Gijs van Kuik

Two-Degree-of-Freedom Active Vibration Control of a Prototyped “Smart” Rotor

This paper studies the load reduction potential of a prototyped “smart” rotor. This is, a rotor where the blades are equipped with a number of control devices that locally change the lift profile on the blade, combined with appropriate sensors and controllers. Experimental models, using dedicated system identification techniques, are developed of a scaled rotating two-bladed “smart” rotor of which each blade is equipped with trailing-edge flaps and strain sensors. A feedback controller based on H∞-loop shaping combined with a fixed-structure feedforward control are designed that minimizes the root bending moment in the flapping direction of the two blades. We evaluated the performance using a number of different realistic load scenarios. We show that with appropriate control techniques the variance of the load signals can be reduced up to 90%.

2D CFD Simulation of Dynamic Stall on a Vertical Axis Wind Turbine: Verification and Validation with PIV Measurements

After a decrease of interest in the 1990’s, the research on Vertical Axis Wind Turbines (VAWT) a has reappeared in the last years as a result of the its increasing application in the built environment, where VAWTs present several advantages over Horizontal Axis Wind Turbines (HAWT). The VAWT has an inherent unsteady aerodynamic behavior due to the variation of angle of attack with the angle of rotation µ, perceived velocity and consequentially Reynolds number. The phenomenon of dynamic stall is then an inherent efiect of the operation of a Vertical Axis Wind Turbine (VAWT) at low tip speed ratios (‚), having a signiflcant impact in both loads and power. The complexity of the unsteady aerodynamics of the VAWT makes it extremely attractive to be analyzed using Computational Fluid Dynamics (CFD) models, where an approximation of the continuity and momentum equations of the Navier-Stokes equations set is solved. The complexity of the problem and the need for new design approaches for VAWT for the built environment has driven the authors of this work to focus the research of CFD modeling of VAWT not in the perspective of creating one large academic model to test a particular situation, but to develop a work that would: † verify the sensitivity of the model to its grid reflnement (space and time), † evaluate the difierences between the difierent commonly used turbulence models (Laminar, Spalart i Allmaras and k i †), and † be evaluated using Particle Image Velocimetry (PIV) experimental data, thus determining the suitability of this data for model validation. The 2D model created represents the middle section of a single bladed VAWT with inflnite aspect ratio. The model simulates the experimental work b of ∞ow fleld measurement using Particle Image Velocimetry (PIV) by Sim~ao Ferreira et al 1 for a single bladed VAWT, for two difierent reference Reynolds numbers of Re = 52000 and Re = 70000 for three tip speed ratios: ‚ = 2;3;4. The results show the suitability of the PIV data for the validation of the model, the unsuitability of the application of a single turbulent model and the high sensitivity of the model to grid reflnement.

Wind Tunnel Hotwire Measurements, Flow Visualization and Thrust Measurement of a VAWT in Skew

The results of experimental research on the wake and induced flow around a vertical axis wind turbine (VAWT) in skew are presented. The previous research on VAWTs in skew is limited because this operation mode has only recently been found to be significant in the operation of VAWTs in the built environment. These results contain hotwire measurements of the incoming flow and wake of a VAWT in nonskewed and skewed flow. The high sampling rate of the hotwire data allows the effects of blade passing to be identified. Flow visualization of the tip vortices is also presented. Thrust measurements of the rotor were performed to understand the effect of skew on thrust variation and to compare with analytical predictions. Copyright

Measurement of Tip Vortex Paths in the Wake of a HAWT Under Yawed Flow Conditions

Tip vortex locations have been measured in the wake of a model rotor in both axial flow and yaw using quantitative flow visualization. For each setting, the axial force coefficient has been derived, as well, from measurements. The results agree well with those previously published on the Delft University of Technology model rotor. The main interest is to determine the tip vortex pitch, wake skew angle, wake expansion, and to physically interpret the data. The results also help to validate and construct models. The tip vortex location data complement the existing skewed wake velocity data from hot-wire anemometry, making it a valuable experimental database.

Active Aerodynamic Blade Distributed Flap Control Design Procedure for Load Reduction on the UpWind 5MW Wind Turbine

This paper develops a system identification approach and procedure that is employed for distributed control system design for large wind turbine load reduction applications. The primary goal of the study is to identify the process that can be used with multiple sensor inputs of varying types (such as aerodynamic or structural) that can be used to construct state-space models compatible with MIMO modern control techniques (such as LQR, LQG, H1, robust control, etc.). As an initial step, this study employs LQR applied to multiple flap actuators on each blade as control inputs and local deflection rates at the flap spanwise locations as measured outputs. Future studies will include a variety of other sensor and actuator locations for both design and analysis with respect to varying wind conditions (such as high turbulence and gust) to help reduce structural loads and fatigue damage. The DU SWAMP aeroservoelastic simulation environment is employed to capture the complexity of the control design scenario. The NREL 5MW UpWind reference wind turbine provides the large wind turbine dynamic characteristics used for the study. Numerical simulations are used to demonstrate the feasibility of the overall approach. This study shows that the distributed controller design can provide load reductions for turbulent wind profiles that represent operation in above-rated power conditions.

