What’s this vortex generator like thing below the wing?
The manufacturer, Big Ass Fans, calls those devices “air fences.” Based on this naming I assume they are most likely there to redirect spanwise flow on the longer airfoils, not to prevent separation. Such a narrow airfoil built to move at a steady speed and loading would probably not suffer from separation issues. They are basically the same as wing fences. 


source:Big Ass Fans website

Wing fences are usually found on swept wings, because that’s what’s responsible for the spanwise flow, and the fan blades don’t appear to be swept. But in the case of a fan there will probably be a certain amount of radial flow due to the spinning motion. After a significant distance this could build up to be significant enough that the fence might help to redirect it.