VORTEX VISUALIZATION AND CHARACTERIZATION IN THE LAMINAR SEPARATION BUBBLE OVER A NACA 0018 AIRFOIL

摘要

Coherent structures that form in a laminar separation bubble over a NACA 0018 airfoil are studied experimentally using time-resolved flow visualization and embedded microphone measurements. Flow visualization reveals the formation of periodic vortices in the aft portion of the separation bubble. Surface pressure measurements synchronized with the visualizations show that these structures produce distinct surface pressure fluctuations, whose character is similar to that expected from a continuous train of near-wall vortices in potential flow. These vortices may merge, which alters the periodic pressure signal as the vortices and hence pressure minima merge, making it possible to detect the passage of individual vortices as well as vortex merging based solely on the analysis of surface pressure signals. Analysis of the experimental results and a complementary potential flow model demonstrates that the merging process is initiated due to the induced orbiting motion of the neighboring vortices, which is promoted by the mean velocity gradient in the shear layer. The results provide insight into vortex dynamics within laminar separation bubbles and provide basis for future experiments based solely on surface pressure measurements.