Edited sentence: Helium II is usefully regarded theoretically as a mixture of normal fluid and superfluid, having a total density equal to the sum of the densities of the two components.In the superfluid component, however, vorticity is restricted to the quantized vortex lines, and there is no viscous dissipation.In turbulence, the normal fluid behaves as a classical fluid, and has a classically turbulent velocity field when a superfluid experiences turbulence.In turbulence, the vortex lines arrange themselves in an irregular fashion, and this is described as a "vortex tangle".Since then, the theoretical understanding of quantum turbulence has posed many challenges, some similar to those of classical fluid mechanics, but also new phenomena peculiar to superfluids and not encountered elsewhere.Some of the theoretical work in this field is quite speculative, and there are a number of areas of divergence between theoretical speculations and what has been obtained experimentally.The idea that a form of turbulence might be possible in a superfluid via the quantized vortex lines was first suggested as a possibility by the theoretician Richard Feynman.The normal part behaves like any other liquid, and the superfluid part flows without resistance.The proportions of the two components change continuously from all normal fluid at the transition temperature (2.172 K) to all superfluid at zero temperature