Whats Going Around Wind Turbines: Turbulent Vortices.

I am researching how wind flows behind wind turbines by analyzing experimental and numerical data. The experimental data has been collected around the turbine that is pictured below; and the numerical model is based on this turbine.



I’m specifically analyzing the structure and evolution of the vortices that are created at the blade tips. Turbines’ blades rotate because the aerodynamic shape of the blades guides incoming wind into creating a pressure difference that, in turn, drives the blades. Objects feel forced to move to lower pressures. Both the blades and the wind are forced into the lower pressure. Some wind wraps around the blades in order to get from the low pressure area. A vortex forms where the wind curls because the low pressure applies a centripetal force – like how a hurricane is a vortex where wind wraps around the hurricane’s low-pressure eye. Vorticity quantifies how much the fluid rotates around a region. In math terms, the vorticity is the curl of the wind velocity. Strong vortices are created at the blade tips; because here the wind can radially bypass the blade, which provides an easier path in the pursuit of lower pressure. This vortex is then pushed downstream by the wind; away from the blades’ low pressure region, the wind continues to curl due to a law of angular momentum conservation. The tip vortices are major sources of turbulence that damage downstream turbines. Consequentially, tip vortex research can lead to improvements in the longevity of turbines in wind farms.


I have made most of my current research progress by using a colleague’s numerical simulation that estimates wind velocity around a wind turbine. I am examining how tip vortices change across time and space. One specific investigation concerns the merging of two adjacent vortices. Since corotational vortices attract each other, the tip vortices sometimes merge together. To quantify this process, I am measuring the distances between adjacent vortices and the areas of those vortices. I hypothesize that by graphing the ratio of their distance to their area as they move downstream, I will find distinct phases in the merging process. I believe that when this ratio of distance to area decreases below a certain level, they will accelerate towards each other much more quickly.

A two dimensional slice of a numerical simulation is depicted below. The color shows the vorticity magnitude; the downstream direction is right, and the ground is located about 100 m below the x axis. The top of the turbine blades would be located on the lower left; here you can see distinct circular tip vortices in red. Farther downstream, the vortices appear to interact, as the vorticity in between the vortices is higher than it is upstream.Tip.png


The image below depicts how these vortices merge into a vortex sheet. This vortex sheet does not always develop, for it depends on the amount of incoming turbulence and on the ratio of the wind speed to the turbine’s rotational velocity. The color scheme has been changed since the last image to emphasize the merging process.Red.png

While I work in Minneapolis, MN, I live in St. Paul. I am staying with 5 Macalester College students. The rent is great: $200/month for the best room in the house (it has great big windows). This is my first time living in a college house, and I like it; for there are usually people around to talk to. I enjoy the camaraderie of the house. The Twin Cities have been very nice. Minneapolis’s culture is a lot like Portland’s. Everyday, I take 6 miles of fabulous biking trails to work. This is a tiring, yet rewarding, ride. When it rains, I use the cities’ good public transportation system. I have gone to fun, free events, including a museum, a jazz festival, a hip hop/spoken word festival, a bike race, a play, fireworks, and a swimming lake. I have been thinking about living here in the future, but the cold winters and the mosquito-infested summers might keep me away. I have, however, successfully fought off the mosquitoes; and I am thoroughly enjoying my time.

At the Beginning


Upon my arrival, I participated with an Orientation for this REU. Through this relationship, I found a network of resources including fellow visiting researchers, an ID card, and a great mentor who helps me with administrative issues. Then, I met the scientists that I collaborate with on my research project. These mentors include the director of the lab and local professors. These mentors have been essential in bringing me up to speed on the technology behind wind energy, which is not studied at my College. Thanks to their generous help, my research can really thrive.

I work at St. Anthony Falls Laboratory, which is an astounding facility. It is located where the Mississippi River runs through downtown Minneapolis. While the urban skyline is beautiful, it pales in comparison to the natural scenery. The lab is located below a magnificent 50-foot waterfall and it is next to roaring rapids; this river dramatically demonstrates the power of fluids. This lab was built below a waterfall so that gravity could be harnessed in bringing the water from above the falls to the experimental facilities within below the falls. As a fluid mechanics laboratory, it uses a lot of water.

Originally, I planned on studying both wave and wind energy. The wave energy experiments, however, are not fully available now; for wave energy is a new research area with a lot of technical issues. Consequentially, I will focus solely upon wind energy. I will specifically study the structure of the wakes that are cast behind wind turbines. As wind moves past a turbine, it becomes slower and more turbulent. If this wake runs across a nearby turbine it will interfere with the turbine’s productivity and longevity. One cannot, however, merely distance turbines far away from each other, because that would increase land, installation, and maintenance costs. Researchers must discover how to balance out these costs and benefits. So, in order to eventually provide insights in designing more efficient wind farms, I will research the structure of a turbine’s wake.