Hi! I hope I'm posting this in the right forum... I was wondering if someone could quickly explain what a "vortex surface" is. I've tried to google it, but I only find papers, which won't help me with a simple explanation. I understand that a vortex surface is an object made from parallel vortex lines. According to the definition I have, these lines must intersect a curve, which is not a vortex line... I'm assuming that means all vortex lines making up the surface have a different vorticity, so that the vortex surface is a surface, where the value of the vorticity is allowed to vary? That conclusion leaves me wondering what the point of such an object would be...? Why is unidirectionality important?