Research - Experimental Research

Coanda Effect

At its broadest level, the Coanda phenomenon can be explained as the deflection of jets by the convex solid surfaces. It is well known that flows have a tendency to become attached to or flow around a convex surface. This phenomenon has been named after Henri Coanda who researched aeronautics. A wall jet flowing over a circular cylinder (Coanda Effect) replaced recently the tail rotor used to prevent the autorotation of a typical helicopter. The NOTAR, as it is called (NO Tail Rotor, see figure), makes a helicopter lighter, safer and quieter.

The NOTAR (NO Tail Rotor)

Oscillatory Blowing

Oscillatory blowing can delay separation and enhance lift. The instabilities inherent in turbulent flow are exploited using small perturbations introduced into the flow by various means.

The parameters governing forced attachment of flow to a flat, inclined surface, were determined by Nishri (1998). The addition of convex curvature is investigated presently using the circular cylinder as a model. In both flows the forcing consisted of periodic oscillations emanating from a narrow slot.

Naturally the flow separates from the surface of a smooth circular cylinder around 70° from the leading stagnation point when the Reynolds number is approximately 40,000.

Periodic oscillations emanating from a slot located some 40° further downstream from the natural separation location altered very significantly the pressure distribution on that surface without affecting the location of separation on the opposite side or the pressure distribution on that opposite surface, apart from the base pressure. The cylinder started to lift and the typical vortex shedding from the cylinder was altered if not entirely eliminated.

Experiments were carried out using both pressure measurements and PIV, they were supplemented by CFD.

Unforced

Forced with periodic oscillations

Computed velocity vectors and pressure contours for the forced and unforced flow around a cylinder. 3D time-dependent FEM/LES at R_e=36,400.