Wednesday, August 28, 2013

Vertical Axis Wind Turbine Particle Image Velocimetry

After many months of mathematical modeling in MATLAB, design in SolidWorks, and prototyping with the 3D printer, I finally managed to develop a working scale model Vertical Axis Wind Turbine (VAWT) for use in the Wind Energy and Turbulence Lab to recreate some of the analysis done by Dabiri on potential energy density improvements for VAWT arrays.

The project was constrained by quite a few factors - size, form factor, aspect ratio, solidity, tip speed ratio, internal motor resistance, etc. and it wasn't until after two fluid dynamic classes that I was really able to approach the problem in an analytical way. I wound up doing a lot of airfoil research and settled on a asymmetric GOE-series airfoil that was able to provide a fair amount of lift over a large percentage of the arc. This is important, since with a VAWT, each airfoil is only in a position to be contributing lift during about a third of the time, so you really have to milk it for all it is worth.

Our research lab uses a wind tunnel and particle image velocimetry (PIV) as our primary experimental tool. The wind tunnel is pretty big, but you can only have one experiment in there at a time. As I'm an undergraduate researcher, I was at the top of the list for using the wind tunnel, right after guests, doctoral candidates, and master candidates. So, after about four months, I managed to get in and get trained on the tunnel and PIV by Betsy.

In short, the PIV process is pretty straight forward. A smoke machine pumps smoke (microscopic oil particles called seeds) into the wind tunnel, a terrifyingly high-powered laser illuminates a plane of the seeds, and an array of expensive cameras takes photos of how the seeds move from one micro-second to the next. Throw all of the photos into some processing software and it creates the averaged vector fields of how the overall airflow is moving, plus some post-processing in MATLAB. The devil of course, is in the details.

Makin sure that everything was set up correctly took a lot of time. Aligning the lasers, focusing the cameras, eliminating reflections from the laser (to not burn out the cameras), etc. took a long time. For every minute of experiment that I ran, there was about 100 minutes of setup time.

Above is a work-in-progress picture of the focusing rig inside of the tunnel. The VAWT is painted black to minimize reflections. Behind the VAWT at mid plane is a piece of paper: this is where the laser plane will be shining, so we will be getting information about the airflow in a thin sheet there. For this experiment I analyzed in front of the VAWT and a few planes behind the VAWT, all at the same height and orientation.

I have yet to do a full analysis on the results, but the great thing about PIV is that it makes some pretty pictures. Since Betsy is a pro at PIV, she made the following images for me.

 So, what are we looking at? This first image is the overall velocity of particles moving downstream in the tunnel (downstream is positive U, looking downstream to the left is positive V, and up is positive W). Since the VAWT is spinning about the V axis you would think that it would create a skewed wake in the direction of its spin, and indeed the wake is clearly skewed to the right.

The next image shows the flow moving to the left and right of the VAWT, again with flow being just as we'd expect: the spinning rotor directs flow in very specific directions (note the negative signs indicating flow to the right).

Finally, this image shows the in and out of plane components, or the flow moving up and down. This shows the complex interactions that are happening as the bulk flow interacts with the mast, airfoils, motor mount, etc. 

Hopefully I will get to further analyze this data; ultimately I'd love to write a publishable paper on it, but we'll see if that works out. Overall, it was a fantastic learning experience to be trained on and then carry out my own PIV experiment!

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