What causes the vortex on the tip of this vertical stabilizer (CFD Simulation)
The subtitle of the video states:


  This video shows a high-fidelity CFD simulation of flow control applied to realistic wing profiles using PHASTA and ParaView Catalyst. Work done by Michel Rasquin from Argonne and Ken Jansen from UC Boulder.


Realistic wing profiles sounds like: we're actually looking at a wing in a vertical orientation - or in a horizontal orientation from a viewpoint that causes some confusion. Looking at a wing at some Angle of Attack that creates lift, and therefore the visualised flow pattern. And then the two vortices are caused by the end effect of finite wing span.