laminar flow
Laminar flow is smooth, orderly fluid motion in which the fluid slides along in neat parallel layers that do not mix. Picture honey pouring in a clear ribbon, or the glassy first centimetre of water leaving a gently opened tap before it breaks up. It answers the question: what does calm, well-behaved flow look like, and when does a fluid move that way?
Precisely, in laminar flow (the name comes from lamina, meaning a thin layer) each particle of fluid follows a smooth path called a streamline, and neighbouring streamlines run alongside one another without crossing or tumbling. The layers may move at different speeds, sliding past each other, but they stay in their lanes. Whether a flow is laminar depends on a balance between the fluid's inertia, which stirs up disorder, and its viscosity, which smooths things out; laminar flow tends to occur at low speeds, in narrow channels, or in thick, viscous fluids. Engineers judge this balance with a single number called the Reynolds number, and low values mean laminar flow.
Laminar flow is quiet, predictable, and easy to calculate, which is why it matters in fine plumbing, blood flow in small vessels, lubrication, and microfluidic chips. One honest point: it is the opposite of turbulence, and a flow can switch from laminar to turbulent as it speeds up. The thin smooth layer of laminar air or water clinging to a surface, the boundary layer, is important precisely because just beyond it the flow may be churning.
Open a tap just a little and the water comes out as a clear, glassy column, that is laminar flow. Open it fully and the stream turns cloudy and chaotic as it becomes turbulent.
Laminar flow moves in smooth, non-mixing layers; it favours low speeds and viscous fluids.
Laminar flow is the smooth opposite of turbulence. The same fluid can flow laminarly when slow and turbulently when fast; the switch depends on speed, size, and viscosity.