Flow regimes describe the nature of fluid flow. There are two basic flow regimes for flow of a single-phase fluid: laminar flow and turbulent flow. Laminar flow is characterized by little mixing of the flowing fluid and a parabolic velocity profile. Turbulent flow involves complete mixing of the fluid and a more uniform velocity profile. Laminar flow has been shown by experiment to exist at Re < 2,000 and turbulent flow at Re > 4,000. Reynolds numbers between 2,000 and 4,000 are in a transition zone, and thus the flow may be either laminar or turbulent.