Image
Helping to Settle a Debate

The first motion picture in history was of a galloping horse captured by Eadweard Muybridge in 1878. The technique was developed in part to settle a popularly debated question regarding whether all four hooves of a galloping horse left the ground at the same time. The human eye is unable to break down the gallop action, and up until that time artists had painted horses with at least one hoof always on the ground. With this new technique, Muybridge unquestionably proved that, for brief moments, all four hooves of a horse are off the ground.

Today, even though wind energy researchers cannot see the airflow around wind turbines, they know that wakes shed from upstream wind turbines lead to reduced power production and increased loading on downstream turbines, driving up the cost of energy. Similar to the question of the galloping horse, wind researchers want to know how wakes form so they can develop effective solutions to mitigate the adverse impacts of the upstream wakes on the downstream turbine performance.

Wind turbines are now so large that iterative experiments in the field are prohibitively expensive. Instead, researchers must rely on models to simulate potential solutions using powerful computing resources. Successful models must accurately capture the physical environment, and thus require validation and calibration with high-quality field measurement data.

To address the need for a “motion picture” of wake formation, researchers at the U.S. Department of Energy’s Sandia National Laboratories (SNL) are developing the SNL Wake Imaging System (SWIS) to provide detailed wake velocity data in the field. The system employs a velocity measurement technique termed Doppler global velocimetry (DGV), a method more suited for large-area measurements of coherent turbulent structures than previously developed methods such as lidar and particle image velocimetry. The DGV method provides velocity data by measuring the Doppler shift frequency of light scattered from aerosol particles within a flow field illuminated by a laser-light sheet.

Scaled Demonstration

The objective of the SWIS is to measure the near-wake flow structures of the turbines installed at SNL’s Scaled Wind Farm Technology (SWiFT) facility in Lubbock, Texas. The DGV measurement technique implemented by SWIS would be risky and costly to implement in the challenging field environment of the SWiFT facility. Instead, SNL researchers completed a half-scale demonstration of the technique at their lab in Albuquerque, New Mexico, to quickly and cost-effectively gather and analyze performance data and refine system components. These data help demonstrate that the system can be successfully scaled-up to larger fields of view for deployment at the SWiFT facility.

In the coming years, SWIS hardware will be deployed at the SWiFT facility as part of a comprehensive near-wake experiment involving multiple measurement platforms to capture inflow, structural and aerodynamic rotor data, and wake formation just downstream of a wind turbine. When using the DGV technique, the measured velocity component and quality of the measurement depends on the specific configuration of the laser sheet, cameras, and viewing region. As a result of the complicated measurement dependence on the hardware configuration, and the need to meet the validation requirements at many locations in a wind turbine wake, SNL developed a simulation tool based on the half-scale demonstration results capable of modeling the SWIS physics to better predict and anticipate the system’s performance when deployed at the SWiFT facility.