Introduction - If you have any usage issues, please Google them yourself
Spatial discretization (3)
Temporal discretization (2)
Convergence, Consistency, Stability (2)
Irregularity (Random and chaotic nature of flow)
Increased exchange of momentum (Diffusivity - spreading rate of jets, boundary layers etc.)
Large Reynolds numbers
Dissipation of kinetic energy to internal energy
Wide range of time and length scales
Almost all practical flows are turbulent