Streamwise vortices in a turbulent wall jet flowing over acircular cylinder

The deflection of a jet by a convex solid surface such as a circular cylinder is often referred to as the Coanda effect. This effect is of interest in aeronautics and hydronautics because of its high effectiveness in exerting a force on the surface. It can be applied to enhance the low speed maneuverability of submersible vehicles because it is more efficient and quiet than conventional thrusters. Although substantial understanding of the curved wall-jet was achieved in recent years, the differences in the flow characteristics, turbulence intensity and the turbulent structure between a wall jet flowing over a straight surface and a convex one are still not explained. Nor is the mechanism of flow separation from the curved surface. A wall jet is a complex turbulent shear flow even in the absence of streamwise curvature because it possesses two dominant instability modes, the inflectional instability in the outer region and the viscous instability near the surface. The curved wall jets are even more complicated because they are susceptible to yet another instability mechanism that is associated with a centrifugal force that interacts with the outer flow when the surface is convex and with the inner flow when the surface is concave. Active or passive flow control that exploits the nature of the prevailing instabilities can operate on any of those mentioned above.