Measurement and Modelling of Tip Vortex Paths in the Wake of a HAWT under Yawed Flow Conditions

Tip vortex locations have been measured in the wake of a model rotor in both axial flow and yaw using quantitative flow visualisation. For each setting, the axial force coefficient has been derived as well from measurements. The results seem to agree well with those previously published on the Delft University of Technology model rotor. The main interest is to determine the tip vortex pitch, wake skew angle and wake expansion and to physically interpret the data. The results should also help to validate and construct models. The tip vortex locations data complement the existing skewed wake velocity data from hot-wire anemometry, making it a valuable experimental database.

Impact of Higher Fidelity Models on Simulation of Active Aerodynamic Load Control For Fatigue Damage Reduction

Active aerodynamic load control of wind turbine blades is being investigated by the wind energy research community and shows great promise, especially for reduction of turbine fatigue damage in blades and nearby components. For much of this work, full system aeroelastic codes have been used to simulate the operation of the activel y controlled rotors. Research activities in this area continually push the limits of the models and assumptions within the codes. This paper demonstrates capabilities of a full system aeroelastic code recently developed by researchers at the Delft Universi ty Wind Energy Research Institute with the intent to provide a capability to serve the active aerodynamic control research effort, The code, called DU_SWAMP, includes higher fidelity structural models and unsteady aerodynamics effects which represent improvement over capabilities used previously by researchers at Sandia National Laboratories. The work represented by this paper includes model verification comparisons between a standard wind industry code, FAST, and DU_SWAMP. Finally, two different types of a ctive aerodynamic control approaches are implemented in order to demonstrate the fidelity simulation capability of the new code.

On the use of velocity data for load estimation of a VAWT in dynamic stall

This paper focuses on evaluating the feasibility of estimating loads on vertical axis wind turbine blades in dynamic stall with velocity data acquired with Particle Image Velocimetry. The study uses numerical simulation data of a 2D Vertical Axis Wind Turbine in dynamic stall to verify sources of error and uncertainty and estimate the accuracy of the method. The integration of the forces from the velocity field overcomes the difficulties and limitations presented by pressure sensors for estimating the local section loads, but adds the difficulty in determining the correct velocity field and its time and spatial derivatives. The analysis also evaluates the use of phase-locked average data as an estimator of average loads.

Performance of a High Tip Speed Ratio H-Darrieus in the Skewed Flow on a Roof

Small wind turbines sited on a flat roof have good opportunities to become widespread. They operate in the accelerated wind above the roof and deliver the power where it is needed. Since the power produced offsets that which would otherwise be bought from the utility, they reduce energy demand and bills from the utility. Furthermore excess power can be sold back to the utility, thus producing income as well. Flow over a building separates at the roof leading edge at a certain angle. Wind turbines sited well above the roof thus operate in skewed flow. H-Darrieus operating at (flat) roofs just recently start to be at public interest, operation of an H-Darrieus in skewed flow is thus not discussed in literature until now. To examine this, a model of an H-Darrieus with high Tip Speed Ratio (λ) in skewed flow is developed. The model is based on multiple stream-tube theory: a combination of axial momentum and blade element theory on an actuator plate representation of the rotor, which is divided into multiple stream-tubes. The model shows that, for an H-Darrieus designed for skewed flow, the optimal power output in skewed flow can be up to two times the power output in normal - perpendicular to the H-Darrieus axis- flow. The spatial dimension of the H-Darrieus is responsible for this.Copyright

Model Validation and Simulated Fatigue Load Alleviation of SNL Smart Rotor Experiment.

In this paper an individual flap controller (IFC) design for smart rotors is presented and compared to the model identification of the Sandia National Labs Smart Rotor experiment. The controller design has been carried out using an in-house aeroservoelastic software - DU_SWAT. The root bending moment response due to a step input of the flap deflection has been linearized. Based on this linearization a Coleman transform combined with a proportionality controller has been used to eliminate the cyclic components in the root bending moment. It was shown that IFC can reduce the 1P mode similar to individual pitch control, while only modest flap deflections of below 5