Session T04: Wind Turbine Aerodynamics

Phase Averaged Flow Statistics of a Wind Turbine's Wake at High Reynolds Numbers

Wind turbine wakes are dominated by a wide range of turbulent structures, with the helical tip vortex being one of the most recognizable. To better understand the tip vortex's contributions on the wake's evolution, a wind tunnel study was conducted at a Reynolds number of 4 million with a 20cm diameter model turbine. High Reynolds number conditions were achieved using a pressurized wind tunnel facility, the High Reynolds number Test Facility (HRTF), at Princeton University. The tip vortex has been shown to be affected by a turbine's rotation, such as a more energetic tip vortex for smaller tip speed ratios. Therefore, experiments were conducted at tip speed ratios of 4 and 7. Velocity measurements were obtained via a nanoscale hot-wire anemometer, to reduce spatial filtering of the velocity signal. The tip vortex is expected to primarily dominate the flow behavior in the near wake region, so measurements were restricted to 0.75 to 2 diameters downstream of the turbine. Time-averaged statistics are insufficient to completely capture the turbulence contributions of the tip vortex, so phase averaged statistics, such as the variance, are presented. The phase averaged results support the inverse relationship between tip speed ratio and tip vortex strength and capture tip vortex breakup, an essential component for understanding bulk wake recovery.   

Understanding 3D flows around rotating blades – how to infer the angle of attack in the field?

Estimating and measuring the aerodynamic performance of wind turbine blades can be complex, because the local wind encountering the rotating blade is the vectorial combination of the unsteady and turbulent wind speed and the rotational speed of the blade, modified by the induction through the rotor. This means, in the frame of reference of the rotating blade, that the inflow gradually changes as it approaches the blade, hence no reference wind speed and direction can be uniquely defined. And yet, to improve aerodynamic models of rotor blades, their aerodynamic performance needs to be compared with 2D non-rotating measurements or simulations, for which the inflow conditions are well defined. An experimental method is therefore required to compare aerodynamic measurements, such as pressure distributions, from a rotating blade to fixed 2D sections. In this work, we present such a method, which utilises pressure measurements from a newly-developed measurement system that can easily be attached to a wind turbine blade. It includes an array of absolute pressure sensors, which allow the pressure distribution to be obtained using corrections from an inertial measurement unit. The inflow conditions are inferred with a hybrid model combining potential flow theory and differential pressure measurements at the leading edge. In this presentation, after introducing the measurement system and the experimental method, its effectiveness will be demonstrated using measurement data from an operating wind turbine.   

Effects of inflow conditions on wind turbine wakes at high Reynolds numbers

Wind turbines are exposed to widely varying inflow conditions that depend on the local boundary layer meteorology. Typically, these inflow conditions are characterised by varying degrees of mean velocity shear and turbulence intensity, which affect the performance and durability of the turbine as well as the downstream evolution of the wake. These effects are challenging to study in the field due to the large scales involved, and most wind tunnel experiments are conducted at low Reynolds numbers. Here, we present results from high Reynolds number experiments in the Variable Density Turbulence Tunnel (VDTT) at the Max Planck Institute for Dynamics and Self-Organization. This wind tunnel uses pressurized SF₆ as the working fluid to achieve diameter-based Reynolds numbers up to Re_D = 10⁷ on a small-scale turbine. In this specific study, Re_D = 3 x 10⁶. Because the VDTT achieves high Reynolds numbers at low velocities, high tip speed ratios can be achieved at reasonable rotation rates. An active grid with 111 individually-controllable paddles is used to generate inflow profiles with varying degrees of velocity shear and turbulence intensity. Hot-wire measurements reveal the downstream evolution of the wake behind a MoWiTo 0.6 model turbine.   

Effects of inflow turbulence on wind turbine loads and wakes for blade-resolved simulations

Wind turbines in atmospheric boundary layer (ABL) routinely encounter varying wind speeds and turbulence. To optimize the design of turbine blades, it is of great importance to understand the complex interaction between wind turbines and ABL. In this work, the effects of inflow turbulence on the unsteady aerodynamic loads, frequency statistics and wake characteristics are investigated for the NREL Phase VI turbine under axial-flow conditions. Two methods are employed for generating inflow turbulence. The first approach defines turbulence conditions at the inlet and estimates the turbulent kinetic energy decay from the inlet to the turbine location. The second method employs synthetic turbulence generator to inject resolved turbulence. The simulations are performed using Shear-Stress Transport (SST) and Improved Delayed Detached Eddy Simulation (IDDES) turbulence models. Comparison of the power spectral density (PSD) based on the thrust coefficient as well as the resulting Strouhal number is performed. The effects on velocity deficit and turbulent intensity are also examined both in the near- and far-field wake regions.   

