Hairpin vortices created by a boundary layer trip wire. Smoke in air.
Reference: Perry, A.E., Lim, T.T. & Teh, E.W. (1981) A visual study of turbulent spots, JOURNAL OF FLUID MECHANICS, Vol 104 , 1981, 387 - 405.
Visit T. T. Lim's web site at http://serve.me.nus.edu.sg/limtt/.
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