The resistance to sliding flow between fluid layers can lead to irregular flow known as turbulence. The amount of turbulence in a fluid depends on the velocity and general characteristics of the fluid. If the layers of a fluid slide easily over one another, the fluid is said to exhibit laminar flow and has little turbulence. If the velocity is high enough, the local flow of the fluid may include circular currents that impede the sliding of the layers of the fluid. The Reynolds number is a quantity that serves as an indicator for the type of flow a fluid will exhibit. This number depends on viscosity h, density r, velocity v, and the diameter D of the tube:
