Session L03: Aerodynamics: Wind Energy

Nonsteady Load Responses to Daytime Atmospheric Turbulence Eddies on the DOE 1.5 MW Wind Turbine at NREL

Field data collected from the NREL/GE 1.5MW wind turbine (WT) and met tower (MetT) at the NREL Wind Technology Center near Boulder, CO June-October 2018 were analyzed to quantify the impacts of turbulence eddies on the load responses measured from sensors on the main shaft, blade and tower. The passage of individual mountain-generated eddies from the met tower to the WT were critically determined by correlating the optimal time shifts in signal between MetT and nacelle anemometer with mean advection time. Loading responses from mountain eddy passage were compared with atmospheric eddies from the north/south, unimpeded by the mountains, and found to be statistically similar. Whereas time variations in torque and horizontal eddy velocity were strongly correlated, the time response of out-of-plane bending moments on the main shaft, directly forcing the main bearing, were uncorrelated with horizontal eddy velocity. This result validates a previous LES study that concluded that, whereas power fluctuations respond primarily to advective eddy velocity, the main bearing responds to asymmetry in alignment between WT rotor and turbulence eddy passage. Surprisingly, the nacelle anemometer produced statistics very similar to those obtained with the MetT.   

Study of 3D Backwards Facing Step to Improve Wind Turbine Blade Performance

A thin layer of tape is typically applied to the leading edge of wind turbine blades for protection from wind, erosion, debris, etc. However, the addition of the thin layer creates a backward facing step (BFS) that is known to decrease the blade performance, which reduces the turbine's electrical power output. A novel tape design with serrated edges, termed 3D-BFS, was tested by the National Renewable Energy Lab (NREL) and showed a 6-7% increase in power production relative to the traditional tape. In another study, our team tested the performance of the 3D-BFS in a wind tunnel via wake survey. The results showed that the 3D-BFS were able to produce strong coherent structures in the far wake region as well as reduced the drag relative to a traditional BFS. The current study looks to replicate the experimental results with computational methods to enable a search for new 3D-BFS configurations with the goal of finding an optimal configuration. This work uses a commercial computational fluid dynamics (CFD) package (Star-CCM+). This talk will compare the CFD and experimental results in addition to examining new parameters that were inaccessible experimentally.   

Author not Attending

The yaw control strategy of wind turbines for wake-redirection is used to avert the turbine wake to minimize the velocity deficit footprint on the downstream turbines in attempt to increase the collective power generation of wind farms. The wake behind a yawed wind turbine undergoes a deflection and deformation process from an elliptical structure to a curled wake. To maximize the benefits of the wake-redirection approach, a better understanding of the transforming wake structure is essential. In the present study, the curled wake generated by various yaw inputs were investigated to characterize the wake deflection and its deformation. The large-eddy simulation and the actuator disk model augmented with rotation were used to simulate the wake behind a yawed wind turbine exposed to an atmospheric flow. The profiles of velocity and turbulent intensity within the wake region were compared with the previous experimental data. The transition from an elliptical structure to the kidney-shape was found to be attributed to the wake rotation and the counter-rotating vortex pair generated by the yawed rotor.

    

Upwind kinetic energy contributions to far wake meandering in wind turbines

The formation of wake meandering, a far wake large-scale oscillation, has been hypothesized to be due to disparate mechanisms: (1) large structures in the atmospheric boundary layer and (2) turbine bluff body effects. We identify energy contributions to wake meandering by quantifying the transfer from upwind scales. We employ large-eddy simulation and designate cases with different turbine blade parameterizations and upwind conditions to isolate specific upwind scales and interactions with wake meandering. The specific kinetic energy of a scale and interscale energy transfer is quantified through a methodology using triple and mode decomposition. Simulations with uniform inflow and the actuator surface model with and without nacelle model reveal different energy transfer behavior in the near wake due to the hub vortex. Subsequent neutral atmosphere boundary layer inflow conditions within a domain large enough to capture coherent structures are enforced on actuator disk and surface parameterizations. Several upwind scales are modulated by the wind turbine and shown to have significant effects on wake meandering.   

