Vortex tube
= Vortex tube
A <vortex tube> is a bundle of vortex lines bounded by a surface to which <vorticity> is tangent. <Circulation> measures its <vorticity> flux. Stretching a nearly incompressible tube reduces its cross-sectional area and increases its <vorticity>. The <Burgers vortex> is a local model in which this concentration balances viscous diffusion.