The tendency of a fluid jet to attach to and follow the contour of a nearby surface due to entrainment, momentum exchange and resulting pressure differences; the phenomenon depends on jet momentum, proximity to the surface, curvature, viscosity and turbulence.

Principle

Attachment occurs because the jet entrains surrounding fluid, reducing pressure adjacent to the jet on the side of the surface; small deviations toward the surface are amplified, causing the jet to remain attached and follow curvature until geometric or flow conditions cause separation.

Demonstration

Illustrative scenario → A horizontal air jet from a diffuser placed close to a curved ceiling adheres to the ceiling and travels along it, delivering air across a larger area than a free jet aimed downward. Changing diffuser velocity or moving it slightly away from the surface causes detachment and a different distribution.

Misapplication

Explaining attachment solely by a generic 'suction' or by invoking Bernoulli without reference to entrainment and momentum balance, or assuming Coanda will occur for any jet and surface regardless of scale and turbulence. The error is ignoring the dependence on momentum, geometry and boundary conditions.

Consequence

Coanda behaviour alters airflow patterns near façades, diffusers and vents, affecting ventilation effectiveness, thermal comfort, pollutant dispersion and entrainment; it can be exploited to shape flows but is sensitive to operating conditions and surface properties.

Reversal

At very low jet momentum, with strong turbulence, high surface roughness, or when the curvature induces adverse pressure gradients beyond the jet's ability to follow, attachment fails and separation occurs; in those conditions Coanda‑based design assumptions are invalid.

Boundary

Clearly within: free jets or jets issuing near solid surfaces with characteristic momentum and proximity that permit entrainment and pressure differences. Boundary case: slow, highly diffusive jets close to porous or rough surfaces where partial attachment occurs. Clearly outside: bulk confined flows far from bounding surfaces or two‑dimensional laminar shear flows without a distinct jet impinging near a surface.

Semantic Tension

Control ↔ Sensitivity — the Coanda effect can be engineered to guide flows (control) but is highly sensitive to changes in speed, geometry and surface condition (sensitivity), making reliable exploitation conditional.

Synthesis

Coanda attachment is an emergent outcome of entrainment and momentum distribution, not an independent force: designers can use it to steer jets, but doing so requires controlling velocity, proximity and surface geometry to avoid unexpected detachment.