DISTORTION OF THE STAGNATION-POINT FLOW DUE TO CROSS-STREAM VORTICITYIN THE EXTERNAL FLOW

The structure of the stagnation-point flow in the presence of weak ste ady cross-stream vorticity in the external flow is investigated. A spe cific case of the two-dimensional basic forward stagnation-point flow past a circular cylinder is considered with the external three-dimensi onal vortical disturbance taken to be periodic in the spanwise directi on with a wavelength lambda less than or equal to lambda*(N) = pi D/( Re-D)(1/2) where D is the diameter of the cylinder and Re-D is the flo w Reynolds number. It is shown that the presence of weak but finite st reamwise vorticity, with lambda less than or equal to lambda*(N) in t he external flow, can be supported by the flow in the stagnation zone, leading to a substructure of counterrotating streamwise eddies in the boundary layer. The magnitude of the streamwise vorticity in the boun dary layer is found to match with that in the external flow for lambda less than or equal to lambda*(N); it is Of much smaller order for la mbda > lambda*(N), which corresponds to a disturbance of the type con sidered by Hammerlin (1955).