Analysis of the transition process around laminar separation bubbles (LSB‘s) in a towing tank using time-resolved 3D particle tracking techniques

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The performance of various lifting surfaces at relatively low Reynolds numbers is known to be affected significantly by laminar boundary layer separation. The examples of the relevant application range from man-made systems, such as turbofan engine blades and tail surfaces on commercial aircraft, wind turbine blades, as well as to nature’s flyers and swimmers. The laminar boundary layer separation typically takes place on the suction side of the airfoil and leads to the formation of a separated shear layer. The flow can then undergo rapid transition and be reattached to the airfoil surface in the mean sense forming a Laminar Separation Bubble (LSB). As LSB’s alter the circulation of the airfoil and consequently the lift and drag, the overall flow development therefore depends on the formation and dynamics of the LSB, which are the main focal points of the present proposal. Despite a significant research progress made to date towards understanding LSB dynamics, a number of critical gaps in knowledge of both fundamental importance remain to be addressed, which serves as a motivation for the present proposal. As the free stream turbulence level has a significant effect on the formation of LSB and the transition process, the experiments will be performed in a towing channel whose free stream turbulence can be varied between zero and two percent. Furthermore, sophisticated three-dimensional time-resolved flow measurement techniques will be used to resolve all flow processes in space and time with high special resolution and low measurement uncertainty. It is expected that the studies will significantly improve the current state of knowledge and that open research questions can be reliably answered.