Wind turbine wakes lead to lower power production and increased loading on downstream turbines from the reduced incoming velocity and increased turbulence contained within the wakes. Detailed measurement will improve the understanding of the wakes needed to improve wake modeling and consequently support reducing cost of energy (COE) at the plant level. Sandia National Lab is developing a wake-imaging system to provide detailed wake velocity measurement capabilities. The project will demonstrate the viability of measuring the formation and development of turbulent flow structures near the rotor at temporal and spatial scales not accessible by current techniques such as Lidar and Particle Image Velocimetry (PIV).
Current scanning Lidar is capable of measuring only the average velocity and turbulence statics over a large area along the path of a scanning beam. This results in poor spatial resolution due to the pointwise measurement and the nature of the Lidar. Also, it does not offer an instantaneous realization of turbulent structures, which is key to understanding turbulence physics in detail.
PIV is based on displacement measurements of small tracer particles entrained into the flow field. The area is illuminated by a thin laser sheet and an instantaneous snapshot of the flow field is captured with a camera. The result is a series of 2D flow fields with very high spatial resolution. A stereoscopic technique can be used to capture the velocity in 3D over the entire area of interest. PIV is capable of providing quantitative information on coherent flow structures, improving flow field understanding. Unfortunately the working distance is limited with PIV which has relegated its use primarily to wind tunnels where high particle seeding density can be achieved.
A method more suited for large area measurements of coherent turbulent structures is Doppler Global Velocimetry (DGV) or Planar Doppler Velocimetry. The DGV methods uses a combination of PIV and Lidar like techniques, and measures the Doppler shift of light scattered from tracer particles within a flow field illuminated by a laser sheet. The Doppler shift is derived at each pixel of an image using an iodine vapor cell as a molecular filter that converts the frequency variations of the scattered light into intensity variations. PIV results can be obtained simultaneously.The resolution of the DGV is much higher and thus more suited for larger areas of interest. DGV was in the past developed by Dr. Meyers at NASA. Sandia National Lab is using Dr. Meyers as an expert advisor for the development of the system.
DGV will be used by the wake-imaging system with the goal of deployment at the Scaled Wind Farm Technology facility to measure the three-dimensional coherent wake structures over relevant time scales with a high spatial resolution. This will produce a novel measurement system capable of providing the needed experimental data to understand fundamental wind-turbine wake phenomena and to validate computational codes and design tools.
Sandia National Lab
Jonathan White, (505) 284-5400.
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