Laminar flow, (from the Latin lamina — a plate or strip), an ordered flow of a liquid or gas in which the liquid (gas) moves as if in layers parallel to the direction of flow. Laminar flow is observed either in very viscous liquids or in flows that take place at sufficiently low velocities, and also when a liquid flows slowly around bodies of small size. In particular, laminar flow occurs in narrow (capillary) tubes, in the layer of lubricant in bearings, in the thin boundary layer that forms near the surface of bodies when a liquid or gas flows around them, and so on. As the velocity of motion of a given liquid increases, laminar flow at some moment passes into turbulent flow. All its properties then change substantially, in particular the structure of the stream, the velocity profile and the law of resistance. The flow regime of a liquid is characterised by the Reynolds number Re. When the value of Re is less than the critical number Recr, the flow of the liquid is laminar; if Re > Recr, the flow becomes turbulent. The value of Recr depends on the kind of flow considered. Thus for flow in circular pipes Recr ≈ 2300 (if the velocity averaged over the cross-section is taken as the characteristic velocity and the diameter of the pipe as the characteristic size). At Recr < 2300 the flow of the liquid in the pipe is laminar flow. Viscous laminar flow of a liquid in a pipe is governed by Poiseuille's law.