The plot above shows the mean relative difference between the SE and NW sonic wind speeds as a function of the SE sonic wind direction at the 250 m level. The other levels look similar. The plot was obtained by averaging 1-sec sonic data from March 6 through May 16. In this case the max occurs when the NW sonic is in the tower wake, and the minimum occurs when the SE sonic is in the tower wake. As you can see the differences can be huge.
What I find really interesting is the fact that the difference changes sign as you move away from the wake zones. One interpretation might be that the wind speed is slightly higher than the free stream in a region just outside of the wake zone. Is that possible? Has such a thing been observed or modeled?
Hi Rob,
ReplyDeleteWe've been looking into the tower wake issue, too. I think that what we and you are seeing is similar to what's seen in the CFD studies that drive the IEC standards for tower boom length, to ensure that the booms are long enough to be out of both the wake and the accelerations around the sides of the tower. To understand the spatial distributions of these accelerations, it's helpful to look at the simulations shown at http://proceedings.ewea.org/annual2012/allfiles2/1405_EWEA2012presentation.pdf . Figure 9 especially shows some nice velocity contours of these accelerations and wakes. To understand the degree of acceleration, we would need to know the tower solidity. I checked with Dan Wolfe on this last week, and he didn't think those estimates had been made for the tower before. Regardless, it's clear the booms aren't long enough to be outside of these tower effects.