conservative rotor loads on the near wake of a wind turbine

The presence of conservative forces on rotor blades is neglected in the blade element theory and all the numerical methods derived from it (like e.g. the blade element momentum theory and the actuator line technique). This might seem a reasonable simplification of the real flow of rotor blades, since conservative loads, by definition, do not contribute to the power conversion. However, conservative loads originating from the chordwise bound vorticity might affect the tip vortex trajectory, as we discussed in a previous work. In that work we also hypothesized that this effect, in turn, could influence the wake induction and correspondingly the rotor performance. In the current work we extend a standard actuator line model in order to account for the conservative loads at the blade tip. This allows to isolate the influence of conservative forces from other effects. The comparison of numerical results with and without conservative loads enables to confirm qualitatively their relevance for the near wake and the rotor performance. However, an accurate quantitative assessment of the effect still remains out of reach due to the inherent uncertainty of the numerical model.

Aeroelastic Design and LPV Modelling of an Experimental Wind Turbine Blade equipped with Free-floating Flaps

Trailing edge aps located outboard on wind turbine blades have recently shown considerable potential in the alleviation of turbine lifetime dynamic loads. The concept of the free-oating ap is speci_cally interesting for wind turbines, on account of its modularity and enhanced control authority. Such a ap is free to rotate about its axis; camberline control of the free-oating ap allows for aeroelastic control of blade loads. This paper describes the design of a scaled wind turbine blade instrumented with free-oating aps, intended for use in wind tunnel experiments. The nature of the ap introduces a coupled form of utter due to the aeroelastic coupling of ap rigid-body and blade out-of-plane modes; for maximal control authority it is desired to operate close to the utter limit. Analytical and numerical methods are used to perform a utter analysis of the turbine blade. It is shown that the potential ow aeroelastic model can be recast as a continuous-time Linear-Parameter-Varying (LPV) state space model of a low order, for which formal controller design methodologies are readily available.