Semi-circular Rods Used to Control Turbulent Boundary Layer Separation at Cylindrical Surface

Abstract

Flows in the boundary layer are retarded due to skin friction at the body surface. This, in the case of an adverse pressure gradient being present in the flow, leads to boundary layer separation (airplane wings, engine diffusers, axial compressors). To prevent or at least delay this phenomenon, the streamwise momentum of air particles in the boundary layer should be continuously supplemented with the momentum of external flow. Numerous investigations conducted hitherto show that the streamwise vortices generated at the surface appear to be a favorable means of transverse streamwise momentum exchange. Due to such vortices, fluid particles with high momentum are swept toward the surface to mix with the retarded air at the surface, which in turn is swept away from the surface. The mean streamwise momentum of the fluid particles in the boundary layer is thereby increased. Various vortex generators were employed to induce partially helical flow at the boundary layer. Fixed solid vortex generators of both the vane and wing types were used over a long period because of their simplicity and low cost (1-7). They form an array of small plates or wings projected normally (in the former case) or parallel to the surface (in the latter case) at an angle of incidence to the flow.