Investigation on artificially forced instabilitaeten in a two-dimensional laminar boundary layer by means of Particle Image Velocimetry

Abstract

Artificially forced instabilities are studied in the region of linear amplificationby hot-wire measurements and stability calculations. Growth-rates, phase-velocities and perturbation- profiles are investigated experimentally and show a very good agreement with theoretical data. An arrangement for artifical excitation provides the possibility to generate the fundamental and the subharmonic type as well. For the study of boundary layer instabilities a new measurement technique, the Particle Image Velocimetry (PIV), is used in the transition region. The good agreement of the PIV results with data obtained by hot-wire measurements and calculations demonstrates for the first time the applicability of PIV for the investigation of boundary layer instabilities. Furthermore the PIV results show for a constant excitation level, that for the subharmonic type the transition process takes place at lower Reynolds numbers than for the fundamental type. Visualization pictures are obtained in the transition region by utilizing a laser light sheet method.