Super-Large-Scale Flow Visualization with Natural Snow

In an effort to better understand how wind turbines interact with the atmospheric boundary layer and with each other, St. Anthony Falls Laboratory researchers have developed a novel technique to use natural snowfall to visualize the turbulent structures generated down-wind of a the University of Minnesota Eolos 2.5MW wind energy research field station. These turbulent structures in the wake of a turbine can potentially impact both power production capability and mechanical strain or load on turbines by changing the flows throughout a wind farm. Unfortunately, present understanding of these turbulent structures at field scale is limited by a lack of available tools to visualize and study the turbulence and flow variations produced by wind moving past the tower and rotating blades.

Particle image velocimetry (PIV) is a technique typically used in wind tunnels at small scales to obtain measurements of velocity and turbulence. In the wind tunnel, the flow is seeded with small particles which can be observed with a 2D pulsed laser sheet. A camera fixed perpendicular to the laser sheet captures images of the particles as they flow past the laser sheet. The images can then be processed using computational tools to obtain measurements of velocity and turbulence. 

Given the unpredictable nature of field measurements at a utility-scale wind turbine, PIV has often been thought to be impossible for full-scale measurements of wind turbine turbulence. The tracer particles must be seeded uniformly throughout the flow and the light source and camera must be placed in precise locations. However, as flow characteristics can vary with scale, there is a need to perform actual measurements of turbulence on a field-scale wind turbine. 

To address these problems, an Eolos research team, led by Jiarong Hong, assistant professor of mechanical engineering at the University of Minnesota, explored the use of natural snow during a winter snow storm as tracer particles in the wake of the wind turbine at the University of Minnesota’s Eolos Wind Energy Research Field Station. PIV measurements must be taken at night so that the light sheet illuminates only the particles in a 2D plane behind the wind turbine. The light sheet in this particular pilot experiment was provided by a 5 kilowatt search light and a convex reflector that converts the beam from the searchlight into a light sheet that extends up to nearly 40 meters. 

Starting around 2:00 a.m. one morning in February 2013, a team of University of Minnesota researchers collected nearly an hour and a half of PIV data in the form of pictures and videos. The results of this data collection demonstrate the feasibility of using natural snowfall to visualize the full-scale turbulent structures in the wake of a 2.5 megawatt wind turbine.

Future work of the research team will focus on using the pictures and video gathered using this technique to obtain actual measurements of the turbulence generated by a utility scale wind turbine. With advanced understanding of the turbulence produced by wind turbines, researchers hope to enhance wind farm siting and control to improve the efficiency and reliability of wind turbines.