Unsteady Characteristics of Laminar Separation Bubbles; An Experimental and Numerical Investigation

Laminar separation bubbles may occur in a wide range of engineering applications such as turbomachinery flows, wind turbines, hydrofoils etc. Much attention has been given to their effect on the flow over airfoils because of the importance for an accurate prediction of lift, drag and heat transfer. In the aeronautical world, laminar separation bubbles have traditionally been of con- cern for the aerodynamics of sailplanes and of small aircrafts. However, recent measurements stress their importance for commercial transport aircrafts in slow flight, where laminar bubbles strongly affect the flow on high-lift devices. In spite of their importance for the global characteristics of the flow, the prediction of the behavior of laminar bubbles is still not satisfactory, especially close to stall conditions. The main objective of this thesis is to investigate the unsteady features of laminar separation bubbles with the ultimate goal of an improved prediction of their behavior during the design process, especially close to stall conditions. This objective was defined during the first part of the investigation, when the stalling behavior of a multi-element slotted high-lift configuration was analyzed. The experimental investigation of this multi-element configuration clearly showed the importance of a correct prediction of the bursting of the bubble present on the flap. This bursting determined in fact the stalling characteristics of the whole configuration. Existing design tools proved inadequate to the task of predicting bursting occurrence. Furthermore, Navier-Stokes calculations of the multi-element con¯guration showed that unsteady effects were crucial in order to predict successfully the behavior of the bubble