Improvements to the Actuator Disk Concept for Modelling Horizontal Axis Wind Turbines

The present study validates design improvements to the actuator disk wind turbine model at high Reynolds numbers through experimental comparisons of the wakes of a horizontal axis wind turbine and matched porous disks. The experiments are conducted in the High Reynolds Number Test Facility at Princeton University, where diameter-based Reynolds numbers on the order of one million are achieved at low velocities using pressurized air as the working fluid. The rotor and disks are matched based on conventional metrics; the Reynolds number and thrust coefficient, in addition to matching rotor length scales and radial solidity profile. Wake hot-wire measurements reveal a near perfect match to the rotor in terms of mean streamwise velocity deficit can be achieved at distances as close as 1 diameter downstream by matching the radial solidity profile, a significant improvement over the 5 to 6 diameter distance of previous models. Furthermore, by matching disk features to the length scale of the rotor's tip chord length, the amplitude of the turbulent fluctuations throughout the disk wake can be matched to the rotor, again improving over previous models. However, specific wakes features, such as the rotor tip vortex, are notably absent.   

The aerodynamic modeling of wind turbine blades using different levels of fidelity

We study the aerodynamic modeling of wind turbine blades. We will explain the difference between blade-element momentum theory, vorticity-based methods, actuator lines and blade resolved simulations. We show that even on simple simulations of uniform inflow with constant rotational speed, these methods provide different predictions of aerodynamic quantities along the blade. These differences translate to differences in thrust and power prediction of more than 50% in some cases. We focus on the fundamental assumptions behind the different approaches to explain where the differences are coming from. Most differences come from the spanwise distribution of vorticity/forces from blade elements. Tip and root corrections also play an important role and can influence the results significantly.

    

Effects of rotation on the wake of a wind turbine at high Reynolds numbers

A better understanding of wind turbine wakes can improve wind farm planning, leading to improvements in farm efficiency. To reveal the effects of rotation on the wake of a wind turbine, a wind tunnel study was conducted at a range of tip speed ratios, 4   

Predicting turbulence statistics of wind flow over oceanic waves using data-driven convolutional neural networks

To efficiently assess the wave effect on the wind field, we developed an autoencoder convolutional neural network (CNN) based on data obtained from large-eddy simulation (LES) of wind turbulence over complex surface waves. We performed simulations for wind fields of four characteristic wind velocities and two wave conditions to produce the training and validation data. is then The CNN was then trained using the LES results of two wind velocities with various wave conditions and employed to reconstruct the time-averaged velocity field, wave-induced fluctuation, and turbulent kinetic energy of the wind field over oceanic waves. The validation study results show good agreement between the CNN predictions and LES for the turbulence statistics of the wind flow in the selected oceanic environment.   

Realizing the potential of periodically surging turbines for increased power production

In full-scale applications, wind and tidal turbines must contend with unsteady flow conditions. In particular, a turbine encountering streamwise unsteadiness may experience either enhancements or losses in its time-averaged power production, depending on the nature of the perturbation and the characteristics of the turbine. In this work, we seek to identify and model the mechanisms responsible for these divergent outcomes. First, single-point velocity and pressure measurements were conducted upstream and downstream of a periodically surging horizontal-axis wind turbine in a fan-array wind tunnel. Surge-velocity amplitudes up to 23% of the wind speed were examined. Then, a turbine surging through amplitudes of up to 40% of the mean flow speed was studied using two-dimensional particle-image velocimetry in an optical towing tank. Flow kinematics were measured both on the turbine blades and in the wake up to 15 diameters downstream of the turbine. These two datasets facilitated the development of analytical models that highlighted factors responsible for the observed changes in time-averaged power. These results can inform the design and control of turbines operating in unsteady conditions for increased efficiency and robustness against flow disturbances.   

High Reynolds number wind turbine experiments in the Variable Density Turbulence Tunnel

Wind turbines operate at high Reynolds numbers that are challenging to reproduce in controlled laboratory settings. Therefore, experiments are often conducted on scaled-down turbine models at vastly reduced Reynolds numbers. However, a change in the Reynolds number changes the nature of the turbulence and can thus affect both the blade-level aerodynamics and the evolution of the wake. Traditional wind tunnels increase the Reynolds number by increasing the velocity, but this reduces the tip speed ratio. In order to maintain reasonably high tip speed ratios, unrealistically high rotational rates are then needed. Here, we present a new experimental setup that is being developed in the Variable Density Turbulence Tunnel (VDTT) at the Max Planck Institute for Dynamics and Self-organisation. This wind tunnel uses pressurized SF6 to achieve Reynolds numbers on the order of Re_D = 10^7 on a small-scale turbine. Because the VDTT achieves high Reynolds numbers at low velocities, high tip speed ratios can be achieved at reasonable rotation rates. Lagrangian particle tracking allows for extended wake studies. An active grid will be used to alter the inflow turbulence intensity.