ON DISTURBANCES EXCITED BY A POINT-SOURCE IN AN AXISYMMETRICAL LAMINAR SEPARATION BUBBLE

The generation and evolution of small-amplitude disturbances in a lami nar boundary-layer separation bubble downstream of an axisymmetric bac kward-facing step were investigated experimentally. The measured data were obtained for 3D-disturbances excited by a time-harmonic point sou rce placed on the surface of the axisymmetric body. The experimental r esults are compared with theoretical predictions based on linear stabi lity theory of parallel flow and on the excitation of 3D-disturbances by a time-harmonic Dirac point source at the wall of the axisymmetric body. Fairly good agreement was found between the experimental and the oretical results in the separation region. The data obtained downstrea m of the source position indicate that, in this plane, the main amplit ude characteristics of the wave-packet excited by the point source do not differ very much from those of the axisymmetric mode alone as gene rated by an external axisymmetric sound field. Furthermore, it seems t hat the exact location of the source is not very important for the amp lification of the wave-packet in the separation region.