Session L14: Wind Energy: Experiments

LiDAR measurements and modeling of onshore wind farms on flat and complex terrains

In order to understand and model interactions between the atmospheric boundary layer and multi-MW onshore wind farms, two experimental campaigns were carried out: the first one for a relatively flat terrain in Panhandle, TX, for which wind farm performance is mainly driven by the atmospheric stability, and a second site in Colorado characterized by a complex topography and low-level jets. Wind velocity measurements were performed through a scanning wind LiDAR, which are coupled with SCADA and meteorological data. LiDAR measurements of wind-turbine wakes are post-processed, clustered and ensemble-averaged based on atmospheric stability regime and operative conditions of the wind turbines. The resulting statistics of the wake velocity fields are used for tuning of a parabolic RANS model. Specifically, LiDAR data are leveraged for tuning the RANS turbulence closure for different atmospheric stability regimes and estimating thrust force over the blade span by coupling the LiDAR data and results of the respective RANS simulations. The RANS model is assessed against power production of individual turbines recorded through the SCADA showing a satisfactory agreement.
  

Abstract Withdrawn

The effects of rotor solidity on the aerodynamic performance of a vertical-axis wind turbine model were systematically investigated in this work. An H-rotor design was configured to obtain rotor solidities (σ_N) in the range 0.45 to 1.12 by changing the number of airfoil blades (N) from 2 to 5 in separate sets of experiments. The models were tested in a compressed-air wind tunnel facility over a large Reynolds number (Re_D) range of 0.5 × 10⁶ to 5 × 10⁶ (based on the rotor diameter) and tip-speed-ratio (λ) range of 0.75 to 2.5. The power co-efficient (C_p) was characterized through time-resolved measurements of the central rotary shaft torque and rotation rate. These results help understand the scaling of C_p with Re_D, σ_N, λ, and identify regimes where C_p achieves Re_D invariance. Overall, experimental data of the present kind is invaluable to the development of modeling tools that robustly capture the complex unsteady aerodynamic effects that underlie vertical-axis rotors.   

Defining the Characteristic Velocity for the Reynolds Number in Turbine Experiments

The Reynolds number can significantly affect the performance of lift-based wind and water turbines, particularly at laboratory-scale. It is, therefore, important to use a consistent and physically-meaningful definition for the length and velocity scales. In these applications, the most meaningful length scale is the blade chord. For simplicity, common definitions of the Reynolds number take the characteristic velocity as either the tangential speed of the blade or the free-stream velocity. However, the flow over the blade can vary with rotational position and depends on the flow induction, free-stream velocity, and tangential blade speed. To explore the most meaningful choice of the characteristic velocity, this study tests a cross-flow current turbine over a range of Reynolds numbers. The Reynolds number is varied first by changing the temperature of the water, which alters the viscosity. The turbine is tested again at constant temperature and varying free-stream velocity. The implications of the choice of characteristic velocity scale are explored by determining the conditions under which performance is invariant between the two experiments.   

PIV of the aerodynamics of an airfoil subjected to icing

Wind turbines located in cold climates face several issues due to ice accretion. These include power losses, mechanical failure and changes in the aerodynamic behaviour. While the effect on symmetric airfoils and airfoils intended for aviation has been covered in the literature, airfoils designed for wind turbines have received limited attention. Therefore, particle image velocimetry experiments have been conducted to investigate the flow field over a NREL S826 airfoil, with different ice accretions at the leading edge. The three accretions represent different icing conditions, namely rime, glaze and mixed conditions. The mixed icing has a horn geometry, while the others have a streamlined geometry. Experiments were also conducted on the clean airfoil for comparison. Different angles of attack were used, ranging from -4deg to 16deg for all geometries. From the measurements, mean velocity, turbulent kinetic energy, mean vorticity and instantaneous swirl have been calculated. The results demonstrate how the separation bubble changes with angle of attack for the horn accretion. A separation bubble over the streamlined accretions at high angle of attack was also found. The shear layer between the recirculating region and the freestream are also described for all experimental cases.   

Reducing Dynamic Stall Effects and Load Fluctuations on Wind Turbine Blades using Trailing Edge Flap

Dynamic stall on wind turbine blades often leads to severe fatigue and high load fluctuations that tend to decrease the lifespan of the blades. In this study the influence of trailing edge flap (TEF) on dynamic stall effects are investigated on an oscillating S833 airfoil with a chord length of 178 mm at a Reynolds number of 1.8×10⁵ and a reduced frequency of k= 0.06 and 0.1. Surface pressure measurements along with strain at the blade support are collected for all cases inside a 0.61 m square wind tunnel . The static lift and moment coefficients will be presented for different pitch and flap angles. The lift and moments curves are then presented for the dynamic stall cases for a mean pitch angle of 0° and 10° to represent stall onset and deep stall cases. The flap is also oscillating at the same pitch frequency but with different phase lags to study the influence of flap motion. The curves clearly show the leading edge vortex formation and convection and how it is influenced by the TEF. The coefficient of pressure for different cases are also presented in a contour plot to illustrate how the pressure along the airfoil chord changes for the pitching cycle. Finally a conclusion is presented on how the TEF can control the load fluctuation experienced by the blades.