Coandă Effect
A fluid jet emerging from a nozzle curves to follow a nearby convex surface instead of continuing straight. Adjust the jet speed, surface curvature, and gap distance to see the transition between attached and detached flow.
About this lab
The Coandă effect, named after Romanian inventor Henri Coandă, describes the tendency of a fluid jet to stay attached to a convex surface placed near it. When a jet exits a nozzle near a curved wall, the entrainment of surrounding fluid between the jet and the surface creates a low-pressure region that bends the jet toward the surface. Once attached, the jet follows the curvature of the wall, deflecting far from its original trajectory. This effect is exploited in fluidic devices, aircraft circulation control, and even the way a spoon deflects a stream of water from a faucet.
The attachment is governed by a balance between the centrifugal force of the curved flow and the pressure difference created by entrainment. For a jet of velocity U following a surface of radius R, the radial pressure gradient is approximately Δp ≈ ρU²/R, where ρ is fluid density. Attachment breaks down when the adverse pressure gradient along the surface becomes too steep—at high jet velocities relative to curvature, or when the gap between nozzle and surface is too large for entrainment to bridge. The critical gap distance scales roughly with the jet width and the Reynolds number of the flow.
This simulation traces particles emitted from a nozzle, computing their trajectories under a simplified potential flow model with a circulation term that represents the entrainment-driven attachment. The velocity field near the surface includes a radial pressure gradient consistent with curved streamline flow. When parameters exceed the detachment threshold, the entrainment term weakens and particles follow ballistic paths. The transition between attached and detached states is visible as a sudden change in flow pattern, analogous to the stall/attachment bifurcation observed in real Coandă flows.