# Laminar flow Laminar flow is fluid moving in orderly parallel layers, each sliding smoothly over its neighbour with no mixing between them except by molecular [[Diffusion]]. It is the regime where [[Viscosity]] is in charge, and where the equations of [[Fluid_dynamics]] are actually solvable. It is also, on human scales, the exception — the [[Reynolds_number]] of everyday air and [[Water]] flows is far too high, which is why [[Turbulence]] is the default and laminar flow has to be arranged. ## Order that is also reversible Because layers do not exchange fluid, laminar flow is nearly reversible: the classic demonstration puts dye into glycerine between concentric cylinders, turns the inner one several times until the dye smears into nothing, then turns it back and the dye reassembles. Nothing was mixed — only sheared, and unshearing undid it. That is impossible in a [[Turbulence|turbulent]] flow, and it is a sharp way to see that mixing is not the same as motion. The [[Taylor–Couette_flow]] apparatus is that same geometry pushed until order breaks. ## Living in the low-Reynolds world Microbes, [[Porous_medium|porous media]], microfluidic chips, and lubrication films all run laminar. In a microchannel two streams can flow side by side for centimetres and mix only by [[Diffusion]], which is a nuisance for chemistry and a gift for anything requiring precise interfaces — a [[WT!HealthForce_Center_of_Excellence|diagnostic]] workhorse. Blood in small vessels is laminar, and the [[Viscosity|Hagen–Poiseuille]] fourth-power law that governs it means a small narrowing has an outsized effect on flow, which is why arterial geometry matters clinically. **Microsim brief (next pass):** Two dye streams in a channel with a Reynolds slider — parallel and unmixed at low Re, wobbling then breaking up as it rises. Add a reverse button for the unmixing demonstration. **On the spine:** [[Turbulence]] · [[Reynolds_number]] · [[Viscosity]] · [[Taylor–Couette_flow]] · [[Diffusion]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Laminar_flow) : [Wikitube](https://en.wikitube.io/wiki/Laminar_flow) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_WT!Thury_Hydrodynamics_Compendium]], [[PORTAL_Physics]]. --- *Fluid seed wave · 2026-09-10 · seed register · microsim layer pending.*