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In Barcelona’s El Prat Airport I saw some ceiling fans and noticed that they have on the lower part of the wings something that looks like a wingtip device or vortex generator.

As far as I understand vortex generators, they prevent the boundary later to detach / move the separation point. But that’s no issue below an airfoil, is it?

ceiling fan with unknown feature below the airfoil

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    $\begingroup$ Can it be related to protection against sharp edges for maintenance people? $\endgroup$
    – Afe
    Commented Mar 31, 2019 at 13:12
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    $\begingroup$ They are seldom maintained or paid any heed to. The last couple of times I had a connection through BCN or VLC (which has the same fans), some of them were turning in the wrong direction (trailing edge into the flow)... $\endgroup$
    – AEhere supports Monica
    Commented Mar 31, 2019 at 15:48

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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.

enter image description here 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.

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    $\begingroup$ What a name for a company. Thanks! $\endgroup$
    – Florian
    Commented Mar 31, 2019 at 22:59
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    $\begingroup$ Hilarious, isn’t it. If you zoom in it’s written right on the fan in your pic $\endgroup$
    – TomMcW
    Commented Mar 31, 2019 at 23:01
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    $\begingroup$ Since this fan uses 8 blades a blade will experience significant wake turbulence from the blade in front of it. Due to the rotational nature of the fan the wake turbulence will have a span wise component. The wing fences will limit this. The "winglets" are meant to reduce the vortices at the tip to also mitigate the wake turbulence problem. This kind of ties in to why we don't just add blades to a propeller to get more thrust. You don't normally see this much engineering in a ceiling fan. $\endgroup$
    – DLH
    Commented Apr 1, 2019 at 20:49

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