1 Dynamics of Separation Bubbles Formed on Rounded Edges

The front part of many bluff bodies contains rounded edges, which lead to the formation of separation bubbles. The separated flow inside these bubbles is highly unstable causing unsteadiness and turbulence. The ability to predict and control the dynamical behaviour of these separations is of great practical importance. The existence of separation bubbles on rounded front edges of bluff bodies contributes to the overall drag of the body. Its reduction is closely related to the control of the eddying wake flow caused by flow separation. While the influence of adverse pressure gradients on separation bubbles has been studied extensively, studies on the influence of flow geometry remain scarce. The aim of this study is to get a better understanding of the physical mechanisms which control this phenomenon on curved bodies Complementary measurements based on flow visualizations, P.I.V. technique and hot-film data acquisition are presented to analyse the dynamic behaviour of separation bubbles. Generic bluff-body models with variable front edge radius and aspect ratio were placed in the test-section of a low-speed water tunnel. The flow velocities of the order of 0.1 m/s allow the confrontation of the time-resolved P.I.V. measurements with the flow visualizations and hot-film measurements. All results confirm the strong influence of the front edge radius on the break-up length of the separation bubble. In addition the results show that the dynamic behaviour of the separation bubble depends on the aspect ratio of the bluff-body. Fo