The large variation of lift and drag around a lifting profile, as a consequence of the flow transition and/or separation, has been widely investigated in recent years. The accurate characterization of the flow separation and the laminar-turbulent transition evolution is a primary input for surface design and flow control, while accurate measurements resolved in space and time are still not available. To this aim, the skin-friction estimation provides useful insights in the understanding of laminar and turbulent boundary layer evolution and in the investigation of separation and laminar-turbulent transition. Here we deal this subject with an experimental approach whose focus is on the skin-friction data derived from time histories of temperature maps obtained using Temperature Sensitive Paint (TSP). We report about a NACA 0015 hydrofoil tested in a water cavitating tunnel (CEIMM facility) at angles of attack AoA between 1° and 10° and Re = 180000, observed at a frame rate of 1 kHz via a Photron SAX camera. A region of recirculating flow was identified confined between laminar separation and turbulent reattachment points, which is commonly referred to as Laminar Separation Bubble (LSB). We extract wall quantities which allow for a topological description of the flow at the wall and shed a light in complex interactions of flow structures with the surface.
The focus is placed on the topology of the coherent structures that appear in the reverse flow inside the recirculating region at different AoA. We conjecture that disturbances from the shear layer, which develop before the incoming laminar-turbulent transition, appear as impinging jets in the reverse flow region. Then, by adopting a potential vortex model as corresponding induced flow field and coupling it with a model of the flow in the reverse region, the raise, development and set of the associated coherent structures at the wall is described and compared with the corresponding skin friction topology extracted from temperature maps. Test cases are shown about the interaction of such impinging jets against the separation line at AoA = 10° and the development of omega-shaped, hairpin-like vortices in the reverse flow region at AoA = 3°. These flow structures appear opposite to the classical hairpin scheme of turbulent boundary layer, i.e. faster flow far from the wall is pushed down against the separation line towards the leading edge, thus inducing intermittent, highly energetic bursting phenomena involving the LSB dynamics. Further examples of coherent structures will be reported in the final contribution
