# Coandă effect A jet of fluid will follow a nearby curved surface rather than continue straight. Hold the back of a spoon to a tap and the [[Water|stream]] bends to hug it and pulls the spoon in — a visible [[Force]] with no contact on the far side. The jet entrains fluid from both sides; the surface limits supply on one side, pressure there drops, and the jet is pushed against the wall and stays attached — a [[Positive_feedback|self-reinforcing]] attachment named for Henri Coandă. ## Attachment, and letting go The jet holds on until the surface curves away faster than the [[Boundary_layer]] can follow, at which point it separates — the same separation event that ends lift at stall, arriving here from the other direction. How long it clings depends on curvature, jet [[Velocity]], and the [[Reynolds_number]], and small changes flip it, which is what makes the effect useful as a switch. Fluidic devices exploit exactly this [[Multistability|bistability]] to route flow with no moving parts and therefore nothing to wear out — a [[Reliability_engineering|reliability]] argument as much as a fluid one. ## Uses Blown flaps and circulation-control wings direct engine air over a curved trailing edge to hold flow attached at angles that would otherwise stall, buying lift for short-field [[Aviation]] performance. Bladeless fans and many air knives are Coandă devices. So are burner flame stabilisers, [[Heat_transfer|heat]]-recovery jets, and some ventilation diffusers on the [[WT!IT_Center_of_Excellence|building-systems]] side. The same attachment explains why a poorly designed teapot dribbles down its own spout instead of pouring cleanly. **Microsim brief (next pass):** Jet emerging beside a curved wall whose radius the viewer drags; the jet clings, then detaches audibly-visibly past a critical curvature. Second surface added to show bistable switching. **On the spine:** [[Boundary_layer]] · [[Bernoulli's_principle]] · [[Potential_flow]] · [[Vortex_generator]] · [[Fluid_dynamics]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Coandă_effect) : [Wikitube](https://en.wikitube.io/wiki/Coandă_effect) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_WT!Thury_Hydrodynamics_Compendium]], [[PORTAL_Physics]], [[PORTAL_Aviation]]. --- *Fluid seed wave · 2026-09-10 · seed register · microsim layer pending.*