What is the physical meaning of turbulent viscosity?

Generally speaking, viscosity is caused by mutual diffusion between molecules. Macroscopically, the flow rate is fast and slow, and microscopically, the irregular movement of molecules runs everywhere. Some molecules in the fast and slow flow rate are equivalent to exchange, the fast one slows down as a whole, and the slow one becomes faster as a whole, so it is sticky. Turbulence can also be understood in a similar way, taking fluid micelles as molecules.

Turbulence is characterized by the speed of the whole body, but the microscopic fluid micelles also run like molecules at sixes and sevens, so the whole body also has a viscous effect similar to molecular thermal motion, so it is quite appropriate to use the viscosity coefficient to express the influence of turbulence.

The basic mechanism of turbulence is vortex diffusion, that is, the random motion of vortex-driven fluid particles leads to strong momentum transfer rate, which makes the apparent viscosity much larger than that at molecular level. According to the expression format of Newton's viscosity law, the eddy viscosity can be deduced.