Study of boundary layer crucial in flow-related energy methods

Energy can come from a multitude of sources. Many methods of extracting energy from our environment involve the combustion of fuels. Solar panels turn the energy from solar radiation into electrical energy. Energy can also come from motion in the form of kinetic energy: the energy from moving air can be extracted using wind turbines, and the energy from moving water can be extracted using turbines in rivers or oceans.

Assistant professor Leonardo P. Chamorro studies turbulence and boundary layer flows: two subjects very relevant to wind energy and hydrokinetic energy (extracting energy from moving water). The boundary layer of any fluid flow is the portion of the flow that feels the effect of the surface it is flowing against. In the atmosphere, for example, the wind feels drag as it moves against the stationary ground. This drag stops having an effect on the wind flow at some higher point in the atmosphere, which defines the limit of the boundary layer. This layer exists in other flows as well, including those of river currents and ocean tides. The complexities of this layer have a great effect on turbines operating within it, and understanding these complexities is part of the drive behind this area of Chamorro’s work.

The first energy application of his studies is in wind energy. The limit of the boundary layer in the atmosphere is approximately one kilometer off of the ground. Large wind turbines average about 100 meters high; they operate where turbulent boundary layer effects are significant. Chamorro and his group, the Renewable Energy and Turbulent Environment Group (RE-TE-G), are studying how to reduce the undue stress exerted on turbines by turbulence in the boundary layer, so as to make them more efficient and reliable.

"I do experiments in the field with various topologies," Chamorro said. "I have 16 turbines of 3.2 meter rotordiameter. This is bridging what we use in wind tunnel experiments, which are very small, and the actual turbines you would find on a wind farm which are 80-100m tall, in that these are easier to manipulate. Then I measure flow turbulence, scalars, and turbine quantities to characterize what’s happening, and then I study how this flow turbulence affects the turbines’ performance."

Chamorro’s group is also addressing environmental questions, such as wind farms’ effect on pollinators. As many wind turbines are installed on farmland, ensuring that they won’t adversely affect the pollination of the crops is crucial.

"Bees are critical for our crops, and we know that bees are affected by turbulence," Chamorro said. "To better understand how turbulence and its structure across scales modulate the behavior of the bees, we are setting the infrastructure to track individual bees with various cameras both in a standard environment and in a wind farm environment. Then we will study how their trajectories and behaviors change with the flow turbulence generated by a wind farm."

Hydrokinetic energy is a form of alternative energy that is slowly gaining traction. It is slightly newer and less developed than other methods such as wind energy or solar energy, but is the perfect application for Chamorro’s study of turbulent boundary layer flows.

"Hydrokinetic is like a little brother of wind energy," Chamorro said. "A special case, but it uses the same principles. If you put a turbine in a fast-flowing river, the electricity is produced from the kinetic energy of the current."

This concept can be used anywhere there is moving water. This includes tidal motion in the ocean, and wave motions on the surface. And because the current is affected by the drag it feels from the bottom and banks of the body of water, the complexities of boundary layer flow apply to the study of hydrokinetic turbines. In an ME470 senior design project, Chamorro is overseeing a group of students whose project is to build a hydrokinetic device. But in his own research, his current goal is also focused on the environmental safety of hydrokinetic energy.

"Even if it is clean energy, we don’t know how it can alter the dynamics of the ecosystem and the topology of the surrounding area," Chamorro said. "Are the fish going to be killed? What about the nutrient distributions in the river bottom or ocean bottom? What are the effects of the enhanced mixing in the water? We don’t know yet, but these are very important questions besides the technological aspect of the problem."