The Coanda effect describes the tendency of a jet to follow the contours of an adjacent boundary even when this boundary curves away from the initial jet axis. This effect either arises due to a) the pressure gradient perpendicular to a curved streamline, or b) differential entrainment and the development of a partial vacuum.
In the first case it may be shown using the Eiuler equation (or the Bernoulli Equation in an incompressible fluid) that a curved streamtube experiences a net force towards the center of curvature (Figure 1a). Since no component of the viscous force acts perpendicular to a streamtube, this implies that P1 > P2. It is this pressure gradient that causes the jet to be deflected from its initial axis, and accounts for the so-called Coanda effect.
Alternatively, if a jet is discharged in the vicinity of a solid boundary (Figure 1b), the entrainment of fluid into the jet will be restricted on one side (A). This creates a partial vacuum so that Pa < Pb, and consequently the jet attaches to the flow boundary.
This effect is particularly apparent in 2-D jets. Schlichting (1968) provides further details of flows near solid boundaries.
Schlichting, H. (1968) Boundary Layer Theory. McGraw-Hill Inc., New York.