Investigation of wind plant wake effects at the AWAKEN field campaign

Wind plant wakes characterized by lower momentum flow with increased turbulent kinetic energy (TKE) have been shown to propagate for several kilometers and interact with neighboring plants. To better understand these interactions, an Oklahoma site hosting five wind plants is heavily instrumented as part of the American Wake Experiment (AWAKEN) field campaign. Upstream, interior-plant, and downstream flows are probed using a suite of remote sensing instruments to quantify wake properties such as the magnitude and extent of momentum deficit and TKE increase. The effects of atmospheric inflow (wind speed, turbulence, stability, etc.) are investigated. Under certain conditions, the impact of an upstream plant on its downstream neighbor can be observed using the performance data recorded by the downstream plant. In addition to providing insight into plant wake behavior, the results of this study will be used as a benchmark case for wind plant simulations.   

Characteristics of a secondary wake steering from two tandem wind turbines under yaw misalignment

The wake structure of a yawed wind turbine undergoes deflection and deformation, resulting in an additional wake steering effect from the downstream wind turbine, known as the "secondary steering". Previous studies reported a substantial influence of the counter-rotating vortex pair generated by the yawed upstream turbine on the downstream turbine wake. However, further studies on the impact of the upstream turbines on the wake deflection and deformation incurred by the downstream turbines still remain largely unexplored. In the present study, a numerical investigation was performed to characterize the flow dominated by the secondary steering effect. Large-eddy simulation coupled with an actuator disk model with rotation was used to simulate the wind turbine pair submerged in an atmospheric boundary layer. In the downstream turbine, the velocity deficit revealed a deflected wake centerline, evincing the occurrence of the secondary steering effect. Moreover, the deformed wake of the downstream turbine, influenced by the yawed upstream turbine wake, was further examined to investigate the primary driver of the secondary steering mechanism.   

Combined Experimental-Analytical Predictions of Thrust, Power and Wake Development of a Yaw-Misaligned Horizontal Axis Wind Turbine at High Reynolds numbers

This study examines yaw effects on horizontal axis wind turbine (HAWT) thrust, power, and wake development through a combined experimental-analytical approach. The recently proposed extension to classical actuator disk theory of Heck et al. (J. Fluid Mech. 2023) is examined using complementary experimental data. The experiments are conducted in the High Reynolds number Test Facility at Princeton University, where field-relevant Reynolds numbers (Re_D = 4 × 10⁶) and tip-speed ratios (4 ≤ λ ≤ 7) are achieved by scaling pressure in place of velocity. Yaw angles spanning -45° ≤ γ ≤ 45° are explored, with results showing excellent agreement between measured and model-predicted power outputs. Notably, the model results are fully predictive, with consideration given to thrust coefficient control as function of yaw through the tip-speed ratio, demonstrating a distinct advantage over previous empirical methods (e.g., cos^α(γ) scaling laws). Furthermore, the experimental wake measurements indicate yawing the turbine deflects the wake significantly, while also reducing the wake width and maximum velocity deficit. The results help identify promising directions for future model development, towards the goal of developing a robust, yet simple model for use in real-time wind farm control.   

An Experimental Study on the Detrimental Effects of Rainfall on the Aerodynamic Performance of a Wind Turbine Blade Model

An experimental investigation was conducted to examine the detrimental effects of rainfall on the aerodynamic performance of wind turbine blades under various test conditions. The experimental study was performed with a typical wind turbine airfoil/blade model mounted inside a unique Icing Research Tunnel of Iowa State University (i.e., ISU-IRT).  In addition to using high-sensitive load cells to measure the variations of aerodynamic forces (i.e., both life and drag forces) acting on the airfoil/blade model as a function of the incoming airflow speed and rainfall rate, a high-speed imaging system was also used to record the dynamic impingement of airborne raindrops and transient behavior of the wind-driven water runback over the surface of the test model. A high-solution digital PIV system was also used to quantify the evolution of the airflow characteristics over the surface of the airfoil/blade model induced by raindrop impingement and its correlations with rainfall-induced aerodynamic degradation under various test conditions. By applying a superhydrophobic coating to coat the surface of the airfoil/blade t model, the effects of the surface wettability of the airfoil/blade model on the rainfall-induced aerodynamic degradation were also evaluated quantitatively in comparison to those of the uncoated case.