CAN SOMEONE EXPLAIN TO ME WHAT MIXING LENGTH THEORY IS?

THANKS

Read a good fluid dynamics book

I guess nobody in this forum knows....

It's a simplified (i.e., algebraic) model for turbulent viscosity, proposed by Prandtl as an analogy with kinetic theory of gases on how turbulent eddies exchange momentum. Eddy viscosity is computed from a "mixing length" and the velocity gradient. The mixing length is essentially the size of the turbulent eddies, which can be computed as a function of y+.

And yes, this is found in any good fluid mechanics text - see Viscous Fluid Flow by White, e.g.