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Kind of a Drag

August 10, 2011

The key to good experiments is keeping extraneous variables under control. It's important to reduce the influence of one trial on the others, so in chemistry you make certain that you clean your beakers to prevent contamination. In physical systems, spatial separation is often the key.

Separation is especially important when your system elements are in a fluid, since disturbances in fluid flow can persist for great distances. This is true even when the fluid is as rarefied as air. A recent study by scientists at the Structure and Motion Laboratory, The Royal Veterinary College, University of London, investigated the flocking behavior of pigeons.[1-2]

Many species of birds fly in flocks for the obvious social and navigational advantages. Some flock formations, such as the commonly observed "V" shape, have an aerodynamic benefit that's apparent in the indicators of energy expenditure, such as wingbeat frequency. The University of London scientists did a remarkable series of observations that used GPS position sensors and accelerometers mounted in instrumentation packages on pigeon backs. A video of an experiment can be viewed online.[3]

The video shows eighteen instrumented pigeons that were tracked during a June, 2010, flight. Wing flaps were inferred from the accelerometer data. The data showed that pigeons, which don't fly in "V" formation, have increased wing flap frequency when they are flying near, and particularly behind, other birds.

A similar problem exists in wind farms, where turbines upwind from others on the farm can affect the performance of downwind turbines. Even when there's considerable spacing of turbines, turbulence effects are still seen.[4]

As I reported in a previous article (Bigger than a Butterfly, January 11, 2011), the wind turbines of the Horns Rev wind farm on the coast of Denmark are spaced seven diameters apart, but the downstream turbines generate about a quarter less power than the front row turbines.

Wind Park in Saxony, Germany

Wind Park in Saxony, Germany. Photo by "Eclipse.sx" via Wikimedia Commons.


Scientists at the California Institute of Technology have been investigating a vertical axis wind turbine design that appears to solve problems of horizontal turbines. Vertical turbines have the advantage that they don't need a yaw mechanism to point them towards the wind. What's especially interesting is that the Caltech approach puts these wind turbines close together in a way that improves overall efficiency. Says Caltech professor, John Dabiri, who is also director of Caltech's Center for Bioinspired Engineering,
"Because conventional, propeller-style wind turbines must be spaced far apart to avoid interfering with one another aerodynamically, much of the wind energy that enters a wind farm is never tapped. In effect, modern wind farms are the equivalent of sloppy eaters. To compensate, they're built taller and larger to access better winds."[5]

Instead of focusing on making individual turbines more efficient, the Caltech approach, which is described in the Journal of Renewable & Sustainable Energy, is to make the farm itself more efficient.[5-9] It also tries to lower capital and maintenance cost by making the turbines smaller and closer to the ground.[5] Winds at higher altitude have much higher speeds than those close to the ground, but all variables are subject to trade-offs when you take a systems approach to the problems of extracting cost-efficient wind energy.

Caltech vertical-axis wind turbines

The Caltech Field Laboratory for Optimized Wind Energy (FLOWE) and its array of vertical-axis wind turbines. Note human in lower right hand corner for scale. (Screen capture from a YouTube video) [9].


Only six vertical turbines were used in the study reported in the Journal of Renewable & Sustainable Energy, but Caltech's Field Laboratory for Optimized Wind Energy (FLOWE), shown above, now has twenty-four ten-meter tall, 1.2-meter-wide vertical-axis wind turbines. This facility is located on two acres in northern Los Angeles County. One trick to increased efficiency was to have turbines rotate in opposite directions to their neighbors, which apparently allows a constructive interference that reduces drag.[6]

One test in which the turbines were placed four turbine diameters apart (5 meters, or about 16 feet) completely eliminated the aerodynamic interference between turbines. This is possible for horizontal turbines only at a twenty diameter spacing. This produced a power density of 21-47 watts per square meter, far in excess of the horizontal turbine efficiency of 2-3 watts per square meter, or an order of magnitude difference.[6]

Small vertical wind turbines mitigate other problems of the large horizontal turbines. There's less noise, fewer bird and bat strikes, and the facilities are more aesthetic.

Kind of a Drag is a song by The Buckinghams that attained number one ranking on the Billboard Hot 100 for two weeks in February, 1967. This coincided with my brief career as a top 40 disk jockey for a small AM radio station.

AM radio disk jockey, 1967

Yes, an authentic photograph, except for the psychedelic coloring.

The photograph dates to 1967 when I was about twenty years old.

I would never have survived in the Clear Channel era)


References:

  1. Geoffrey Spedding, "Aerodynamics: The cost of flight in flocks," Nature, vol. 474, no. 7352 (June 23, 2011), pp. 458f.
  2. James R. Usherwood, Marinos Stavrou, John C. Lowe, Kyle Roskilly and Alan M. Wilson, "Flying in a flock comes at a cost in pigeons,"Nature, vol. 474, no. 7352 (June 23, 2011), pp. 494-497.
  3. Video of a fight of 18 pigeons flying in a flock measured with back-mounted GPS and Inertial Measurement Units, Supplement to above paper, via Nature.
  4. Editorial - Turbines and turbulence, Nature, vol. 468, no. 7327 (23 December 2010), p. 1001.
  5. Charles E. Blue, "Bold new approach to wind 'farm' design may provide efficiency gains," American Institute of Physics Press Release, July 13, 2011.
  6. Kathy Svitil and Deborah Williams-Hedges, "Wind-turbine Placement Produces Tenfold Power Increase, Caltech Researchers Say," California Institute of Technology Press Release, July 13, 2011.
  7. J. O. Dabiri, "Potential order-of-magnitude enhancement of wind farm power density via counter-rotating vertical-axis wind turbine arrays," Journal of Renewable and Sustainable Energy, vol. 3, no. 4 (July 1, 2011), Document No. 043104 (12 pages).
  8. J. O. Dabiri, "Potential order-of-magnitude enhancement of wind farm power density via counter-rotating vertical-axis wind turbine arrays," arXiv Preprint Server, June 24, 2011.
  9. John Dabiri discussing wind research and aerial footage of the CalTech Field Laboratory for Optimized Wind Energy, YouTube video.

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