  Why the temperature gradient in the vortex tube deviates significantly from
the adiabatic gradient is a very important but unresolved issue in vortex tube
research. In compressible fluids, the vorticity of a fluid parcel that is
different from its environment can affect its own pressure or density, just
like temperature. Gases near the wall have very high vorticity, and when they
detach from the wall, their higher vorticity compared to the environment makes
their density lower than the environment. Driven by vorticity, some fluid
parcels undergo multiple collisions near the axis, achieving temperature
separation.
