Tower wakes and accelerations

We can easily understand that a meteorological tower will affect the measurements taken within the wake of the tower. Here at the BAO, we are suspicious of measurements on the SE boom when the winds are from the NW, and similarly, we look for wakes in the NW boom data when the winds are from the SE. However, we should also remember that flow around a tower like the BAO can also generate accelerations for certain wind directions as well around the tower as well. The IEC standard 61400-12-1 Annex G specifies boom lengths (as a function of tower dimension and tower solidity) to ensure that measurements are taken outside of the zones of these wakes and accelerations.

CFD simulations (caveat: neutral stability) for flows around towers like the BAO can illustrate these issues. An example is shown below, from Stickland et al. 2012 (EWEA), for simulations of flow around the FINO3 tower (*not* the BAO).

As flow approaches from the left, the flow decelerates in an induction zone (cool colors are flow less than 100% inflow speed), but also accelerates (warm colors) around the sides of the tower.

As a reminder, here is what our tower looks like. Last week, I checked with Dan Wolfe regarding the tower solidity (as the degree of acceleration and deceleration depends on tower solidity), and such measurements have not yet been done for the BAO.

 
But what do our data show? Rob has pointed out below that the ratios of the sonics on the two booms show some evidence of both wakes and accelerations. Paul Quelet at CU has also looked at this issue, comparing the sonics to each other and to the profiling windcube (WC) lidars at the lidar supersite 135 m south of the tower. Because of the lidar comparison, Paul looks at 2-min averages of the tower sonic data at 50 m, 100 m, and 150 m:

It's interesting that the NW boom, on average, is always faster than the SE boom data, even when the NW boom should be waked. When the SE boom is waked (wind directions ~ 330), the differences are large as would be expected.

Then, when comparing the tower sonics to the lidars south of the tower, some other interesting differences emerge. Let's look at the NW boom data first:

We're comparing the WC to the NW boom sonic wind speeds. When the flow is from the south-east, the NW boom is clearly in the wake and the lidar speeds are much higher. Conversely, when the winds are from the NW, the WC measures low...but at 130 m away to the south, the WC should not be sampling tower wake. So when we see the WC measuring "low" at wind directions ~ 320, are we actually seeing the sonics measuring accelerations around the tower? Or is the tower wake propagating further downwind than we would expect?

The picture from the SE boom is different. First, the data are noisier, with lower wind speeds generally introducing more variability into these 2-min averages.
  

Second, the really puzzling thing is that the WC is still seeing higher winds than the sonic when the winds are from the south-east. The sonic isn't waked from this direction - why would the sonic wind speeds be low (or the WC winds be high)? Further, the wake effect we would expect to see when the winds are from the northwest isn't distinct.

Paul is continuing to work on this analysis, using higher time resolution data and stratifying these effects by atmospheric stability.

A major caveat to this quick analysis is that the sonic data, as yet, have not been corrected for any tilt in the booms. A systematic tilt issue could modify these results significantly. 

Thanks, Paul, for all your work on this so far!